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CUBISM
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  1. CUBISM or How I Learned to Stop Worrying and Love the Cube J.D. Smith JDS 1

  2. Today’s Menu • SINGS IRS Program Refresher • CUBISM — your friendly neighborhood cube-builder. • From here to there • Assembling cubes from BCD inputs. • ...and back again • Spectral maps and line extraction. • Leftovers — casserole anyone? JDS 2

  3. SINGS IRS Program • LL radial strips: 4–16x11 steps, both slits at once: • 76.2 hrs; 936.7 MB. • SL: 1x9, subslits SL1 & SL2 pointed separately: • Nuclear: 16.1 hrs; 224.6 MB. • Extra-Nuclear: 36.6 hrs; 224.6 MB. • SH & LH : 3x5 (SH: 2 cycles): • Nuclear: 61.3 hours; 561.6 MB. • Extra-Nuclear: 62.1 hours;549.1 MB. JDS 3

  4. Mapping Data • The IRS spectral maps range in size from 15 to 176 DCE’s per slit. • Each galaxy’s nucleus is covered by ~85 individual slit pointings. • SL maps will offer “satellite” wings, since each sub-slit is mapped sequentially over the same area. • LL maps will have non-uniform sub-slitcoverage. JDS 4

  5. Background • CUBISM is a tool for constructing spectral cubes (,, — one pixel per location/wavelength) from both low and hi-res IRS spectral mapping data sets, the release-form of which will consist of: • The Cube-Builder Back End. • CubeView and CubeProject Interface Components. • Written entirely in IDL†, drawing on existing elements of SCOREX. †Except for one auto-compiled, fault-tolerant component to be written in C for speed. Currently targets: IDLv5.5. Will target: IDLv5.6. JDS 5

  6. Development Synopsis • Low-Res  High-Res, progressing through three stages: 1. Rough cube, no de-fringing, error weighting, adjustable grid spacing, or refined image combination. 2. Proper weighting for combination from pixels at different pointings and spectral orders. 3. Possible additions: tunable grid spacing, optimized slit-image combination. • Simulated data was/is critical. • SIM data progressed alongside the cube builder to provide increasingly robust checks. JDS 6

  7. Cube Builder • Assemble cubes directly from BCD data set, minimizing interpolation — pixels take shortest route possible from BCD to cube. • Single cube pixel: • Data from different pointings (e.g. half-slit offsets) • Data from different orders on the array (bonus order/high resolution modules) • Provides “pixel accountability.” • Controls error “blooming” caused by unnecessary correlating operations (e.g. fractional pixel shifts or rotation). JDS 7

  8. Drizzle Cube Builder • Permits flexible grid spacing. • Will accommodate pointing uncertainties and geometric distortions in the slit image (if measured). • Allows Resolution vs. S/N tradeoffs — exploring alternative cube “sky” grid spacing. • Flux–conserving, drizzle-like (with many more clip operations, but unit drop size). • Separate cube foreach sub-slit. JDS 8

  9. Cube Building — Details 1. A few to dozens of BCD’s are read into a cube “project”. Headers are fully parsed. 2. Logical row/column numbers, plate scale, slit and slit dimensions stored for locating samples on the mapping grid. 3. Calibration set object loaded from disk • Cached order position, wavelength solution, PR clipping areas and polygons, etc. • User-selectable. New aperture clips automatically stored. JDS 9

  10. Cube Building — Details 4. Build order and aperture selected (normalized “slit-length”, linearly varying across order). 5. Spatial dimensions of the cube calculated from map step coordinates & slit size. 6. Pixel-based WAVSAMP samples obtained from calibration object for chosen aperture. 7. Merge data for combining wavelength-overlapping orders pre-computed from order(s) selected and calibration inputs. JDS 10

  11. Cube Building — Details 8. Wavelength dimension of the cube computed from merge data. 9. Accounting list: BCD pixel  Cube pixels, with overlap areas. Slow to compute: cached (big). Associated with BCD record (compute individually). 10. Each order and each PR considered in turn. Partial pixels de-rotated (slit-rotation, position-angle deviations within map, etc.), offset and clipped to sky grid (can’t trust pointing reconstruction). JDS 11

  12. Cube Building — Details 11. Accounting cube filled and (possibly) merged with existing accounts. 12. Reverse accounting histogram constructed from merged accounts: Cube Pixel  BCD Pixels. 13. There’s no step 13. 14. Reverse accounts used to combine appropriate fractions of contributing BCD pixels in cube. Total area also accumulated. 15. Area-weighted cube complete. JDS 12

  13. Cube Builder JDS 13

  14. Maps and 1D Spectra • Both spectral maps (e.g. [Ne II], PAH 11.3µm) and (non-BQD) 1D spectral extractions (e.g. resolved nucleus) can be made directly from the cube. • Flexible map creation: arbitrary band-pass. • Simple square aperture, expanding aperture, or full parallel aperture photometry 1D extractions: • Can be tailored for changing extraction environment. • Potential for automatic aperture selection (brightest knot). JDS 14

  15. Archival Format • Discussions with FITS spec-writers led to: • FITS Cube/Error Cube/Flag Cube in subsequent FITS Extensions (-headers — STScI convention). • Wavelength map for each cube plane in a single column of a binary table in a separate extension, pointed to by PS3_{0,1} with CTYPE3: ‘WAVE-TAB'. • Wavelength sampling still unresolved: • Linearize? • Small or vanishing overlap… • Other archive product formats TBD. JDS 15

  16. Archival Format JDS 16

  17. From SIM to Cube JDS 17

  18. From SIM to Cube JDS 18

  19. From SIM to Cube JDS 19

  20. From SIM to Cube JDS 20

  21. Interface • CubeView & CubeProject — defer to demo. • Fundamental Interface Doctrine: All cube operations can be performed without the interface (scripting): • E.g. after 100 cubes have been delivered, the calibration data gets a major update: re-do them all by hand? No, simply script it. JDS 21

  22. CUBISM Schedule • How we’re doing: • Overall, quite well. • Cube improvements more involved, unexpected “deeper” issues discovered  interface lagged. • February, 2002: First internal alpha team release. • December, 2004: First full release to the SSC (to coincide with first data cubes available), with SSC-scientist level documentation. JDS 22

  23. The Leftovers • MIPS SED. • Assorted cube improvements planned. • Interface gravy. • Unknowns: • Fringing. • Deep cube building issues. • Questions and uncertainties for the SSC. JDS 23

  24. MIPS Coordination • MIPS SED mode can produce spectral maps too! • In coordination with the MIPS team, MIPS SED data will also be processed and displayed in CUBISM. • Analogous to a single Low-Res IRS order (e.g. SL1), but with more involved detector issues (offsets, varying gains, etc.). • MIPS-specific issues will be factored out (G. Bendo). JDS 24

  25. Cube Improvements • Position-based offsets: bizarre combinations, like SL1_cen, followed by SL_cen at another epoch (and PA). Exact telescope/calibration agreement on slit center required. • Position optimization — treating reconstructed positions in aggregate. • Full error treatment, and alternative BCD pixels  Cube pixel combination choices (area weighted: A, A2, A2t, clipped, median, etc.) JDS 25

  26. Cube Improvements • Auto-aperture selection (guided by SIMIII). • Noise floor for data rejection. • User definable bad pixels. • Updated and optimal extractions: • Expanding aperture. • In-place sky removal ala parallel aperture photometry. • Images stacked according to line fit (e.g. de-blending line pair). JDS 26

  27. Interface Additions • Pixel-account querying and feedback. • User-definable bad pixels. • Saved extraction band-pass sets. • Graphical manual/auto spectral aperture selector. • Circular aperture, expanding cube aperture extraction selector. JDS 27

  28. Spectral vs. Spatial • Pixel-level ambiguity between spatial & spectral resolution (slicers undersample). • Plan: carry 1n & 2n for comparison. JDS 28

  29. Fringing • IRS has fringes (<15% peak-peak) in both high-resolution modules at R~50, originating in the detector substrate. • Photometric accuracy of line maps impacted. • Multiple de-fringing efforts underway. • 1D vs. 2D de-fringing, in or after the IRS pipeline? • CUBISM favors 2D de-fringers, but will evaluate all options when the smoke clears after launch. JDS 29

  30. Questions & Uncertainties • For the SSC: • Flux calibration: different for extended objects (sensitive to slit throughput function)? • Reformulated WAVSAMP: pixel-based? • Error plane reliability and simulations. • Wait and see: • Pointing inaccuracies: Drift, Reconstruction errors. • Background subtraction strategy (SV). JDS 30

  31. Fin

  32. Raw SCORE (SH Prototype) Atmospheric Emission Spectrum JDS 33