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37.4 Spectrum Noise and Structure
Dead or Noisy Diodes
Each diode of the linear array (containing 512 diodes, diodes 7-506 are science diodes) was independently monitored via its own electronics chain. Diodes could exhibit anomalous behavior or fail. These diodes were grouped together as dead or noisy diodes. Diodes that showed anomalous behavior over an extended time were turned off for science observations. In practice, the threshold voltage for anomalous diode was set to a high value so that it did not detect electrons from the photocathode. The GHRS calibration software corrects for known anomalous diodes. If anomalous absorption features are present in the calibrated data, new noisy or dead diodes may be at fault. The non-standard thresholds for detector diodes are listed in Table 37.4.
Problematic or Otherwise Important GHRS Diodes
Science Diode #1
|
Diode #
1-5122
|
AMP/CH3
|
Threshold4
|
Comments
|
|---|
|
Detector (Side) 1
|
|
|
1
|
24/07
|
46
|
Large background diode (50% peak +3)
|
|
|
2
|
24/08
|
44
|
Large background diode (50% +3)
|
|
|
3
|
27/07
|
120
|
Gold coat radiation diode
|
|
|
4
|
00/08
|
255
|
Diode not connected by design
|
|
85
|
91
|
30/00
|
|
Dead-BDT5 r0h, r5h change May 29, 1995 (95149 SMS)
|
|
123
|
129
|
22/05
|
50
|
Threshold 60% stops noise
|
|
262
|
268
|
15/12
|
255
|
Dead electronics-in BDT
|
|
273
|
279
|
11/04
|
255
|
Bad contact-crosstalk when contact made. In crosstalk table
|
|
436
|
442
|
00/15
|
52
|
Threshold 60% stops noise
|
|
445
|
451
|
00/03
|
44
|
Bad 4096 bit-in bad diode table (BDT)
|
|
487
|
493
|
08/07
|
255
|
Very noisy-in BDT
|
|
|
510
|
04/08
|
120
|
Gold coat radiation diode-not functional
|
|
|
511
|
04/07
|
42
|
Large background diode (50% peak +3)
|
|
|
512
|
08/08
|
47
|
Large background diode (50% +3)
|
|
Detector (Side) 2
|
|
|
1
|
24/07
|
46
|
Large background diode (50% +2)
|
|
|
2
|
24/08
|
44
|
Large background diode (50% +2)
|
|
|
3
|
27/07
|
120
|
Gold coat radiation diode
|
|
|
4
|
00/08
|
255
|
Diode not connected by design
|
|
51
|
57
|
25/05
|
255
|
Bad diode-added to BDT 26 Feb 96.
|
|
80
|
86
|
30/14
|
|
Threshold 60% stops noise April 20, 1992, r0h
|
|
104
|
110
|
27/11
|
255
|
Bad diode-in BDT
|
|
140
|
146
|
24/13
|
40
|
Threshold 80%, crosstalk noise
|
|
144
|
150
|
25/12
|
255
|
Bad diode in BDT
|
|
146
|
152
|
16/09
|
49
|
Threshold 50%, occasionally noisy
|
|
168
|
174
|
24/10
|
43
|
Threshold 50%
|
|
237
|
243
|
16/00
|
44
|
Threshold 50% April 20, 1992; threshold 100% May 15, 1995
|
|
263
|
269
|
|
|
Flaky: intermittently strong (see GHRS ISR 080).
|
|
273
|
279
|
11/04
|
255
|
Bad contact-crosstalk April 20, 1992
|
|
293
|
299
|
|
|
Flaky: intermittently weak (see GHRS ISR 080).
|
|
313
|
319
|
|
|
Flaky: intermittently strong (see GHRS ISR 080).
|
|
339
|
345
|
|
|
Flaky: intermittently weak (see GHRS ISR 080).
|
|
342
|
348
|
11/10
|
|
Threshold 100% April 20, 1992
|
|
355
|
361
|
|
|
Flaky: intermittently weak (see GHRS ISR 080).
|
|
423
|
429
|
|
|
Flaky: intermittently weak (see GHRS ISR 080).
|
|
426
|
432
|
|
|
Flaky: intermittently weak (see GHRS ISR 080).
|
|
440
|
446
|
00/14
|
41
|
Threshold 50%
|
|
441
|
447
|
01/02
|
|
Threshold 70% April 20, 1992
|
|
442
|
448
|
02/13
|
44
|
Bad 16384 bit-in bad diode table
|
|
|
510
|
04/08
|
120
|
Gold coat radiation diode
|
|
|
511
|
04/07
|
43
|
Large background diode (50% peak +2)
|
|
|
512
|
08/08
|
41
|
Large background diode (50% peak +2)
|
The easiest way to tell a bad diode from an absorption line or blemish is by the width, flat profile, and location. The width will depend on your comb addition, sub-stepping strategy (see "File Sizes" on page 35-5), and which file you are in. Dead diodes in data taken with the default comb addition will be 4-pixels wide in raw data and 16-pixels wide in default (quarter-stepped, merged) calibrated data. Dead or flaky diodes will be generally flatter than an absorption line. A dead diode, in raw data, is illustrated in Figure 37.9.
Flaky Diodes
In 1996 we noticed, first in some Side 2 CVZ RAPID mode data, that there were anomalous absorptions (so-called flaky diodes) in the data. Eventually we determined that increases in the GHRS pre-amp temperature resulted in non-linear amplification of input charge from diodes, effectively resulting in the occasional appearance of flaky diodes in spectra, as if a change in the threshold voltage had changed the response of a few Side 2 diodes. This phenomenon, also apparent in non-CVZ data as GHRS usage become heavier, is discussed in more detail in GHRS ISR 78. The same effect was not noticed in Side 1 spectra. Table 37.4 lists diodes that are occasionally seen as flaky in data after 1996. If you think you have seen flaky diodes, check the width of the features and which pixels are affected. The first low raw pixel in each feature should agree with the science diode value in Table 37.4 above. 
Blemishes
Scratches, pits, and other microscopic imperfections in the detector window and on the photocathode surface are referred to as blemishes. The magnitude of blemishes upon spectra depends on how the spectrum illuminates the photocathode near a blemish. Many blemishes have spatial structures and depths that would make them difficult to distinguish from real stellar or interstellar features, see Figure 37.9. Therefore, it is difficult to automatically correct data for the effect of blemishes. In the absence of independent information, individual subexposures can be displayed in diode space to identify non-real spectral features. The calibration code does not correct for blemishes. However, the data quality file (.cqh) contains data quality values marking which pixels are affected by known blemishes. See Table 37.10 for information about data quality flags.
Spikes
Large spikes and unrealistic flux levels were found in a subset of Fall 1995 calibrated GHRS data. The affected observations were all made using the Ech-A grating in orders 35 and 36. Fortunately, this problem only affects a handful of observations in a few proposals. The solution to the problem is to recalibrate with a vignetting file with the groups in ascending line position order (e.g., fc11607fz.r2h, or more recent).
Also the stuck bit at Side 2 science diode position 442 will look like a spike as shown in Figure 37.9.
Granularity
As we noted in the previous chapter, the first-order diode-to-diode response variations are removed in calhrs. There are also granularity effects produced in the photocathode faceplates of the detectors. No calibration is provided for these because determining them is impractical. However, grating G140L has been so characterized because its bandpass is large (see GHRS ISR 076 for details on how this was derived). For the other gratings, features such as FP-SPLIT and COMB addition were provided to allow for improved signal to noise in the final spectrum.
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