Calibration of AGASC 1.7

Introduction

Below 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) > 1.5 in the flight AGASC (version 1.6). This results in predicted values being up to 3 magnitudes fainter, and exceeding 1 magnitude brighter than the actual ACA observed magnitude. To correct this problem, we propose re-populating the MAG_ACA and MAG_ACA_ERR arrays in the AGASC, and populating the RSV1-3 arrays, for all stars with available B(Tycho) - V(Tycho) color, originating from the Tycho-2 catalog, using a calibration based on the V-Band magnitude, the actual Tycho-2 color, and the new color, V - i, from the AAVSO (American Association of Variable Star Observers) Photometric All-Sky Survey (APASS) DR9 catalog. It is important to recalibrate these stars as often they act as spoilers. In AGASC 1.7, the RSV1-3 arrays contain the new V(APASS)-i(APPAS) color, V(APASS) magnitude, and a flag indicating if MAG_ACA and MAG_ACA_ERR arrays have been updated compared to AGASC 1.6, respectively.

The following Jupyter notebooks contain details of matching the AGASC 1.6 and APASS catalogs and details of the proposed AGASC 1.7 re-calibration method.

Further details are given in the readme file, /data/agasc1p7/readme.txt

Data Products

New predictions for MAG_ACA are determined from a database of 60600 guide stars observed from the start of the mission until day 2018:150. Only stars that were tracked for more than 98% of time at a position within 5 arcsec from their catalog position, had at least 500 ACA magnitude measurements, valid Tycho V magnitudes, valid V(Tycho) - B(Tycho) colors, and valid V(APASS) - i(APASS) colors were used. These criteria reduce the number of stars used for re-calibration to 24319.

The properties of these 24319 guide stars were compared with the properties of two testing star sets: acquisition stars (excluding any stars that were used also as guide stars), and all stars with shaped MAG_ACA distribution (i.e. MAG_ACA distribution matching that of the guide stars) observed before day 2018:150. All sets show similar behaviour in the V(Tycho) - B(Tycho) (or COLOR2) vs. V(APASS) - i(APASS) (or COLOR3) vs. MAG_ACA(1.6) - V(Tycho) parameter space, as demonstrated in the figure below.

Analysis and Results

The expected ACA magnitude is calculated as a function of the Tycho V-Band magnitude, and Tycho COLOR2 for stars located to the left of the "calibration belt" marked in the figure below with the dashed lines, or a new CX coordinate calculated as a combination of COLOR2 and COLOR3 for stars within the "calibration belt". This figure contains only the guide stars used to develop the re-calibration model.

Below is the observed offset of Tycho magnitude from ACA magnitude against COLOR2 (i.e. VT-BT) or CX, as well as the fits used for magnitude calibration and associated magnitude errors.

The data were split into bins and we applied a 1d interpolation models to the binned data. These models were used to derive recalibrated MAG_ACA values. To estimate errors on the new predictions, we derived standard deviations in each bin and computed final MAG_ACA_ERR for each star by applying error proparagion formula sqrt(std**2 + VT_err**2), where VT_err is the error on the catalog V(Tycho) measurement, MAG_ERR.

The improvement in ACA_MAG prediction is presented below. The new calibration performs accurately well past the point were AGASC 1.6 fails. For BT-VT < 1.5 the calibrations are essentially equivalent. For BT-VT > 1.5, however, there is no contest.

Validation and Testing

The below plot shows a histogram of changes to MAG_ACA between AGASC 1.6 and 1.7. The histogram peaks at dmag = 0, as expected, indicating no or very small change to MAG_ACA with the new calibration. Histogram wings extend to values of -1 and 3 indicating mag correction to stars that till now were belived to be fainter/brighter than in reality.

SSAWG discussed the proposed re-calibration at multiple meetings between April and September, 2018, and approved it on Aug 18, 2018 (see 2018-04-11, 2018-04-18, 2018-05-02, 2018-07-17, 2018-08-15, 2018-09-19).

Final validation and testing has been performed independently by SOT and FOT. The teams confirmed that between AGASC 1.6 and 1.7:
- no stars are missing or were added,
- no changes in RA, DEC,
- all values in all columns except MAG_ACA, MAG_ACA_ERR and RSV1-3 are the same,
- there are only expected differences in MAG_ACA, MAG_ACA_ERR, RSV1-3 columns,
- there are only expected differences in the header comment cards.



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Last modified:09/18/18