Re: Calculation of mag errors

[Andrew Bennett <andrew.bennett@ns.sympatico.ca>]

On Mon, 7 Mar 2005 18:49:12 -0500, Stupendous Man
<richmond@stupendous.cis.rit.edu> wrote:

>
>  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.

Great! I was always too lazy to attempt to program this ...

I noticed when I went through your code that you don't
add a term for the uncertainty in estimating the
background level that gets subtracted from each
pixel. Years ago, I  went through your code, which was
then quite different from the current version, and, as I
remember it, it did add a term for this. I distinctly
remember taking exception to the way you
calculated the variance of the median. I also
distinctly remember not posting to the list about it on the
grounds that the difference was too small to make
a fuss about.

Now that you have done all the dirty work of
setting up the statistics for the annulus, is this
a good time to revisit estimating this error?
One might be able to estimate things like
background curvature - if too big, go to a
smaller annulus and accept the higher random
error in exchange for the smaller systematic
error. I never tried to do this: I couldn't see how
to do it in a manner that was not horribly
sensitive to nearby confusing stars.

Andrew Bennett, Avondale Vineyard, NS, Canada