[Aavso-photometry] Ensemble photometry - revised

[arne]

Jim Roe wrote:
> I'm trying to understand and learn how to do "ensemble photometry" 
> because I have come to believe that it will produce more accurate and 
> precise results.  Right?  I have re-visited threads on the list, 
> Googled, read several papers but I have yet to find a clear, concise 
> definition of the technique.  E.g.,
> 
> Honeycutt (PASP 104:435-440, June 1992) states: "A simple (and often 
> quite adequate) technique for CCD ensemble photometry is to calculate 
> the difference between the instrumental magnitude for the program star 
> and a comparison magnitude obtained from the sum of the intensities of 
> perhaps a dozen of the brighter stars which appear in each exposure of 
> the series."  This doesn't seem right to me, perhaps it is a typo that 
> was mentioned in an earlier post.
> 
> In another paper on the web by Hayes-Gehrke 
> <http://www.astro.umd.edu/~avondale/extra/Variability.html>
> it is stated "Ensemble averages make this technique even more reliable. 
> Given at least a few dozen stars in the image, the assumption is made 
> that the average magnitude of these stars would remain constant from 
> frame to frame, regardless of the type or behavior of the individual 
> stars. "  This makes more sense to me.
> 
> Finally, The MaximDl manual says: "If you tag more than one reference 
> star, you must set the magnitude for each; the results will be 
> averaged."  Does this, then, constitute "ensemble photometry?"
> 
> If so, and I would just as soon continue using Maxim given my $400 
> investment therein, several questions arise.
> 
> Which extra reference stars should I include?  Presumably only from 
> those for which photometry exists, but sometimes (often) there are 
> photometry stars that are much fainter than the program star and, unless 
> I expose for good S/N on them, their measurement errors will be greater 
> and it would seem adding them to the "ensemble" will degrade the whole 
> system.
> 
> Would I consider all of the individual reference stars to be check stars 
> to be compared to the ensemble average?
> 
Dang - one *important* word got left out.  Here is a revised reply.
------------------------------
We are not recommending ensemble photometry submission until the
new upload formats are announced (shortly; how shortly depends on
the conclusion of our beta testing period, so I can't give a
definitive date).

The basic concept is that you have multiple comparison stars in your
image, so why not use all of them to derive a magnitude for the
target object?  You can think of it as (V-C1), (V-C2), etc., where you
average these values together.  That reduces the effect of some bad
measurement of any comparison star, variability of the comparison stars,
systematic effects like transformation, etc.  So it is good, right?
The devil lies in the details.  There are multiple methods of
creating such an ensemble solution, from using flux space or magnitude
space, averaging (V-C) or forming a master comparison star, how
many stars form the ensemble, how do you weight the individual
measurements, etc.  In addition, what magnitudes you use for the
comparison stars impacts the final solution, and we cannot deconvolve
this effect sometime later like we can if you use just a single
comparison star.

The new format gives a method for submitting ensemble results, and includes
a way in which we can adjust the photometry later.  I hope it works!  In
addition, the new comparison star database photometry is precise enough
that choice of ensemble stars should not affect solutions dramatically.  So
we are getting closer to accepting ensemble results.
Arne