Re: Software to perform inhomogeneous ensemble photometry

[Michael Koppelman <lolife@bitstream.net>]

In regards to this:

>   I suppose I could make a column for "uncertainty in magnitude"
> in my reports, with the same value 0.004 for each measurement.
>
>   Yes, it would be even better to figure out some sort of
> uncertainty for each individual measurement, based on a combination
> of the SNR for that measurement and the overall nightly scatter,
> but I don't know how to do that properly.

...the way I do it is just adding the 1/SNR and the comp star 1-sigma 
in quadrature:

err = sqrt( (1/SNR)^2 + comp_sigma^2)

Why I find this useful is you can see when clouds rolled through or bad 
tracking occurred and such because in those cases the low SNR causes 
that term to dominate. Not all data points are necessarily created 
equal.

In response to this:

>   Hmmm.  Are you sure that you meant to write "the standard deviation
> of any given comp stars is the same as for all the other comp stars?"
> If I have two comp stars, a bright one and a faint one, I would
> expect that the standard deviation from the mean magnitude would be
> smaller for the bright star.

This and the zeropoint question are related. For each frame Mira makes 
a solution based on the inputted magnitudes of the comp stars. It does 
a least squares on the differences between the instrumental magnitudes 
and the given magnitudes. The effectively spreads the error out among 
all the comp stars. For each frame, each star will end up being the 
same distance away from it's "true" magnitude. This creates the 
situation where all the comp stars end up with the same 1-sigma. The 
zeropoint error stamped in the header is the uncertainty in the least 
squares fit.

Your software sounds cool. It compiled and installed no problem on Mac 
OS X. I haven't played with it yet...

Cheers,
Michael Koppelman
http://www.lolife.com/astronomy/