Photometer Spatial Calibration

1. Photometer Spatial Calibration D. Lutz, P. Popesso Photometer spatial cal

2. Focus & Central pointing position • FMILT: Focus ok… • Pray for Herschel focus • Offset phot/spec measured Photometer spatial cal

3. Enlarged z adjustment range of OGSE compared to CQM test was sufficient • All focus sequences analysed • Inconsistencies between peak maximum and width minimum removed after proper consideration of non-telecentric behaviour of test optics • Optimum focus: ILT1 blue: +2.9mm, ILT2: data useless, ILT3 blue: +4.2mm, ILT3, red: consistent with blue • For those ILT tests done with slightly different z (2mm…5mm): Effect on PSF width negligible Photometer spatial cal

4. Offset to Spectrometer [Center of blue photometer array to central spec module] Note: 3.25 deg tilt wrt XY stage, at direction consistent with other array misalignments (see below) Photometer spatial cal

5. FOV distortion • ILT3 was crucial to obtain the necessary data • Now excellent characterization for the ILT case: Bolometer arrays ↔ OGSE XY stage • Need modeling for transfer XY ↔ Herschel sky – N.Geis, ongoing – see his presentation Photometer spatial cal

6. ILT3 rasters with 1.5mm hole at 7 chopper positions • First process each raster to a 4D array Signal=f(p,q,x,y) • Changed peak fitting from ‘image plane’ to ‘raster plane’. This significantly improved quality of fits: No need for flatfield, simple treatment of dubious pixels (drop), easily include edge pixels. Photometer spatial cal

7. ‘Raw’ projection of pixels into xy plane Bad pixels Bear crosstalk removes some peaks and makes them coincident with peak of crosstalking pixel Photometer spatial cal

8. Cleanup (1) • Remove all pixels with • Low signal • High noise • Known bear crosstalk > 0.3 * true signal • Side result: input for a ‘bad pixel list’ Photometer spatial cal

9. Cleanup (2) • Some neighboring pixels remain with overlapping projection! Mostly in red but also one case in blue. • Some smaller local distortions of red array projection are reproducible between different tests – not noise • Indication for more bear-crosstalk like behavior? Photometer spatial cal

10. Cleanup (3) Lost telemetry packets (cases in both blue and red) confuse 4D array assembly and subsequent fit – discard affected region Photometer spatial cal

11. Examples of cleaned datasets Total: 13620 points blue 3273 points red Photometer spatial cal

12. Step 1: Derive inter-matrix gaps • Assume individual matrices are perfect 750μm pitch grids • For each chopper position separately, fit a model combining relative shift/tilt of matrices with a 2nd order distortion • Typical RMS residues 0.027 xy mm (blue) 0.07 (red). See above overplot of observation (black asterisk) and fit (red cross) • Matrix offset/tilt well reproduced at different chopper positions • Red gap 1.34 pixel, blue gaps of same order • Has been converted to SubarrayArray calfiles Photometer spatial cal

13. Step 2: Global distortion fit • Keep fixed matrix offsets from the values obtained in step 1 → u,v, array coordinate for each pixel • Fit to all data second order polynomials x,y=f(u,v,chopper angle). Higher fit orders not significantly better • RMS residues in y direction 0.041 xy mm (blue) 0.073 (red), better in x direction • Has been converted to PhotArrayInstrument calfiles (for ILT situation, and crudely scaled to sky but with ILT distortions) Photometer spatial cal

14. Measured trails for different pixels • Trails with chopper position are very slightly curved • Different slopes indicate distortion or rotation Photometer spatial cal

15. Distortion/Rotation between Chopper angle ±2.5deg (from fit) Photometer spatial cal

16. Projected pixel sizes (cf point source/extended source fluxcal) Photometer spatial cal

17. Some useful misalignment numbers • Angle between chopper and xy stage: 1.38 deg • Angle between chopper and blue array: 1.98deg • Angle between chopper and red array: 2.58deg • Angle between xy stage and blue array: 3.36deg • Angle between xy stage and red array: 3.96deg (all with a grain of salt given they are only approximating the distortions…) Photometer spatial cal

18. Summary Distortion • ILT3 was the decisive step in getting good data • Calfiles have been defined • All these nice results (~1/20 pix accuracy) refer to the XY stage, however. Ongoing modelling to transfer to sky! Photometer spatial cal

19. Point Spread Function • ILT setting: PSF is close to model… • Need better model (Telescope, filter response) for in-orbit situation. Started with NG… • Not addressed here: PSF smearing during fast line scans Photometer spatial cal

20. Basic PSF width results PSF width close to expectations. Biggest deviation ~13% for blue and smallest hole Simulation: Bessel function for ILT setup, consider wide filterbands, proper spatial sampling → PSF rings smeared quite a bit! Photometer spatial cal

21. PSFs from finely sampled rasters (PRF in Spitzer speak) Photometer spatial cal

22. PSF/PRF profile • Slightly wider core / excess at 1-3 gaussian sigmas • No indication for unexpected large scale wings (though difficult to measure given real structure of OGSE setup and background subtraction issues) Photometer spatial cal

23. PACS 70μm map of OGSE hole setup `Shadows’ are artefacts from high-pass filter in reduction Real (very faint) PSF spike? Aligned with array, not with xy. Real structure on the OGSE setup! Photometer spatial cal

24. Slight elongation/’astigmatism’ Changes with focus Highest peak, Narrowest PSF  Roundest PSF Photometer spatial cal

25. Straylight/Crosstalk/Ghosts • `Bear crosstalk’: How far can it be minimized? • Blue streaks: Origin inside PACS • Origin of fainter irregular ghosts: ? Photometer spatial cal

26. Bear Crosstalk • Important for pipeline: • If severe: • Throw away pixels? • Two independent measurements for crosstalking pixel? • If less severe: • Correct by matrix? Photometer spatial cal

27. Bear Crosstalk (2) • How far can it be minimized (for full red array) by tuning voltages (cf. KO presentation at ICC #28) • Is there a similar effect between neighboring pixels in red array (1 pixel also in blue)? Almost coincident symbols - neighboring pixels with same spatial response??? Photometer spatial cal

28. Blue Streaks • Source near corner of blue array (not seen in red array) • % to few % effect • Clearly originating inside PACS – expected also in orbit • Has been mapped out • No immediate pipeline module - Later ‘flag and drop’ for scan maps with large dynamics Photometer spatial cal

29. Weak ambiguous/irregular ghosts Streak like or different? PACS or OGSE? Not clear. Photometer spatial cal

30. Need to confirm in orbit (using bright point sources)! Photometer spatial cal

31. Ghost/Signal on dead pixels - Curiosity? … No equivalent effect seen on good pixels… Photometer spatial cal

32. Some in-orbit spatial calibration issues • Have to work around pointing system accuracy/stability – simple duplication of successful XY stage procedures will not automatically work. • Need for ‘astrometric’ and PSF standards (bright!) that are not considered explicitly in current HCalSG portfolio which is mainly fluxcal driven Photometer spatial cal

33. The End Lockman Hole GOODS North Photometer spatial cal