PHOTOMETRY FOR DUMMIES: METHOD #2
Bruce L. Gary, Hereford Arizona Observatory (G95)
Last updated 2005.01.02
0.15 mag SE,
Using
Several Tycho or UCAC2 Reference Stars (5 Minutes)
This method is a slight elaboration of the previous one and achieves it
improved performance by using several UCAC2 or Tycho stars as
"reference stars." As stated before, be sure you have UCAC2 files on
your hard disk and configure your planetarium program to support this
catalog.
1) Look for at least 4 stars in your image that aren't seriously
saturated (i.e.,
maximum counts <35,000).
2) Use your planetarium program to identify these stars.
3) Use the planetarium program to determine the UCAC2 magnitudes for
these stars. If Vmag >10.5, add 0.25 mag.
4) Set the photometry aperture so that
most of the "light" from the reference stars fall within the signal
circle, and check that
no interfering stars are in the sky background reference annuli.
5) Select the Photometry Tool (in MaxIm DL click Analyze/Photometry).
6) Set the mode to Object and left-click the asteroid.
7) Set the mode to Reference, and for each UCAC2 reference star
left-click it and enter the UCAC2 V-magnitude for that star.
8) Click View, and save the results to a CSV-file.
9) View the CSV-file (using your favorite utility, such as Total
Commander), and read the asteroid's magnitude.
Note 1: Since the UCAC2 catalog exhibits magnitude errors that
appear to be random at a given star region it's possible to reduce the
UCAC2 calibration uncertainty, SEc, by using N reference stars the
calibration achieves a systematic uncertainty approximately given by
0.20 / SQRT(N). If you use 4 UCAC2 stars, then you may achieve 0.10 mag
SEc. This overlooks the fact that color terms are still missing from
the analysis.
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