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
T
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.
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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-fold1, 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 row3 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
Repeat for:
Event counter = X, Y, Z, W, VE, EV & OR
SGM threshold =130000 counts
Integration period = 1.0 seconds
Collect 5 samples
Set MAMA parameters
At completion set:
Counter = VE
SGM threshold =130,000 counts
Integration period = 1.0 seconds
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
Disable MAMA folds
4 Folds
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
5 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
5 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
6 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
6 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
6 Folds (iii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
8
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
7 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
7 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
8 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
8 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
8 Folds (iii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
9 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
9 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
10 Folds (i)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
10 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
10 Folds (iii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
11 Folds (i)
Collect 5 VE samples
9
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
11 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
12 Folds (ii)
Collect 5 VE samples
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
Enable MAMA folds
Collect 5 EVsamples
Collect 5VE samples
Switch off HITM lamp
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
Select event counter
Collect 5 samples
Restore MAMA settings
Schematic Key
Column folds
Row folds
C2 C3 C4 C5 C6 R2 R3 R4 R5 R6
= Enabled
10
= Disabled
11