Re: Calculation of mag errors

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

  As a first step to fixing the error in calculating uncertainty
in instrumental magnitude, I have added a new option to the
"phot" program within XVista.  This is the program which takes
a list of stellar position and a corrected image, extracts
instrumental magnitudes via aperture photometry, and prints out
the results.  

  The old version calculated uncertainty by separately determining
the variance in the sky background, the readnoise, and the light
from the star itself, then adding them all up.  That method
underestimates the error if a mean sky level has been subtracted
from the image.

  The new version takes the approach Mike G. suggested: it calculates
the scatter in pixel values within an annulus around each star,
and uses _this_ to estimate the variance in the background
(a combination of sky and readnoise and dark current).  
It adds in the variance due to the star, then compares to 
the signal from the star to estimate an uncertainty.

  I tested the new routine on some Mark IV images (with
names like hvra3302755.fits, in case you want to check yourself).
The uncertainty in instrumental magnitudes for stars
of roughly V=12 was -- with the old method -- about 0.020 mag.
The new code produces values around 0.050 mag, much closer
to the values Andrew Bennett and I detemined manually.

  The "phot" program now has a new command-line option
to choose this mode of operation; by default, it will still
do things the old way (backwards compatibility is a good thing).
I have created a new release of the XVista package to incorporate
this change: XVista-0.1.3.  You can find it at

      http://spiff.rit.edu/tass/xvista/

  The next step is for me to make a few very small changes
to the pipeline code so that the "phot" program is called
with this new option (if the user chooses to do so, of course).
That could be done by the weekend.

  Tom has requested that I make another change to the pipeline,
which would take much more work:

> As you know, the present pipeline requires a V and I measurement to output
> data.
> 
> This is a shame because we lose bright I stars as soon as the V value
> falls below about mag 15.  This provides only a very narrow range for
> tracking very  red stars.
> 
> Would it be possible to modify the pipeline so as to output all the
> measurements?

  In order to do this, I would have to modify the code which
performs the nightly photometric solution so that it accepts
stars which have only a single V-band or I-band measurement,
without a matching detection in the other band.  In theory,
this doesn't sound difficult -- just assume some reasonable
color -- but in practice, it may involve some complex
changes to the bookkeeping.   I don't know how long that
would take.

  Just wanted to keep people up to date ....


                                       Michael Richmond