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13Mon. Not. R. Astron. Soc.000, 1–19 () Printed 19 September 2018 (MN LATEX style file v2.2)

Flux calibration of the Herschel⋆-SPIRE photometer

G. J. Bendo1, M. J. Griffin2, J. J. Bock3, L. Conversi4, C. D. Dowell3, T. Lim5, N. Lu6,C. E. North2, A. Papageorgiou2, C. P. Pearson5,7, M. Pohlen8, E. T. Polehampton5,B. Schulz6, D. L. Shupe6, B. Sibthorpe9,10, L. D. Spencer2, B. M. Swinyard5,11,I. Valtchanov4, C. K. Xu61 UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road,Manchester M13 9PL, United Kingdom2 School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, United Kingdom3 NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA4 Herschel Science Centre, ESAC, ESA, PO Box 78, Villanueva dela Canada, 28691 Madrid, Spain5 Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom6 NASA Herschel Science Center, IPAC, 770 South Wilson Avenue, Pasadena, CA 91125, USA7 Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK8 Gemini Observatory, Northern Operations Center, 670 N. Aohoku Place, Hilo, HI 96720, USA9 UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, United Kingdom10 SRON Netherlands Institute for Space Research, NL-9747 AD Groningen, The Netherlands11 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom

ABSTRACTWe describe the procedure used to flux calibrate the three-band submillimetre photometer inthe Spectral and Photometric Imaging REceiver (SPIRE) instrument on theHerschelSpaceObservatory. This includes the equations describing the calibration scheme, a justification forusing Neptune as the primary calibration source, a description of the observations and dataprocessing procedures used to derive flux calibration parameters (for converting from voltageto flux density) for every bolometer in each array, an analysis of the error budget in the fluxcalibration for the individual bolometers, and tests of theflux calibration on observations ofprimary and secondary calibrators. The procedure for deriving the flux calibration parame-ters is divided into two parts. In the first part, we use observations of astronomical sources inconjunction with the operation of the photometer internal calibration source to derive the un-scaled derivatives of the flux calibration curves. To scale the calibration curves in Jy beam−1

V−1, we then use observations of Neptune in which the beam of eachbolometer is mappedusing Neptune observed in a very fine scan pattern. The total instrumental uncertainties inthe flux calibration for most individual bolometers is∼ 0.5%, although a few bolometershave uncertainties of∼ 1 − 5% because of issues with the Neptune observations. Based onapplication of the flux calibration parameters to Neptune observations performed using typi-cal scan map observing modes, we determined that measurements from each array as a wholehave instrumental uncertainties of 1.5%. This is considerably less than the absolute calibrationuncertainty associated with the model of Neptune, which is estimated at 4%.

Key words: instrumentation: photometers

1 INTRODUCTION

The Spectral and Photometric Imaging REceiver (SPIRE;Griffin et al. 2010), is one of the three instruments on theHerschelSpace Observatory (Pilbratt et al. 2010). It has separate photomet-

⋆ Herschel is an ESA space observatory with science instruments providedby European-led Principal Investigator consortia and withimportant partic-ipation from NASA.

ric and spectroscopic imaging sub-instruments that take advantageof Herschel’scapabilities for submillimetre observations. The pho-tometer has three individual arrays of feedhorn-coupled bolometersusing Neutron Transmutation Doped (NTD) germanium thermis-tors (Turner et al. 2001) cooled to approximately 0.3 K by an inter-nal helium-3 refrigerator. The arrays cover three broad passbandscentered at approximately 250, 350 and 500µm and contain 139,88, and 43 bolometers, respectively. The photometer is primarily

c© RAS

2 G. J. Bendo et al.

used in a scan map mode that produces images in a single observa-tion ranging from4× 4 arcmin to several square degrees.

A flux calibration method appropriate for broadband sub-millimetre photometric instruments is described in Griffinet al.(2013, submitted). This method involves the conversion of lin-earised bolometer voltage signals to monochromatic flux densitiesor sky surface brightness values using measurements of a calibra-tion standard and knowledge of the relevant instrument properties.The purpose of this paper is to describe the detailed implementationof this flux calibration scheme in the case of the SPIRE photometer.The resulting parameters describing the calibration curves are usedby the flux conversion module within the SPIRE photometer dataprocessing pipeline Griffin et al. (2008); Dowell et al. (2010).

Section 2 gives an overview of the equations (including non-linearity corrections) describing the conversion of the measuredbolometer output voltage to flux density. Section 3 providesinfor-mation on the use of Neptune as the primary flux standard, includ-ing the calculation of the Neptune flux density. Descriptions of theobservations that were performed to derive the terms in the fluxcalibration equations are given in Section 4, and Sections 5and 6provide details on the analysis used to derive the calibration param-eters for every bolometer in the SPIRE photometer arrays. Separatecalibration terms are derived for the two standard bias voltage set-tings used for photometer observations: the nominal settings (usedfor most observations) which are optimised for sources fainter than200 Jy beam−1, and the bright source settings, which are intendedto be used for sources brighter than 200 Jy beam−1. A discussionof problematic bolometers and how they were handled is giveninSection 7. Sources of uncertainty in the flux calibration curves arediscussed in Section 8, while Section 9 describes tests of the de-rived flux calibration curves using observations of primaryand sec-ondary calibration sources. Section 10 provides a detailedsummaryof the results from the assessment of the flux calibration. Through-out this paper, individual bolometers are referred to by their SPIREarray designations PSW, PMW, and PLW for Photometer (Short,Medium, and Long) Wavelength (with the wavelength correspond-ing to 250, 350, and 500µm, respectively) followed by row letterand column number (e.g., PSWE2).

2 CONVERSION OF BOLOMETER VOLTAGE TO FLUXDENSITY

The SPIRE detectors, being bolometers, respond not to the ab-sorbed photon rate but to the amount of power that they absorb.As noted by Griffin et al. (2013, submitted), the absorbed power isproportional to the spectral response function (SRF) weighted fluxdensity, which is the flux density weighted by the overall SRFandintegrated across the passband. This is given by

SMeas =

∫

νS(ν)F (ν)η(ν)dν∫

νF (ν)η(ν)dν

, (1)

where F (ν) is the SRF andη(ν) is the aperture efficiency.The SPIRE photometer SRFs, which were measured by Fouriertransform spectroscopy, are shown in Figure 7 of Griffin etal. (2013, submitted) and in the SPIRE Observers Manual(Herschel Space Observatory 2011)12, and they are also available

12 The SPIRE Observers’ Manual is available athttp://herschel.esac.esa.int/Docs/SPIRE/pdf/spireom.pdf .

in tabular form within the Herschel Interactive ProcessingEnviron-ment (HIPE; Ott 2010).

For NTD bolometers, the small-signal responsivity (variationof output voltage with absorbed radiant power) depends on the totalvoltage across the bolometer in a manner which is approximatelylinear over a wide range of background loading and bath tem-perature conditions (Griffin 2007)13 The relation between a smallchange in the in-beam SRF-weighted flux density,dSMeas, and thecorresponding change in the voltage across the bolometer,dV , canbe expressed as

dSMeas

dV= f(V ) (2)

wheref(V ) is allowed to depart from linearity. We find that thedifferential responsivity for the SPIRE bolometers can be well rep-resented by

f(V ) = K1 +K2

V −K3

, (3)

as is demonstrated in Section 5.2. In this equation,K1, K2, andK3 are constants specific to each bolometer in each bias voltagesetting.K1 has units of Jy V−1, K2 has units of Jy, andK3 hasunits of V. The conversion between a measured voltageVm andthe corresponding SRF-weighted flux density is obtained from theintegral off(V ) given by

SMeas =

∫ Vm

V0

f(V )dV, (4)

whereV0 is the operating point voltage (the signal that would bemeasured when viewing dark sky) andVm is the measured voltage.The result of this integral is

SMeas = K1(Vm − V0) +K2 ln(

Vm −K3

V0 −K3

)

. (5)

The SPIRE calibration scheme is thus based on deriving theK-parameters that describef(V ). For calibration off(V ), thephotometer internal calibration source (PCal; Pisano et al. 2005)is used to provide a repeatable small change in power illuminatingthe detector. The inverse of the corresponding detector response,∆VP , for a given bolometer voltage,V , is directly proportional tof(V ). The relation can be written as

1

∆VP (V )= Af(V ) = AK1 +

AK2

V −K3

, (6)

whereA is a constant. The shape of the functionf(V ) can thusbe determined by observing changes in the signal from uniformPCal flashes while varying the effective bolometer operating pointvoltageV . This could be done by changing the temperature of thehelium-3 bath, but for SPIRE, it is more straightforward to varyV by viewing regions with different in-beam flux densities. Itisimportant to note that in this exercise that the flux density does notactually need to be known; the purpose of exposing the detectorto a range of source brightnesses is merely to vary the operatingpoint voltage of the detectors over the range of interest so as tocharacterisef(V ).

Having determined the shape off(V ) from these PCal mea-surements, the absolute value of the constantA can be found fromobservations of a source with a known SRF-weighted flux density,

13 This document is available at http://herschel.esac.esa.int/twiki/pub/Public/SpireCalibrationWeb/SPIREDetectorParameterSensitivity Issue1 Nov 14 2007.pdf .

c© RAS, MNRAS000, 1–19

Flux calibration of the Herschel-SPIRE photometer3

Table 1.Values ofKMonP

Array λ0 KMonP

(µm)

PSW 250 1.0102PMW 350 1.0095PLW 500 1.0056

which then allows for determining the numerical values of theK-parameters. Following Equation 1, the SRF-weighted flux densityfor a calibration source with a spectrumSC(ν) is given by

SC = KBeam(ν)

[

∫

νSC(ν)F (ν)η(ν)dν)∫

νF (ν)η(ν)dν

]

, (7)

whereKBeam(ν) is a correction factor for possible partial resolu-tion of the calibrator by the telescope beam.

The equation relating the SRF-weighted flux density of thecalibration source,SC , to the scaling termA can be written as

A =1

SC

∫ VON

VOFF

1

∆VP (V )dV (8)

whereVON is the voltage measured when the bolometer is pointedat the calibrator andVOFF is the voltage measured off-source.

After determining theK-parameters in this way, the measuredSRF-weighted flux density for an unknown source can be foundfrom the corresponding measured bolometer voltage. Conversionof the SRF-weighted flux density to a monochromatic flux den-sity requires a choice of the frequencyν0 at which the flux densityis to be calculated, knowledge of the instrument SRF, an assump-tion concerning the source spectral shape, and the stipulation thatthe source and calibrator have the same spatial characteristics toensure that the conversion from in-beam flux density to absorbeddetector power is the same for the source and calibration observa-tions. When adopting a point-like source as the calibrator,as is thecase for SPIRE, it is also appropriate to quote source flux densitiesbased on the assumption that the source is also point-like.

The convention adopted forHerschelis to quote monochro-matic flux density values under the assumption thatνS(ν) is con-stant (i.e. the spectral indexα is -1). The conversion between theSRF-weighted flux density and the monochromatic flux densityS(ν0) is given by Griffin et al. (2013, submitted) as

S(ν0) = KMonP (α, ν0)SMeas, (9)

whereKMonP is computed forα = −1 and the chosen value ofν0 using

KMonP (α, ν0) =

∫

νF (ν)η(ν)dν

∫

ν

(

νν0

)αF (ν)η(ν)dν

. (10)

Values forKMonP and the wavelengthsλo corresponding toνo aregiven in Table 1.

For SPIRE, the flux densities produced by the data pipelineare quoted at frequencies corresponding to standard wavelengths of250, 350, and 500µm. The emission from most sources observedby SPIRE is from dust, and the continuum spectrum is not welldescribed by a spectral index of -1. The derivation of the neces-sary colour correction functions to account for the different spec-tral shapes is described by Griffin et al. (2013, submitted),andthe functions for SPIRE are given in the SPIRE Observers’ Man-ual (Herschel Space Observatory 2011). The accurate calibration

of extended and semi-extended emission also requires a detailedknowledge of the beam profile and aperture efficiency, and this isalso discussed by Griffin et al. (2013, submitted) and in the SPIREObserver’s Manual.

3 PRIMARY CALIBRATION STANDARD (NEPTUNE)

The planet Neptune is used as the primary calibration sourcefor theSPIRE photometer. It has a well-understood submillimetre spec-trum, it is almost completely point-like in the SPIRE beams (theangular diameter of Neptune is∼ 2 arcsec, while the 250, 350, and500µm arrays have full-width at half-maxima (FWHM) of 18, 25,and 36 arcsec, respectively), and it is sufficiently bright to providehigh S/N when observed but not so bright as to introduce any sig-nificant non-linear response from the detectors. We adopt a modelof the disk-averaged brightness temperature spectrum of Neptunebased on the ESA-4 version of the planetary atmosphere modelfirstpublished by Moreno (1998)14. The absolute photometric uncer-tainty of this model is±4% (R. Moreno, private communication),with the true brightness temperature being enclosed withinthosebounds. The absolute uncertainty is mainly attributed to the uncer-tainty in molecular absorption coefficients (∼ 1%) and the adoptedtemperature structure (∼ 3%). The Neptune brightness temperaturespectrum in the SPIRE range is shown in Figure 1. The absorptionfeatures are caused by CO, and the emission lines are caused byHCN.

To calculate the solid angle of Neptune for a given observationwe use an equatorial radius,req of 24,766 km and polar radiusrpof 24,342 km, both of which are based on the analysis of Voyagerdata by Lindal (1992). These are similar to the values used intheground-based observations by Hildebrand et al. (1985), Orton et al.(1986), and Griffin & Orton (1993).

In calculating the planetary angular sizes and solid angles, acorrection is applied for the inclination of the planet’s axis at thetime of observation. The apparent polar radiusrp−a is given byMarth (1897) as

rp−a = req[

1− e2 cos2(φ)] 1

2 . (11)

In this equation,φ is the latitude of the sub-Herschelpoint, andeis the planet’s eccentricity, which can be calculated using

e =

[

r2eq − r2pr2p

] 1

2

. (12)

The observed planetary disc is taken to have a geometric meanra-dius,rgm, given by

rgm = [reqrp−a]1

2 . (13)

TheHerschel-planet distance, is obtained from the NASA JPLHorizons ephemeris system (Giorgini et al. 1996)15, and the ob-served angular radius and solid angle are calculated accordingly.The Neptune flux density spectrum,SC(ν) at theHerscheltele-scope aperture is computed from the solid angle and the disk-averaged brightness temperature spectrum. Over a period ofoneyear, the brightness of Neptune exhibits±7% variations due to theseasonally varyingHerschel-Neptune distance. However, because

14 The ESA-4 models for Uranus and Neptune are available atftp://ftp.sciops.esa.int/pub/hsc-calibration/PlanetaryModels/ESA4/ .15 The ephemeris can be accessed at http://ssd.jpl.nasa.gov/?horizons .

c© RAS, MNRAS000, 1–19

4 G. J. Bendo et al.

60

70

80

90

TB (

K)

300 600 900 1200 1500 1800ν (GHz)

1000 500 400 300 200λ (µm)

Figure 1. The disk-averaged model brightness temperature of Neptuneas a function of frequency and wavelength.

Neptune is only observable when it crosses through one of twovis-ibility windows, Neptune varied by only±2% in brightness whenit was observable byHerschel. Typical flux densities at the nominalSPIRE wavelengths are 160, 100, and 60 Jy at 250, 350, and 500µm, respectively. Figure 2 shows the model spectrum of Neptuneon Herscheloperational day (OD) 168 along with the SRFs of thethree SPIRE photometer bands. The slope of the Neptunian submil-limetre continuum is somewhat less steep than that of a blackbodybecause of the increase in brightness temperature with decreasingfrequency, with lower frequencies probing deeper and warmer partsof the troposphere. The continuum spectral indices (α as given byS(ν) ∝ να, averaged across the bands) are 1.26, 1.39, and 1.45 forthe 250, 350, and 500µm bands, respectively.

4 OVERVIEW OF DERIVATION OF CALIBRATIONTERMS

Following the procedure outlined in Section 2, the derivation of theflux calibration was broken into two steps. The first step was toidentify the shape off(V ) as given by Equation 3. In these ob-servations, the telescope stared at a series of regions withdifferentsurface brightness values (thus providing a range of in-beam fluxdensities). With the stationary telescope position providing a fixedphotometric background, and therefore a fixed operating point volt-age, PCal flashes were applied to generate a small additionalmod-ulation of the detector signal. This gave both a measurementof thevoltage (V ) and of the change in voltage relative to a small changein signal (dSMeas/dV ). For the reason explained in Section 2, itwas important that the observations sample a series of regions withwidely varying surface brightnesses to provide a sufficiently widerange of operating voltages. We used observations of brightex-tended emission in Sgr A as well as several nearby backgroundregions to derive the unscaled calibration curves for the nominalbias detector settings (which are used for almost all observations).For the bright source mode, which is used for only a few sourcesbrighter than∼ 200 Jy beam−1, we used pointings near Sgr B2.

Further details on the observations and data analysis are given inSection 5.

Observations of the primary calibration source Neptune byeach and every bolometer were needed to set the absolute scale forthe calibration parameters. We observed the planet using a special“fine scan” map mode in which every bolometer was scanned overthe target with a finely-spaced grid. We then fitted two-dimensionalGaussian functions to the timeline data for each bolometer to deter-mine the peak voltage (the difference between the on-targetvoltageVON and the off-target voltageVOFF ) measured by each bolome-ter when centered on Neptune. These peak voltages were then usedalong with the Neptune model flux densities to scale the flux cali-bration curves. Details are given in Section 6.

The measured flux density depends on the value ofV0 that isused. IdeallyV0, which corresponds to the bolometer voltage whenviewing blank sky, would be a constant over the life of the mis-sion. However most of the radiant power incident on the SPIREbolometers is from the warm telescope, and because the telescopetemperature varies by several K depending on the season and thesolar aspect angle, unique values ofV0 cannot be defined. For theSPIRE pipeline we have adopted a nominal set of values based ondark sky observations early in the mission and with a representa-tive telescope temperature. TheV0 values for the nominal modewere derived for each bolometer from the median voltages mea-sured in dark sky observation 1342182454, which were performedduring Operational Day (OD) 98 (19 Aug 2009), and theV0 val-ues for the bright source mode were derived from data taken duringobservation 1342185829 on OD 153 (14 Oct 2009). SPIRE datataken in other observations normally have slightly different temper-atures and therefore may have photometric offsets, so SPIREdataalone cannot be used to make absolute estimates of the sky inten-sity. These offsets are ultimately subtracted off in the mapmakingprocedure.

c© RAS, MNRAS000, 1–19

Flux calibration of the Herschel-SPIRE photometer5

0

50

100

150

200

250

S (

Jy)

0.0

0.2

0.4

0.6

0.8

1.0

SR

F

300 600 900 1200 1500 1800ν (GHz)

1000 500 400 300 200λ (µm)

Figure 2. The model spectrum of Neptune on OD 168 (29 Oct 2009) in black along with the SRFs of the three SPIRE photometer bands (fromftp://ftp.sciops.esa.int/pub/hsc-calibration/SPIRE/PHOT/Filters/), with the blue, green, and red curves corresponding to the 250, 350, and 500µm bands,respectively.

5 DEFINING THE SHAPE OF UNSCALED FLUXCALIBRATION CURVE

5.1 Observing procedure

To determine the unscaled versions of theK parameters, we per-formed a series of staring observations on fields with varying sur-face brightness levels as PCal flashes are applied. During each 1min observation, PCal was alternately turned on for 1.5 sec andoff for 1.5 sec for a total of 20 cycles. For the nominal detectorsettings, the targets for these pointings consisted of a grid of loca-tions centered on Sgr A and an additional strip of pointings runningroughly perpendicular to the plane of the Galaxy as shown in Fig-ure 3. The locations were selected using estimates of the expectedsurface brightness in the SPIRE bands based on the Caltech Sub-millimeter Observatory 350µm data from Bally et al. (2010). Thepointings around Sgr A provided data for the high signal end of thecalibration curve, while the strip to the northwest provided data forlower signals. The observations were planned taking into accountthat, when the target was visible, the4 × 8 arcmin SPIRE arrayswere oriented so that the long (y) axis was approximately alignedwith right ascension and the short (z) axis aligned with declination.A key requirement of these observations was to point every detec-tor at least once at the region where the signal is>

∼75% of the peak

surface brightness of Sgr A. This region is roughly0.75 × 2 ar-cmin in size with its major axis aligned slightly counter-clockwiseof north-south. Based on simulating observations of this region, weadopted a grid of observations consisting of 7 columns spaced by36 arcsec(the distance in the z direction between two rows inthePMW array) and 6 rows spaced by 42 arcsec. In the strip of point-ings to the north of Sgr A, we used 20 pointings spaced by 36 arc-sec. In the total set of observations, the surface brightness from thebackground measured by each bolometer was predicted to varybya factor of∼ 10 and to approximately equal or exceed the surfacebrightness of Neptune at the high end of the range.

For the bright detector settings, the targets consisted of agrid

of locations centered on Sgr B2 as shown in Figure 4. These lo-cations were also selected using estimates of the expected surfacebrightness in the SPIRE wavebands based on CSO 350µm datafrom Bally et al. (2010). We took into account that, when Sgr B2was visible, the array would be oriented so that the long axiswouldbe aligned with right ascension and the short axis would be alignedwith declination, just as was the case with Sgr A. As with the Sgr Aobservations, it was necessary to point every detector at least onceat the0.6 × 1.4 arcmin region with a surface brightness that is atleast 75% the peak surface brightness of Sgr B2. We used a gridof pointings identical to that used for Sgr A. During these observa-tions, the in-beam flux density was expected to vary by a factor of35-40 for the typical bolometer in each array.

These observations of Sgr A and Sgr B2 were performed onOD 153 (14 Oct 2009). In addition, we also used all other usablecalibration data taken between OD 116 (07 Sep 2009; the first datewhen usable observations with PCal flashes had been performedafter the current bias voltage levels were set for the nominal andbright source modes) and OD 424 (12 Jul 2010; the last date whenobservations with PCal flashes were performed). This included ob-servations of dark sky and observations in which point-likecalibra-tion sources (Mars, Uranus, Neptune, 3 Juno, 4 Vesta, Alpha Boo,Gamma Dra, and VY CMa) were observed for various calibrationtests. These data were used to constrain the low signal end ofthecalibration curves.

5.2 Derivation of the unscaled flux calibration curve

Examples of the PCal flash data obtained in the nominal and brightsource voltage modes are shown in Figures 5 and 6. In the nominalmode data, the PCal flash provides a change in voltage which islarge compared to the noise level but represents a small change inV (approximately a 0.03 mV change to a 3.2 mV measurement). Inthe bright source mode data, the magnitude of the PCal flash maybe equivalent to or smaller than the variations in the background

c© RAS, MNRAS000, 1–19

6 G. J. Bendo et al.

17:47:00 17:46:00 17:45:00Right Ascension (J2000)

-29:20

-29:10

-29:00

-28:50

Dec

linat

ion

(J20

00)

101 102

Jy/beam

Figure 3. The SPIRE 250µm image of Sgr A∗ from Herschelwith green8× 4 arcmin regions overlaid to show the locations where the SPIRE arrays pointedduring the nominal voltage mode observations with PCal flashes. The image was created with data from parallel-mode scan map observations 1342204102and 1342204103 that were processed using a modified version of the standard parallel data processing pipeline and the fluxcalibration parameters that werederived in this paper. The displayed area is40× 40 arcmin with north up and east to the left.

signal, as shown in the example. This occurred mainly in the cen-tre of Sgr B2, which is very bright compared to PCal but relativelycompact. Pointing jitter, even though at a low level, resulted in vari-ations in the background signal that were large relative to PCal.

The techniques used to measure the change in signal∆V at abackground signalV for both the nominal and bright source volt-age modes were similar. The steps described here were appliedto the signal from each bolometer in each observation with PCalflashes. We first fitted lines to each segment of the square wave.The gaps between the lines fitted to sequential steps in the squarewave gives us individual∆V measurements. We then calculated

the mean and standard deviation of∆V from each observation foreach bolometer after removing statistical outliers (data more than5σ from the mean). The correspondingV values were obtained indifferent ways for each voltage bias mode. For the nominal modedata, we measured the mean and standard deviation inV using allof the data for each bolometer in each observation, as the relativelysmall variation in the signal induced by the PCal flash ultimately re-sulted in a relatively low standard deviation inV . In the case of thebright source mode data, we used the midpoint between the steps inthe square wave as individualV measurements and then calculated

c© RAS, MNRAS000, 1–19

Flux calibration of the Herschel-SPIRE photometer7

17:48:00 17:47:30 17:47:00Right Ascension (J2000)

-28:30

-28:25

-28:20

-28:15

Dec

linat

ion

(J20

00)

102 103

Jy/beam

Figure 4. The SPIRE 250µm image of Sgr B2 fromHerschelwith green8× 4 arcmin regions overlaid to show the locations where the SPIRE arrays pointedduring the bright source mode observations with PCal flashes. The image was created with data from parallel-mode scan mapobservations 1342184474 and1342184475 that were processed using a modified version of the standard parallel data processing pipeline and the flux calibration parameters that were derivedin this paper. The displayed area is20× 20 arcmin with north up and east to the left.

the mean and standard deviation ofV for each bolometer in eachobservation.

We then fitted Equation 3 to theV and∆V data. In the nom-inal voltage data, we excluded data with unusually high uncertain-ties in ∆V (> 10−3 mV), which may originate from instanceswhere the prior statistical filtering steps have not succeeded in re-moving data containing glitches. Example plots, includingbest fit-ting functions, are shown in Figure 7. The dotted lines show theaverage on-source and off-source voltage levels measured for Nep-tune in the fine scan observations described in Section 6.

Table 2 gives the typical signal range in Jy beam−1 over

which the calibration applies (calculated after performing the scal-ing steps in Section 6 first). The ranges were calculated in two steps.First, we determined for each bolometer the minimum and maxi-mum signals measured during the observations with PCal flashes.Then for each array, we determined the median of the maxima andthe median of the minima. The flux calibration should be well-constrained within these signal ranges, but measurements outsideof these ranges should be treated with caution. Note that therangeextends below 0 Jy beam−1. This is because the background signalin dark sky may sometimes drop below below 0 as a result of ther-mal drift in the telescope and instrument; minimum values below

c© RAS, MNRAS000, 1–19

8 G. J. Bendo et al.

0 20 40 60Time (s)

3.19

3.20

3.21

3.22

3.23V

olta

ge (

mV

)

PSWE2 (nominal)

Figure 5. An example of an observation with PCal flashes in the nominal voltage mode. These are data taken by bolometer PSWE2 in a staring observation ofSgr A* with ID 1342185872. The black squares show the individual measurements; the grey line joining the squares is used to aid in the visualisation of howthe signal varied over time. Higher voltage values correspond to the signal measured when PCal was off, while lower voltage values correspond to the signalobserved when PCal was on. The dashed red line shows the mean voltage measured during the observations ((3.2109 ± 0.0004) × 10−3 V), and the solidred lines show the functions fitted to the timeline segments when PCal was on or off. The∆V for these data was measured as−(2.821± 0.003) × 10−5 V.

0 20 40 60Time (s)

1.148

1.150

1.152

1.154

Vol

tage

(m

V)

PSWE2 (bright)

Figure 6. An example of an observation with PCal flashes in the bright source mode. These are data taken by bolometer PSWE2 in a staring observation ofSgr B2 with ID 1342185949. The black squares show the individual measurements; the grey line joining the squares is used to aid in the visualisation of howthe signal varied over time. Higher voltage values correspond to the signal measured when PCal was off, while lower voltage values correspond to the signalobserved when PCal was on. The solid red lines show the functions fitted to each segment of the square wave during the observations. The mean voltage anduncertainty measured during the observations, based on themeasured voltages for each step in the signal, is(1.1515 ± 0.0004) × 10−3 V). The mean anduncertainty in the∆V for these data were measured as−(3.18± 0.04)× 10−6 V.

c© RAS, MNRAS000, 1–19

Flux calibration of the Herschel-SPIRE photometer9

2.6 2.8 3.0 3.2 3.4V (mV)

-50

-40

-301/

∆V (

1/m

V)

PSWE2 (nominal)

0.8 0.9 1.0 1.1 1.2 1.3V (mV)

-500

-400

-300

1/∆V

(1/

mV

)

PSWE2 (bright)

Figure 7. PCal flash data for PSWE2. Data are shown for both bias modes.Lower voltages correspond to viewing brighter regions of sky. The solidline shows the function described by Equation 3 that was fitted to the data.The dotted lines show the mean peak and background signal measured forNeptune in the fine scan observations described in Section 6.

Table 2.Representative signal ranges from zero point covered by PCal flashdata (in Jy beam−1)

Array Nominal Mode Bright Source ModeMinimum Maximum Minimum Maximum

Values Values Values Values

PSW -20 400 -30 2710PMW -20 360 -30 2200PLW -40 270 -50 1650

0 arise from these variations in the telescope background. Usersshould attach no physical interpretation to background signals be-low 0 measured in SPIRE data, as SPIRE is only designed to mea-sure signals relative to the background within fields and nottheabsolute sky brightness.

6 SCALING THE FLUX CALIBRATION CURVE

6.1 Observing procedure

In the Neptune fine scan observations, each bolometer was scannedover the planet in a series of parallel scan legs with∼ 2− 3 arcsecoffsets from each other. Scans were performed in four directions:two scans were aligned with the y-axis in the instrument plane,and two scans aligned with the z-axis in the instrument plane. Al-together, the centre of Neptune passed within∼ 1 arcsec of eachdetector four times. The nominal voltage mode observations(ob-servation numbers 1342186522-1342186525) were performedonOD 168 (29 Oct 2009), while the bright source mode observations(observation numbers 1342187438, 1342187439, 1342187507, and1342187508) were performed on OD 201-202 (01-02 Dec 2009).

6.2 Measuring the peak signal in the timeline data

To measure the peak signal from Neptune and the backgroundsignal, we used a Levenberg-Marquardt algorithm to fit two-dimensional elliptical Gaussian functions to the timelinedata takenduring each fine scan observation by each bolometer. The advan-tage that timeline-based beam fitting has over map-based beam fit-ting is that the timeline data contain the signal measurements assampled at their precise pointed positions whereas the map data areaffected by smearing effects related to pixelisation. The limitingfactor in the positional accuracy of the timeline measurements isthe relative pointing uncertainty of the telescope, which is <

∼2 arc-

sec in the data used for this analysis (Pilbratt et al. 2010).In con-trast, when the timeline data are converted into maps, the signalfrom an individual sample is assigned to a square pixel area (or, asis the case with some mapmakers using drizzle methods, the signalis divided over multiple pixels), so some spatial information is lost.Moreover, measurements passing across the edges of map pixelsare effectively shifted in position to the centres of the mappix-els. Hence, mapmaking has the effect of smoothing the data, whichsuppresses the peak signal from unresolved sources. In using thetimeline data, we avoid these smoothing effects, thus allowing formore precise measurements of the peak signals.

We performed beam fitting on the data from each fine scanobservation for each individual bolometer. We first selected all datafrom all bolometer samples that fell within a target radius and back-ground annular area based on central locations selected from mapdata. The position of the target aperture did not need to be accu-rately defined, as tests with this method have demonstrated thatthe central position can be offset from the target position by halfof the beam FWHM and still produce statistically similar results.After the data for the fit were selected, the central positionof thesource was treated as a free parameter in the fit. The target radiuswas set to extend to the location of the minima between the peakand the first diffraction ring of the beam profile, thus encompassingthe central part of the beam that most closely resembles a Gaussianfunction. The radii were 22, 30, and 42 arcsec for the 250, 350, and500µm arrays, respectively. The background annulus was set to be350 to 400 arcsec in radius, which is large enough that signalfromthe outer diffraction rings is not detectable within the area. In thenominal bias setting analysis, we used data from all scan legs thatfell within the target and background apertures. In the bright sourcemode, however, we found significant drift in the background signal,which caused problems when attempting to fit the background.Inthat case, we used all data points that fell within the targetaperture,but we only selected data falling in the background aperturefromscan legs that also passed through the target aperture.

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10 G. J. Bendo et al.

After selecting the data to be fit, we used the data in the back-ground annulus to measure and subtract an initial background. Wethen fitted a two dimensional elliptical Gaussian function to the datain both the target aperture and the background annulus. The sevenfree parameters in this fit were the peak voltage (the difference be-tween the on- and off-source voltages, which is a negative valuebecause an increase in signal corresponds to a decrease in voltage),the right ascension and declination of the peak position, the majorand minor axes of the beam, the position angle of the axes, andthebackground.

Example radial profiles of the data from within the target aper-tures and fits to those data for the nominal and bright source modedata are shown in Figures 8 and 9. Although the beam is well fit-ted by Gaussian functions in these examples, the data may deviatefrom the fit at the∼ 1% level near the peak of the beam. We alsoexplored using alternate functions (e.g. sinc2 functions, polynomialfunctions) to determine the peak of the beam, but we found that thepeak signal varied by∼ 1% when slight alterations were made tothe fitting method (such as changing the radius within which datawere selected and adjusting the exact shape of the fitted function).Given the simplicity of using Gaussian functions and the lack ofany advantages (in particular, the lack of any improvement in accu-racy) in using other functions to fit the data, we based the calibra-tion on fitting Gaussian functions.

For each bolometer in each bias voltage setting, we generallyobtained a set of four Neptune peak voltage measurements withcorresponding background voltage measurements. We used the un-scaledK-parameters from the PCal flash analysis in Section 5.2 inEquation 5 to determine the change in voltage between the back-ground and Neptune for each bolometer for each observation.Toproduce the scaling termsA, we then divided the measured changein signal by the Neptune model flux densities (with beam correc-tions applied to account for the finite angular size of Neptune) givenin Table 3 and by theKMonP values given in Table 1. For eachbolometer in each voltage mode, the unscaledK1 andK2 param-eters are divided by the mean of theA terms from the four obser-vations, and the standard deviation in the fourA terms is used tocalculate the uncertainties in theK1 andK2 parameters. Figure 10shows examples of the resulting scaled calibration curves from thenominal and bright source modes. See Section 8.2 for the analysison the fractional uncertainties related to the scaling terms.

6.2.1 Truncated detector signals in Neptune fine scan data

In the nominal voltage bias data, 17 detectors in the PSW array and6 detectors in the PMW array measured signals from Neptune thatwere truncated in at least one of the observations. In the SPIREon-board electronics, an individual voltage offset is applied to eachbolometer signal to ensure that its output voltage remains withinthe range of the Analogue to Digital Converter (ADC). The offsetsare fixed at the start of each observation and depend on the exacttelescope background and sky brightness. Neptune is sufficientlybright that its large signal level drove a few bolometer voltage levelsoutside the ADC range, leading to signal truncation. In the analysisof these fine scan data, we needed to alter our analysis for thesetruncated bolometers.

We examined the effects of the signal truncation by experi-menting with how the peak signal measured by the “good” bolome-ters changed as we progressively removed data from the timelines(with “good” bolometers defined as those where the Neptune time-lines are not truncated in any of the nominal-voltage fine scan ob-servations and where the bolometers are not identified as problem-

0 5 10 15 20 25Radius (arcsec)

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Figure 8. Examples of the signal measured from Neptune in the fine scanobservation 1342186522, which used the nominal voltage bias setting. Thered line shows the best fitting function to the data. Data falling within thebackground annulus were included in the fit but are not included in this plot.

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Table 3.Neptune flux densities on dates of fine scan observations

Operation Observations Voltage SRF-weighted Flux Densities Beam Correction Factorsa SRF-weighted Flux DensitiesDay Bias Mode without Beam Correction (Jy) with Beam Correction (Jy)

250µm 350µm 500µm 250µm 350µm 500µm 250µm 350µm 500µm

168 (29 Oct 2009) 1342186522 nominal 163.48 102.85 61.26 0.99419 0.99684 0.99853 162.53 102.52 61.17134218652313421865241342186525

201 (01 Dec 2009) 1342187507 bright 157.45 99.06 59.00 0.99440 0.99696 0.99859 156.57 98.75 58.921342187508

202 (02 Dec 2009) 1342187438 bright 157.28 98.95 58.94 0.99441 0.99696 0.99859 156.40 98.65 58.851342187439

a The beam correction factors account for the finite angular size of Neptune and are based on a Gaussian main beam coupling tothe emission from planetarydisc (Ulich & Haas 1976).

atic as described in Section 7). For each “good” bolometer ineachnominal voltage observation, we first fitted all of the data, then ar-tificially truncated the signal by removing data above a set frac-tion Ractual of the best fitting peak voltage value, and then we re-peated the fit. The functions fitted to the data were elliptical Gaus-sian function where the axes and position angle were fixed to themean values measured for the individual bolometers in the brightsource mode timeline data16. We found that fixing the dimensionsof the beam in this analysis produced the results with the lowestvariance. We then calculated the mean and standard deviation inthe ratio of the best fitting peak voltage for the truncated data com-pared to the value measured when all data were used. These ratiosare the correction factors for the peak fits to bolometers that sat-urate on Neptune. As the fitted peak value may change when thedata are removed from the peak, we also calculated the ratio of thelowest voltage values (which correspond to the highest signal val-ues) in the fitted data to the peak voltage derived from the fit tothose data (which we label asRmeas). Also note that the results ofthis analysis apply specifically to the elliptical Gaussianfunctionsfit to the beams and may not necessarily be applicable when otherfunctions are fit to the data.

Table 4 lists the correction factors derived using the aboveanalyses. The peak voltage measurements from the truncateddatawere divided by correction factors that were found by interpolatingbetween the values listed in Table 4. Table 5 lists the meanRmeas

values measured in the fine scan observations for each bolometerwhere the Neptune signal is truncated as well as the mean cor-rection that is applied for each bolometer. Because offset settingssometimes changed between fine scan observations, not all oftheNeptune data from the bolometers listed in Table 5 were truncatedin all of the observations. These specific cases have been flagged inthe table. Additionally, data from PMWB4, PMWC7, and PMWB8were each completely truncated in one of the fine scan observa-tions. In these cases, we ignored data for any bolometer fromanyobservation in which all of the data were truncated, appliedcorrec-tions when only the peak was truncated, and used the direct peakmeasurements when the data are not truncated.

16 To assess the appropriateness of using bright source mode beam dimen-sions to represent nominal mode beams in both the PSW and PMW data,we compared the bright source and nominal voltage mode measurementsof the major axis, minor axis, and position angle for each non-problematicbolometer where the signal from Neptune was not truncated. The major andminor axes typically vary by<∼2 % between the modes, and the positionangles of the axes only vary by<∼10◦.

Table 4.Correction factors for peak voltagea measurements for bolometerswhere the Neptune fine scan data is truncated

Array Ractualb Correctionc Rmeas

d

PSW 0.900 1.012 ± 0.005 0.8900.800 1.017 ± 0.007 0.7860.700 1.023 ± 0.009 0.6840.600 1.025 ± 0.011 0.5860.500 1.018 ± 0.014 0.4910.400 1.005 ± 0.017 0.3980.300 0.977 ± 0.021 0.3070.200 0.929 ± 0.030 0.215

PMW 0.900 1.008 ± 0.007 0.8930.800 1.012 ± 0.011 0.7900.700 1.014 ± 0.018 0.6900.600 1.012 ± 0.020 0.5930.500 1.006 ± 0.024 0.4970.400 0.991 ± 0.029 0.4040.300 0.963 ± 0.033 0.312

a The peak voltage is defined as the difference between the on-target andoff-target voltages.b In the “good” bolometer data used to derive the corrections in this table,Ractual is the ratio of the lowest voltage values used for fitting the data tothe best fitting peak voltage (when all data were used).c The best fitting peak voltage measurement from the truncatedsignal datashould be divided by these correction factors.d Rmeas represents the ratio between the lowest voltage values usedforfitting the data to the peak voltage determined from fitting those data.

7 PROBLEMATIC BOLOMETERS

For some bolometers that have been identified as dead, noisy,orslow, it was still possible to measureV and∆V values from thePCal flash data and the amplitude of the voltage from the Neptunefine scan data, which allows us to derive calibration curves.How-ever, for others, we lacked usable measurements from eitherthePCal flash data or the fine scan data, and so we cannot create empir-ical calibration curves for these bolometers. For these bolometers,we needed to insert alternateK values as placeholders into the fluxcalibration table to avoid problems when executing the flux calibra-tion software. We used the values derived from the bolometermod-els that are listed in version 2-3 of the flux calibration table, whichare derived from models of the bolometer responsivities readjustedbased on early observations of Ceres and Alpha Boo. Table 6 liststhe bolometers that are labelled as dead, noisy, or slow as well as

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V)

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Figure 9. Examples of the signal measured from Neptune in the fine scanobservation 1342187438, which used the bright source bias setting. Thered line shows the best fitting function to the data. Data falling within thebackground annulus were included in the fit but are not included in this plot.

2.6 2.8 3.0 3.2 3.4V (mV)

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eam

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Figure 10. Calibration curves for PSWE2. Data are shown for both biasmodes.

an indication of whether empirical calibration curves could be de-rived for the bolometers. Separate attempts were made to calibratebolometers in the nominal and bright source voltage modes, but weobtained identical results in terms of being able to derive calibra-tion curves.

All of the subsequent tests that we performed with the newflux calibration values did not include any of the bolometersin Ta-ble 6. They are currently flagged as bad bolometers in the timelinedata and are not used by default in the mapmaking, and so it wouldbe inappropriate to include them in the tests.

8 CALIBRATION UNCERTAINTY BUDGET FORINDIVIDUAL BOLOMETERS

The conversion between detector signal and astronomical signalfor individual bolometers in the SPIRE photometer arrays involvesfour sources of uncertainty. Two of these sources of uncertainty aresystematic effects across all bolometers in each array and are in-dependent of the measurements used to derive theK-parameters.These systematic effects are the absolute uncertainty in the Nep-tune model flux density and the uncertainty related to uncertaintiesin the SPIRE SRFs. The uncertainty in the Neptune model flux den-sity is 4%. The uncertainty related to the SPIRE SRFs is dominatedby uncertainties in the positions of the band edges; uncertainties inthe shapes of the SRFs have less of an effect. Griffin et al. (2013,submitted) determined that the uncertainty in the scaling of the in-

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Table 5.Bolometers with truncated Neptune signal data in nominal voltagemode

Bolometer Mean MeanRmeas

a Correctionb

PSWA4 0.200 0.919PSWB5 0.544 1.023PSWB11 0.844 1.014PSWC4 0.787 1.017PSWC6 0.621 1.025PSWD2 0.394 1.004PSWD4 0.853 1.014PSWD12 0.641 1.025PSWE6 0.815 1.015PSWE10 0.435 1.011PSWE14 0.298 0.973PSWF4c 0.144 0.879PSWF12 0.323 0.983PSWG1 0.254 0.952PSWG3 0.762 1.018PSWH2 0.358 0.995PSWJ14 0.552 1.023PMWB1c 0.385 0.986PMWB8c 0.368 0.982PMWC2c 0.386 0.987PMWD3 0.756 1.013PMWD12 0.754 1.013PMWF3 0.728 1.014

a Rmeas represents the ratio between the lowest voltage values usedforfitting the data to the peak voltage determined from fitting those data.b The best fitting peak voltage measurement from the truncatedsignal datashould be divided by these correction factors.c For these bolometers, the signal from Neptune is truncated in some butnot all of the observations. The numbers here are for only theobservationsin which the peak signal from Neptune is truncated but the backgroundsignal is not truncated.

Table 6.Calibration status of dead, noisy, or slow bolometers

Bolometer Problem Empirical Calibrationa

PSWA10 Slow YPSWA11 Slow NPSWA13 Slow NPSWC12 Dead NPSWD15 Dead NPSWF9 Noisy YPSWG8 Dead NPSWG11 Dead NPMWA13 Slow NPMWB11 Noisy YPMWD1 Dead YPMWD6 Noisy YPMWE8 Noisy YPLWA6 Dead NPLWC9 Noisy Y

a An entry of “Y” indicates that an empirical calibration was derived usingthe PCal flash and Neptune fine scan data. An entry of “N” indicates thatthis was not possible.

Table 7.Median fractional uncertainties related to fits from PCal flash data

Array Median Fractional UncertaintyNominal Bright Source

Mode Mode

PSW 0.00021 0.0060PMW 0.00022 0.0052PLW 0.00017 0.0020

dividual bolometers resulting from uncertainties in the SRF peaksat 1.6%.

The other two sources of uncertainty in the the flux calibrationof the individual bolometers are associated with the measurementsused to derive theK-parameters: uncertainties from fitting the PCalflash data (Section 5.2) and uncertainties from determiningthe scal-ing terms (Section 6). We quantify these uncertainties below.

8.1 Uncertainty from fitting the PCal flash data

In theory, it should be possible to derive uncertainties in the un-scaled versions of theK-parameters from the fits to the data. How-ever, degeneracy problems arose when attempting to fit Equation 3to the PCal flash data; small variations in the input data could resultin significant variations in theK-parameters, including significantvariations in the relative magnitudes of the first and secondtermson the right side of Equation 3. We therefore could not directly cal-culate uncertainties for these terms.

Instead, we calculated how the uncertainties in the PCal flashmeasurements would affect the final calibration curve usingaMonte Carlo technique. In a series of 1000 trials, random noisescaled by the uncertainties in the magnitude of the PCal flashmea-surements was added to the original PCal flash data. Equation3was then fitted to the data, and theK-parameters were rescaled us-ing theA parameters from the analysis in Section 6. The standarddeviation in all of these calibration curves was measured atevenlyspaced locations in voltage. This standard deviation curvewas thenused as the uncertainty in the flux densitySMeas resulting from theuncertainties in the PCal flash measurements.

Examples of the resulting fractional uncertainty curves areshown in Figure 11. Because Neptune was used to derive the scal-ing terms, the uncertainty curves drop to∼ 0 at locations that cor-respond to the flux densities of Neptune. Table 7 states the medianfractional uncertainties measured for the curves within the regionscovered by the PCal flash data.

For the nominal voltage mode, the median fractional uncer-tainties from the fits to the PCal flash data are very small comparedto the uncertainties related to the variance in the peak flux mea-surements of Neptune (see Section 8.2). This is mainly becausethe large amount of PCal flash data that is available has tightlyconstrained the calibration curves. The uncertainties forthe brightsource mode data are higher because fewer observations withPCalflashes were performed, and these median fractional uncertaintiesare comparable to the median fractional uncertainties in the scalingterms based on the Neptune data.

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0 100 200 300 400S (Jy/beam)

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Figure 11. Uncertainties in the calibration curves related to the fits to thePCal flash data for PSWE2. Curves are shown for both bias modes.

8.2 Uncertainty in the scaling terms for the calibrationcurves

As described in Section 6, the fine scan observations produced a to-tal of four peak voltage measurements of Neptune for each bolome-ter. The standard deviation in theA values from these four measure-ments was used to derive the uncertainty in the flux calibration foreach detector. Tables 8 and 9 give the mean and maximum uncer-tainty in the scaling terms based on the results for all bolometers ineach array and in each bias voltage mode.

The nominal mode fractional uncertainties are reported forall non-problematic bolometers (bolometers not identifiedas dead,noisy, or slow in Table 6) and then for subsets of the non-problematic bolometers where the signal from Neptune in thefinescan data was either truncated (listed in Table 5) or not truncated.The fractional uncertainties for the bolometers with untruncatedNeptune data typically stayed below 1%. The uncertainties for thebolometers with truncated Neptune data, however, are typically∼ 2% and may approach 5%, reflecting the inherent uncertaintiesin the correction factors for the truncation. The worst individualbolometers were PSWF4, which had high uncertainties because theNeptune data were severely trucated in two of the fine scan obser-vations, and PMWB8, which had high uncertainties partly becausethe Neptune data were strongly truncated and partly becauseone ofthe fine scan observations produced no usable data for the bolome-ter. The uncertainties reported in Table 8 are generally much higherthan the uncertainties from the fits to the PCal flash data listed in

Table 7, which means that these uncertainties in scaling thecal-ibration curves dominate the overall instrumental flux calibrationuncertainties for individual bolometers.

For the bright source bias setting, the median uncertainties inthe Neptune measurements are< 1%. For the 250 and 350µmarrays, the uncertainties from fitting Equation 3 to the PCalflashdata is the dominant instrumental source of uncertainty forindivid-ual bolometers, as can be seen by comparing the median fractionaluncertainties in Tables 9 and 7. However, the uncertaintiesin thescaling terms measured for the 500µm bolometers are greater thanthe uncertainties from fitting the PCal flash data.

9 TESTS OF THE FLUX CALIBRATION

While the assessments in Section 8 quantify the uncertainties inthe calibration of individual bolometers, they are not indicative ofthe flux calibration uncertainties applicable to a source observed bymultiple bolometers in standard scan map observations. We testedthe point source flux calibration scheme using scan map observa-tional data of Neptune, Uranus, and the star Gamma Dra. Uranuswas used for these tests because its flux density is known as accu-rately as Neptune’s. Gamma Dra is a K5III star (Perryman & ESA1997) used as a SPIRE secondary calibrator that is observed regu-larly and that is visible at all times of the year. While the flux den-sity of the star is not known as accurately as the flux densities ofNeptune and Uranus, the large amount of data acquired for GammaDra during the course of the mission makes it particularly usefulfor examining the consistency and long-term stability of the fluxcalibration. None the less, the tests with Neptune are the most im-portant mainly because they demonstrate the repeatabilityof fluxdensity measurements of the primary calibration source.

We used the standard SPIRE scan map data processingpipeline in HIPE version 10.0.620 and the default calibration prod-ucts from version 81 of the calibration tree. We removed the tem-perature drift removal from the pipeline because it is dependent onthe flux calibration, but to compensate for this, we used the baselineremoval module to remove the temperature drift and other varia-tions in the background. The data used for these tests were standardscan map (large scan map), standard small scan map, and standardparallel observing mode data for the sources. Almost all small andlarge scan map observations were performed using the mediumornominal scan speed (30 arcsec s−1), although one or two large scanmap observation of each target were performed using the fastscanspeed (60 arcsec s−1), two of the Gamma Dra parallel mode obser-vations used the fast scan speed, and the other two Gamma Dra par-allel mode observations used the slow scan speed (20 arcsec s−1).All available observations of these sources that were performed be-tween OD 100 (the date of the first calibration observations afterthe bias voltages were set to their current levels) and OD 1434 (thelast date in the mission when SPIRE photometer observationsofflux calibration sources were performed) were used. Flux densitieswere measured by performing fits to the timeline data using a sim-ilar method described in Section 6, but we used all unflagged datafrom all bolometers in each array instead of the data from singlebolometers.

9.1 Tests with Neptune data

The flux density of Neptune varies over time because of changes inthe distance between it and the Earth, so we cannot simply report

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Table 8.Uncertainties related to variance in peak flux measurementsof Neptune for the nominal mode

Array Fractional Uncertainties(all non-problematic (non-problematic bolometers (non-problematic bolometers

bolometers) with truncated Neptune data) with no truncatedNeptune data)Median Maximum Median Maximum Median Maximum

PSW 0.0059 0.047 0.022 0.047 0.0049 0.023PMW 0.0042 0.045 0.016 0.045 0.0039 0.019PLW 0.0052 0.012 a a 0.0052 0.012

a None of the PLW bolometers were truncated during the Neptunefine scan observations.

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Neptune500µm(bright)

Figure 12.Histograms of the measured to model flux densities for Neptune. The grey data represent measurements made in small scan map mode, and the reddata represent measurements made in large scan map mode.

Table 9. Uncertainties related to variance in peak flux measurementsofNeptune for the bright source mode

Array Fractional Uncertaintiesa

Median Maximum

PSW 0.0032 0.0073PMW 0.0023 0.012PLW 0.0038 0.014

a The values reported here exclude the uncertainties for the dead, noisy, andslow bolometers listed in Table 6.

statistical results on the flux densities. Instead, we divided the mea-sured flux densities by the model flux densities so that data fromdifferent ODs could be compared together. These ratios are shown

in Figure 12 and Table 10, and Figure 13 shows how the ratios varyover time.

The measured/model flux density ratios for both the nominalbias mode and bright source mode data generally lie∼ 0.5% or∼ 1σ below unity, although a slightly larger systematic offset isseen in the nominal mode data. Given that the peak of the beamcannot be fit to an accuracy better than∼ 1%, this performance isactually very good. We measured some systematic offsets of∼ 1%between the median 350µm ratios for the small scan maps andthe ratios for the large scan maps, but these offsets are∼ 1σ. Theymay be the consequence of the minor differences in coverage of thebeam in the small and large scan maps. Some large scan map ratiosare offset from unity by> 0.01, but for any observation, only onewaveband deviates by this amount.

The small scan map observations were performed more fre-quently than the large scan map observations and also do not in-clude any notable outliers, so we used the small scan map mea-

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Figure 13.Ratios of the measured to model flux densities for Neptune plotted as a function of OD. The grey circles are measurements made in small scan mapdata, and the red squares are measurements made in large scanmap data. The uncertainties from the fits are equivalent to orsmaller than the symbols in theseplots.

Table 10.Measured/model flux density ratios for Neptune

Waveband Measured/Model Flux Density Ratios Change in Ratios between(µm) Nominal Mode Bright Source Mode OD 100 and OD 1450a

All Small Large All Small Large Nominal BrightMaps Scan Maps Scan Maps Maps Scan Maps Scan Maps Mode Source Mode

250 0.993 0.994 0.992 0.997 0.997 0.994 0.000 −0.011±0.005 ±0.005 ±0.007 ±0.005 ±0.004 ±0.007 ±0.003 ±0.003

350 0.993 0.991 1.003 0.996 0.994 1.001 −0.010 −0.008±0.008 ±0.005 ±0.009 ±0.007 ±0.005 ±0.007 ±0.003 ±0.005

500 0.997 0.998 0.994 0.998 0.998 0.998 −0.006 −0.005±0.004 ±0.004 ±0.003 ±0.004 ±0.003 ±0.006 ±0.003 ±0.003

a These values are calculated by fitting lines to the measured/model ratios for the small scan map data and then calculatingthe difference in the best fitting line between OD 100 and OD 1450.

sured/model ratios to examine long-term trends in the reproduca-bility of the Neptune flux density. We examined this by fittinglinesto the data and then calculating the difference in the best fitting linebetween OD 100 and OD 1450 (the approximate time range dur-ing which SPIRE was operational). Except for the nominal mode

250 µm data, all small scan map ratios decrease by∼ 0.7% be-tween these ODs. The significance of this decrease is typically be-low the3σ level. Based on these data alone, it is unclear whetherthis is an issue with the Neptune models, a long-term change inthe sensitivity of SPIRE, or some other misdiagnosed systematic

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effect. Note that the trends in the bright source mode data are de-pendent upon just three data points before OD 400, although thesimilarity between the trends seen in the nominal and brightsourcemode data imply that the ratios are changing in data taken usingboth bias modes.

9.2 Tests with Uranus data

As with the Neptune data, we present statistics on the ratio of themeasured flux densities to the model flux densities for Uranusbe-cause the distance to Uranus and hence the observed flux den-sity will vary over time. The ratios are shown in Figures 14-15 and Table 11. The model flux densities are derived from theESA-4 version of the Uranus planetary atmosphere model (Or-ton, private communication). We did not use data from observation1342233337 because of quality control issues related to calculatingthe correct voltage offsets for that specific observation.

We systematically measure flux densities that are 2-3% lowerthan the model flux densities for Uranus, indicating that theNep-tune and Uranus models are consistent to within 3%. This lieswithin the 4% uncertainties of the Uranus and Neptune models.A statistically-significant difference is seen between the350 µmlarge and small scan map ratios, which again reflects differences inthe coverage of the beam peak. Also, the observations on OD 967(1342237551) produced a measurement/model ratio that is∼ 2%lower in all three bands than the ratios measured in any otherob-servations. The reason for this discrepancy is unclear, as the obser-vations are no different from any others in terms of the observationset-up or quality. If we exclude the OD 967 data from our dataset,the standard deviation in the measured/model ratios for thesmallscan map data decreases to 0.004, but the results are not otherwisesignificantly affected.

Ignoring the OD 967 data, we see a 1.0% decrease in the mea-sured/model ratio for the 250µm data between OD 100 and OD1450 (although this is only measured at the∼ 3σ level) but nosignificant change in the 350 and 500µm ratios. The changes inthe ratios for Uranus and Neptune bright source mode observationsare similar for all three bands, which implies that it could be aninstrument-related effect, albeit an effect that is barelystatisticallysignificant.

9.3 Tests with Gamma Dra data

The flux density of Gamma Dra is not expected to vary over time,so we report statistics on the flux densities themselves (with nocolour corrections) in Table 12, show histograms of the flux densi-ties in Figure 16, and show the flux densities as a function of timein Figure 17. We changed the timeline-based fitting method sothatit would fit circular two-dimensional Gaussian functions insteadof elliptical Gaussian functions, as tests with simulated sourceswith the same brightness as Gamma Dra demonstrated that mea-surements based on circular Gaussian functions produced slightlymore accurate and more precise results. One of the observations(1342238335 from OD 989) produced results with spuriously highuncertainties and is excluded from this analysis. The PSF wassparsely sampled in the parallel mode observations using the fastscan rate, so we made several changes to the technique used tofitGaussian functions to the PSFs to optimise it for these data17

17 For the parallel mode data taken at the fast scan speed, we processed thedata with the wavelet deglitcher disabled; in version 10.0.620 of HIPE, this

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Figure 14.Histograms of the measured to model flux densities for Uranus.The grey data represent measurements made in small scan map mode, andthe red data represent measurements made in large scan map mode.

Gamma Dra is much fainter than Neptune and Uranus, sothe measurements show more dispersion, particularly at 350and500 µm. Unlike the Neptune and Uranus data, no statistically-significant differences are seen between the large and smallscanmap measurements. However, the 0.5% effects seen for Neptunewould be undetectable in the Gamma Dra data with its lowersignal-to-noise ratio. The Gamma Dra timeline data exhibitrela-tively more scatter near the peak of the source than the Neptuneor Uranus timeline data, so it is also possible that minor coveragedifferences between the small and large scan map measurementsdo not significantly affect the function fit to the data. The paral-lel mode measurements show no statistically-significant differencefrom the large and small scan map measurements, although oneofthe fast scan speed parallel map measurements at 500µm is∼ 4σlower than the median measured in other data (probably because ofthe lower signal-to-noise ratio in these data and the faintness of thesource at 500µm).

We did not measure any statistically significant change in anyof the data between OD 100 and OD 1450. The Gamma Dra data

module was misidentifying Gamma Dra as a glitch. Disabling the deglitcherresulted in excess noise in the background annulus, so in fitting a PSF to thedata, we only measured a median signal in the background annulus dataand fixed the background level during the fit rather than usingthe data inthe background annulus in the fit and treating the backgroundlevel as afree parameter. We also fixed the FWHM of the PSF to the geometric meanvalues given by Herschel Space Observatory (2011), which mitigated issueswith the sparse sampling of the beam in these data.

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18 G. J. Bendo et al.

Table 11.Measured/model flux density ratios for Uranus

Waveband Measured/Model Flux Density Ratios Change in Ratios between(µm) All Small Large OD 100 and OD 1450a

250 0.982 ± 0.006 0.982 ± 0.006 0.985± 0.004 −0.010± 0.003350 0.973 ± 0.010 0.970 ± 0.006 0.987± 0.004 −0.003± 0.004500 0.971 ± 0.004 0.970 ± 0.005 0.972± 0.002 −0.003± 0.003

a These values are calculated by fitting lines to the measured/model ratios for the small scan mapdata (excluding the data point from OD 967) and then calculating the difference in the best fittingline between OD 100 and OD 1450.

Table 12.Measured flux densities for Gamma Dra

Waveband Flux Densities (Jy)a Fractional Change in(µm) All Small Large Parallel Flux Density between

Scan Maps Scan Maps Scan Maps Mode Maps OD 100 and OD 1450b

250 0.266 ± 0.003 0.266± 0.003 0.266± 0.003 0.267 ± 0.003 0.005± 0.005350 0.142 ± 0.004 0.142± 0.003 0.144± 0.005 0.144 ± 0.004 0.009± 0.008500 0.073 ± 0.004 0.072± 0.004 0.074± 0.003 0.76± 0.10 −0.011 ± 0.021

a These flux densities include no colour corrections.b These values are calculated by fitting lines to the measured/model ratios for the small scan map data and thencalculating the difference in the best fitting line between OD 100 and OD 1450.

may lack the sensitivity needed to detect the∼ 0.7% decrease indetector response that may be implied by the Neptune data. How-ever, the measured increase of 0.5% in the 250µm data is∼ 2σgreater than the expected 0.7% decrease. This suggests thatthechange in the Neptune and Uranus measured/model flux densityratios at 250µm may actually be related to issues with modellingtemporal changes in flux densities for the planets, althoughthe ev-idence for this is tenuous.

10 SUMMARY OF THE ASSESSMENT OF THE FLUXCALIBRATION

We have outlined the methods by which Neptune is used as the pri-mary flux standard for theHerschel-SPIRE photometer, includinga detailed assessment of the overall error budget associated withtransferring the Neptune calibration to an unknown point source.

The flux calibration for all individual bolometers has beenthoroughly assessed. The relative uncertainties are typically ∼

0.5% for most bolometers in both calibration modes. However, be-cause of the problems with the truncated signal during the Neptuneobservations, the uncertainties for some individual bolometers inthe nominal bias mode are∼ 1− 5%.

The primary assessment of the flux calibration uncertaintiesfor each array as a whole is based on the Neptune data. We wereable to measure the flux density of Neptune to within 1.5% of themodel flux density in all three bands and using both voltage biasmodes. This uncertainty includes both the systematic offset be-tween the measured and model flux densities and the1σ disper-sion in the measurements. As all Neptune data were used to calcu-late this uncertainty, it encompasses any possible temporal changesin the detector sensitivity during the mission and any variabilityin the brightness of Neptune not accounted for by the models,al-though the evidence for either is inconclusive. The uncertainty alsoincludes the variations in measurements between differentobserv-ing modes, which is mainly a consequence of minor differences inthe coverage. We therefore conclude that 1.5% can be adoptedas

the relative calibration uncertainty for the SPIRE photometer ar-rays. The overall error budget must also include the 4% absoluteuncertainty ascribed to the Neptune model, and any statistical orother uncertainties associated with a particular measurement.

ACKNOWLEDGMENTS

We thank the reviewer for the helpful comments on this paper.SPIRE has been developed by a consortium of institutes led byCardiff Univ. (UK) and including: Univ. Lethbridge (Canada);NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy);IAC (Spain); Stockholm Observatory (Sweden); Imperial CollegeLondon, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); andCaltech, JPL, NHSC, Univ. Colorado (USA). This developmenthas been supported by national funding agencies: CSA (Canada);NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN(Spain); SNSB (Sweden); STFC, UKSA (UK); and NASA (USA).HIPE is a joint development by the Herschel Science Ground Seg-ment Consortium, consisting of ESA, the NASA Herschel ScienceCenter, and the HIFI, PACS and SPIRE consortia.

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Figure 15.Ratios of the measured to model flux densities for Uranus plottedas a function of OD. The grey circles are measurements made insmall scanmap data, and the red squares are measurements made in large scan mapdata. The uncertainties from the fits are equivalent to or smaller than thesymbols in these plots.

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Figure 16.Histograms of the measured to model flux densities for Uranus.These flux densities do not include colour corrections. The grey data rep-resent measurements made in small scan map data, the red datarepresentmeasurements made in large scan map data, and the green data representmeasurements made in parallel mode map data.

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Figure 17. Flux densities for Gamma Dra plotted as a function of OD.These flux densities do not include colour corrections. The grey circles aremeasurements made in small scan map data, the red squares aremeasure-ments made in large scan map data, and the green triangles aremeasure-ments made in parallel mode map data.

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