Summary
This document describes the effect of radial spoiling in star selection. One initial step in building a list of candidate guide stars is to exclude candidate stars with a nearby neighbor star (so-called radial spoilers). In this study I have done this initial screening for a large number of fields (10000) distributed isotropically over the sky, using several different exclusion radii. For fields with fewer than 10 candidate guide stars, there is very little dependence on the exclusion radius. For crowded fields, a larger radius decreases the available set of candidate guide stars, but never drops the number below ~10. Based on these simulations, there appears to be little advantage to changing the OFLS spoiler radius parameters. A better avenue is to explore allowing the OFLS to choose stars as faint as 10.5 mag if needed. This is already done on a regular basis with SFE-generated star catalogs.Simulation
- Stars were extracted from the AGASC (using mp_get_agasc) at approximately 10,000 locations distributed uniformly over the celestial sphere. The grid locations were selected in Galactic coordinates, so that the entire Galactic plane (B = 0) was sampled. For each field, a radius of 0.8 degrees was extracted. This matches the total area of the ACA CCD. (The CCD is square, while the extracted region is a circle, but this should not affect the results).
- Stars fainter than 13.5 mag (ACA magnitude) were completely thrown out, which means they were not considered as spoiler stars.
- Candidate guide stars were selected based on:
- MAG <= 10.2 (or 10.5, see below)
- ASPQ1 <= 1
- ASPQ2 = 0, ASPQ3 = 0, CLASS = 0
- A candidate guide star was considered spoiled if
- Three values of spoil_rad were used: 9, 11, and 13 pixels.
- For all cases I used slope = 0.7
- For each field I tabulated the number of candidate guide stars (based strictly on the MAG <= 10.2, ASPQ1 etc criteria), and the number of spoiled guide stars for the three cases of spoil_rad.
- In addition, the number of candidate guides stars with MAG <= 10.5 was calculated.
- DIST < CLIP(mag_cand - mag_spoil)*slope + spoil_rad
where CLIP(x) = x if (x < 0), CLIP(x) = 0 if (x >= 0)
Results
The plots below show the distribution of the number of unspoiled candidate guide stars. The top plot shows the fraction of fields with the given number of unspoiled candidate stars, and the bottom shows the cumulative distribution of fields with less than or equal the given number of candidate stars.. The green line corresponds to spoil_rad=13 pixels, blue is spoil_rad=11 pixels, and red is spoil_rad=9 pixels. The black line shows the distribution for candidates selected with MAG <= 10.5, with spoil_rad=13.
The clear result is that in the fields with few candidate guide stars, there is very little dependence on the value of spoil_rad. This makes intuitive sense, since those fields tend to be sparse. A much more effective way to increase the number of candidate stars is to bump up the magnitude limit a bit. This is exactly what the ACA team does with the SFE in cases when catalogs if there are insufficient guide or acquisition stars.
Another result comes out which may not have been so obvious: In crowded fields, using spoil_rad=13 and including stars down to 13.5 mag, there is no problem finding sufficient unspoiled candidate guide stars. This is shown in the scatter plots below, showing the number of unspoiled candidate stars versus total number of stars in each field (brighter than 13.5 mag). As expected, when the total number of stars goes up dramatically, there is only a modest increase in the number of available guide stars. This is essentially due to crowding. The important result is that there are very few crowded fields with fewer than ~10 candidate guide stars, especially if you allow stars down to 10.5 mag.
Tom Aldcroft
Last modified:03/30/01