1. CODED MASK IMAGING WITH ASTROSAT SCANNING SKY MONITOR (SSM) Mask Design Camera Simulation Image Reconstruction 1-d 2-d, single camera 2-d, multiple camera continuous rotation Sky Simulation D. Bhattacharya & B.T. Ravishankar, RRI, Bangalore Detector MASK

2. CODED MASK IMAGING CONCEPT Multiple pin-hole MASK Mask casts shadow on detector plane Shift of shadow pattern encodes source location Cross correlation of mask pattern with shadow recovers shift and locates sources Designed for pointed-mode observation

3. Mask Design Goals 1. Clean autocorrelation function (min. sidelobes) 2. Maximum transparency (photon collection) Adopted 63-element pseudo-noise Hadamard Set URA Six different patterns, one-dimensional coding Open Fraction = 50% Flat sidelobe response in cyclic autocorretation

4. RANDOM URA

5. Six mask patterns for the SSM Patterns joined side-by-side. Gives limited resolution in non-coded direction

6. Camera Simulation Set up Camera Geometry Set up source locations (θ x, θ y within FOV) Monte-Carlo simulate photon strikes on mask Those striking open elements propagated into the camera Photons striking the shield discarded x-locations and wire nos. recorded for those striking the detector Add position uncertainties due to finite resoln. Add background noise

7. Image Reconstruction (coding direction) Bin detector counts at mask resolution Split FOV into sky elements Identify detector and mask sections for each sky elem Perform cross correlation of detector counts with (a) mask and (b) complement of mask Diff. (a)-(b) yields source strength at that sky element Coding Noise due to sky element cross-talk. Model and remove iteratively

14. Image Reconstruction (2-d, single camera) 6 mask patterns, 8 wires : 48 basic 1-d reconstructions 55 allowed combinations of these 48 reconstructions correspond to different source locations in the uncoded direction Look for max. S/N among these 55 combinations, resulting in recognition of source location in uncoded direction Final resolution (nominal): 10 arcmin x 2 deg Eventual refined 2-d location by using 2 cameras with crossed FOV

15. Reconstruction: Raw Coding Noise Removed

16. ASTROSAT SSM Coded Mask Development: Present Status Mask Design : Completed Camera Simulation: Software developed, documented. Further refinements after fabrication. Image Reconstruction 1-d: Basic software ready, including IROS. Documentated. Richardson-Lucy evaluation on. 2-d, single camera: Capability demonstrated. Effects of some non-idealities investigated. 2-d, multiple camera: Not yet taken up Continuous rotation: Investigations started Sky Simulation: Not yet taken up

17. ASTROSAT SSM Coded Mask Development : Next six months (April- September 2002) Finalise current version of imaging software and complete documentation Finish evaluation of Richardson-Lucy procedure Devise reconstruction algorithms for continuously rotating camera. Investigate alternative camera design. Begin development of tools for Sky Simulation Current person-power: 1+1

18. Shadow Pattern Source location: x-elem=60, y-elem=27 Pattern 3 Pattern 4

20. Reconstructio n: Raw Coding Noise Removed