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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)

1 Science Diode: diodes 7-506 used for science observations.

2 Diode: the entire (1-512) diode array.

3 AMP/CH: Amplifier/channel, onboard electronic location of diode.

4 Threshold: Discriminator threshold voltage setting for channel.

5 BDT: Bad diode table.

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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