Recalibration of AGASC1.3 from ACA MAG FITS from ACA DATA to Produce
AGASC1.4

Paul J. Green, John Grimes, & Kris Eriksen
31 Jan 2000

See /data/mp2/ACA_cal/dec99/ for data, code, and notes.
This file is /proj/asc/www/LOCAL/mp/html/agasc1p4.html

Occasionally, a poorly estimated AGASC magnitude has resulted in failure to acquire a star. A larger problem is that the padding on star magnitude errors in the OFLS Characteristics, means that too many stars are being rejected from consideration, so that targets are rejected, or have a number of guide stars that is less than optimal. In Aug 1999 PCAD and aspect camera (ACA) exercises (for IRU Cal observations and M7 ACA Field Distortion) provided about 100 realtime observed ACA magnitudes with which to compare the current star catalog AGASC1.3

All the tracked stars are compared below to AGASC1.3, which shows that a mean offset of about 0.26mag in the sense that AGASC1.3 magnitudes (MAGACA) are too bright. 

	  ACA-MAGACA COLOR1
 NUM          103      103
 MED        0.187    0.573
 MEAN       0.259    0.657
 SIGMA      0.353    0.572
 MIN       -1.214   -0.126
 MAX        1.432    1.832
Here are further details. In reality, this is a strong function of color (usually cataloged B-V): the mean offset is 0.13mag for bluer (B-V < 0.7) stars, but is 0.41mag for red stars. Here is a plot of the difference between observed ACA mag and AGASC1.4 MAG_ACA values, as a function of (B-V) color.

Here I plot only stars with a measured color or spectral type. Note that many stars in AGASC have no cataloged colors, for which we assume (B-V)=0.7mag. The mean colors for the 103 stars here is 0.66, bearing out this assumption.

NEW ACA DATA More recently, Jeff Shirer et al. produced, using GRETA, data for 876 star observations while in Normal Pointing Mode and while on Kalman filtering. Magnitudes more than +/- 0.5 from the mean were excluded as outliers. The y and z angles for the stars are corrected for dither, but they are not statistical averages over the entire observation. The angles are instantaneous values, 60 seconds after going to Kalman filtering. John Grimes used the Y and Z angles, along with the OBS timing information to compare to SOE files, to find the correct corresponding AGASC star.

Star data is now easily found by AGASC_ID using a tool that spews out chosen columns for each AGASC_ID:
/data/mpcrit1/bin/getagasc.pl <AGASC_ID> Removal of repeat observations of the same star cull the list down to 271 unique stars ACA observations, matched to cataloged AGASC stars. For a primer on how MAG_ACA values are calculated, see MAG_ACA update pseudocode. Essentially, I fit (ACA - V) as a function of (B-V). Based on this new ACA data, I've done polynomial fits to predict ACA mags for all stars, to recalibrate the MAG_ACA values. New fit results are as follows: 

 C0      C1      C2      C3      C4      Reduced Chisq
-0.079   .       .       .       .       0.193
 0.542   -0.730  .       .       .       7.829E-02
 0.355   -0.022  -0.442  .       .       6.918E-02     CHOSEN FIT
 0.359   -0.060  -0.382  -0.025  .       6.943E-02
 0.355    0.162  -1.113   0.704  -0.224  6.955E-02
Note that the older fit was similar: 
 C0      C1      C2      C3      C4      Reduced Chisq
0.347    0.099  -0.482  .       .       7.84321E-02     CHOSEN FIT

So the most reasonable fit is a quadratic. The difference between AGASC1.4 magnitudes and observed ACA magnitudes as a function of color for this sample is shown below, both for a simple offset (0th order fit) and quadratic (2d order fit).

While the mean difference is 0 above, the 1-sigma dispersion in the plot is 0.26mag. This figure illustrates the inescapable dispersion involved in a transformation based only on B,V colors to a camera system with a much redder bandpass.

Finally, the new fit shows no signs of being magnitude dependent:

Three important points:

1) A few percent of stars will be rather extreme outliers (a magnitude or more off) even when colors are known. This may be due to extinction, or unusual spectral features, or variability. The outliers in this dataset have Observed-Predicted extremes of -1.2 and +1.7 mag.
2) I made the fit and plot above using stars with cataloged (B-V) colors. Many stars in AGASC (nearly all that are fainter than 10th) have no cataloged colors, so a color of (B-V)=0.7 is assumed. The dispersion in a observed-predicted plot for such stars will be significantly larger. However, some 90% of stars bright enough for guide star selection are Tycho stars with measured colors.
3) The new calibration will improve things in 2 ways. The OFLS will not tend to pick as many red stars as it does now, because the effect of color has exaggerated the predicted brightness of stars in AGASC1.3. The predicted magnitudes of stars picked will be closer to the actual magnitudes, ESPECIALLY for red stars.
4) We may now change the values in the OFLS CHARACTERISTICS file from their preflight values: 
  
       ODB_MAG_ACA_NSIG  = 3,                    
       ODB_MAG_ACA_ERR_RAN = 0.34,               
       ODB_MAG_ACA_ERR_SYS = 0.5,                
to
       ODB_MAG_ACA_NSIG  = 3,                    
       ODB_MAG_ACA_ERR_RAN = 0.26,               
       ODB_MAG_ACA_ERR_SYS = 0,                

The OFLS SSA MAG_ACA bright and faint thresholds are calculated as
     MAG_ACA_BRT_THR= MAG_ACA - DELTA_THRESH
     MAG_ACA_FNT_THR= MAG_ACA + DELTA_THRESH
where
	DELTA_THRESH   =  MAG_ACA_ERR_SYS + 
  MAG_ACA_NSIG*sqrt(MAG_ACA_ERR*MAG_ACA_ERR + MAG_ACA_ERR_RAN*MAG_ACA_ERR_RAN)

The OFLS SSA simply uses MAG_ACA_BRT_THR
and MAG_ACA_FNT_THR as filter thresholds for consideration
of guide stars.  No star is accepted for which either
of these conditions holds

	MAG_ACA_BRT_THR < ODB_BRT_MAG_AC
	MAG_ACA_FNT_THR > ODB_FNT_MAG_AC

The fraction of stars from the GRETA sample that have |Predicted-Observed| greater than 3sigma is 3percent. This shows, as expected, that the distribution is not Gaussian. The proposed change to the CHARACTERISTICS will decrease the typical value of DELTA_THRESH from 1.5mag to 0.8mag. A corresponding change must be to reduce the current wide limits from 

  
       ODB_BRT_MAG_AC    = 4.5,     
       ODB_FNT_MAG_AC    = 11.3,   
       ODB_BRT_MAG_G     = 4.5,     
       ODB_FNT_MAG_G     = 11.3,   
to
       ODB_BRT_MAG_AC    = 5.2,     
       ODB_FNT_MAG_AC    = 11.0,   
       ODB_BRT_MAG_G     = 5.2,     
       ODB_FNT_MAG_G     = 11.0,
Narrowing those limits somewhat reduces the number of stars available for use as guide stars, but the change is needed to prevent stars that are too bright or faint from being selected.

UPDATE of AGASC1.3 to AGASC1.4

Here are details on how MAG_ACA and spoilercode updates are performed to a copy of AGASC in place.

NOTE that the data format remains unchanged from AGASC1.2

RELEVANT STATISTICS for AGASC1.4

Stellar Surface Density The average stellar surface density of unspoiled stars brighter than MAG_ACA=10.2 with color information ((ASPQ1=0, CLASS=0, C1_CATID.ne.0) is 9.5 stars per square degree in AGASC1.4 Near the galactic poles (b>80deg), where the stellar surface density is lowest, there are 4.1 stars per square degree. The desired figure of merit (FOM) of 5.1 per square degree over 95% of the sky is thus not quite achievable with these selection criteria from current catalogs, and may not ever be (i.e. we are already nearly complete). The current Chandra guide star selection includes stars without TYC colors or PPM SpType information, which boosts the surface density, but at this limiting ACA magnitude, such colors are available for 98% of stars. 
  
 b1	b2	Nstars  Ndeg^2	stars/deg^2
		w/colors	w/colors
----  -----     -----   -----   --------------
 80	90	1285	313.3	4.10
 55	65	8405	1797	4.68	
 25	35	24951	3113	8.02	
-90	90	393623	41253	9.54
Changes in Selected Guide Stars from AGASC1.3 A key measure of the effects of this update is the change in selected stars between AGASC1.3 and AGASC1.4. Roughly 1/3 of the Cycle 1 observations will have different star sets chosen by the CXC MP Star Fidlight Evaluator (mp_sfe) for the new AGASC 1.4 mags. How do the stars chosen in these fields differ? 
  
All Fields	AGASC1.3	AGASC1.4
		---------	---------
mean mag:	8.448		8.630
mean fom:	0.15308		0.31221
min fom:	0.13669		0.13721
max  fom:	0.32571		0.32755
  
Number of
guide stars:	AGASC1.3	AGASC1.4
		---------	---------
5		634  (94.9%)	 610  (91.3%)
4		 22  ( 3.3%)	  33  ( 4.9%)
3		  9  ( 1.3%)	  17  ( 2.5%)
3		  3  ( 0.4%)	   8  ( 1.2%)
FOMs and the number of suitable stars per field brighter than MAG_ACA=10.2 looks slightly worse in AGASC1.4 Since AGASC1.4 is simply more realistic, that's nature's fault, not ours. Here's the breakdown on the number of individual target fields whose stars change between AGASC1.3 and AGASC1.4: 
  
0 stars changed: 434 ( 65.0%)
1 stars changed: 175 ( 26.2%)
2 stars changed:  47 (  7.0%)
3 stars changed:  12 (  1.8%)
4 stars changed:   0 (  0.0%)
5 stars changed:   0 (  0.0%)

total fields analyzed:  668
total stars analyzed:	3285

FUTURE AGASC UPDATES MAY INCLUDE

1) Incorporation of stars from the Extrastars catalog.
2) Incorporation of new (actual) MAG_ACA values from ACOSC.
3) The most likely major catalog merge would involve additions between about 9th and 12th magnitude from the USNOA-2.0 catalog.