Instrument Detectors
STIS has three large-format (1024 x 1024 pixel) detectors; one CCD and two Multi-Anode Microchannel Array (MAMA) detectors. The CCD has a 52 x 52 arcsecond quare field of view and operates from ~2000 to 10,300 Å. The MAMAs operate in the Near and Far UltraViolet and both have a 25 x 25 arcsecond square field of view. The NUV operates from 1600 to 3100 Å while the FUV operates from 1150 to 1700 Å.
Below are summaries of the properties of each detector, but for more information, please see the STIS Instrument Handbook.
The STIS/CCD is a low-noise device capable of high sensitivity in the visible and the near UV. It is a thinned, backside-illuminated device manufactured by Scientific Imaging Technologies (SITe). In order to provide a near-UV imaging performance, the CCD was backside-treated and coated with a wide-band anti-reflectance coating. The process produces acceptable near-UV quantum efficiency (QE) without compromising the high QE of the visible bandpass.
The CCD has only one operating mode, ACCUM, for imaging and spectroscopy. Electrons which accumulate in the CCD wells are read out and converted to data numbers by the analog-to-digital converter. The conversion factor can be selected by the CCDGAIN parameter. While the default is CCDGAIN=1, a CCDGAIN=4 value can be chosen if CCD saturation is a concern. Scientific data are obtained on 1024 x 1024 pixels for a full readout. The CCD supports both on-chip binning and the use of sub-arrays.
| Characteristic | CCD Performance |
|---|---|
| Architecture | Thinned, backside illuminated |
| Wavelength range | 2000-11,000 Å |
| Pixel format | 1024 x 1024 illuminated pixels |
| Field of view | 52 x 52 arcseconds |
| Pixel size | 21 x 21 µm |
| Pixel plate scale | 0.05071 arcseconds |
| Quantum efficiency | ~ 20% @ 3000 Å ~ 67% @ 6000 Å ~ 29% @ 9000 Å |
| Dark count at -83° C | 0.003 e- sec-1 pix-1 |
| Read noise | 4.4-5.3 e- rms at GAIN=1* 7.3-7.8 e- rms at GAIN=4 |
| Full well | 144,000 e- over the inner portion of the detector 120,000 e- over the outer portion of the detector |
| Saturation limit | 33,000 e- at GAIN=1 144,000 e- at GAIN=4 |
*When STIS began operating with "Side 2" electronics in July 2001, CCD read noise increased from 4.4 to 5.3 e- for GAIN=1, and from 7.3 to 7.75 e- for GAIN=4. ETC calculations use the most recent estimates of the read noise.
** See also ISR 98-31: STIS CCD Performance Monitor
On-board STIS, there are two Multi-Anode MicroChannel Array (MAMA) detectors operating in the FUV and NUV. The STIS MAMA detectors are photon-counting devices which process events serially. In MAMA ACCUM mode, photons are accumulated into a 2048 x 2048 array in the STIS data buffer memory as they are received. In TIME-TAG mode, the MAMA produces an event stream of data points with a time resolution of 125 microseconds.
| Characteristic | FUV-MAMA | NUV-MAMA |
|---|---|---|
| Photocathode | CsI | Cs2Te |
| Wavelength range | 1150-1700 Å | 1600-3100 Å |
| Pixel format | 1024 x 1024 | 1024 x 1024 |
| Pixel size | 25 x 25 µm | 25 x 25 µm |
| Image mode pixel plate scale | 0.0243 x 0.0246 arcseconds (clear) 0.0244 x 0.0247 arcseconds (filtered) |
0.0245 x 0.0248 arcseconds |
| Field of view | 25.1 x 25.3 arcseconds | 25.1 x 25.4 arcseconds |
| Quantum efficiency | 25% @ 1216 Å | 10% @ 2537 Å |
| Dark count | 5 x 10-6 to 1 x 10-5 counts sec-1 pix-1 | 8 x 10-4 to 1.7 x 10-3 counts sec-1 pix-1 |
| Global count-rate linearity limit 1 | 285,000 counts sec-1 | 285,000 counts sec-1 |
| Local count-rate linearity limita | ~220 counts sec-1 pix-1 | ~340 counts sec-1 pix-1 |
MAMA Bright Object Protection
The MAMAs are subject to catastrophic damage at high global and local count rates and cannot be used to observe sources which exceed the defined safety limits Observers will have to adjust their observing strategies if a target exceeds these limits. The STIS Imaging and Spectroscopic Exposure Time Calculators issue a warning if an applicable limit is exceeded.