The Cryogenic Camera
The characteristics of the camera are summarised in the table below. The main improvements implemented in S-Cam 2 concern the increased resolving power (now ~8 at 500nm), the higher maximum count rate sustained by each of the electronics channels (now 5 KHz per pixel), the simplified alignment procedure, and a number of improvements in the Graphical User Interface (GUI) software. The camera design is based on a bottom loading 4He cryostat hosting an adsorption ³He cooler. A dedicated optical collimator unit interfaces the camera focal plane to the Nasmyth focus of the Ground High Resolution Imaging Laboratory of the William Herschel Telescope, in La Palma.
A plate scale of 0.6 arcsec per pixel was selected to match the telescope point spread function, which is typically around one arcsec. The optical unit is based on a reflective section and on a lens objective. Two filter wheels are available with neutral density and narrow band filters. The neutral density filters are needed to hold down the count rate for bright objects (such as flux standard stars) which would otherwise exceed the 5Khz per pixel maximum event rate of the array. The front-end electronics is based on 36 charge sensitive preamplifiers and related shaping stages operating at room temperature. These permit pixels to be biassed and read-out individually. Each channel has a peak detection unit and an analog-to-digital converter, allowing pulse-height analysis to be performed on every detected photon. Accurate time-tagging of these events is crucial, and is achieved using a commercial GPS receiver with an accuracy of ~ 5 microseconds. The electronics system is connected to a dedicated data acquisition computer, which is in turn connected to the control PC on which the GUI is run. This PC would typically be operated from within the telescope control room. Data storage takes place on the same unit.
|Band-pass: ||350-650 nm (at 10% photon collecting efficiency)|
|Data per detected event: ||Wavelength, arrival time, pixel #|
|Resolving power: ||of order 10 at 300 nm |
|Event time resolution: ||5 microseconds (GPS) |
|Maximum count rate: ||5 kHz/pixel |
|Camera field of view: ||4.0 × 4.0 arcsec |
|Instrument installation: ||Nasmyth focus (f/11) - WHT |
|Observation time: ||In excess of 10 hours (cooler hold time) |
|Camera focus adjustment: ||Telescope secondary mirror and dedicated optical unit |
|Camera Guiding: ||Telescope facility (autoguider camera) |
|Filter wheel: ||2 sets of 8 filters on 2 independent wheels |
|On-line data analysis: ||'Quick-view' SW on control PC |
|Data storage format: ||FITS format (via control PC) |
Because the detector has such high responsivity and detection efficiency, it is crucial to reject thermal infrared photons if the resolving power of the instrument is to be optimised. The expected sky background flux at the Nasmyth focus is not a major problem (producing about 50 counts per second per pixel), but the thermal radiation emitted by warm components in the field of view has a major degrading effect on the system performance. To counter this, S-Cam 2 uses two KG2 glass filters of different thicknesses, cooled to 12 and 2 K respectively. A third silica element is located in front of the focal plane array and maintained at 0.32 K. The optical entrance window of the cryostat is sapphire, with standard ARC's on one side and a multilayer IR filter on the other. These filters reduce the overall camera throughput in the nominal band-pass to about 25%. Further work is on-going to improve the efficiency of the IR filters.
One possible solution to the IR contamination problem lies in the development of blocking grids. This 9×10 prototype grid was milled from a gold-coated sapphire chip using a focussed-ion beam. The grid bars are 100 nm wide.
||The STJ Detector Array
||Results: Crab Pulsar
Last Update: 17 Feb 2005