Aperture Photometry with CCD
Images using IRAF
Kevin Krisciunas
Images must be taken in a sensible manner. Ask advice
from experienced observers. But remember Wallerstein’s
Rule: “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 and
high in the sky, you better observe standards low in the
sky and high in the sky so that you can measure the
effect of the atmosphere.
If your program stars are very red or very blue, you better
observe standard stars that are even redder or bluer,
so that you can interpolate transformations rather than
extrapolate them. Or you may realize that you need
non-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 have
one very blue star like
Rubin 149 and a number
of redder stars, allowing
you to determine photometric
color terms easily.
Here north is down, east to the left.
For crowded fields you have to reduce your photometry
with DAOPHOT or DOPHOT. This involves determining
the 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 photometry
can often be much more straightforward. And there
are some advantages:
If the focus slowly degrades over the course of the
night or you have slight tracking errors (giving
stellar images that are not round), with a software
aperture of radius 8 or 10 pixels, you can still put
more 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 using
an annulus (say from radius 12 to 20 pixels) centered
on each star. IRAF allows you to easily get around
faint stars or cosmic ray hits in the sky annulus.
Say you observe with UBVRI filters. The goal is to
convert the instrumental magnitudes of the standards
to a photometric system such as Landolt’s. One
assumes that the CCD detector is linear, namely that
the arrival of twice as many photons produces twice
as many countable electrons. Some cameras are
demonstrably non-linear at high count rates, such as
the Las Campanas 1-m camera. In that case the
observer has to make sure that one doesn’t exceed the
recommended 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 standards
to some photometric system using linear equations such as these
for 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. ubvri
are instrumental magnitudes from IRAF. k’s are
extintinction coefficients for the atmosphere. X = “air
mass”. CT’s are “color terms”. zp’s are photometric zeropoints.
The “air mass” X is the path length through the atmosphere
toward the field you are observing, compared to the
path length through the atmosphere toward the zenith.
Most observing is done more than 30 degrees above the
horizon (zenith angle 60 degrees). We can use a plane
atmosphere approximation and take X = secant of the
zenith angle. So – it is determined from spherical
trigonometry.
Typical extinction coefficients at Cerro Tololo are
ku = 0.51, kb = 0.26, kv = 0.15, kr = 0.11, and ki = 0.06
magnitudes per air mass. But – these can vary +/- 20 %
from clear night to clear night. The U-band coefficient
might vary 20 percent over the course of a single night.
If you have a night’s worth of images taken in a sensible
fashion and they are properly flat fielded, IRAF will allow
you to determine the extinction coefficients, color terms,
and zero points. Then, observations of stars and supernovae
of unknown brightness, made with the same telescope
and camera on that night, can be transformed to the same
photometric system.
Rev up IRAF. Type “noao” then “digiphot” then
“apphot”.
It helps to have a hardcopy of a log of the images for
a particular night. Of course it is sensible to put the
images for each night into separate subdirectories.
Know your FITS header parameters of interest, for
example Universal Time, object name, filter, exposure time,
and airmass. The parameter names vary from observatory
to 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 electrons
per ADU), and names of key FITS header parameters. This
example is for the CTIO 0.9-m telescope.
This example is for the Las Campanas 1-m telescope
as it was in 2012:
epar fitskypars
To set the sky fitting algorithm (“median” or “mode” better
than “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 can
decide later which aperture is best for the night in
question.
Now we’re ready to obtain some aperture magnitudes with
IRAF’s apphot task “phot”. You put the little circle on a
star in your SAOimage (ds9) window and hit the space bar.
This lists the image name, the pixel coordinates of the
stars, and starting in column 5 the instrumental aperture
magnitudes for radius = 6, 7, 8, 9, and 10 pixels.
Say a particular field was imaged in the U, B, V, R, and I
filters, 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 contain
these five lines for this set of images:
obj216 0 0
obj217 -3 5
obj218 -4 7
obj219 -6 9
obj220 -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 aperture
photometry on a given field using “mknobsfile”. Parameters
are set via “epar mknobsfile” then executing the action
with “:go”
We will also be using “fitparams” and “evalfit”.
Example for
CTIO 0.9-m
telescope:
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).
This example of the parameters for mknobsfile is
for the Las Campanas 1-m telescope as it was in
2012. 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 each
filter, one uses an editor and creates a single file such as
stds.raw . Then one can solve for the coefficients for
all the photometric transformations using program
“fitparams”.
Example for CTIO 0.9-m telescope, which uses
five filters (U, B, V, R, I).
Example for Las Campanas 1-m telescope, which uses
6 filters (u, B, V,g, r, i)
Note: set parameter “interac” to “no”.
File ubvri is derived from Landolt (1992, 2007) or some
other list of magnitudes and/or colors. Here the variable
BV is the B-V color, UB is the U-B color, etc.
IRAF insists that the names of your standards in your
raw data file match the ID’s in your catalog file.
Beginning and end of
the file
ubvri_ext.config
which is used to
specify the transformation to the standard
photometric system.
One can give default
starting values for
certain parameters.
Here, for example,
V+BV means V mag
from ubvri + B-V
color, which equals
B magnitude.
We should point out that if all the observations were
taken at just about the elevation angle, there is very
little range of air mass for the observations. In that
case one should just adopt sensible mean values of
extinction for the site and use simpler transformation
equations, solving only for zeropoints and color terms.
Example of B-band fit
for Nov 26, 2005, photometry
with 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 doing
ground 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 catalog
file of magnitudes and colors of standards (ubvri)
you can use program “evalfit” to apply the derived
transformations to all the observations of the
standards. The output file gives the differences
between your UBVRI magnitudes and those of
Landolt.
If your residuals change steadily from -0.05 to +0.05
over the course of the night, that indicates the
photometric zeropoint was slowly changing over the
night. A truly photometric night should show random
small pluses and minuses in these residuals vs. time.
Example of parameters for IRAF program evalfit in the
photcal package. This applies the photometry calibration
from fitparams to photometry done in the same software
aperture for program fields. This was used for photometry
obtained with the Las Campanas 1-m telescope.
Portion of file stds.calib (output of task “evalfit”). Starting
in column 4, every 3rd column gives “catalog value minus
our value”. This example is data taken with the CTIO 0.9-m
telescope (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.