10.6 On-Orbit Protection Procedures

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.

10.6.1 FUV Bright Object Protection

There are five levels of protection for the COS FUV detector:

1. At the lowest level are the count-rate screening limits imposed on observers in order to provide a margin of safety for the instrument. The screening limits (Table 10.1) are set at 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.
2. At the next level, within COS the "Take Data Flag" (TDF) is monitored during an exposure. If an event occurs that causes the TDF to drop (such as loss of lock on a guide star, which could lead to the telescope drifting), then the exposure will continue with the COS external shutter closed. Subsequent exposures in the visit may also be lost.
3. Next comes local rate monitoring. It is possible to permanently damage a localized region of the micro-channel plates without necessarily exceeding the global rate limits. This could occur if an object with bright emission lines were observed, for example. At the beginning of each exposure, the COS flight software bins the FUV spectrum by 4 pixels in x and 1024 in y; if the count rate in any bin exceeds 1000 counts per 15 s, the external shutter is closed and the calibration lamps turned off. All subsequent exposures until the next grating change or target acquisition are lost.
4. Global rate monitoring is next. The COS flight software continuously monitors the total event rate for both FUV detector segments. If the rate for either segment exceeds 600,000 counts in 10 s, the high voltage to both segments is turned off. Special commanding is required to turn on the FUV detector high voltage, so subsequent FUV observations will be lost, and the HST schedule will be disrupted.
5. At the highest level, the instrument is protected by software that senses an over-current condition in the high-voltage power supply; if triggered, the software shuts down the high voltage.

10.6.2 NUV Bright Object Protection

Similar protections also apply to the NUV detector:

1. At the lowest level are the count-rate screening limits imposed on observers to provide a margin of safety for the instrument. The screening limits (Table 10.1) are set at 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.
2. At the next level, within COS the "Take Data Flag" (TDF) is monitored during an exposure. If an event occurs that causes the TDF to drop (such as loss of lock on a guide star, which could lead to the telescope’s drifting), then the exposure will continue with the COS external shutter closed. Subsequent exposures in the visit may also be lost.
3. 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 were imaged or had a spectrum with bright emission lines, 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.
4. Global rate monitoring is next. The COS flight software continuously monitors the total event rate for the NUV MAMA. If the total count rate exceeds 20,000 in 0.1 s the high voltage to the MAMA is turned off, the external shutter is closed, and the calibration lamps are turned off. The NUV detector can resume operations only after a safe-mode recovery procedure, so subsequent NUV exposures will be lost, and the HST schedule disrupted.
5. At the highest level, the NUV MAMA is protected by the detector electronics. The Bright-Scene Detector (BSD) monitors the output of every 2 anode wires across the detector, corresponding to every 32nd row of pixels. The wires are parallel to the dispersion axis. If the total count rate exceeds 17,000 in 138 ms, then the high voltage is turned off. COS can resume operations only after a safe-mode recovery procedure. BSD differs from global-rate monitoring in two ways: it is done in hardware, not software, and what is measured is not a digitized count rate, but current in the anode grid wires.