7.4 The MAMA Detectors

7.4.1 MAMA Properties

There are two MAMA detectors: the STIS/FUV-MAMA provides coverage from 1150 to 1700 Å and the STIS/NUV-MAMA provides coverage from 1600 to 3100 Å (with lower response below 1600 Å). The STIS MAMA detectors are photon-counting devices that process events serially. They can be used to take data in either an accumulate (ACCUM) mode in which a time-integrated image is produced, or in a time series (TIME-TAG) mode in which the detector location and time of arrival of each photon are recorded as an event stream (see Section 11.1.2 and Section 11.1.3, respectively). The primary benefits afforded by the STIS MAMAs, in comparison with previous HST UV spectroscopic detectors such as those of the GHRS and FOS, are high spatial resolution, two-dimensional imaging over a relatively large field of view, and low background for point sources. The MAMA detector was developed by J. Timothy and R. Bybee for X-ray and UV imaging applications. The properties of the STIS MAMA detectors are summarized in Table 7.7 and described in detail in Timothy, JATIS, 2, 030901, 2016.


Table 7.7: STIS MAMA Detector Performance Characteristics.

Characteristic

FUV-MAMA Performance

NUV-MAMA Performance

Photocathode

CsI

Cs2Te

Wavelength range

1150–1700 Å

1600–3100 Å

Pixel format

1024 × 1024

1024 × 1024

Pixel size

25 × 25 micrometers2

25 × 25 micrometers2

Image mode
pixel plate scale

0.0245 × 0.0247 arcsec2 (clear)
0.0246 × 0.0247 arcsec2 (filtered)

0.0245 × 0.0248 arcsec2

Field of view

25.1 × 25.3 arcsec2 (clear)
25.2 × 25.3 arcsec2 (filtered)

25.1 × 25.4 arcsec2

Quantum efficiency

24% @ 1216 Å

9% @ 2537 Å

Dark count

5 × 106 to 6 × 104 counts/s/pix

0.001 to 0.002 counts/s/pix

Global count rate
linearity limit1

285,000 counts/s

285,000 counts/s

Local count rate
linearity limit1 2

~220 counts/s/pix

~340 counts/s/pix

1 Rate at which counting shows 10% deviation from linearity. These count rates are well above the bright object screening limits.

2 The MAMA Linearity reference file was changed to use the 1% deviation from linearity values of ~22 counts/s/pix (FUV-MAMA) and ~34 counts/s/pix (NUV-MAMA) in September 1999 to better handle saturated pixels with high count rates that were not being flagged using the 10% deviation from linearity values.


Figure 7.13 and Figure 7.14 illustrate the design of the FUV- and NUV-MAMA, respectively. A photocathode material is deposited on the front surface. The FUV-MAMA has an opaque CsI photocathode deposited directly on the face of the curved microchannel plate (MCP); the NUV-MAMA has a semi-transparent Cs2Te photocathode deposited on the back side of the detector's entrance window.

Target photons strike the photocathode, liberating single photoelectrons which pass into the microchannel plate (MCP). There they are multiplied to a pulse of ~4 × 105 electrons. The pulse is recorded by an anode array behind the photocathode and detected by the MAMA electronics which process it, rejecting false pulses and determining the origin of the photon event on the detector.

The FUV-MAMA has a field electrode (repeller wire) which is used to repel 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 a small increase in the detector PSF halo. The repeller wire is normally on for FUV-MAMA observations (but see Section 7.4.4).

Figure 7.13: Design of the FUV-MAMA.



Figure 7.14: Design of the NUV-MAMA.


7.4.2 MAMA Spectral Response

The spectral responses of the unfiltered FUV- and NUV-MAMAs are illustrated in Figure 5.9. The peak photocathode response of the FUV-MAMA occurs at Lyman-α. Its spectral response is defined by the cutoff of the MgF2 window at 1150 Å at short wavelengths, and by the relatively steep decline of the CsI photocathode at long wavelengths. Out-of-band QE at longer wavelengths (>2000 Å) is <10–6 yielding excellent solar-blind performance. The NUV-MAMA spectral response has a relatively flat maximum (~10%) that encompasses 1800–2600 Å. The photocathode QE declines to ~4% at 3150 Å, while at longer wavelengths the out-of-band QE is ~10–4. (See also Section 5.3.3.)

7.4.3 MAMA Sensitivity

The STIS MAMA detector modes, similarly to those of the STIS CCD, suffer from changes in sensitivity which depend on both temperature and time. A STIS Sensitivity Monitoring program similar to that aimed at characterizing the CCD behavior has been used to characterize the MAMAs (see Section 7.2). The well-known sensitivity of the FUV-MAMA to temperature for the first-order mode G140L has been confirmed to be 0.32%/° C. No significant change with temperature for the NUV-MAMA first-order G230L mode has been registered. Following a correction for the temperature dependence, the sensitivities of the FUV first-order modes decrease linearly with time by wavelength-dependent amounts ranging up to a few percent per year. For the NUV first-order configurations, the sensitivities increased during the first 1.5 years of STIS operations and then began to drop, by wavelength-dependent amounts comparable to those in the FUV. There was a significant slow-down in the decline of the time-dependent sensitivity (TDS) of all STIS observational modes beginning in 2002. All detectors continued to decline during the years STIS was inoperative. For STIS Echelle modes, the TDS corrections adopted are taken to be the same as for the corresponding low-order modes. Note, however, that new photometric throughput tables have been created based on Cycle 17 data. TDS corrections for all STIS spectra are implemented in the data reduction pipeline (see Section 15.1). Pysynphot, and the ETC also include TDS corrections for STIS data. Currently these default to the estimated throughputs for April, 2020.


Figure 7.15: Relative Wavelength-Averaged Sensitivity of STIS MAMA First-Order Low-Resolution Modes G140L and G230L as a Function of Time.


7.4.4 Optical Performance

Both MAMAs exhibit low-level extended wings in their detector point spread functions (PSFs), with the NUV-MAMA PSF being considerably worse. Sample MAMA detector PSFs are shown in Figure 7.16. For those wishing to model their effect on absorption or emission line equivalent-width measurements or coronagraphic observations, the LSFs and detector PSFs are maintained on the STScI STIS website. Data for spectral modes can be found at:

http://www.stsci.edu/hst/instrumentation/stis/performance/spectral-resolution

Links to the plots of the imaging mode PSFs can be found in the table at:

http://www.stsci.edu/hst/instrumentation/stis/performance/image-quality.

Figure 7.16: MAMA Detector PSFs.


The FUV-MAMA includes a repeller wire that establishes a field above the microchannel plate and reflects forward-ejected photoelectrons back into the microchannel pores. The repeller wire is normally on for FUV-MAMA observations, but an improvement to the FUV-MAMA PSF (at the expense of a 35% decrease in sensitivity) can be made by disabling the high voltage to this wire. This procedure is only recommended for observations that use the E140H, and perhaps also the E140M, gratings when used with the 0.1X0.03 aperture (sometimes called the "Jenkins slit") for observations shortward of 1400 Å. In pre-launch testing, resolutions as high as R ~ 220,000 were obtained in observations of a mono-isotopic emission line lamp in highres mode (see Section Highres), and R ~ 200,000 has been achieved on-orbit (Jenkins 2000, BAAS 32, 731, and Jenkins & Tripp 2001, ApJS, 137, 297). Figure 7.17 shows the effect of disabling the repeller voltage for the case of an observation of a line lamp with the E140H CENWAVE=1234 Å setting. Note that the main difference is not the FWHM of the central core, but a decrease in the intensity of the line wings.
Figure 7.17: Average Profile of Calibration Lamp Line at 1234 Å with the Repeller Wire On and Off.



Only a small number of proposals per Cycle (of the order of one to two) requesting the repeller off mode will be accepted. Note that repeller off observations will require special treatment, so a scientific case will need to be made for why these observations are necessary.

Also note that, due to the large PSF of HST in imaging mode (currently 3.5 low-resolution pixels), there is no advantage in performing FUV-MAMA imaging observations with the repeller wire voltage turned off.