If you notice a feature in your data similar to the absorption feature described
above, you should suspect a spurious dead diode in your observations. Tables 31.4
and 31.5 list all diodes that were disabled during the FOS lifetime. Normally, a
diode was disabled and a new dead diode reference file created after the third
report of anomalous behavior by that diode. Although the USEAFTER of the new
reference file was set to the date of the first reported anomaly, your data may contain an earlier, unreported occurrence. If your suspected diode has been subsequently disabled (see Tables 31.4 and 31.5) you may use a later dead diode
reference file to correct your data as long as the alternate reference file does not
also correct diodes that are fully functional in your data. You may also contact the
STScI Help Desk (firstname.lastname@example.org) for further assistance in producing a special dead diode correction table for re-calibrating your observation.
32.9.2 Noisy Diodes
The effect of a noisy (or hot) diode was typically to produce an emission feature extending over a fixed number (NXSTEPS x OVERSCAN) of pixels (typically 20). Figure 32.25 shows an observation where pixels 400 to 420 are affected by a noisy diode. Unlike a dead diode, the profile of the feature need not be particularly flat since the degree of spurious signal generation by the diode may have varied from one 300 millisecond internal readout cycle to another. The effect of a noisy diode cannot be removed by recalibrating the data.
Noisy diodes sometimes appear for only very short intervals within an exposure.
Look at individual groups of your data in the group counts paper products plot as
an initial diagnostic of the possible presence of a noisy diode in your data. You
might see that one or a few groups have substantially greater signal than the rest.
Similarly, for ACCUM mode you can use task deaccum (see section 33.4) to plot
the whole spectrum accumulated in each individual readout interval to isolate a
noisy event to only a few groups.
Cycle 6 Observations: A particularly strong noisy diode appeared around diode 250 (pixel 1000 for quarter-stepped data) in FOS/RD spectra on several occasions after July 1, 1996.
32.9.3 Detector Background (Dark)
The FOS was subject to two types of background effects caused by high energy particles:
The dark contribution in short exposures was dominated by individual events that typically affected individual diodes. Many FOS exposures, and all individual readouts, were not lengthy enough to allow a uniform dark distribution to build up in all pixels at sufficiently high S/N for the pipeline mean correction to have high accuracy. For example, a one-orbit (2000 second) FOS/RD ACCUM exposure (500 seconds effective exposure in each pixel) at low latitude produced a mean dark level of about 5 counts-pixel-1. Random excursions of at least a factor of two about this mean level were to be expected and were often seen.
Further, there were some indications that the geomagnetic model used to scale the reference background file in the calfos pipeline1 may have underestimated the background counts in science data by approximately 10% at low geomagnetic latitudes (< 20 degrees) and by about 20-30% at high geomagnetic latitudes. At present no refinement of these uncertainties is possible without a detailed study of the dark as a function of the ambient geomagnetic field measured by the onboard magnetometer. These uncertainties are not significant for strong sources (you can verify this by comparing the counts in the .c5h and .c7h files), but could cause substantial errors in the derived flux and spectral shape of weaker sources. For example, a V~19 magnitude star with an effective temperature of 10000 K, produced the same count rate at 2700 . as the low latitude mean background of the FOS/RD detector.
On the other hand, FOS ISR 146 examined all dark measures made through July 1, 1996 and essentially confirmed the SV mean levels as a function of geomagnetic latitude, gm. No correlation of the dark rate with geomagnetic longitude (apart from the vicinity of the SAA), solar angle, or solar cycle was found. The mean background from this analysis is:
Mean rates may contain 10-30% uncertainties depending upon geomagnetic latitude. In nearly all cases, these uncertainties in the dark model are dominated by the statistical uncertainties in the estimated mean dark signal. That is, individual particle events dominate the dark count distribution for shorter exposures. The distribution of dark count per pixel begins to even out and resemble the scaled model mean values for exposures >2000 seconds
You can refer to the .c7h file and the FOS paper products for information about
the background subtraction made for your data. The mean dark level is reported in
the exposure summary section of the FOS paper products. When it is a significant
contributor to the total signal, the mean dark count per group is visible in the
group counts plot, as well. Additionally, as described above, you can examine any
unilluminated pixels in the .c4h file as an indicator of the statistical nature of the
actual dark signal in your observation.
32.9.4 Scattered Light
As a single pass spectrometer with blazed, ruled gratings and detectors that were sensitive over large spectral ranges, the FOS was subject to scattered light which originated primarily in the diffraction patterns of the gratings and the entrance apertures, as well as in the micro-roughness of the grating rulings due to their ruled surfaces.
FOS ISR 103 described the calfos grating-scatter correction procedure, first implemented in March, 1994. The correction algorithm determines the mean detected signal for those diodes that are insensitive (or not illuminated) in a given dispersed spectrum and uses this mean as a measure of the wavelength-independent scattered light for the entire spectral range of the grating. This mean scattered signal is subtracted as a constant from all pixels in the spectrum. Only those gratings that have insensitive or unilluminated pixels (see Table 31.6) can be corrected in this fashion. If the wavelength range readout was restricted (e.g., in RAPID mode) it was possible that no data were recorded by the insensitive pixels. No scattered light correction was made in these cases. The algorithm was altered in February, 1996 to use the median, rather than the mean, with the additional proviso that all deviations from the median greater than 4 are eliminated in order to remove the impact of strong signals due to particle hits from the determination.
The calfos scattered light correction is effectively a residual mean background correction. For those gratings for which only a small number of pixels are used to form the mean scattered light correction (e.g., FOS/RD G190H), poor results may occur. Often at low count rates the quality of scattered light correction is obviated by poor photon statistics in the target spectrum. The corrected fluxes often vary about zero or are negative for faint sources.
You can refer to the SCT_VAL group parameter of the .c1h file for information
about the scattered light correction made to your data. The mean correction level
is also reported in the exposure summary section of the FOS paper products.
Comparison of this value with the signal level in the .c4h file gives an indication
of the severity of the scattered light contamination in the spectrum. If you have an
accurate spectral energy distribution of your object at longer wavelengths, you can
model the grating scatter with bspec.
Figure 32.28: Scattered Light -Comparison of GHRS and FOS
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