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Adventures in Classical PSF Subtraction

Adventures in Classical PSF Subtraction

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Adventures in Classical PSF Subtraction

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  1. Adventures in Classical PSF Subtraction C. Grady Eureka Scientific & GSFC & Marshall Perrin STScI Sagan Summer Workshop 2014

  2. Need for Coronagraphy • Circumstellar Disks, exoplanets, stellar companions are often inconveniently close to a bright object (host star) • Exposing sufficiently deeply to detect the object of interest can mean that you overexpose the instrument you are using and swamp the signal of interest • This talk will focus on using a simple coronagraph, and how best to separate the signal of interest from light from the star Sagan Summer Workshop 2014

  3. The PSF • Optical systems, and HST is no exception, typically spread the light from an unresolved source due to diffraction, scattering in the telescope, and in the science instrument, and in some cases within the detector system. • This is termed the point spread function (PSF). • For the majority of circumstellar disks and exoplanets signal in the wings of the PSF>>signal of interest. Sagan Summer Workshop 2014

  4. Classical PSF Subtraction • Simplest of the techniques that will be covered in this hands-on demonstration • Use a suitably chosen other observation as an estimate of the light from the star that you want to get rid of. • Need to match the science observation in terms of factors affecting the shape of the PSF, and those affecting temporal variation in the measured PSF. Sagan Summer Workshop 2014

  5. Shape of HST PSF depends on Source Spectral Energy Distribution Targets for coronagraphic observation are typically on the bright side for direct Imaging. STIS CCD used for coronagraphic imaging Has throughput from 0.2-1.0 microns Effective wavelength of the image is a strong function of Teff. Here we see the observed response for a white dwarf (Feige 110) – bulk of signal is at wavelengths <5000 Å Sagan Summer Workshop 2014

  6. A1V simulation By A1V, the bulk of the signal is in the Conventional optical, but with broad Wings. Sagan Summer Workshop 2014

  7. M2 Simulation By early M, the effective wavelength of the STIS image is 7700 Å. Sensitivity to color of source decreases as bandwidth decreases, but is typical of all 3 HST coronagraphs. JWST will have other issues, namely a sensitivity to thermal emission from The inner disk (IR excess), and the fact that diskless stars will be blue compared to any system with an IRE. Sagan Summer Workshop 2014

  8. And also on factors affecting focus •HST is in a low-Earth orbit and experiences changing thermal conditions. •Scheduling for HST does not include thermal effects •differences between focus conditions for the target and what you are using as an estimate of the PSF result in radial streamers – differences in the dispersed speckles Sagan Summer Workshop 2014

  9. HST STIS Coronagraph • Very broad band (2,000-10,000 Angstroms) • 50 mas/pixel • Pair of occulting wedges to allow choice of occulter width • No apodization, no Lyot masking of diffraction spikes

  10. Case 1: Use the same star, different roll of spacecraft on sky AU Mic – obiw36030_flt.fits Subtracting obiw35030_flt.fits Sagan Summer Workshop 2014

  11. Introduction to IDP3 • For exploring PSF subtraction you need a display-oriented tool that will let you play with registering, scaling, and subtracting one image from another, while allowing you to tweak the display scaling, color, etc. to your preferences. • Various tools used by the HST IDTs, and other teams, but we have chosen IDP3 developed by the NICMOS team • Stobie et al. 2006 • Starting point is file in the detector frame with NO geometric distortion correction or mapping to the sky. • Filenames ending in “_flt” in HST STIS pipeline terminology. Sagan Summer Workshop 2014

  12. Starting IDP3 • Log into the VNC session • Open a terminal (from Applications, System Tools menu ) • > cd Tue/Part1 • > idl • IDL> idp3 • A large window should appear with File, Images, Adjust, Edit, Spitzer, and Help on menu bar • Note that you may need to resize windows and use scroll bars; idp3 was designed for larger screens.

  13. Loading Data Choices • STIS data are typically multiple sub-exposures to facilitate cosmic ray removal. • Can load the first image of a set, or the full set– we will load only the first image. • Under file, go to the load images item • To read the first readout, go to load image on the 2nd menu, • to read in a set, go to Load Multiaccum (NICMOS terminology). Sagan Summer Workshop 2014

  14. Data for Case 1: AU Mic • M dwarf with edge-on debris disk at 10 pc • Uses 1.0 arcsec wide wedge position • Same star at different orientation will be used as PSF reference. • Exposures from two adjacent orbits of Hubble, with the telescope rolled around the optical axis. • 1630 s per exposure, rectangular subarray • Load these files: • obiw35030_flt.fits • obiw36030_flt.fits

  15. Selecting the file to be loaded Sagan Summer Workshop 2014

  16. Showing the images Sagan Summer Workshop 2014

  17. Adjusting the Display • You can adjust the image dynamic range, color table, and scaling (linear, log, square root) using adjust> display menu item. • HST coronagraphic images typically have a large dynamic range, so I find log scaling works well. • You can also shrink the window to the actual data size using the resize display option. Sagan Summer Workshop 2014

  18. Starting Point: The pipeline processed image Raw Data (flt file) linear stretch Sagan Summer Workshop 2014

  19. Raw Data log Stretch Sagan Summer Workshop 2014

  20. Image Subtraction • Make sure you have loaded images obiw35030_flt and obiw36030_flt (IDP3 in this window replaces .fits with 1fits) • all other images should either not be listed in Show Images or should be turned off • Select the image that you want to act on (register to the other image, scale, etc.) by clicking on its name in the show images list. You should see an * to the left of the name. • Decide whether you want this image to be in positive (add) or negative (subtract). Click on the add button and select the entry from the pull down menu. If you select add for one image, you want to select subtract for the other image. • Set obiw35030 to Subtract and obiw36030 to add

  21. The disk Residuals on one side of star indicate that image Registration is needed Sagan Summer Workshop 2014

  22. Image registration Sagan Summer Workshop 2014

  23. Image Registration • In Show Images window select the image to be subtracted as the one with * • Now go to the Adjust>Adjust Position menu • A dialog box will appear – see next slide. • Use the multiple arrow panel to “drive” the image to be subtracted so that there is not a significant dark/light asymmetry in the image and the diffraction spikes are largely nulled • Default increment is 1.0 pixels (0.05” for STIS), • I find 0.05-1 pixel works better • In this case since we were using data for the same star taken in consecutive orbits, we did not need to adjust the flux scaling for the net image.

  24. Adjust Position Tool • Arrow pad moves around the selected image • Offset Increment adjusts the size of the steps in pixels. Set to no more than 0.1

  25. Registering the Images Positive/Negative Structure in the Spkes means you need to Register images using the image offsets. Try 0.1 pixel increment, and push the arrow button until the negative feature merges with the positive and the spike gets much fainter – this is an example of bad registration Sagan Summer Workshop 2014

  26. Good registration – this is roll-differencing (Lowrance et al. 2005) or angular differential imaging in its most primitive form Sagan Summer Workshop 2014

  27. Saving Your image • You can save for use within IDP3 • Or write a file for external use or both • Go to File>Save Display menu, give a file name and path. • Congratulations, you now know the basics of classical PSF subtraction. Sagan Summer Workshop 2014

  28. Saving Your Image Sagan Summer Workshop 2014

  29. HD 181327 v11 – v12 Now for a face-on disk… Sagan Summer Workshop 2014

  30. Strengths and Weaknesses of ADI-like strategies • Best for point source detection and edge-on disks such as AU Mic • Can eliminate any signal which is azimuthally-symmetric over the roll angle – face on disks can be eliminated. • Can end up with a mess if the nebulosity is very structured • Need alternate robust technique for removing the PSF, which conserves flux Sagan Summer Workshop 2014

  31. Case 2: Non-Contemporaneous PSF template observations • Science target HD 181327, mid-F member of β Pic moving group • Disk marginally resolved by Herschel (Lebreton et al. 2012) • Disk resolved in scattered light by NICMOS and ACS (Schneider et al. 2006) • Location of bulk of debris constrains SED modeling; asymmetries can constrain planets. Sagan Summer Workshop 2014

  32. Using archival data for HR 4413 as template HD 181327v12-hr4413 Sagan Summer Workshop 2014

  33. Using HD 134970 as template Sagan Summer Workshop 2014

  34. What you get from such processing • Detection • Bulk of signal in ring – inner edge, outer edge, inclination, can compare with predictions for location of debris belt from FIR data assuming that the grains are large, compact grains (gray or blackbody). • Exterior to ring, additional, azimuthally asymmetric nebulosity • There are residuals which depend on choice of PSF template data, and this is a bright disk. Residuals become more of a nuisance as the surface brightness of the disk decreases. Sagan Summer Workshop 2014

  35. Data for Case 2: HD 181327 • F star with circumstellar ring debris disk • Uses 1.0 arcsec wide wedge position • Deep exposures which are saturated just on either side of the wedge. Ignore those regions. • Files: • obiw11040_flt.fits • obiw12040_flt.fits • And several PSF stars listed on following slides

  36. Case 2 hands-on • Turn OFF the AU Micimages by toggling the on/off buttons • Load files obiw11040_flt.fits – HD 181327 • obiw12040_flt.fits “ • Now try subtracting 12040 from 11040 using the same approach as for AU Mic – one file should be add, the other subtract • You should see very little – roll differencing a face on disk – not a great idea. Sagan Summer Workshop 2014

  37. Case 2 hands-on • Now load some additional PSFs • obiw13040_flt.fits – psf contemporary with the HD 181327 data • obiw17040_flt.fits – same PSF star, taken at a later date • Now try subtracting 13040 from 11040 or 12040(11040 or 12040 add, 13040 subtract) – you will need to flux scale as well as register. • Flux scale adjustment is just below alignment. • Iterate adjusting the alignment and flux scaling until you are satisfied. Sagan Summer Workshop 2014

  38. Case 2 hands-on • Now load some additional PSFs • obiw13040_flt.fits – psf contemporary with HD 181327 • obiw17040_flt.fits – psf taken with the second visit set • Now try subtracting 13040 from 11040 or 12040(11040 or 12040 add, 13040 subtract) – you will need to flux scale as well as register. • obiw93040_flt.fits - psf – another star • obiw97040_flt.fits - psf - another star • Try using one of these observations in place of 13040 or 17040 • If we can get everyone to this stage, we will have a poll to see which PSF data everyone thinks works best. Sagan Summer Workshop 2014

  39. Case 3: Color-matched, contemporaneous PSF data Sagan Summer Workshop 2014

  40. Comments on color-matched, contemporary PSF subtraction • Suppress residuals, since have allowed HST to come to a quasi-equilibrium • Note large number of hot pixels, cosmic ray events, etc. – these can be removed by combining a suite of science target-PSF data where HST is rolled between observations – reduce STIS wedge to a quasi-circular occulted zone with r=0.35”, and can median filter to remove hot pixels. – requires creating masks: too time consuming for demo here. Sagan Summer Workshop 2014

  41. HD 181327 – smaller obscuration Image Credit Schneider et al. 2014 Sagan Summer Workshop 2014

  42. HD 181327 after merging images Image Credit Schneider et al. 2014 Sagan Summer Workshop 2014

  43. Deprojection and compensation for r-2 illumination gradient Image credit Schneider et al. 2014 Sagan Summer Workshop 2014

  44. Further improvements & Summary • Can largely remove the remaining residuals using filtering techniques • Now are at the point that you can begin science analysis – see Stark et al. (2014) for details • PSF subtraction with HST requires choice of suitable template targets, planning the observations so that the template is taken as close in time to the science data as feasible, and straightforward data reduction. Sagan Summer Workshop 2014