ngst/nexus operations jane morrison october 19, 2000 nexus ngst
TRANSCRIPT
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NGST/NEXUS OperationsJane Morrison
October 19, 2000
Nexus
NGST
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NGST at a Glance
Formulation Phase (A/B) Implementation Phase
- Logical successor to HST
- Key part of the Origins Program
99 00 01 02 03 04 05 06 07 08 09
Select Prime PDR/NAR CDR Launch
• 8m primary mirror• 0.6-10+ µm wavelength range• 5 year mission life (10 year goal)• passively cooled to <50K• L2 orbit• Launch 2009
LRD
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Guider is part of NIRCAMNIRCAM: 0.6 to 5.3 micronsCamera composed of 4 sub cameras each covering a field of 2 2 arc minutes.Guiding with:
Large bandpass filters - a portion of sub-camera used for guidingNarrow bandpass filters- entire sub-camera used for guiding.
Guiding with NGST
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Guiding using NIRCAM
10 10 pixel area around guide star is read at 30 Hz. The 512 512 readout area is excluded from science observations 2'
4'
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Requirements of Guide Star Catalog
All Sky catalogAt least 1 arc second positional accuracySufficient density to ensure a 95% probability of finding a suitable guide star.Limiting magnitude corresponding to a 95 % probability of finding NGS > 1
IJ 17.0 JJ 16.2 KJ 15.5
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Guide Star Catalogs for NGSTGuide Star Catalog: GSC-2 supplemented with 2MASS2MASS
•300 million stars and other unresolved objects•For unconfused sources outside the |b| < 10 • J = 15.8 , H = 15.1 and Ks = 14.3• Positional accuracy better than 0. 5 arc seconds
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Guide Star Catalogs for NGST cont.
GSC-2• 2 billion stars, galaxies and other objects•All sky catalog of position, magnitudes, proper motions and colors • Positional accuracy better than 0. 3 arc seconds• Limiting magnitude needed to meet goal of 95% probability of finding a guide star in the FOV of NIRCAM (Spagna) J385 < 19.5 F610 < 18.5
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Areas of Concern
Uncertainty in predicting NIR magnitudes from Photographic J, F and N bands. Contamination of the stellar population with galaxies (problem alleviated somewhat in galactic plane relax magnitude limit for guide star). Contamination by unresolved blends Gaps in Coverage
•Over-exposed regions•Crowded regions
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Summary: Guide Star Catalog for NGST
GSC-2 + 2MASS sufficient density and accuracy to be used as the guide star catalog for NGST.
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Nexus•Pathfinder for NGST - flight demonstration testbed for NGST Technology
Prior to launch, each phase of Nexus will reduce risk & help NGST.Actual Nexus flight/mission will be final validation with enough time to fix problems for NGST.
•Launch Date: late 2004/early 2005.•Mission Orbit: L2.
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Nexus Targets
Nexus must validate critical NGST wavefront sensing and control and guiding procedures.
Search for targets and associated guide stars to ensurethere will be an adequate number of pairs for WS testing.
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Target Area(on sky)
1.3’ 1.3’ 1.3’
WSC Detector Guider Detector
0.92’
0.92
’
0.38’
0.38
’
- area covered by field diversity experiment (50%)
- initial allowable area for guide star
initial location of target
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Catalog Search• The Catalog
– Tycho-2 Catalog– 2.5 million stars with positions, proper motion,
and magnitudes (BT and VT)– Observations from the ESA Hipparcos satellite in
combination with the Astrographic Catalog and 143 other ground-based star catalogs
– Density: 25 stars deg-2 (Galactic poles), 150 stars deg-2 (Galactic plane).
– Positional accuracy: 60 mas– Photometric accuracy: 0.10 mag– Limiting magnitude: K~10-11
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Query: Selection Criteria• Target Stars
– Magnitude: K<10– No nearby neighbors within 27” and
brighter than Kprimary+5• Guide Stars
– Magnitude range: 8<K<14– No nearby neighbors within 5”
• Separation – Annulus with radius=2.6’±0.19’
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Process of Selection• Sort catalog (in declination)• Convert to K magnitude• Reject objects
– Color– Magnitude error above tolerance– No mean position given (no ground-based
position)• Find target objects
– Search for nearby spoiler stars– Search for guide stars– If match found – search for spoiler stars for GS
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Color Transformations
(BT – VT)
(VT –
K)
data from Gunn and Struker (ApJ Supp. 52 June 1983)
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Results of Selection• Number of objects in catalog: 2 539 913• Rejected objects:
– No mean position given 122 543– B-V > 1.3 472 566– B-V < -0.39 12 080– Magnitude error > 0.5m 25 796– Total number rejected 632 985
• Number of objects remaining: 1 906 928• Target/GS pairs found: 82 755
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Histogram of Pairs found
223758-9
491569-10
76837-8
25256-7
7695-6
1844-5
523-4
112-3
# Pairs Magnitude of Target Stars
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Tycho-2 magnitude distribution
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Distribution of Pairs on Sky
# pairs =1016
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# pairs =2525
Distribution of Pairs on Sky
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# pairs =7683
Distribution of Pairs on Sky
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# pairs =22315
Distribution of Pairs on Sky
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# pairs =49156
Distribution of Pairs on Sky
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Conclusions
• We find about 82,000 qualified target pairs (Nqual)• Number of instantaneously available target pairs
(Ninst) is Nqual multiplied by following factors:– 10/360 (orientation)– 0.8 (spectral type)– 1/3 (field-of-regard)
• Ninst=600• Continue with study- Use a catalog with a fainter
limiting magnitude ( GSC-2); this could have a dramatic effect in adding spoilers