Space Telescope Science Institute   4.3.8 UV Light and the HRC CCD  4.4.2 SBC Spectral Response

4.4 The SBC MAMA


4.4.1 MAMA Properties


The ACS MAMA detector is the STIS flight spare STF7. It provides coverage from 1150 Å to 1700 Å. The MAMA detector is a photon-counting device which processes events serially. The ACS MAMA only operates in the accumulate (ACCUM) mode, in which a time-integrated image is produced. Unlike the STIS MAMAs, the ACS does not offer the high-resolution (2048 ×2048) mode or time-tagged data acquisition. The primary benefits afforded by the STIS and ACS MAMAs, in comparison with previous HST UV spectroscopic detectors such as those in the GHRS and FOS, are high spatial resolution, two-dimensional imaging over a relatively large field of view, and low background for point sources.

Figure 4.13: Design of the SBC MAMA.

 

Figure 4.13 illustrates the design of the MAMA, which has an opaque CsI photocathode deposited directly on the face of the curved microchannel plate (MCP). Target photons strike the photocathode, liberating single photoelectrons which pass into the MCP. There they are multiplied to a pulse of ~4×10e-. The pulse is recorded by an anode array behind the photocathode, and detected by the MAMA electronics which rejects false pulses and determines the origin of the photon event on the detector.

The field electrode, or repeller wire, repels electrons emitted away from the microchannel plate back into the channels. This provides an increase in quantum efficiency of the detector at the price of an increase in the detector point spread function halo. The repeller wire voltage is always on for SBC observations.


Table 4.7: SBC detector performance characteristics.
Characteristic
SBC MAMA performance
Photocathode
CsI
Wavelength range
1150 to1700 Å
Pixel format
1024 ×1024
Pixel size
25 ×25 µm
Plate scale
0.034 ×0.030 "/pixel
Field of view
34.6" x 30.8"
Quantum efficiency
19.2% @ 1216 Å
Dark count
1.2 ×10-5 counts/second/pixel
Global count-rate linearity limit1
360,000 counts/second
Local count-rate linearity limit
~350 counts/second/pixel
Visible light DQE
< 1.2 ×10-9 above 400 nm
1Rate at which counting shows 10% deviation from linearity. These count rates are well above the bright-object screening limits.

 4.3.8 UV Light and the HRC CCD  4.4.2 SBC Spectral Response
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