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From: richmond@spiff.Princeton.EDU (Stupendous Man)
Subject: Tips for doing photometry on SN 1993J (long)
Originator: news@nimaster
Nntp-Posting-Host: spiff.princeton.edu
Organization: Princeton University
Date: Thu, 1 Apr 1993 18:09:18 GMT
Irwin Horowitz asked for some tips on doing photometry of
the SN, and since this object IS well within the range of
many of the telescopes out in Net-Land, I thought that perhaps
many people might want to know how they can do "Real Science" (TM)
and help out us professionals who are stuck under clouds :-(
First, use a FILTER! Unfiltered images are not nearly
as useful as those for which we have some idea of the effective
wavelength. My preference is for filters that conform to
the passbands given in an article by Bessell (PASP 102, 1181 (1990)),
but any filter for which you can tell me the effective wavelength
and width is OK. Note that most CCDs are more sensitive in the red,
so you're likely to get higher signal-to-noise with R and I
than with U or B or V.
Second, observe the SN! I would guess that exposures of
around five to ten minutes on an 8-inch, or two minutes on
an 20-inch, will be required to get good S/N, but experiment!
Note the date, time and place of each observation. It's
best if you can make regular observations (every 1/2/3 nights?),
but even three or four scattered observations can be of use.
Third, note all the instruments used: the telescope size, type
and make, the filter(s) used, the CCD (or other detector).
Fourth, make sure that you perform all the necessary steps
of image reduction: for CCD images, this means bias-subtraction,
dark-frame subtraction (if necessary) and flat-fielding.
At this point, you have a reduced image. If you wish, I'd
be happy for you to send it to me and let me take over from here.
If you want to carry on, please do!
Those who want to keep going: refer to the charts I've posted
a few days ago (you can get a copy via anonymous FTP from
astro.princeton.edu, in the "richmond" directory: the file
is called "chart"). Measure the SN and stars A, B and C,
plus any other star you can find mentioned on the chart which
happens to fall in your image.
Fifth, if you use aperture photometry, please try to match the
aperture size to the size of the stellar images. Using an
aperture with radius between the FWHM and 2*FWHM is a good
rule of thumb. Note the size of the aperture you used. The
SN and comparison stars are all muvh brighter than the local
background (at least at the current time), so there shouldn't
be much problem here. Do make sure that you use a LOCAL sky
background when calculating the sum of counts from each object,
rather than a global value for the entire frame. Taking a sky
measurement from an annulus with, say, radii of 10-15 arcseconds
is probably okay.
Now, at this step, you should have relative, uncalibrated magnitudes
of the SN and nearby stars. Again, if you want to stop here and
send me the results, great! If you want to keep going - beware;
it gets tricky. However, in order to make the most of your
observations, you MUST perform some kind of calibration, as follows:
Sixth, immediately before or after your observations of the
SN, take pictures of a field of standard stars from which one
can figure out the exact characteristics of your system. I suggest
using the Landolt field of PG 0918+029 (see the paper by Landolt
in AJ, 104, 340 (1992)). This field contains five stars of
different colors in the magnitude range from 12-15, so it
might be hard to find: the coordinates of the central star
are (2000.0) RA=09 21 34, Dec=+02 48 01 (or, in 1950 coords,
RA=09 18 58, Dec=+03 00 49). Note that this field is
near the celestial equator, faaar from M81 - but it is at roughly
the same RA. Alternatively, you may use the "dipper" asterism
in M67 (Schild, PASP 95, 1021 (1983)), which is easier to find:
it's located in M67, centered on (2000.0) RA=08 51 21, Dec=+11 46 16
(or, in 1950 coords, RA=08 48 37, Dec=+11 57 33).
If you've gone this far, and taken calibration frames, I must
ask you to reduce them just as you did those of the SN, and also
to find the calibration coefficients for your system: that is,
find the relationship between the instrumental magnitude values
for the standard stars and the ACTUAL, calibrated magnitude
values published in the papers mentioned above. For an example
of how you might do this, see the classic article by Hardie
(in "Astronomical Techniques", 1962, ed. W.A. Hiltner, part of
the "Stars and Stellar Systems" series), or that of Harris,
Fitzgerald and Reed (PASP, 93, 507 (1981)).
Calibrating your system accurately is a LOT of work, but,
as I say, it's required to get the most accurate results. I doubt
that one could do photometry to better than 0.05 magnitudes or
so without it. Of course, for our purposes, photometry to
0.1 or even 0.2 magnitudes can help to fill the gaps, and so
calibration is not REQUIRED. Just nice :-)
Seventh - if you've figured out the calibration, apply it
to your results from step 5 above. And pat yourself on the
back - you've done more than many people who publish their
results!
In summary, here are the steps again:
1. Find and filter and use it!
2. Observe SN (as often as possible). Note time, place and date
of observations.
3. Note all instruments used.
4. Reduce CCD frame (bias- and dark-subtraction,
flat-fielding, any other necessary steps)
(stop and send results to Michael?)
5. Perform aperture (or PSF-fitting) photometry, with
local sky values, to SN, plus stars A,B,C and
possible others.
(stop and send results to Michael?)
6. Observe calibration fields, reduce data, determine
calibration constants (extinction and color terms)
7. Apply calibration to instrumental magntiudes
(stop and send results to Michael?)
8. Write paper, submit to AJ :-)
I will be more than happy to answer questions that you might
have, although I would hope that they are ones that can't be
answered by reading the papers mentioned above.
Anyone whose data is used in our publications will of course
receive acknowledgment, and - if the quality and quality of
your data is sufficient (say, >= ten nights of observation
with high S/N and well-described instrument?) - become
co-authors!
Michael
--
----- Michael Richmond
"This is the heart that broke my finger." richmond@astro.princeton.edu
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