Format of data in Mike Gutzwiller's star lists

Michael Gutzwiller
Feb 28, 1997

The star lists start with a header section containing the following fields if the catalog match worked: 

# Analyzing image "E:\TASS\ACQUIRE\432\30T0432.820"
# Image noise level was 65 ADUs
# PSF size was 9x7 pixels
# 1102 stars detected
the lines are:
  1. The path of the raw image file that was analyzed.
  2. The estimated noise sigma of the image after dark and flat correction (one sigma).
  3. The size in pixels of the PSF determined by the psf generation logic. This is also the size of the aperture used for photometric calculations.
  4. The total number of stars in the list for this image.
If the catalog match failed or a psf couln't be found or the e-mail option was selected the header will be slightly different but all header lines will start with a "#".

For each star detected a row is printed as follows: 

 186.877  105.149 220877  596  95.647  -0.234  7.640 0.003
    ^        ^       ^     ^     ^       ^      ^     ^
    1        2       3     4     5       6      7     8
  1. X center (the first pixel in a row is at X = 0)
  2. Y center (the first pixel in a column is at Y = 0)
  3. Total ADU counts over background within aperture
  4. Estimated 1 sigma error of (3)
  5. Right Ascension in degrees
  6. Declination in degrees
  7. Magnitude
  8. Estimated 1 sigma error of (7)
Each field is separated by spaces.

Feb 28, 1997: in reply to questions from Herb Johnson 

> (3) Total ADU counts over background within aperture 
> (4) Estimated 1 sigma error of (3)
>
> Is your point-spread function a Gaussian, over the pixel aperture noted 
> in your header? Then this value (3) is a TOTAL of ADU counts within that 
> aperture, distributed as a Gaussian, right? So value (4) is the standard 
> deviation in ADU's from the Gaussian? But in looking at your first 
> starlist, s30t0483.967: although the ADU counts vary between hundreds 
> and
> thousands, the ADU error is between 439 and 508. Why such a limited value - 
> I'd expect a wider range. I smell a bug......
The PSF is empirically determined for each image. The software first scans the image for peaks >20 sigma above the local background and measures the FWHM in x and y for each. The software then finds the median FWHM in x and y for all the candidates and assumes this is the FWHMs of a "typical" star. The software then throws out any of the candidates from the first step whose FWHM in either x or y varies by more than 25% from the median. This throws out spurious peaks, saturated stars, etc. The remaining candidates are averaged to give the PSF for the image.

The ADU sum is calculated by summing the ADU count within the PSF box above the local sky background value. The ADU sum error is calculated by quadratically summing the following contributions: 

   photon shot noise of star itself = sqrt(ADU sum/gain)
   
   sky background shot noise = noise level * number of pixels in PSF box
   
   sky level noise = noise level * (number of pixels in PSF box)^2
                     ---------------------------------------------
                        number of pixels in annulus around star
Only the first term depends on the star itself, the second is constant for all stars, and the third is usually constant but can vary near the edges or near a bright star.

For Tom and myself the sky background is so high that the noise is dominated by the last two terms except for very bright stars.

That is why the ADU sum errors have such a small range. 

> It might also be of value to note the identified stars that best 
> correspond to the cataloged or referenced stars. This would not 
> be
> a large number: I think for the starlist above vs. the SAO list, only 
> about 40 of the latter's stars are in the field of view. Maybe
> you could add a column for either the catalog list star's name, or
> at least a one-column asterisk for the star best matching a cataloged 
> star. I found some of them by "hand", but I don't know if YOUR program 
> "found" them or others.
I have thought about this and will probably add it in the next version. 
> Along these lines, some figure of merit for "best astrometic fit" to all
> the appropriate catalog *positions* would be useful. Say for the 40 SAO 
> stars identified to be in the field, the differences in position between the 
> SAO and the observed stars has a mean of .5 pixels and a variance (you call 
> it
> 1 sigma) of .02 pixels (or convert it to fractional arc minutes). Or: Michael 
> Richmond apparently simply does a "hit count": the correlary would be
> "35 objects identified as cataloged stars, out of 40 in catalog's field of 
> view" or some such to a given criterion. Incidently, my sense of this work 
> from my radio astronomy experience is that you'll be reporting these kinds 
> of statistics anyway in your paper. I can probably get you a good 
> PROCEDURAL example from the (radio) Ohio (State University) Survey 
> publications in
> the Astrophysics Journal: let me know.
A good idea. There are lots of other figure of merit as well, such as constantness of the sky background, etc. In general in any halfway decent image all SAO stars are seen unless they are too bright and saturate the CCD or A/D converter.

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