NGST/NEXUS OperationsJane Morrison

October 19, 2000

Nexus

NGST

NGST at a Glance

Formulation Phase (A/B) Implementation Phase

- Logical successor to HST

- Key part of the Origins Program

- 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

Guider is part of NIRCAM

NIRCAM: 0.6 to 5.3 microns

Camera 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 guiding

Narrow bandpass filters- entire sub-camera used for guiding.

Guiding with NGST

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'

Requirements of Guide Star Catalog

All Sky catalog

At least 1 arc second positional accuracy

Sufficient 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

Guide Star Catalogs for NGST

Guide Star Catalog: GSC-2 supplemented with 2MASS

2MASS

•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

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

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

Summary: Guide Star Catalog for NGST

GSC-2 + 2MASS sufficient density and accuracy to be used as the guide star catalog for NGST.

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.

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.

Target Area(on sky)

1.3’ 1.3’ 1.3’

WSC Detector Guider Detector

0.92’

0.9

2’

0.38’

0.3

8’

- area covered by field diversity experiment (50%)

- initial allowable area for guide star

initial location of target

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

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’

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

Color Transformations

(BT – VT)

(VT –

K)

data from Gunn and Struker (ApJ Supp. 52 June 1983)

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

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

Tycho-2 magnitude distribution

Distribution of Pairs on Sky

# pairs =1016

# pairs =2525

Distribution of Pairs on Sky

# pairs =7683

Distribution of Pairs on Sky

# pairs =22315

Distribution of Pairs on Sky

# pairs =49156

Distribution of Pairs on Sky

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