Choosing Guide Stars for JWST:
the Suitability of Guide Star Catalog 2 (GSC-2)
Jerry Kriss
With help from:
Jeff Stys
Ed Nelan
Brian McLean
JWST
Guide Star Catalog 2
James Webb Space Telescope
 Collaborative effort between STScI and the Osservatorio Astronomica
di Torino
 The COMPASS source database contains ~2 billion stars, galaxies,
and other objects based on measurements of about 6000 sky-survey
photographic plates
 Three colors: J385 (B band), F610 (R band), and N9/N715 (I band).
 Official release of GSC-2.2.0 is magnitude-limited to J=19.5, F=18.5,
but full catalog contains objects down to J~22 and F~20. (Only J and
F are currently available.)
 Above these limits, magnitudes accurate to ~0.15 mag; below, the
accuracy is ~0.5 mag.
 Positional accuracy is 0.25-0.40 arcsec (11-16 mag), 0.20-0.65 arcsec
(>16 mag).
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Concerns over using GSC-2 for JWST
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 GSC-2 is based on visual plate material. The JWST Fine Guidance
Sensor (FGS) will be a near-infrared camera.
 Catalog completeness as a function of magnitude
 Catalog contamination fraction as a function of magnitude
 Photometric accuracy, especially at the faint end, and the reliability of
transforming photographic magnitudes to the infrared.
 Are there enough stars to assure that JWST can acquire a guide star
with 95% probability at any location and any orientation in the field of
regard? (More stringent than HST, ~80% at any given orientation.)
• Facilitates mosaic surveys and multiple repeat visits
• Enables multi-instrument follow ups
(common for NIRCam  NIRSpec)
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• Simplifies scheduling and operations
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Evaluating GSC-2 using SDSS
 The Sloan Digital Sky Survey (SDSS) has 5 photometric
bands (u’, b’, g’, r’, i’, and z’) that probe much deeper with
more accurate magnitudes than GSC-2 (22.0, 22.2, 22.2,
21.3, and 20.5) in the five bands.
 SDSS object classification is highly accurate. To the full
depth of GSC-2, it is essentially 100% correct in
classifying stars vs. galaxies.
 We used a 2-square degree region at
(,)(J2000) = (190.0, 0.1) to (192.0, 1.1), (l, b) = (301, +62)
from the SDSS Early Data Release to compare to GSC-2.
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James Webb Space Telescope
Classification Accuracy of SDSS
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James Webb Space Telescope
Evaluating GSC-2 using SDSS
 The Sloan Digital Sky Survey (SDSS) has 5 photometric
bands (u’, b’, g’, r’, i’, and z’) that probe much deeper with
more accurate magnitudes than GSC-2 (22.0, 22.2, 22.2,
21.3, and 20.5) in the five bands.
 SDSS object classification is highly accurate. To the full
depth of GSC-2, it is essentially 100% correct in
classifying stars vs. galaxies.
 We used a 2-square degree region at
(,)(J2000) = (190.0, 0.1) to (192.0, 1.1), (l, b) = (301, +62)
from the SDSS Early Data Release to compare to GSC-2.
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 Objects were matched between the two catalogs using
positional coincidence.
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Positional Errors in GSC-2 vs. SDSS
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James Webb Space Telescope
Transforming GSC-2 Magnitudes
to the Infrared
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Errors in the Transformation
Dispersion is 0.4 mag
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Completeness of GSC-2
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James Webb Space Telescope
Completeness of GSC-2
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Using I-band magnitudes when they become available should enhance
completeness by 0.5—1.0 magnitude.
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The Stellar Population at the
Faint Limits of GSC-2
J = 19
J = 17
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Contamination Fraction of GSC-2
Differential # of Objects vs. J385 Magnitude
1200
Unknown
Galaxies
Stars
Total
11.8%
38.6%
1000
# of Objects
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1400
4.4%
800
4.8%
600
2.8%
400
2.9%
4.5%
200
32.0%
12.2%
80.0%
0
13-14
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14-15
15-16
16-17
17-18
18-19
19-20
20-21
21-22
22-23
J385 (1 magnitude bins)
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Contamination Fraction of GSC-2
Integral # of Objects vs. J385 Magnitude
Unknown
Galaxies
Stars
Total
4500
14.3%
14.4%
22
23
4000
7.7%
# of Objects (cumulative)
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5000
3500
3000
5.6%
2500
2000
6.3%
1500
7.1%
1000
9.7%
15.0%
500
48.6%
28.1%
14
15
0
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16
17
18
19
20
21
J385
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Meeting the Requirement for a 95%
Probability of Guide Star Acquisition
 For a 16 Hz sample rate,10 e- read noise, and a required
centroiding accuracy of 2.1 mas per axis, the FGS is likely
to have a limiting magnitude of JAB < 20.
 GSC-2 is as complete as an IR survey down to JAB~17.4;
for 17.4 < JAB < 20, it is ~75% complete.
 Contamination rates (primarily due to galaxies) down to
JAB = 20 are ~8%; adding in the ~2% fraction of close
binaries seen by HST implies an overall contamination
rate of ~10%.
 Given this contamination rate, at least 2, and preferably 3
candidate guide stars should be uplinked for every field
(Isaacs 2001).
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Guide Star Acquisition Probabilities
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