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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:

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jwst fgs guide star studies

JWST FGS Guide Star Studies


Nov. 19, 2009

Sherie Holfeltz


Ed Nelan & Pierre Chayer

jwst guide star id acq
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
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
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
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
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
pattern matching in the id image
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
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
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
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
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
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
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
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
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
mitigating guide star failures
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
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