STIS Instrument Science Report, STIS 98-02R Cycle-7 MAMA Pulse height distribution stability: Fold Analysis Measurement Harry Ferguson, Mark Clampin and Vic Argabright October 26, 1998 ABSTRACT We describe the procedure for obtaining a MAMA detector anode fold analysis. The procedure for cycle-7 is revised from the SMOV procedure (TIR 97-09) to use the line lamps with neutral density filters in an undispersed mode. In all other respects it is the same as the SMOV procedure. This revision corrects minor inconsistencies with the phase-2 proposal 7643, and a typographical error in the last table. 1. Introduction The performance of MAMA microchannel plates can be monitored by analyzing the distribution of anode folds. A charge cloud from the MCP strikes the anode array where the charge is divided among the illuminated anodes. The number of contiguous anodes in either the X or Y anode planes, where the amount of detected charge exceeds the charge amplifier/discriminator threshold, is called the fold number. This is illustrated below in Figure 1. T 1
Figure 1: Schematic showing how the size of the charge cloud incident upon the MAMA s anode array determines the size of the anode fold. A fold analysis measures the statistical distribution of anode fold numbers and, thus, the distribution of charge cloud sizes incident upon the anode. The distribution of charge cloud sizes provides an indication of the stability of the MAMA s pulse height distribution as charge is extracted from the microchannel plate. Encoding of anode events into pixel coordinates is accomplished in the MAMA electronics by dedicated Application Specific Integrated Circuits (ASIC), whose job is to determine a pixel location from an arbitrarily ordered fold. This is a two step process of anode encoding, followed by pixel decoding. Anode encoding entails converting each anode fold into its equivalent two-fold 1, and Pixel decoding is where the equivalent two fold is converted into a corresponding pixel position. Most typical anode folds are in the range of one to four folds for a single anode plane. In the case of the MAMA s 2-D imaging format, where separate anode planes provide the X and Y event coordinates, Vic Argabright has defined a set of 2-D anode fold numbers based on the combinations of row and column folds, summarized below in Table 1. Each of these 2-D fold numbers can be independently enabled, or disabled in the ASICs, via flight software. 1. All anode folds can be reduced to an equivalent two-fold. As Figure 1 illustrates a three-fold, has two possible two-folds. The encoder dithers between the two possible locations during encoding. The four fold can be reduced to a single unambiguous two-fold. Note that one-folds are discarded as nonvalid events. A detailed description of the MAMA ASIC is given in the Ph.D. Thesis, An ASIC for high speed and High resolution decoding of MAMA detectors by B. Kasle (1992),. 2
Table 1. Nomenclature for 2-D Fold numbers. The 2-D Fold number is given by the combination of a column fold number and a row fold number. 2-D Fold number 1-D fold combination 4 Col-2 fold: Row-2 fold 5 Col-2 fold: Row-3 fold + Col-3 fold: Row-2 fold 6 Col-2 fold: Row-4 fold + Col-3 fold: Row-3 fold + Col- 4 fold: Row-2 fold 7 Col-3 fold: Row-4 fold + Col-4 fold: Row-3 fold 8 Col-3 fold: Row-5 fold + Col-4 fold: Row-4 fold + Col-5 fold: Row-3 fold 9 Col-4 fold: Row-5 fold + Col-5 fold: Row-4 fold 10 Col-4 fold: Row- 6 fold + Col-5 fold: Row-5 fold + Col-6 fold: Row-4 fold 11 Col-5 fold: Row-6 fold + Col-6 fold: Row-5 fold 12 Col-6 fold: Row- 6 fold In this report we will outline the procedure for obtaining a fold analysis of the MAMA detectors. This procedure was used in SMOV Proposal 7106 and will become part of the calibration program for monitoring the MAMA detectors during Cycle 7. It should be noted that the most efficient way of performing the fold analysis is to use the engineering diagnostic mode, which is not currently available, as it reduces the time required to collect telemetry and lamp usage. 2. Fold analysis strategy While uniformly illuminating the detector s active area, the various counters are monitored while various combinations of row and column folds are enabled and disabled. The strategy for obtaining a fold analysis comprises the following steps, which are shown schematically in Figure 4: Flat field illumination: Select the appropriate aperture and lamp combination. Normal wavecal procedures should be used for turning on the lamp. The fold analysis for the FUV MAMA uses the combination of aperture F25ND3+MIRROR with the HITM2 lamp operating at 10 ma, to achieve uniform illumination of ~19,000 counts/sec, while for the NUV MAMA the combination of F25ND3+MIRROR with the HITM1 operating at 3.8 ma current is used to obtain ~20,000 counts/sec. In the event it is necessary to change the flat field configurations, contemporaneous measurements should be made with the old and new flat field configurations. 3
Check event counter settings The event counter is cycled through each of the counters X, Y, Z, W, VE, EV, and OR. Five samples are collected for each selection. These measurements are made to check that the event rates are consistent with those measured in previous fold analysis experiments. As the lamps age, illumination levels can be expected to change, however, the ratios between X, Y, Z, W and OR counts should remain constant. In Table 2 we show the results of measurements made during pre-delivery testing at GSFC. Table 2. Nominal fold analysis event counter rates for the FUV and NUV MAMAs. Counter FUV MAMA NUV MAMA X 36486 57012 Y 36860 55808 W 36082 57006 Z 35102 57022 EV 34670 54846 VE 34446 51816 OR 39732 60852 2D-Fold number measurements Anode folds are measured by disabling all anode fold combinations except those required for the fold number and then measuring five Valid Event (VE) samples e.g. to measure 5-Folds, it is necessary to measure five VE samples with column-2 folds and row- 3 folds enabled and five VE samples with column-3 folds and row-2 folds enabled. The full analysis is performed by selectively enabling and disabling, in turn, each of the relevant fold combinations, as summarized in Table 1. This element of the procedure is illustrated schematically in Figure 4. The raw data required to perform the fold calculations can be obtained from OMS by the STIS engineer. Check for lamp drift All of the anode folds are re-enabled and five samples of EV and VE count rates are measured to check that there is no evidence of lamp drift while the experiment was in progress. Check the dark rate The flat field lamp is turned off and the dark rate on X, Y, Z, W, VE, EV and OR, is measured by co-adding five samples for each event type. 4
3. Analysis The fold analysis is performed by generating a histogram in which the X ordinate is the anode fold number and the Y ordinate is the number of measured events divided by EV. EV is independently measured in Step 2. The >12 Folds measurement is determined by 12 EV Folds ( EV) 4 The baseline fold analyses for the FUV and NUV MAMA detectors are shown in Figure 2 and Figure 3. These plots can be used for comparison with the results of SMOV proposal 7106. In general, a loss of MCP gain will manifest itself as shift of the distribution to lower values of 2D folds. However, in cases where the detector has been grossly over-illuminated leading to the production of gas within the detector, the MCP gain distribution may shift to higher 2D fold numbers. Deviations of more than 20% from the previously measured values should be regarded as cause for further investigation. 4. Bright Object Protection Issues This test is potentially dangerous to the MAMA detectors because the Software Global Monitor (SGM) will not function properly when anode folds are disabled. During the measurement of anode folds, the SGM trigger limit is reduced to 150,000 VE /sec with an integration period of 1.0 seconds (due to ground-system restrictions, this number is now 130,000 VE/sec), to help alleviate the problem. However, with most of the anode folds disabled, the disparity between the measured VE count rate and the true OR count rates can be significant. Consequently, the flat field configuration used in this procedure should never be changed. In addition it is recommended that the external shutter should be closed during this test as an additional security measure. 5. Lamp use The cycle-7 implementation of the fold analysis uses the HITM lamps instead of the flatfielding lamps to preserve the flatfielding lamp lifetime. The proposal uses Special Commanding and requires approximately 1 orbit for each detector. The current plan is to run the proposal every 6 months. For the HITM1 lamp, this would add 3.8 ma hours per year of use over the normal use for wavecals and dispersion solutions. For the HITM2, the fold analysis would add 10 ma hours per year. Over five years this projects to 38 ma hours for HITM1 and 100 ma hours for HITM2. For HITM1, this is a small fraction of the current lamp usage for wavecals and dispersion solutions (which totals 64 ma hours as of 1/5/98). For HITM2, the fold analysis is a significant fraction of the lamp usage (which totals 2.6 ma hours as of 11/4/97), but is still not a significant fraction of the total projected lamp lifetime over 5 years. 6. Acknowledgments We wish to thank Steve Kraemer for providing the GSFC reports on Fold Analysis and Jim Younger for making available the science teams s MAMA Fold Analysis SMS. 5
Figure 2: Ground test and SMOV fold analyses for the FUV MAMA. 6
Figure 3: Ground test and SMOV fold analyses for the NUV MAMA 7
Figure 4: Schematic showing the procedure for a MAMA fold analysis. Initialize MAMA settings Switch on HITM lamp Set MAMA Parameters: Event counter = OR SGM threshold =15000 counts Integration period = 0.1 seconds 300 sec ACCUM exposure Select event counter Collect 5 samples Set MAMA parameters Disable MAMA folds Repeat for: Event counter = X, Y, Z, W, VE, EV & OR SGM threshold =130000 counts Integration period = 1.0 seconds At completion set: Counter = VE SGM threshold =130,000 counts Integration period = 1.0 seconds 4 Folds 5 Folds (i) 5 Folds (ii) 6 Folds (i) 6 Folds (ii) 6 Folds (iii) 8
7 Folds (i) 7 Folds (ii) 8 Folds (i) 8 Folds (ii) 8 Folds (iii) 9 Folds (i) 9 Folds (ii) 10 Folds (i) 10 Folds (ii) 10 Folds (iii) 11 Folds (i) 9
11 Folds (ii) 12 Folds (ii) Enable MAMA folds Collect 5 EVsamples Collect 5VE samples Switch off HITM lamp Select event counter Collect 5 samples Restore MAMA settings Repeat for: Event counter = X, Y, Z, W, VE, EV & OR SGM threshold =130000 counts Integration period = 1.0 seconds At completion set: Counter = OR SGM threshold =77000 counts Integration period = 0.1 seconds Schematic Key Column folds Row folds = Enabled = Disabled 10
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