Aperture Photometry with CCD Images using IRAF

Kevin Krisciunas

Images must be taken in a sensible manner. Ask advicefrom experienced observers. But remember Wallerstein’sRule: “Four astronomers, five opinions.”

1)Some people prefer dome flats.2)I prefer sky flats (pointless if there are visible clouds).3)After focusing camera at start, do one standards field in all the filters you will use.4)Finish the night with a standards field.5)Do standards every 90 minutes or so. It is the only way to prove that the night was photometric. The CCD detector is a much better cloud detector than your eyes…

If your program objects are observed low in the sky andhigh in the sky, you better observe standards low in thesky and high in the sky so that you can measure theeffect of the atmosphere.

If your program stars are very red or very blue, you betterobserve standard stars that are even redder or bluer,so that you can interpolate transformations rather thanextrapolate them. Or you may realize that you neednon-linear transformations.

Popular lists of standards:

UBVRI – Landolt (1992), Landolt (2007)

Sloan filters – J. Allyn Smith et al. (2002)

Most field stars are red.Many Landolt fields haveone very blue star likeRubin 149 and a number of redder stars, allowingyou to determine photometriccolor terms easily.

Here north is down, east to the left.

For crowded fields you have to reduce your photometrywith DAOPHOT or DOPHOT. This involves determiningthe point spread function of the instrument. The PSF can vary with position in a frame or from frame to frame.

Plots made with IRAF task “imexam” with options r (for radial plot) or s (for surface plot).

If you have non-crowded fields, aperture photometrycan often be much more straightforward. And thereare some advantages:

If the focus slowly degrades over the course of thenight or you have slight tracking errors (givingstellar images that are not round), with a softwareaperture of radius 8 or 10 pixels, you can still putmore than 98 percent of the light into that aperture.

8 px might be OK for observations high in the sky,but seeing is worse for observations low in the sky.

The underlying sky level is determined by usingan annulus (say from radius 12 to 20 pixels) centeredon each star. IRAF allows you to easily get aroundfaint stars or cosmic ray hits in the sky annulus.

Say you observe with UBVRI filters. The goal is toconvert the instrumental magnitudes of the standardsto a photometric system such as Landolt’s. Oneassumes that the CCD detector is linear, namely thatthe arrival of twice as many photons produces twiceas many countable electrons. Some cameras aredemonstrably non-linear at high count rates, such asthe Las Campanas 1-m camera. In that case theobserver has to make sure that one doesn’t exceed therecommended count limit on the stars of interest, or one must correct for this after the fact (which would be a hassle).

We transform the instrumental magnitudes of the standardsto some photometric system using linear equations such as thesefor the CTIO 0.9-m telescope’s camera.

U = u – kuX + CTu(u-b) + zpu

B = b – kbX + CTb(b-v) + zpb

V = v – kvX + CTv(b-v) + zpv

R = r – krX + CTr(v-r) + zpr

I = i – kiX + CTi(v-i) + zpi

UBVRI are “catalog magnitudes” from Landolt. ubvriare instrumental magnitudes from IRAF. k’s are extintinction coefficients for the atmosphere. X = “airmass”. CT’s are “color terms”. zp’s are photometric zeropoints.

The “air mass” X is the path length through the atmospheretoward the field you are observing, compared to thepath length through the atmosphere toward the zenith.Most observing is done more than 30 degrees above thehorizon (zenith angle 60 degrees). We can use a planeatmosphere approximation and take X = secant of thezenith angle. So – it is determined from sphericaltrigonometry.

Typical extinction coefficients at Cerro Tololo areku = 0.51, kb = 0.26, kv = 0.15, kr = 0.11, and ki = 0.06magnitudes per air mass. But – these can vary +/- 20 %from clear night to clear night. The U-band coefficientmight vary 20 percent over the course of a single night.

If you have a night’s worth of images taken in a sensiblefashion and they are properly flat fielded, IRAF will allowyou to determine the extinction coefficients, color terms,and zero points. Then, observations of stars and supernovaeof unknown brightness, made with the same telescopeand camera on that night, can be transformed to the samephotometric system.

Rev up IRAF. Type “noao” then “digiphot” then“apphot”.

It helps to have a hardcopy of a log of the images fora particular night. Of course it is sensible to put theimages for each night into separate subdirectories.

Know your FITS header parameters of interest, forexample Universal Time, object name, filter, exposure time,and airmass. The parameter names vary from observatoryto observatory. You can double check an image whatever.fits by doing something like this:

> imhead whatever l+ | page

Then you might make a log file doing this:

> hselect *.fits $I,ut,object,filter2,exptime,airmass yes > something.log

epar datapars

To set valid data range, read noise, gain (number of electronsper ADU), and names of key FITS header parameters. Thisexample is for the CTIO 0.9-m telescope.

This example is for the Las Campanas 1-m telescopeas it was in 2012:

epar fitskypars

To set the sky fitting algorithm (“median” or “mode” betterthan “mean”) and the sky annulus parameters.

Exiting an option list is done via “:q”

epar photpars

To specify the list of aperture radii in pixels. You candecide later which aperture is best for the night inquestion.

Now we’re ready to obtain some aperture magnitudes withIRAF’s apphot task “phot”. You put the little circle on astar in your SAOimage (ds9) window and hit the space bar.

This lists the image name, the pixel coordinates of thestars, and starting in column 5 the instrumental aperturemagnitudes for radius = 6, 7, 8, 9, and 10 pixels.

Say a particular field was imaged in the U, B, V, R, and Ifilters, producing a set of files obj216.fits through obj220.fits.The telescope tracking might have drifted a little over time.

A file of the pixel shifts between the images might containthese five lines for this set of images:

obj216 0 0obj217 -3 5obj218 -4 7obj219 -6 9obj220 -8 11

I put all the sets of shifts on a given night into one file, called something like nov26.shifts

Next one exits “apphot” and invokes “photcal” in IRAF.

One can create a text file of the U, B, V, R, I aperturephotometry on a given field using “mknobsfile”. Parametersare set via “epar mknobsfile” then executing the actionwith “:go”

We will also be using “fitparams” and “evalfit”.

File ru149.imsets might contain only one line:

ru149 : obj216 obj217 obj218 obj219 obj220

We decided to use the 5th aperture specified by“photpars” (radius = 10 px).

Example forCTIO 0.9-mtelescope:

This example of the parameters for mknobsfile isfor the Las Campanas 1-m telescope as it was in2012. It’s particularly important to have “idfilter” correct.

Having generated a number of text files with “mknobsfile”containing the aperture magnitudes for each star in eachfilter, one uses an editor and creates a single file such asstds.raw . Then one can solve for the coefficients forall the photometric transformations using program“fitparams”.

Example for CTIO 0.9-m telescope, which usesfive filters (U, B, V, R, I).

Example for Las Campanas 1-m telescope, which uses6 filters (u, B, V,g, r, i)

Note: set parameter “interac” to “no”.

File ubvri is derived from Landolt (1992, 2007) or someother list of magnitudes and/or colors. Here the variableBV is the B-V color, UB is the U-B color, etc.

IRAF insists that the names of your standards in yourraw data file match the ID’s in your catalog file.

Beginning and end ofthe fileubvri_ext.configwhich is used tospecify the transforma-tion to the standardphotometric system.One can give defaultstarting values forcertain parameters.Here, for example,V+BV means V magfrom ubvri + B-Vcolor, which equalsB magnitude.

We should point out that if all the observations weretaken at just about the elevation angle, there is verylittle range of air mass for the observations. In thatcase one should just adopt sensible mean values ofextinction for the site and use simpler transformationequations, solving only for zeropoints and color terms.

Example of B-band fitfor Nov 26, 2005, photometrywith CTIO 0.9-m telescope.

RMS error +/- 0.018 mag

zpb = -2.887 +/- 0.011

kb = 0.279 +/- 0.007 mag/airmass

CTb = -0.102 +/- 0.004

(part of file nov26.out)

On this night we obtained the following.This is about as good as it gets doingground based photometry.

Filter RMS extinction

U +/- 0.044 0.504 (0.032)

B 0.018 0.279 (0.007)

V 0.016 0.160 (0.006)

R 0.012 0.121 (0.004) I 0.021 0.072 (0.008)

Using stds.raw, ubvri_ext.config, and your catalogfile of magnitudes and colors of standards (ubvri)you can use program “evalfit” to apply the derivedtransformations to all the observations of thestandards. The output file gives the differencesbetween your UBVRI magnitudes and those ofLandolt.

If your residuals change steadily from -0.05 to +0.05 over the course of the night, that indicates thephotometric zeropoint was slowly changing over thenight. A truly photometric night should show randomsmall pluses and minuses in these residuals vs. time.

Example of parameters for IRAF program evalfit in thephotcal package. This applies the photometry calibrationfrom fitparams to photometry done in the same softwareaperture for program fields. This was used for photometryobtained with the Las Campanas 1-m telescope.

Portion of file stds.calib (output of task “evalfit”). Startingin column 4, every 3rd column gives “catalog value minus our value”. This example is data taken with the CTIO 0.9-mtelescope (filters U, B, V, R, I).

Finally …

• Obtain the aperture magnitudes for all your research stars for all the frames and filters using “phot”.

• Use “mknobsfile” to create files such as ngc1234.raw (you have to use the same aperture size as for standards)

• Use files such as ubvri_ext.config, nov26.out and IRAF program “evalfit” to apply transformations to convert instrumental magnitudes on research objects to standardized photometric system.