Sensitivity of coded mask telescopes

Abstract

Simple formulae are often used to estimate the sensitivity of coded mask X-ray or gamma-ray telescopes, but these are strictly only applicable if a number of basic assumptions are met. Complications arise, for example, if a grid structure is used to support the mask elements, if the detector spatial resolution is not good enough to completely resolve all the detail in the shadow of the mask or if any of a number of other simplifying conditions are not fulfilled. We derive more general expressions for the Poisson-noise-limited sensitivity of astronomical telescopes using the coded mask technique, noting explicitly in what circumstances they are applicable. The emphasis is on using nomenclature and techniques that result in simple and revealing results. Where no convenient expression is available a procedure is given which allows the calculation of the sensitivity. We consider certain aspects of the optimization of the design of a coded mask telescope and show that when the detector spatial resolution and the mask to detector separation are fixed, the best source location accuracy is obtained when the mask elements are equal in size to the detector pixels.

THE 22 MONTH SWIFT -BAT ALL-SKY HARD X-RAY SURVEY

“…General information on coded mask imaging can be found in Skinner (1995Skinner ( , 2008, Fenimore & Cannon (1981), and Caroli et al (1986).…”

“…From the perspective of pure Poisson counting statistics, the uncertainties are governed primarily by the properties of the coded mask and the background (see Skinner 2008 for details). The expected 5σ noise level can be expressed as (adapting from Skinner 2008 Equations (23) and (25)):…”

IMAGING ANALYSIS OF THE HARD X-RAY TELESCOPE ProtoEXIST2 AND NEW TECHNIQUES FOR HIGH-RESOLUTION CODED-APERTURE TELESCOPES

“…Observations with fixed pointings (circles) are compared with the 40 point dithering motions (squares and diamonds). The results of a fixed pointing with pure Poisson noise set an ideal limit of the system, which is bounded by the imaging factor at low SNRs (80%) (Skinner 2008) and by the coding noise limit at high SNRs. The coding noise is the mask patterninduced noise (Skinner 2008).…”

“…The results of a fixed pointing with pure Poisson noise set an ideal limit of the system, which is bounded by the imaging factor at low SNRs (80%) (Skinner 2008) and by the coding noise limit at high SNRs. The coding noise is the mask patterninduced noise (Skinner 2008). The coding noise limit of the output SNR is about 296 for ProtoEXIST2.…”

“…For an ideal system with perfect pointings and pure Poisson-driven background, we expect a signal-to-noise ratio (SNR) of 2.7. The finite mask to detector pixel ratio (1.8) introduces an imaging factor of 0.8 (Skinner 2008), causing an additional loss in SNR by 20%. Assuming a relatively moderate aspect blurring of 1 (likely an underestimate given the performance of the pointing system), we expect another reduction in SNR by ∼ 20%.…”