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

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

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

RANDOM URA

Six mask patterns for the SSM

Patterns joined side-by-side. Gives limited resolution in non-coded direction

Camera SimulationSet up Camera Geometry

Set up source locations (θx , θy 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

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 ofdetector counts with (a) maskand (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

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

Reconstruction: Raw

Coding Noise Removed

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

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

Shadow PatternSource location: x-elem=60, y-elem=27

Pattern 3 Pattern 4

Reconstruction: Raw

Coding Noise Removed