P h o t o m e t r y O b s e r v a t i o n G u i d e l i n e s
(An observation = 1 frame in each of V, R, and I)
STANDARDS
Aim for 20+ observations of standards for a night; these must have good
counts (say 25K or more) since they set the exinction and transformation
coefficients that apply to all unknowns.
Bright standards may be done as soon as it is too dark for flats and
before it is dark enough for parallax regions.
5-6 observations of 1 or 2 of the very red ones (W359, VB8, GL866) are
needed in order to get second-order transformation terms. A good way is
to do one twice, a set immediately followed by another set, then again
an hour or so later if possible.
The red/blue extinction stars (B-V > 1.2) must be followed from the
meridian to an airmass around X = 2 (or a few tenths greater than the
largest program airmass).
UNKNOWNS
Should be done on the meridian if possible, but certainly at airmass
smaller than 1.5 (unless there are some special, desperate reasons for
going to higher X - since the extinction coefficient is multiplied by
the air mass in the reductions, the smaller the airmass, the smaller
the influence of the exinction coefficient and its error).
On a night of photometry, when doing a parallax region always do a
frame or two of the other color - then that too could be reduced for
magnitude and colors at little cost in observing time.
Unknowns should be observed on three separate nights to check for
reliability in the photometry and possible variability - single
observation on one night should never be considered adequate for
publication.
PRINCIPLES AND PRACTICE
Differential photometry is relative photometry, and you can do it
in small fields like ccd fields or over small disstances of arc -
people who work on eclipsing binaries, e.g., use a standard very
close by, and just get delta m's between the variable and std. If
the extinction coeficient is, say, 0.1 magnitudes, in some color,
and if you observe a star at an airmass say only a tenth of an
airmass different from your standard star, then the effect of the
extinction is only 0.1 x 0.1 or 0.01 magnitudes. Thus the
magnitude of your star is just the delta m between it and your
standard added with the mag of the standard, or more succinctly,
mag(unknwn) = mag(std) + delta mag + 0.01. And if the color of
the two stars is very similar you don't have to worry about
getting onto the standard system (but if there were a large
color difference between standard and unknown there might also be
a small correction tht is a function of color owing to different
color senstivity of detector + filter + telescope + site).
So the reduction is just some simple arithmetic if you know the
extinction coefficient and any color correction, and if the
differences in airmass and color between standard and unknown are
small, you can't make much of an error, even if the extinction is
not well determined.
But real men do all sky photometry, and that is usually what is
meant when you say photometry, and if you do not mean that you (I)
((a few others at least)) say "differential photometry". That
means standards all over the sky, at different sometimes large
airmass differences (as large as 1. say), and then you need
enough standards to reduce your data to some level of accuracy
while getting or assuming extinction coefficients, and getting or
assuming the polynomial coefficients needed to correct the
instrumental magnitudes to "the" standard system. At the 1-meter
we really need to have several measurements of one or more
extremely red stars to make it all work properly.
A p e r t u r e P h o t o m e t r y R e d u c t i o n s
BIAS SUBTRACTION
CCD #6 has no overscan region, so bias subtraction must be done manually.
Each set of frames must be preceded and followed by a separate bias frame.
Use MISTAT to find the mean counts for each bias frame, then for each bias
"pair", find the average of the two means and use ICSUB to subtract that
value from each of the frames in the set bracketed by the pair.
OTHER PREPROCESSING
The remainder of the preprocessing proceeds as for astrometry, up to and
including RASTAT. However, during the CPOS phase, be sure to include a
few extra well-exposed stars wherever possible, especially when the object
of interest is faint. This will help later in judging the "curve of growth"
with increasing apertures, and in the case of faint objects, is essential to
making aperture corrections.
DOFOTO
At this point, each .DST frame should have a corresponding .CEN file. Use
the routine DOFOTO to obtain a set of corresponding .MAGS files. DOFOTO
is a front-end to the Figaro routine FOTO. FOTO handles only one frame at
a time and allows a maximum of 7 aperture sizes. DOFOTO handles multiple
frames, allows 2 or 3 passes through FOTO (i.e., 14 or 21 aperture sizes),
and calls the C program APDIFFS, which lists the differences in magnitudes
between successive aperture sizes (type "dofoto" by itself for the syntax
and usage details). At this point, the option for a third pass through FOTO
(apertures 15-21) should NOT be used (more on this later). Obtain landscape
hardcopies of all the .MAGS files with the routine MPRINT (mprint *.mags).
SELECTING APERTURE MAGNITUDES
Here's where things can become somewhat subjective. Go through each .MAGS
listing and observe the curve of growth by examining the "Aperture Deltas".
Here is a sample (truncated--it actually extends out to 14-13):
*** Aperture Deltas ***
Object 2-1 3-2 4-3 5-4 6-5 7-6 8-7 9-8 10-9 ...
1 -0.978 -0.304 -0.125 -0.057 -0.029 -0.019 -0.011 -0.006 -0.005 ...
2 -0.914 -0.295 -0.118 -0.055 -0.028 -0.017 -0.011 -0.007 -0.005 ...
3 -0.892 -0.306 -0.121 -0.058 -0.031 -0.018 -0.011 -0.007 -0.005 ...
4 -0.914 -0.297 -0.119 -0.054 -0.029 -0.017 -0.011 -0.008 -0.005 ...
5 -0.894 -0.307 -0.121 -0.055 -0.029 -0.019 -0.012 -0.009 -0.004 ...
Notice that as the aperture sizes increase, the differences in magnitudes
between successive apertures decreases. Choose the aperture for which
the delta is less than or equal to -0.010. In this case, that would be
aperture 9. In this example, the only object of interest is object 1.
However, by selecting additional objects as suggested above, it's possible
to see that the curve of growth is fairly consistent for all the stars
chosen. On the hardcopy, circle in pen the magnitude listed for the object
of interest under the appropriate aperture heading.
APERTURE CORRECTIONS
Here's another (truncated) sample:
*** Aperture Deltas ***
Object 2-1 3-2 4-3 5-4 6-5 7-6 8-7 9-8 10-9 ...
1 -0.901 -0.301 -0.105 -0.044 -0.024 -0.015 -0.011 -0.008 -0.010 ...
2 -0.916 -0.283 -0.099 -0.044 -0.022 -0.014 -0.008 -0.006 -0.005 ...
3 -1.125 -0.304 -0.111 -0.047 -0.024 -0.014 -0.009 -0.007 -0.005 ...
In this case, things aren't so consistent--the curve of growth for object 1
is not keeping pace with the other two. It would be tempting to choose
aperture 9 and move on, however, the "majority rule" would suggest aperture
8 instead. By inspecting the actual magnitudes (or by foreknowledge of the
situation), the discrepancy can be attributed to the relative faintness of
object 1. In this case, a magnitude correction is probably called for. The
steps for making the correction are as follows:
1) Find the "majority rule" aperture (this may often be dictated by
a single well-exposed star, chosen expressly for this purpose).
Call this apX.
2) Find the aperture size that best represents the FWHM radius of the
image. Call this apY. For example, with a pixel size of 0.57 arcsec,
if the recorded seeing value is 2.0 arcsec, that makes a FWHM diameter
of 3.5 pixels, or a FWHM radius of ~1.8 pixels, so choose apY = 2.
3) For all the well-exposed objects (i.e., those used in making the
"majority rule" determination), calculate mag(apX) - mag(apY), and
then the average of these deltas.
4) Add this average delta value to the mag(apY) of the object of interest.
The result is the corrected magnitude.
5) Record the result and the calculations involved in pen on the .MAGS
hardcopy for that frame.
THE "3" OPTION
In the event of especially poor seeing, the aperture deltas may not "bottom out"
to -0.010, even with an aperture radius of 14 pixels. In that case, it's possible
to extend the aperture range by rerunning DOFOTO on the frame(s) involved using
the "3" option. This a judgment call. If extending the range by only one or two
more apertures helps, it may be ok to use the result, but data with such poor seeing
should probably be called into question. The "3" option may perhaps be useful only
to gauge the severity of the situation.
THE CCDPHOT INPUT FILE
Make a copy of the obslog file from the observing run involved, and edit it so
that it contains only the information for the photometry frames. Sets of frames
(i.e., V, R, I frames at the "same" airmass) should be separated by blank lines.
For example,
240 4.0 10:56:17 +07:01:59 10:52:22.5 1.2739 W359 PBL 240 I 4
242 20.0 10:56:17 +07:01:59 11:00:25.3 1.2739 W359 PBL 242 R 20
243 80.0 10:56:17 +07:01:59 11:04:46.6 1.2746 W359 PBL 243 V 80
262 11.0 10:56:17 +07:02:02 13:02:42.4 1.5141 W359 PBL 262 I 11
263 55.0 10:56:17 +07:02:03 13:06:41.0 1.5321 W359 PBL 263 R 55
264 240.0 10:56:17 +07:02:03 13:09:44.5 1.5466 W359 PBL 264 V 240
220 15.0 09:23:25 -45:24:06 09:27:12.1 1.0310 E4-A PBL 220 I 15
222 12.0 09:23:25 -45:24:06 09:31:44.8 1.0313 E4-A PBL 222 R 12
223 12.0 09:23:25 -45:24:06 09:34:16.7 1.0316 E4-A PBL 223 V 12
Use the routine PREPHOT to create a template for the CCDPHOT input file. Let's
say the edited obslog copy is called "obscopy" and we want to call the CCDPHOT
input file "pbl2.dat". The template is created with the command
prephot obscopy pbl2.dat
The resulting template will look like this:
nn, 1.2741, vv.vvv, 80, rr.rrr, 20, ii.iii, 4, bb.bbb, 0, 'W359'
nn, 1.5309, vv.vvv, 240, rr.rrr, 55, ii.iii, 11, bb.bbb, 0, 'W359'
nn, 1.0313, vv.vvv, 12, rr.rrr, 12, ii.iii, 15, bb.bbb, 0, 'E4-A'
Edit this file as follows:
1) segregate the standards from the unknowns, with the standards
at the top.
2) replace the fields "nn", "vv.vvv", "rr.rrr", etc., with each object's
index, V magnitude, R magnitude, etc., respectively.
3) insert at the very top a line of the form
'RUN#:DDMONYY', NSTDS
where "NSTDS" is the number of standard observations, for example
'PBL2:03MAR96', 016
4) insert above the start of the unknown observations a line containing the
number of unknowns, for example
08
CCDPHOT
In the directory containing the CCDPHOT input file, make a copy of the file
~/dat/foto/stds.dat. You are now ready to run CCDPHOT. Type
ccdphot
and reply to the prompts.