Magnitude errors in AGASC 1.4

I have extracted the star magnitudes derived by the CXC aspect pipeline for 585 stars observed between Jan. 4, 2000 and Apr. 28, 2000. Repeat observations of the same star are not included. These magnitudes are taken from archived aspect level 1 "GSPROPS" (guide star properties) files. They have been calculated based on the summed pixel values in good images, meaning those images without any bad status flags or outlying centroid values.

The plots below show the magnitude error (observed magnitude - AGASC magnitude) versus three independent variables: color (B-V), AGASC magnitude, and time. In each case I also plot a line corresponding to zero error and a 0.8 mag error. The latter quantity corresponds roughly to the systematic error used in the MAY1000 and MAY1400 loads.

Comments

The color plot shows that the uncertainty increases for redder stars, but that the new AGASC magnitudes successfully keep the mean error near zero.
The plot versus magnitude shows the expected increase in uncertainty for fainter stars
The plot versus time shows no dramatic change in magnitude errors with time, although in the last month there does appear to be an excess of stars which are brighter than expected.

Discussion

In these plots, it is key to note that the systematic error used in commanding during January - April 2000 was approximately 1.5 mags (i.e. faint magnitude limit ~ AGASC mag + 1.5). The fact that only 3 of 585 stars are more than 0.8 mag fainter than the prediction does not seem to be a selection effect. Only 15 of 585 (2.6%) magnitude errors are greater than 0.6 mag. The changes to the OFLS CHARACTERISTICS appear justified on the basis of these data.

Nevertheless, the large number of failed ACQ and GUIDE star acquisitions for the MAY1000 and MAY1400 loads means something was happening. A more careful look at the frequency of failed acquisitions during this time period is a first step.

Details

The analysis is on the HEAD LAN in

/home/aldcroft/Aspect/flt/guide_fid

The key files are:

get_gs_fid.pl		Drive arc5gl to get all gsprops and fidprops files
gsprops_1.3.fits	Star data using original gsprops ACA_MAG.  Mix of 1.3 and 1.4
gsprops_1.4.fits	Star data using AGASC 1.4 magnitudes (780 star points)
gsprops_1.4_uniq.fits	Same as gsprops_1.4.fits, but no repeated stars
gsprops_all.fits	Raw merge of all gsprops.  Includes many repeats.
mag_err.gif		Plot of observed - catalog magnitudes
mag_err.ps		Plot of observed - catalog magnitudes
mags.dat		AGASC magnitudes for the 780 stars
plot_mags.pro		Make mag_err.ps plot

Tom Aldcroft

Last modified: Tue May 16 17:32:59 EDT 2000