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Sub-pixel dithering with IRAC

Sub-pixel dithering with IRAC . All of the IRAC channels are undersampled even if the diffraction limit is 6 microns. If better than requirements then severely under-sampled. Original observing strategy was not optimized for sub-pixel dithering. “Raw” IRAC. 4 Position Dither + Drizzle.

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Sub-pixel dithering with IRAC

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  1. Sub-pixel dithering with IRAC • All of the IRAC channels are undersampled even if the diffraction limit is 6 microns. • If better than requirements then severely under-sampled. • Original observing strategy was not optimized for sub-pixel dithering

  2. “Raw” IRAC 4 Position Dither + Drizzle Smoothed to Nyquist 3.6 micron IRAC band

  3. Dithering Requirements • Prefer 4 dither positions: (0,1/2)(1/2,0)(1/2,1/2)(0,0) • Multiple exposures/dither make CR rejection easier • Dither position errors larger than 1/10 pixel increase noise

  4. Current Observing Mode • 5 dither positions • 1 exposure at each position • 30 seconds integration time • High dynamic range • Extra short integration for each exposure

  5. Option 1 • 4 dither positions • 3 exposures at each position • 12 second integration time • High dynamic range • Needs good pointing accuracy to achieve optimum S/N • 5% less time than proposal • Requires good pointing (0.1 pixel rms)

  6. Option 2 • 12 dither positions • 1 exposure per position • 12 second integrations • No high dynamic range • Effect • Increased overhead • Much less sensitive to pointing errors • 17% increase in time

  7. Observing Summary

  8. Recommendation • Adopt Option 1 • Verify early if pointing is good enough • If yes, then stay with option 1 • If not good enough, evaluate whether option 2 is worth the observing time or if option 4 is deep enough.

  9. IRAC pipeline enhancement • Self Calibration (Fixsen et al. 1990) • This allows us to calibrate our data using our data. • Flat Field • Dark Current • Bias • Use variation in source structure and dither to deconvolve source/flat field/ bias effects.

  10. Why do self calibration? • Can deal with the unexpected much better/faster than the pipeline • Can use as a backup to the pipeline • Our data sets are well suited to it • Lots of source variation • Bright sources • Large overlapping mosaics • Can piggyback off of GOODS calibration • They plan to use this as their primary method

  11. Self-calibration Problem • Could have a problem with the PSF is undersampled • Intra-pixel sensitivity variation • Requires simulation

  12. IRAC Post-Pipeline • Assuming SSC BCD is the starting point • Use the IRAF/STSDAS dither package to: • Register the images against each other • Remove residual cosmic rays • Correct for geometric distortion • Form mosaics with improved PSF sampling (drizzling) • Additionally, use short exposures to correct for saturation • Use off-galaxy regions to determine the background

  13. Pre-Launch Tasks • Obtain geometric distortion model for mosaicing • Simulate selfcal with test data • GOODS will be doing this. We should help (Regan) • Simulate Drizzle PSF reconstruction with mosaicing with real PSFs • One month of effort (Regan) • Simulate Fourier Transform method with real PSFs • optional, requires software to be written

  14. Post-Launch Tasks • If we accept SSC calibration: • All reduction will use IRAF CL/shell scripts • Initially the reduction will take 2 weeks per galaxy (Regan) • Includes determining the parameters and creating the scripts • Once scripts are written and tested mosaic creation will take one day/galaxy (anyone)

  15. Color Maps • Starting with 14 broadband images • BVRI- JHK- 4 IRAC bands- 3 MIPS bands • Resolution varies by a factor of 40 • Wavelength varies by a factor of 80 • 182 color maps can be generated • Many are redundant but not all • Requires more work/thought on how we want to create these

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