1. Markus Nielbock (MPIA) – Herschel Pointing PACS ICC Meeting #38 Herschel Pointing Summary and Recent Developments Markus Nielbock (MPIA Heidelberg) with input from Herschel Pointing WG and IOPR #2 MPE, 31 st May 2011

2. Herschel Pointing Overview ● apparent systematic pointing offsets ● STR focal length parameter update ● STR1/STR2 alignment ● CP raster (gyro nodding) ● a few words on astrometric accuracy Markus Nielbock (MPIA) – Herschel Pointing

3. Herschel Pointing History Markus Nielbock (MPIA) – Herschel Pointing ● speed bumps in PACS scan maps were observed ● STR CCD hot pixels found responsible (not cooled) ● applied Peltier cooling from +13°C to -10°C (finally on OD 320, final SIAM) ● “... was not anticipated to be used during the mission.” (Galileo official, IOPR #2) ● concerns about possible impact on pointing accuracy already raised early

4. Herschel Pointing Current situation (1) Markus Nielbock (MPIA) – Herschel Pointing ● no apparent change of offsets, but scatter in y and z differ ● scatter in z non-Gaussian (individual offsets of up to 7”) ● chopped/nodded and scan maps agree → not caused by telescope nodding

5. Herschel Pointing Current situation (2) Markus Nielbock (MPIA) – Herschel Pointing ● affects wavelength calibration and line profile of PACS spectrometer ● influence on flux entering the central spatial pixel

6. Herschel Pointing Systematic pointing shifts (1) Markus Nielbock (MPIA) – Herschel Pointing

7. Herschel Pointing Systematic pointing shifts (2) Markus Nielbock (MPIA) – Herschel Pointing ● apparent seasonal variation ● no offsets observed in astrometry ● not due to lack of coordinate precision (uncertainty: 4 mas) ● pointing also depending on sky range (e.g. when Herschel observes Taurus region)

8. Herschel Pointing STR guide star pattern Markus Nielbock (MPIA) – Herschel Pointing ● large offsets when guide star pattern is highly asymmetric ● wrong plate scale leads to wrong pointing fit ● focal length parameter in ACS was not changed after CCD temperature change!

9. Herschel Pointing STR focal length (1) Markus Nielbock (MPIA) – Herschel Pointing STR CCD temperature change ● focal length in ACS to determine plate scale not adequate (never was!) ● explains occasional large pointing offsets ● error cancels out for homogeneous guide star distribution ● would also explain pointing scatter in general → APE improvement possible

10. Herschel Pointing STR focal length (2) Markus Nielbock (MPIA) – Herschel Pointing ● simulation of influence of incorrect focal length parameter on APE ● reproduces the current pointing behaviour very well

11. Herschel Pointing STR focal length (3) Markus Nielbock (MPIA) – Herschel Pointing ● simulation of influence of corrected focal length parameter on APE ● resulting rms after 1-dim focal length parameter correction for both y and z: 0.7” ● APE improvement to approx. 1” (no STR interlacing) can be expected ● even further improvement possible if 2-dim correction is applied ● however: industry (Galileo) very reluctant

12. Herschel Pointing Correction programme Markus Nielbock (MPIA) – Herschel Pointing ● STR focal length parameter update scheduled for OD 762 (14 June 2011) ● pointing behaviour investigation on OD 763 or 764 ● there is hope that SRPE also improves (important for chopped spectroscopy) ● possible 2-dim correction of ACS/STR to be discussed afterwards

13. Herschel Pointing STR1/STR2 alignment Markus Nielbock (MPIA) – Herschel Pointing ● two STR systems, not well aligned ● autonomous switch from STR1 to STR2, if STR1 is declared unhealthy ● thermally induced pointing drift + pointing offset to STR1 ● rare occurences: possible loss of observations was considered to be more ● affordable than time consuming STR2 pointing calibration programme ● major ACS/STR failure during Christmas period changed that opinion ● dedicated pointing calibration programme done of ODs 731-733 ● comprised > 40 pointing stars + pointing drift analysis after STR switch

14. Herschel Pointing STR1/STR2 alignment Markus Nielbock (MPIA) – Herschel Pointing Δy = 5.47” Δz = 13.06” APE = 1.62” ● update of on-board STR1/STR2 rotation matrix to be implemented by FDS

15. Herschel Pointing CP raster – The initial problem Markus Nielbock (MPIA) – Herschel Pointing ● low telescope nodding accuracy ● NOD A and NOD B do not agree → SPEC signal recovery impaired ● Question: Can the reproducibility be improved by pointing under gyro control? CP raster / gyro nodding Disadvantages: ● not throughly tested during CoP ● large observing overheads due to proper ● gyro calibration

16. Herschel Pointing CP raster – Photometer tests – OD 516 Markus Nielbock (MPIA) – Herschel Pointing

17. Herschel Pointing CP raster – Photometer tests – OD 649 Markus Nielbock (MPIA) – Herschel Pointing

18. Herschel Pointing CP raster – First spectrometer test OD 716 Markus Nielbock (MPIA) – Herschel Pointing fine pointing module 11 CP raster Signal evolution

19. Herschel Pointing CP raster – First spectrometer test OD 716 Markus Nielbock (MPIA) – Herschel Pointing fine pointing module 12 CP raster Signal evolution

20. Herschel Pointing CP raster – First spectrometer test OD 716 Markus Nielbock (MPIA) – Herschel Pointing fine pointing module 13 CP raster Signal evolution

21. Herschel Pointing CP raster – Repetition on OD 740 Markus Nielbock (MPIA) – Herschel Pointing fine pointing CP raster

22. Herschel Pointing CP raster – Conclusions Markus Nielbock (MPIA) – Herschel Pointing ● altogether CP raster not too promising ● very complicated – error prone at FDS level, manual interaction needed ● large observing overheads, still no guarantee for success ● calibrated gyro drift could jump at any time ● deadline for possible release fixed for OT2 call (early June) ● cannot be met ● Is it worth to put more effort into investigating this mode? ● (lessons learned for future missions)

23. Herschel Pointing Astrometry Markus Nielbock (MPIA) – Herschel Pointing ● extracted obs time from frames ● determined pm shift of stars manually ● photAssignRaDec() ● correctRaDec4Sso(...,linear=0,...)