1. Bradley/Markley p1 SNAP Fine Guidance 11/16/2001 G&C SNAP Fine Guidance Art Bradley/ spacecraft System Engineering Services Landis Markley/ NASA/GSFC Nov. 16, 2001

2. Bradley/Markley p2 SNAP Fine Guidance 11/16/2001 G&C Observing strategy  An acquisition strategy is proposed to conduct field mapping and spectroscopy.  Field mapping is performed with a dwell scan algorithm — Simple flight code and command I/F –Primary eigenvector –Secondary eigenvector –Number of dwell points –Jerk pulse width –Constant rate interval –Primary eigenvector jerk pulse magnitude –Secondary eigenvector jerk pulse magnitude –Dwell time  Fine guidance CCD damps drift rate during shutter closure.

3. Bradley/Markley p3 SNAP Fine Guidance 11/16/2001 G&C Stellar distribution  Star tables of candidate Guide Stars provided for planning purposes only.  Two 25 deg 2 fields have been compiled with centers at 16h25m +57º and 4h30m -52º  Started with HST GSC.  Supplemented with latest Yale Bright Star Catalog & removed duplicates.  Supplemented with USNO A2.0 to get beyond 18m v & removed duplicates. A2.0 stars only populated in the 4ºx4º central region.

4. Bradley/Markley p4 SNAP Fine Guidance 11/16/2001 G&C For the purposes of this study, 16 sq deg regions were selected near the NEP and SEP, to determine guide star probability. Northern region  RA from 16h17m to 16h33m  DEC from +55º to +59º Southern region  RA from 4h22m to 4h38m  DEC from -54º to -50º

5. Bradley/Markley p5 SNAP Fine Guidance 11/16/2001 G&C Stellar distribution near NEP

6. Bradley/Markley p6 SNAP Fine Guidance 11/16/2001 G&C Stellar distribution near NEP

7. Bradley/Markley p7 SNAP Fine Guidance 11/16/2001 G&C Stellar distribution near SEP GSC population from 15.5 to 16m v not strong, should have used A2.0 in that region.

8. Bradley/Markley p8 SNAP Fine Guidance 11/16/2001 G&C Stellar distribution near SEP GSC population from 15.5 to 16m v not strong, should have used A2.0 in that region.

9. Bradley/Markley p9 SNAP Fine Guidance 11/16/2001 G&C FGS Design – Area  4 geometric arrangements were examined — 200x200 arcsec- one CCD — 400x400 arcsec- four CCD’s — 200x400 arcsec- two CCD’s column stacked — 400x200 arcsec- two CCD’s row stacked

10. Bradley/Markley p10 SNAP Fine Guidance 11/16/2001 G&C FGS Design – placement

11. Bradley/Markley p11 SNAP Fine Guidance 11/16/2001 G&C FGS Design – Guide Star Probability  Determined with 100 arcsec increments in Right Ascension and Declination.  NEP m v range is 5.5 to 18.2  SEP m v range is 6.1 to 17.6 NEPSEP 200x200 asec95.77%97% 400x400 asec99.995%100% 200x400 asec99.8%99.9% 400x200 asec99.8%99.3%

12. Bradley/Markley p12 SNAP Fine Guidance 11/16/2001 G&C FGS Design – Guide Star Probability  Metric will be reduced for near neighbor’s and exclusion areas around CCD edge resulting from attitude Star Tracker error.

13. Bradley/Markley p13 SNAP Fine Guidance 11/16/2001 G&C FGS Design – Histograms

14. Bradley/Markley p14 SNAP Fine Guidance 11/16/2001 G&C

15. Bradley/Markley p15 SNAP Fine Guidance 11/16/2001 G&C

16. Bradley/Markley p16 SNAP Fine Guidance 11/16/2001 G&C

17. Bradley/Markley p17 SNAP Fine Guidance 11/16/2001 G&C

18. Bradley/Markley p18 SNAP Fine Guidance 11/16/2001 G&C

19. Bradley/Markley p19 SNAP Fine Guidance 11/16/2001 G&C

20. Bradley/Markley p20 SNAP Fine Guidance 11/16/2001 G&C

21. Bradley/Markley p21 SNAP Fine Guidance 11/16/2001 G&C FGS Design – Roll Control  CCD’s placed on opposite ends of the focal plane can provide: — optional roll control — disturbance detection –Small focal plane X-Y disturbances will be greatly magnified in the roll axis. –This indicator (even if not used for control), could assist in post facto image analysis and on-board Loss-Of-Lock detection.

22. Bradley/Markley p22 SNAP Fine Guidance 11/16/2001 G&C Recommendation  200x400 arcsec CCD’s on opposite ends of the focal plane  Redundancy  Roll control   95% Guide Star probability after star elimination.

23. Bradley/Markley p23 SNAP Fine Guidance 11/16/2001 G&C