[vsnet-chat 1355] Magnitude estimates from unfiltered CCD images

[aah@nofs.navy.mil]

Brian replied to Gianluca, and while I agree in most detail, there are a couple of points
Brian neglected to mention.

USNOA-1.0 r magnitudes are roughly on the Kron-Cousins R system, and so should be better
than GSC for unfiltered CCDs.  The zero point on a plate-to-plate basis flops around,
and the nonlinear transformation common to all photographic plates was not handled well
due to the lack of photometric standards.  Still, differential magnitudes between two
stars in the same field should be relatively ok.  USNO-A2.0 is much better in this
regard.  Dave used Tycho to set the bright end, and set the faint end on about 300 fields
using deeper BVRI photometry.  He then did an iterative adjustment on all the plates
using the plate overlap region so that the magnitude system is self-consistent.  USNOA-2.0
should be formally announced very soon (if it hasn't been already), and is the catalog
of choice IMHO.

Brian suggested using R magnitudes from the many Landolt standards to set the zero point
for your unfiltered images.  The technique is reasonable, but he did not mention that
you need to make sure you adjust the instrumental magnitudes for possible differing
exposure times between your program frames and the standard star frames,
and that if the standards are at a different airmass, atmospheric
extinction needs to be considered.  Since your unfiltered observations cannot directly
be compared with anyone else's observations, probably just using some of the many
BVRI stars in Brian's master LONEOS list would be sufficient to set the zero point, and
are more likely to be near the asteroid/variable field and therefore not need airmass
corrections.  In any case, I would determine your unfiltered, but with zero point applied,
magnitudes for a couple of stars in your program field and report the GSC/USNO ID along
with their magnitudes in any report (like "I used GSC xxxx with R=13.0 and GSC yyyy 
with R=12.8 as comparison stars"), so that others can match your system as closely as
possible on subsequent observations.

Gainluci also asked about public domain programs for doing CCD photometry.  Two I've
found are:

MUNIDOS at http://monoceros.physics.muni.cz/~rudolfn/
EZPhot at http://vsnet.mtco.com/~jgunn/

I have no idea how well either of these two programs work.

Arne

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