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.
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 Cs
2Te 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
× 10
5 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).
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 MgF
2 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.)
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
−0.30%/
°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 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).
Synphot,
pysynphot, and the
ETC also include TDS corrections for STIS data. Currently these default to the estimated throughputs for April, 2014.
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 Web site. Data for spectral modes can be found at:
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
“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.
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.
