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In last few years, SiPMs have emerged as a viable alternative to PMTs used in the imaging atmospheric Cherenkov telescopes. In addition to their higher photon detection efficiency, SiPMs provide attractive features like possible increase in observation duty cycle owing to their safe operation under partial moonlight conditions. Design and development of 256 pixel SiPM based camera for a 4m class Cherenkov telescope is currently at an advanced stage. This camera is proposed to cover a field of view of 5 • × 5 • , with a pixel size of ∼ 0.3 • . The camera being developed, is planned to be mounted in the focal plane of one of the vertex elements of TACTIC telescope system which is currently operational at Mt Abu, in the north-western part of India. The associated camera electronics will also be mounted in focal plane of telescope behind the SiPM pixels. The camera will have modular structure, with each module consisting of 16 pixel sensors and the associated front end electronics. The signal generated from the pixels on registration of a Cherenkov event will be passed to "back-end" electronics for trigger generation, digitization @1GSPS and the subsequent data recording. A 16-pixel prototype module has already been developed and tested in our laboratory. A "mini-camera" consisting of 64 pixels has also been assembled and is currently at advanced stage of testing. After completion of the successful testing of the "mini-camera", field tests at the telescope site will be conducted. Salient features of the SiPM based camera, results from the tests conducted by us and status report will be presented.

The alignment and X-ray imaging performance of the Advanced X-ray Astrophysics Facility(AXAF) Verification Engineering Test Article-I(VETA-I) was measured by the VETA-I X-Ray Detection System(VXDS). The VXDS was based on the X-ray detection system utilized in the AXAF Technology Mirror Assembly(TMA) program, upgraded to meet the more stringent requirements of the VETA-I test program. The VXDS includes two types of X-ray detectors; 1) a High Resolution Imager(HRI) which provides X-ray imaging capabilities and 2) sealed and flow proportional counters which, in conjunction with apertures of various types and precision translation stages, provide the most accurate measurement of VETA-I performance. Herein we give an overview of the VXDS hardware including X-ray detectors, translation stages, apertures, proportional counters and flow counter gas supply system and associated electronics. We also describe the installation of the VXDS into the Marshall Space Flight Center(MSFC) X-Ray Calibration Facility(XRCF). We discuss in detail the design and performance of those elements of the VXDS which have not been discussed elsewhere; translation systems, flow counter gas supply system, apertures and thermal monitoring system.

The X-ray detection system used to calibrate the Advanced X-ray Astrophysics Facility (AXAF) mirrors will include gas flow and sealed proportional counters. To meet the ultimate 1% goal of the calibration project, the transmission and uniformity of the windows must be well known for the soft X-ray wavelengths involved. Various window materials for use with proportional counters are examined for transmission at X-ray wavelengths in the range of 0.1 to 59 keV. These include the usual window materials (polypropylene and beryllium), as well as materials only recently employed for detector applications (polyimide and diamond). The transmission uniformity of beryllium at 1.49 keV is examined with a microchannel plate detector, producing a "shadowgraph" of the window material illuminated with soft X-rays. This technique allows us to investigate nonuniformities on a spatial scale of about .2 mm.