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Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 10: Bright-Object Protection > 10.7 On-Orbit Protection Procedures

10.7
Should an overly-bright object be observed with COS, on-board software will typically act to protect the instrument from damage. The most serious response is to reduce the high voltage of the affected detector; subsequent observations will not take place until COS undergoes a safe-mode recovery procedure that is run from the ground. Activating any of the instrument protection levels listed below is regarded as a serious breach of our health and safety screening procedures and is cause for an investigation. Observers are responsible for ensuring that their observations do not endanger the instrument.
FUV Bright Object Protection
There are five levels of protection for the COS FUV detector:
1.
At the lowest level are the screening limits imposed on observers in order to provide a margin of safety for the instrument. The screening limits (Table 10.1) are set a factor of two or more below actual risk levels, and we expect observers to work with us to ensure these limits are adhered to. They are determined by estimating the expected count rate from an object, both globally over the detector and locally in an emission line if appropriate. The COS ETC is the tool used for this check.
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NUV Bright Object Protection
Similar protections also apply to the NUV detector:
1.
At the lowest level are the screening limits imposed on observers to provide a margin of safety for the instrument. The screening limits (Table 10.1) are set a factor of two or more below actual risk levels, and we expect observers to work with us to ensure these limits are adhered to. They are determined by estimating the expected count rate from an object, both globally over the detector, and locally in an emission line if appropriate. The COS ETC is the tool used for this check.
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Next comes local rate monitoring. It is possible to permanently damage a small region of a micro-channel plate without exceeding the global rate limits. This could occur if an object with bright emission lines were observed, for example. Before each observation, the flight software takes a 0.3 s exposure, bins it in “superpixels” of 4 4 pixels each, and analyzes it in two passes. During the first pass, the flight software checks that each superpixel does not exceed the threshold values of 225 counts and 390 counts for imaging and spectroscopic observations, respectively. During the second pass, the software steps a box (of 1 2 superpixels for spectroscopic exposures and 2 2 pixels for imaging exposures) across the image, checking that the same limits are not exceeded in the larger area. The purpose of the second pass is to ensure that bright sources at the edge of the superpixels are not missed. This 0.3 s exposure is not recorded. If the local rate limit is exceeded, the COS flight software closes the external shutter and all subsequent exposures until the next grating change or target acquisition are lost.
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Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 10: Bright-Object Protection > 10.7 On-Orbit Protection Procedures

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