FM ILT Spectrometer Wavelength Calibration Status Report H. Feuchtgruber

PACS ICC Meeting #29 1/2 Oct 2007

Wavelength Calibration 1

FM ILT Spectrometer Wavelength Calibration

Status Report

H. Feuchtgruber



Overview of Measurements (1)

• Nov/Dec 2006: FMILT1 - 2 short scans of water vapour cell and few laser lines measured (initial calibration done, report available)• Mar 2007: FM ILT2

- Measured FIR-Laser lines [µm] (5 days):- CH3OH: [70.511638, 77.405660, 96.522408, 118.834107,

163.03352, 170.57637] x 5 chopper angles;- 13CH3OH: [115.82324, 203.63577] x 5 chopper angles;

- CH318OH: [134.6+-0.1, 162.647, 219.801] x 5 chopper

angles;- Gas cell measurements (with Kapton foil) : - H2O: (27 hours) One Measurement consists of full scan vapour + full scan vacuum” Filter A (C=[0.1pF(blue), 1pF(red)] x 3 chopper angles

Filter B (C=[0.1pF(blue), 0.2pF(red)] x 3 chopper angles - CO: (3 hours) Grating range scan pair: vacuum + CO gas at 1 chopper angle



Overview of Measurements (2)

• June 2007: FMILT3 Gas cell measurements (without Kapton foil): - H2O: (9 hours) One Measurement consists of full scan vapour + full scan vacuum Filter A (C=[0.1pF(blue), 1pF(red)] x 1 chopper angle Filter B (C=[0.1pF(blue), 0.2pF(red)] x 1 chopper angle



FIR Laser Setup

• TUFIR window

• Extended, but structured source

with substantial contrast• Substantial laser power

variation (recorded within setup)

• Scan parameters Range: ~100-120 steps

(up&down) Stepsize: 100 Readouts per ramp: 64 Ramps per position: 3



Gas Absorption Setup

• Focus window• Extended and

controlled background source

• Source T=600°C (H2O) and T=400°C (CO)

• Absorption path ~56cm

• p= 9mBar (H2O) and p=70mBar (CO)

• Scan parameters- Range: 32000-1064000- Stepsize: 200- Readouts per ramp: 64- Readouts per position: 3



Calibration Method (1)

=; ==0.6253; gLHe=117.175µm; n=[1,2,3] ; pix=[1..16]

= 0 + (p1-1)*(gratpos/dgrat) + p2*(gratpos/dgrat)2 + p3*(gratpos/dgrat)3

with dgrat = 23301 [steps/]

δpixn = mean[δpixn,FILT1(module)]



Calibration Method (2)

• Begin with calibration from FILT1 (few lines measured only)• Calculate up/down corrected and divided spectra (gas/vacuum)

for each pixel:

Fforeground(λ) + Tgas(λ)*Fbackground(λ)Ratio = Fforeground(λ) + Fbackground(λ)

with Fforeground (λ) (20…50) x Fbackground (λ)

• Calculate model spectra (at expected PACS spectral resolution) for the wavelength range under investigation and define spectral intervals for correlation analysis

• Determine λ-calibration offsets for each interval• Store 3rd order polynomial parameters fitted to offsets vs.

(gratpos/dgrat) for each pixel new = old + polynomial fit• Iterate previous steps (at least twice)



Results• A semi-automatic method to calibrate each pixel in each

array has been developed• λ-calibration 3rd order polynomials for all individual

spectrometer pixels have been derived• Separate sets of polynomials have been calculated for

the 3 main PACS spectrometer bands and the “extra” band (2nd order via filter A: 54-73µm)

• CO line positions are compliant with calibration based on water

• Laser positions largely compliant with calibration, however fewsingle pixels show still some significant deviations to be corrected in a case by case analysis

• Laser lines may show peak-center variations due to actual position of laser spots within the slit.

• This is the initial uplink calibration for flight



Data Quality (1)

Typical data used during correlation analysis

single pixel



Data Quality (2)

single pixel

rebinned module



Accuracies

• PCD V8.0 (sec. 4.2.1): Required accuracy:

“Peak position to within 10-20% of a spectral resolution element”

• In general the requirement is met throughout all bands however at band borders, due to leakage effects and lower S/N the calibration accuracy (in terms of σ over all pixels) is closer to 20% of a spectral resolution element, while in band centers, σ values even better than 10% are obtained. σ=stdev(all_pixels residual λ-shift vs. model)

• Absolute verification against Laser and CO lines• Relative verification by combining all pixels• Be aware: relative position of point sources within slit can

have significant effect



• Depth of grating scans (normalized to line scan AOT):3rd order (2.5h), 2nd’ order (2.5h), 2nd order (2.8h),1st order (3.6h); (time1st= 58steps × 0.75s × 2updown × 3numres × 2vapvac × 25modules / 3600s)• HDO lines are detected (abundance ~3e-4; partial pressure 2.7e-6 bar)• CO spectra show residual lines from water contamination

Faintest lines ~3e-18W/m2

Remarks



Verification with

Laser Lines

Expected Line centersand slit widths overplotted



Next Steps / Open Work

• Investigate/recalibrate few outlying pixels found during verification against Laser lines. The reason is essentially the poor S/N in the measurements during FM ILT 2.

• Calibrate –L/+L chopper throws, spot checks indicate that there is no significant change in calibration…

• Provide Report and Calfiles (version as of this report provided for PACS spectrometer pipeline)

• Check calibration and eventually recalibrate for FMILT 3 measurements



Wavelength Calibration in Flight (1)

• Observing modes:– Use standard SpecRangeScan AOT– Use Wave_Cal mode with pointing request

• Suitable target list :

- PICC-ME-TN-013, ”Use of late type stars for PACS wavelength calibration”, (D. Lutz)

- “Photometric and Spectroscopic Calibration of Herschel Instruments with Planets and Satellites”, (Th. Encrenaz, R. Moreno, A. Coustenis)

– Uranus(H2O), Neptune(H2O, CO), Saturn(H2O, NH3, PH3)– Jupiter (H2O, NH3, PH3 ; fills entire 5x5 field of view !)



Wavelength Calibration in Flight (2)

• Target Visibilities in their 1st two windows (HSPOT):

Start PV

Start Routine Ops

Mars 18-Aug-09 – 9-Dec-09

FM ILT Spectrometer Wavelength Calibration Status Report H. Feuchtgruber

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