JWST FGS Guide Star Studies

1. JWST FGS Guide Star Studies TIPS/JIM Nov. 19, 2009 Sherie Holfeltz with Ed Nelan & Pierre Chayer

2. JWST Guide Star ID & ACQ • The following steps are part of the JWST guide star identification and acquisition process: • The ground system must provide a local catalog containing the positions and expected electron count rates, as measured by the FGS, for the guide star and several reference objects • An Identification image is taken and the observed scene is pattern-matched to the uplinked catalog • If ID or ACQ fails for one guide star candidate, try the next guide star candidate and its reference objects. Up to 3 guide star candidates may be tried • Several studies have been undertaken, touching on many aspects of the guide star ID process

3. The Uplinked Local Catalog • JWST will use GSC-II + 2MASS for guide stars • Deepest all-sky survey available • Optical catalog (B, R, I eff= 0.47, 0.64, 0.85 m) • FGS operates from 0.6 - 5m • For stars without 2MASS mags, GSC-II mags will be transformed into NIR Flux of M0 Star at J = 18.5 GSC-II pass bands 2MASS pass bands FGS pass band

4. Magnitude Transformations Transformations derived for stars at high galactic latitudes (|b|>40o) • cross correlated GSC-II with 2MASS • Fit polynomials to color-color diagrams

5. Testing The Magnitude Transformations • Tested by comparing predicted NIR mags to observed mags from CFHT & UKIDSS/LAS • Allowed testing of GSC-II down to the faint limit of the FGS • Transformations sufficiently accurate when applied to stars; mean mag ~ 0 and 1- errors < 0.4, except for Ks-band predictions when GSC-II lacks I-band magnitude

6. Predicted NIR Magnitudes • GSC-II matched to UKIDSS/LAS • GSC-II optical mags median B,R,I = 19.5, 18.1, 17.0faint cutoffs for B,R,I = 22.5, 20.8, 18.5 • GSC-II optical mags  NIR • Predicted compared to observed magnitudes • Predicted:median J,H,K = 16.4, 15.8, 15.742% predicted JAB < 18 • Observed:32% observed JAB < 18

7. Pattern Matching in the ID Image • FGS identifies the guide star in its FOV by matching the ground supplied predicted scene (red) with the observed scene (blue). • Challenges include: • pointing error • s/c jitter and drift • cosmic rays • missing objects • surprise objects • mis-classed objects • catalog contamination

8. Missing Objects • ~10% of GSC-II objects are artifacts, not real objects • No match in SDSS or UKIDSS and are not seen in HST images • Tendency to be detected in only one GSC-II pass band • Usually near (faint) plate limit • No way to identify them as artifacts based solely on GSC-II data • Faint blue objects may drop out in NIR B,R,I = (20.5, 18.8, 17.1) I-Band Image R-Band Image

9. Surprise Objects • GSC-II is ~69% complete down to JAB < 19.5 for both stars and non-stars, based on cross correlating with ~400 deg2 of UKIDSS/LAS catalog at high galactic latitudes. For every two objects predicted to be in the FGS FOV, there will be one additional surprise object • Objects classified as non-stars in GSC-II out number the stars by 2 to 4 times at high galactic latitudes (|b| > 30) • These objects are bright enough to be seen in the guide star ID image • If not accounted for in the predicted scene, the FGS may fail to identify the guide star due to pattern match confusion

10. GSC-II Non-Stars This study had two goals: • Characterize the GSC-II objects classified as non-stars to evaluate their affect on the FGS’s ability to identify the guide star • Evaluate whether or not the nature of the non-stars could be understood in a meaningful way on an object-by-object basis using only GSC-II parameters

11. Characterizing GSC-II Non-Stars • Archival HST (ACS/WFC) images used to study the non-stars • Objects detected in ACS images were matched to GSC-II catalog • Sizes and shapes from the GSC-II catalog were compared to those measured from the HST images • GSC-II sizes and shapes were not found to be predictive of non-star characteristics: • GSC-II size was strongly correlated with brightness • Dispersion of GSC-II eccentricity strongly correlated with faintness • Size and shape distributions similar for stars & non-stars; most objects of both types have small size measures • Non-stars should be used as reference objects for guide star ID pattern match

12. Catalog Errors • Contamination, e.g., binary stars • JWST will use GSC-II at its faint end, where catalog contamination rate is estimated to be ~12 - 15% • Object type mis-classification • GSC-II mis-classifies ~25% of its stars as non-stars based on SDSS and UKIDSS/LAS data cross correlated with GSC-II • A visual inspection of GSC-II objects matched to HST images (at high galactic latitudes) estimated that up to 20% of both GSC-II stars and non-stars are mis-classed • Mis-classifications more prevalent at the faint end of the catalog, where JWST will be operating • Catalog artifacts • ~10% of GSC-II objects are artifacts • Errors in optical-to-NIR magnitude transformations • Garbage in, garbage out • Non-preferred transformation methods

13. Up to 3 Guide Star Candidates? • Allowing 3 candidate guide stars (if available), each with an 85% success rate, yields a combined success rate of 99.7% • Will we routinely have 3 candidate guide stars available? • Guide star availability studies • Confirmed previous assumption that the Poisson distribution is a reasonable approximation to the distribution of stars in GSC-II over local regions of the sky • # of GSC-II guide stars per FGS FOV at high galactic latitudes • # of 2MASS guide stars per FGS FOV at low galactic latitudes

14. GSC-II Guide Star Availability at High Galactic Latitudes • Virtual FGS FOV scanned over ~170 deg2 of GSC-II at |b|  45 • # stars / FGS FOV: min = 0, max = 12, mean = 2.7, median = 3 • FGS has two FOVs

15. 2MASS Guide Star Availability at Low Galactic Latitudes • Virtual FGS FOV scanned over ~103 deg2 of 2MASSat |b|  30 • # stars / FGS FOV: min = 0, max = 377, mean = 28.2, median = 15(preliminary results) • FGS has two FOVs

16. Mitigating Guide Star Failures • Allowing up to 3 candidate guide stars improves success rate • Including non-stars as reference objects enhances the probability of success • Candidate guide stars with I-band photometry should be chosen preferentially over those lacking I-band photometry • Flight software ID algorithm should be tested against realistic conditions • Most JWST GSC-II work to date focused on high galactic latitude fields. Availability and selection of guide stars in other areas needs to be studied: • in the disk of the galaxy (2MASS; underway) • optically opaque star forming regions • crowded fields • near bright (V<6) stars that may be targets for coronagraphy

17. Relevant Reports • The Areal Density of the 2MASS Catalog at Low Galactic LatitudesHolfeltz, Nelan, Chayer 2009, in progress • Comparison of GSC2.3 and UKIDSS LAS at High Galactic LatitudesJWST-STScI-001668, SM-12Holfeltz, Chayer, Nelan 2009, in review • Characterizing Non-Stars in GSC2.3JWST-STScI-001641, SM-12Holfeltz, Chayer, Nelan 2009, in review • The Distribution of Stars in GSC2.3 at High Galactic Latitudes, Part 1Holfeltz, Chayer, Nelan, 2009 • Algorithms for Transforming GSC-II Magnitudes into the NIRJWST-STScI-001410 Chayer, Nelan, 2008