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

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

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

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

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

Predicted NIR Magnitudes GSC-II matched to UKIDSS/LAS GSC-II optical mags median B,R,I = 19.5, 18.1, 17.0 faint 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, 15.7 42% predicted JAB < 18 Observed: 32% observed JAB < 18

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

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

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

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

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

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

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

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

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

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

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