IntroductionBelow is presented a proposed re-calibration of the AXAF Guide and Acquisition Star Catalog (AGASC). This calibration is motivated by inaccurate values of MAG_ACA, the expected magnitude in the ACA bandpass, at B(Tycho)-V(Tycho) > 0.9 in the flight AGASC (version 1.5). This error results in predicted values exceeding half a magnitude brighter than the actual ACA observed magnitude. The below plot shows this offset. See the previous report for more details. To correct this problem, we propose re-populating the MAG_ACA and MAG_ACA_ERR arrays in the AGASC for all stars originating from the Tycho-2 catalog (about 99% of the usable acq/guide stars) using a calibration based on the V-Band magnitude and the actual Tycho-2 color.Data ProductsNew predictions for MAG_ACA are determined from a database of 31219 stars acquisition attempts from the past five years of operations. Only stars that were positively identified by the OBC and having valid Tycho-2 magnitude and color information were used. This criteria reduces the number of stars used for re-calibration to 30238 ( Note that this statistic indicates a dominance of Tycho-2 stars over acquisition candidates in addition to fantastic camera performance. ). Previous calibrations have used flight data in the form of guide star magnitudes from a GRETA script. While the acquisition process uses only a single data point (the OBC magnitude at the moment the star is acquired), the GRETA data was averaged over an entire observation. It is good and useful to show that these are equivalent. Below are plots of the acquisition and guide star magnitude offsets for 1587 observations. Median values are discretized to 1/16 mag. The center of each distribution is less than 5/1000 magnitudes away from zero.Analysis and ResultsThe expected ACA magnitude is calculated as a function of the Tycho V-Band magnitude and Tycho color (VT-BT). Below is a the observed offset of Tycho magnitude from ACA magnitude against VT-BT, as well as the fit to be used for calibration.
scolerr= COLOR1_ERR/100. For AGASC 1.6, the catalog error is calculated as follows:
scolerr= COLOR2_ERR/100. In addition to simply recalculating MAG_ACA_ERR, we now have the means to see how the catalog empirically matches the ACA response by looking at the spread of observed stars. For each color bin defined above we characterize the aca response error as the mean deviation from the expected error.
obs_err = MAG_ACA - obs_mag So for each star in the catalog the AGASC 1.6 MAG_ACA_ERR is calculated by adding the appropriate response error (depending on color only) to the propagated catalog error. Below are three scatter plots that show the effect of the above calculations. The first plot is the AGASC 1.5 MAG_ACA_ERR vs. COLOR2. Note that stars with COLOR2 > 1.76 (1.5/0.85) have errors set to 0.87 mag. The second plot shows the catalog MAG_ACA_ERROR calculated for AGASC 1.6. The third plot shows the combination of the catalog error and the response error. Aspect Information main page Comments or questions: Aspect Help Last modified:12/27/13 |