Space Telescope Imaging Spectrograph Instrument Handbook for Cycle 17

7.5 MAMA Operation and Feasibility Considerations

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7.5.1 MAMA Saturation-Overflowing the 16 Bit Buffer

The MAMAs are photon-counting detectors: as each photon is recorded, it is placed into buffer memory. The STIS buffer memory stores values as 16-bit integers; hence the maximum number it can accommodate is 65,536 counts per pixel in a given ACCUM mode observation. When accumulated counts per pixel exceed this number, the values will wrap. As an example, if you are counting at 25 counts sec^-1 pixel^-1, you will reach the MAMA saturation limit in ~44 minutes.

Keep accumulated counts per pixel^-1 below this value, by breaking individual exposures into multiple identical exposures (see also Section 11.2.4), each of which is short enough that fewer than 65,536 counts are accumulated per pixel. There is no read noise for MAMA observations, so no penalty is paid in lost signal-to-noise ratio when exposures are split. There is only a small overhead for each MAMA exposure (Chapter 9).

Keep the accumulated counts per pixel below 65,536, by breaking single exposures into multiple exposures, as needed.

7.5.2 MAMA Darks

The STIS MAMA detectors have intrinsically very low dark currents. Dark currents measured during ground testing were less than 10 counts/sec for the FUV-MAMA and less than 30 counts/sec for the NUV-MAMA over the whole detector. For the FUV-MAMA, this exceptionally low dark current was initially achieved on orbit. For the NUV MAMA, charged particle impacts on the MgF₂ faceplate cause a higher background that results in a dark current of 800-2000 counts per second, varying both with temperature and the past thermal history of the detector. This particular phenomenon is not present for the FUV-MAMA, but the dark current for that detector now also varies with time, temperature and position. The different dark-current behaviors of the detectors are discussed in more detail below, and up-to-date information can be found on the "Monitoring" page of the STIS web site:

http://www.stsci.edu/hst/stis/performance/monitoring/.

NUV-MAMA Dark Current

Most of the dark current in the NUV-MAMA comes from phosphorescence of impurities in the MgF₂ detector faceplate. A simple model of the phenomenon was developed by Jenkins and Kimble that envisions a population of impurity sites each having three levels: (1) a ground state, (2) an excited energy level which can decay immediately to the ground state, and (3) a meta-stable level that is at an energy slightly below the one that can emit radiation. The meta-stable states are initially populated by charged particle impacts that mostly occur during passages through the South Atlantic Anomaly (SAA). Hours or days later, the electrons trapped in these meta-stable states are thermally excited to an unstable upper level and then emit a photon as they decay to the ground state. This thermal excitation rate is proportional to e^-E/kT, where E is the energy difference between the levels. The behavior of the count rate vs. temperature leads to an estimate of 1.1 eV for E.

While this temperature dependent function does a good job of predicting the short term response of the NUV dark current to temperature changes, the longer term response is more complex. In equilibrium, the number of decays will match the number of excitation events. A sudden temperature increase would then result in an initial rapid increase in the dark rate, as the meta-stable states are more easily depopulated at higher temperatures. However, after several days, the population of meta-stable states would reach a new equilibrium, resulting in a dark rate that, while higher than the equilibrium rate was at the cooler temperature, is significantly lower than the short term response to the same temperature increase. The predicted large temporary increases in the dark rate were observed after the initial STIS installation, and also after SM3a (see Figure 7.16).

Figure 7.16: The NUV-MAMA Dark Rate as Measured From Dark Monitor Exposures Between 1997 and 2004.


 
Note the large, but temporary, increases in the dark rate after initial installation of STIS and after SM3a (near MJD 51500), when the detector was turned back on after being cold for several weeks. Also note the long term fluctuations in the mean dark current.

 

At a fixed detector temperature of 30º C, the time-constant for the dark current to reach an equilibrium value is about 8 days. In practice because the MAMA high-voltage power supplies have to be shut down during SAA impacted orbits, the detector temperature varies from about 27º C to 40º C on a roughly daily time scale, and the dark current never reaches equilibrium. In addition, the real rate of excitations caused by charged particle impacts is not directly measurable and is expected to vary unpredictably on all time scales. Even the time averaged excitation rate may differ considerably from year to year, depending on the low earth orbit radiation environment.

During the last nine months of STIS operations prior to the failure in August 2004, the typical NUV-MAMA dark current ranged between 8×10^-4 and 1.6×10^-3 counts/pixel/second (Figure 7.17). Changes in the low earth orbit radiation environment may affect the rate at which the meta-stable impurity states are populated. This leads to an additional 10% uncertainty in the predicted dark current. For Cycle 17 planning purposes we will assume that the STIS NUV-MAMA dark current will be 10% higher than it was during 2003-2004; we will assume that it will vary between 8.5×10^-4 and 1.7x10^-3 counts/pixel/second. For use in the STIS Exposure Time Calculator (ETC) we will adopt a mean rate of 1.3×10^-3 counts/pixel/second.

Figure 7.17: NUV Dark Current vs. OM2CAT Temperature Between November 2003 and August 2004.


 

To predict the NUV dark current and subtract it as part of standard processing, the OTFR pipeline and the calstis software use a simple temperature dependent relation:

Both norm and Tmin are slowly varying functions of time that are empirically adjusted to give a good match to the observed dark rate, and which are tabulated in the temperature dependent dark correction table (tdc) reference file. The temperature for a given observation is taken from the OM2CAT telemetry value, which is included in the extension header of each MAMA observation. This approach usually predicts the dark rate with 5 to 10% accuracy, although the error in individual cases may be substantially larger.

Because 99% of the NUV-MAMA dark current is due to photons hitting the detector, it is appropriate to apply the flat field prior to subtracting the dark current. The dark current varies slowly across the face of the detector, being about 1.25 times higher near the lower left corner (AXIS1, AXIS2 < 300) than at the center. This shape varies with time and temperature enough that subtraction of the scaled dark reference may leave a residual, spatially varying dark current. This is easiest to remove by fitting a low-order two-dimensional function to the background vs. pixel position.

FUV-MAMA Dark Current

The FUV-MAMA dark current is substantially lower than that of the NUV-MAMA. Initially values as low as 7 counts/sec across the face of the detector (7 × 10^-6 counts/sec/pix) could be routinely expected. However, there is also an intermittent glow that covers a large fraction of the detector (see Figure 7.18). The source of the dark current is not phosphorescence but is intrinsic to the micro-channel plate array (it was seen in ground testing). This glow can substantially increase the dark current over a large fraction of the detector, and this leads to count rates of up to 300 counts/sec integrated across the face of the detector.

An example of the dark current variation across the detector can be seen in Figure 7.18, which is the sum of a number of 1380-second dark frames taken during periods of high dark current. The dark current in the lower right quadrant (pixels [900:1000,10:110] in IRAF notation) appears to be stable to within 10% over time. The dark current in the upper left quadrant (pixels [200:400,600:800]) varies with time and temperature. The total dark current can be approximated by the sum of a constant dark current plus a "glow" image, scaled to the net rate in the upper left quadrant.

Figure 7.18: Dark Current Variation Across Detector. The region in the upper left quadrant has the higher dark current.


 

During the first two years of STIS operations, this glow was only present intermittently, but since mid-1999 it has been present more often than not (see Figure 7.17). The glow increases with the amount of time the detector high-voltage has been on since the last SAA passage. The rate of increase is greater when the detector is warmer, and it has also been increasing from year to year, even under otherwise comparable conditions. The dark monitor measurements for the last 10 months of STIS operations from November 2003 to August 2004 are shown in Figure 7.19.

Figure 7.19: Dark Monitor Measurements for the Last 10 Months of STIS Operations.


 
The average count rate for a region in the upper left of the FUV detector centered on the bright glow (pixels [200:400,600:800]) is plotted as a function of the observation date for the FUV-MAMA dark monitor observations. Note that the brightness of the glow has increased substantially over time.

 

In Figure 7.19, the FUV-MAMA dark current is shown for dark monitor observations taken between 2003-Nov-03 and 2004-Aug-03. Each day, the MAMA high voltage is turned on for the block of HST orbits that is unaffected by SAA passages, and as the detector warms up, the glow increases. Filled circles show the count rate in a region centered on the peak of the dark glow (pixels [200:400,600:800]), while the open circles show the average rate for the entire detector. Note that the lower right corner of the detector (pixels [900:1000,10:110]), denoted by plus signs, shows little or no increase over the initial low dark current values.

Because the physical origin of the FUV detector glow is poorly understood, it is difficult to predict what the behavior will be during Cycle 17. It appears that heating or cooling the FUV-MAMA detector affects the rate at which the glow increases. For planning purposes, we will assume that in the center of the glow region the mean dark current will start with a value near 6×10^-6 counts/pixel/second at the beginning of each SAA-free block of orbits, but will then increase at a rate of about 1×10^-4 counts/pixel/second/HST-orbit, reaching peak values as high as 6×10^-4 counts/pixel/second. In the dark corner of the detector, the count rate will remain very low, with a rate near 6.3×10^-6 counts/pixel/sec. For use in the STIS Exposure Time Calculator (ETC), we will adopt 1.5×10^-4 as representative of the expected rate averaged over the whole detector.

During Cycle 12, the D1 apertures were implemented in order to make it easier to place a faint target at a position with a reduced glow. See Section 4.2.3 for further details on their use.

Dark current for the FUV-MAMA is not currently subtracted by the pipeline. However, the "glow" images are available from the STIS Web site, at:

http://www.stsci.edu/hst/stis/design/detectors/fuvDarkGlow.html

and can be used for off-line reduction.

Because the dark current is so low in the MAMA detectors, a typical STIS FUV-MAMA observation will have less than one count per pixel from the dark. It is good to keep this in mind when reducing the data, as various standard measures of background (the median for example) are not good estimates when the data are quantized into just a few values. The best way to estimate the background is to identify hot pixels using the standard reference files, then use an unclipped mean for the remaining pixels in a source-free region of the image.

7.5.3 MAMA Signal-to-Noise Ratio Limitations

MAMA detectors are capable of delivering signal-to-noise ratios of the order of 100:1 per spectral resolution element or even higher. Tests in orbit have demonstrated that such high S/N is possible with STIS (Kaiser et al., 1998, Proc. SPIE, 3356, 415; Gilliland, STIS ISR 1998-16.)

High S/N observations of several standard stars were obtained during STIS commissioning, and they were reduced with flats obtained during preflight testing of the detectors. Signal-to-noise ratios of 125 and 150 per spectral resolution element (for an 11 pixel extraction height in the cross dispersion direction) were achieved for the FUV- and NUV-MAMA observations, respectively; see Chapter 12 for a more detailed discussion.

For targets observed at a fixed position on the detector, the signal-to-noise ratio is limited by systematic uncertainties in the small-scale spatial and spectral response of the detector. The MAMA flats show a fixed pattern that is a combination of several effects including beating between the MCP array and the anode pixel array, variations in the charge-cloud structure at the anode, and low-level capacitive cross-coupling between the fine anode elements. Intrinsic pixel-to-pixel variations are 3.9% and 2.8% rms for the FUV- and NUV-MAMA, respectively, in 1024 × 1024 pixel format. In the highres 2048 × 2048 format (see Chapter 11) the intrinsic variations are much larger. This fixed pattern appears to be stable at the 1-2% level. The structure of the flat may vary slightly for different modes due to different incidence angles of the incoming photons on the microchannel-plate pores.

Observing strategies for achieving spectral S/N higher than ~50:1 are discussed in Chapter 12. For echelle mode spectra, observers may want to consider the use of the FP-SPLIT slits; for first-order mode observations, they may wish to dither the target along the slit.

Since MAMA observations can be binned in post-observation data processing with no additional signal-to-noise price, the option to obtain MAMA observations with unequal binning (e.g. BINAXIS1=1, BINAXIS2=2) were disabled starting in Cycle 8.

7.5.4 MAMA Non-linearity

Global

The MAMA detectors begin to experience non-linearity (photon impact rate not equal to photon count rate) at global (across the entire detector) count rates of 200,000 counts sec^-1. The non-linearity reaches 10% at 300,000 counts sec^-1 and can be approximately corrected in post-observation data processing. Additionally, the MAMA detectors plus processing software are not able to count reliably at rates exceeding 285,000 count sec^-1. For this reason and to protect the detectors, observations beyond this rate are not allowed (see Section 7.7, below).

Local

The MAMA detectors remain linear to better than 1% in their counting up to ~22 counts sec^-1 pixel^-1 for the FUV-MAMA and 34 counts sec^-1 pixel^-1 for the NUV-MAMA. At higher rates, they experience local (at a given pixel) non-linearity. The non-linearity effect is image dependent-that is, the non-linearity observed at a given pixel depends on the photon rate affecting neighboring pixels. This property makes it impossible to correct reliably for the local non-linearity in post-observation data processing. In addition, the MAMA detectors are subject to damage at high local count rates (see Section 7.7).