[vsnet-chat 2488] Re: vsnet-chat 2485

[Brian Skiff <bas@lowell.edu>]

     In vsnet-chat 2485, Mati Morel asks about what led to the choice of stars
measured by the galaxy photometry observers.  This was driven pretty solely
by stars that appeared in the photometer apertures used to measure the galaxy
itself.  This was so that their light could be subtracted out of the light
from the galaxy + stars.  Thus stars outside the relatively small photometer
diaphragms used (rarely over 4' diameter) were ignored in most cases in
determining the galaxy magnitudes and colors.  I am presenting data from a
specific set of publications, not an inventory of all stars superposed on
galaxies.  Thus the recent lists are just a part of that, but do provide a
starting point for supernova photometry in the galaxies that have been
observed in this way.
     In the specific cases of NGC 300 and other Sculptor group galaxies, I am
indeed aware that there is additional photometry by Alcaino & Liller for stars
in these fields.  All such stars _not_superposed_ on the galaxies themselves
have long since been included in my main photometric reference file.  Have a
look at it, Mati!  Alcaino & Liller unfortunately do not specify what 
photometer apertures they used to measure stars superposed on the galaxies,
and one can suspect since small telescopes were involved that they could have
been fairly large, leading to possibly erroneous results.  The galaxy observers
by contrast measured the stars with the smallest diaphragm available to them
(typically 8"-10"), and measured the galaxy background carefully along the
same isophote in each case.  I suspect, for instance, that the V magitude A&L
give for GSC 6413-1094 is possibly too bright (not enough galaxy background
subtracted out), and their B-V and V-I colors are somewhat inconsistent, too,
(V-I too blue by ~0.1 mag. relative to their B-V), again suggesting problems
with background subtraction.  
     As I noted in the previous lists, the Longo & de Vaucouleurs collection
is itself not error-free, and given the difficulties in doing single-channel
photometry on superposed stars, I felt free to omit stars where the colors
were plainly wacko (non-physical).  Clearly a lot of this work should be done
over again using a CCD, since the star profiles and background can be modelled
more adequately.  As I intimated in my first post, this is another area where
amateurs using standard filters could contribute.

The big list and biblio are at:
http://ftp.lowell.edu/pub/bas/starcats/loneos.phot.gz
http://ftp.lowell.edu/pub/bas/starcats/loneos.ref


\Brian

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