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 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, 17.0 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, % 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 ~ %
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 2MASS at |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 Latitudes Holfeltz, Nelan, Chayer 2009, in progress
Comparison of GSC2.3 and UKIDSS LAS at High Galactic Latitudes JWST-STScI , SM-12 Holfeltz, Chayer, Nelan 2009, in review
Characterizing Non-Stars in GSC2.3 JWST-STScI , 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 Chayer, Nelan, 2008