Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

11.5 Safety First: Bright Object Protection

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COS users are required to check their targets when preparing their Phase II programs to determine that they are safe to observe. The specific procedures to be followed will be provided at the time Cycle 17 proposals are selected. Here we provide the technical information about bright object protection for COS.

Photon-counting detectors are vulnerable to physical damage or degradation if illuminated with too much light at one time. Well before that flux level is reached, excess light leads to poor results because the electronics cannot handle the high event rates (the dead-time correction). In the case of the COS FUV detector, the very high gains mean that over-illumination of an area on the detector leads to charge depletion that can permanently impair the sensitivity of the detector at that point. This is also true to a lesser degree for the NUV MAMA detector.

For all these reasons COS has stringent count-rate limits that all observations must conform to. Similar procedures are used for the STIS and ACS/SBC MAMA detectors.

11.5.1 Limiting Magnitudes and Bright Object Limits

Like STIS, COS has a set of bright limit restrictions that will preclude some objects from being observed. Since the throughput of COS is considerably higher than that of STIS, particularly in the FUV, it may be necessary to observe some bright sources with STIS rather than COS, or to use the bright object aperture with COS. COS has two general types of bright limits: global and local. For the FUV detector, the global bright limit is ~60,000 ct s^-1 segment^-1, and the local count rate limit is ~1.67 ct s^-1 pix^-1 (100 ct s^-1 resel^-1). For the NUV detector, the global bright limit is ~170,000 ct s^-1, and the local count rate limit is 500 ct s^-1, measured over 4 pixels. The NUV global count rate limit can be increased slightly at the expense of doppler compensation during the course of the exposure.

Table 11.1 contains approximate estimates of the bright limit fluxes for the medium- and low-resolution COS modes at several wavelengths. Below each flux limit we also list the approximate corresponding visual magnitude of an unreddened O9 V star. The final operational screening limits set by STScI may be more restrictive than those listed in Table 11.1.

Table 11.1: COS Count Rate Limits.

Detector	Mode	Type of limit	Limiting count rate (sec-1)
FUV	TIME-TAG	global	21,000 per segment
		local	100 per resel
	ACCUM	global	60,000 per segment
		local	100 per resel
NUV	TIME-TAG	global	21,000
		local	200 per pixel
	ACCUM	global	170,000
		local	200 per pixel

Table 11.2: Local and Global Flux Limits for COS

Detector	Wavelength	Local limit (FEFU)1		Global limit (FEFU)
		M gratings	L grating	M gratings	L grating
FUV	1300	6600 9.8	4100 12.3	530 14.5	300 14.7
	1600	20,000 7.9	8600 10.8	570 14.0
NUV	1800	82,000 8.1	10,000 10.3	14,000 10.0	1200 11.4
	2300	73,000 7.4	6300 9.9	11,000 9.4
	2800	80,000 6.6	8300 9.1	11,000 8.9

1Second value listed is the equivalent V magnitude of an O9V star.

Limiting Magnitudes for NUV Imaging

The following are the V magnitudes of an O5 star that is safe to observe with COS using NUV imaging. Therefore anything brighter for which measured UV flux data are not available is judged to be unsafe to observe.

• PSA with MIRRORA: V = 19.14
• PSA with MIRRORB: V = 16.13
• BOA with MIRRORA: V = 14.14
• BOA with MIRRORB: V = 11.13

Limiting Fluxes for NUV Imaging

The following fluxes are the highest permissible for a flat-spectrum source being acquired in ACQ/IMAGE mode:

• PSA with MIRRORA: 2 FEFU.
• PSA with MIRRORB: 30 FEFU.
• BOA with MIRRORA: 400 FEFU.
• BOA with MIRRORB: 6,000 FEFU.

11.5.2 Bright Object Protection Procedures

Any of the instrument protection levels shown below being activated is regarded as a serious breach of our instrument health and safety screening procedures and is cause for an investigation. Several of these conditions lead to a situation in which COS shuts itself down and subsequent observations do not take place until the instrument goes through a safe-mode recovery procedure that is run from the ground. Observers are responsible for ensuring that their observations do not cause an on-orbit problem with the instrument.

FUV Bright Object Protection

There are five levels of protection for the COS FUV XDL 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 (see Section 11.5.2) are set at about a factor of two 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 estimating 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), then the COS external shutter is commanded closed. If this occurs, only that one exposure is 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. The flight software in COS analyzes the FUV spectrum to ensure that local count rates do not exceed a threshold value. The limit is set at 100 events sec^-1 per resel. If the local rate limit is exceeded, the COS flight software closes the external shutter and turns off the calibration lamps.
4. Global rate monitoring is next. The COS flight software monitors the total event rate for both FUV detector segments. If the rate for either segment exceeds a threshold, the high voltage to the detector is set to its lowest value, internal lamps are turned off, and the external shutter is closed. The detector high voltage cannot be turned up again until special commanding is executed, and so if the global rate check is violated subsequent COS observations are likely to be lost.
5. At the highest level, the instrument is protected by the software sensing an overcurrent condition in the high voltage; this shuts down the high voltage entirely.

NUV Bright Object Protection

Similar protections also apply to the NUV MAMA:

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 (see Table 11.3) are set at about a factor of two 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 estimating 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), then the COS external shutter is commanded closed. If this occurs, only that one exposure is 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. The flight software in COS analyzes the NUV spectrum and takes a short exposure to check for groups of pixels exceeding a threshold value. This short exposure is not recorded. If the local rate limit is exceeded, the COS flight software closes the external shutter and turns off the calibration lamps. Again, if this occurs only the one exposure is lost.
4. Global rate monitoring is next. The COS flight software monitors the total event rate for the NUV MAMA. If the total count rate exceeds 77,000 in 0.1 sec the high voltage to the MAMA is turned off, the external shutter is closed, and the calibration lamps are turned off. COS can resume operations only after a safemode recovery procedure.
5. At the highest level, the NUV MAMA is protected by the detector electronics. If the detected count rate exceeds 77,000 in 138 msec, then the high voltage to the MAMA is turned off, the external shutter is closed, and the calibration lamps are turned off. COS can resume operations only after a safemode recovery procedure. This "Bright Scene Detection" procedure differs from the global rate monitoring in two ways: BSD is done in hardware, not software, and what is measured is not a digitized count rate but instead current from a grid of wires over the MAMA detector.

Screening Limits

Screening limits are the count rate limits that we at STScI expect observers to adhere to, in order to provide a margin of safety in instrument operations. Screening limits are of two kinds - global and local - and both limits must be adhered to. The COS screening limits are shown in Table 11.3.

Bear in mind that these are "screening limits," which means that if a target is predicted to cause counts in excess of these rates, then a more thorough check must be made. There are two higher limits that are important. First, a factor of two above the screening limits is the practical operation limit, the level we will not knowingly allow an observation to exceed, so as to provide a margin of safety for COS. In addition, if the FUV detector is used in TIME-TAG mode, significant data drop-outs occur when the count rate exceeds 21,000 per segment. The highest of these rate limits are those specified in the HST Constraints and Restrictions Document (CARD). If the CARD limits are exceeded on-orbit, the software and hardware within COS turn off the high voltage to the detector and COS goes into safe mode. This requires a safe-mode recovery procedure that must be executed from the ground, and no COS observations can be executed until that recovery is carried out.

Table 11.3: COS Count Rate Screening Limits.

Detector	Source type1	Type of limit	Limiting count rate2
FUV	predictable	global	15,000 per segment;
		local	40 per resel3
	irregular	global	6,000 per segment
		local	40 per resel3
NUV	predictable	global	30,000 per stripe
		local	80 per pixel
	irregular	global	12,000 per stripe
		local	80 per pixel

1"Predictable" means the brightness of the source can be reliably predicted for the time of observation to within 0.5 magnitude. 2Entries are counts per second. 3An FUV resel is 6 pixels wide by 10 high.

If a target is too bright to observe in the Primary Science Aperture (PSA), it may be possible to observe it with the Bright Object Aperture (BOA), which attenuates flux by a factor of approximately 200. However, the neutral density filter in the BOA also degrades the optical quality of the source image, reducing the effective resolving power for a point source by a factor of 2 to 3.

If a target is safe to observe in the PSA but is too bright for a straightforward acquisition with ACQ/IMAGE mode in the NUV channel, it is possible to acquire with an attenuating mirror, with the BOA, or with both. The target may also be acquired in dispersed light, which is explained in Section 7.6 and Section 7.7.

Risks from Nearby Objects

It is not sufficient for just a potential target to be safe to observe, because nearby bright objects can pose a risk as well. There are three scenarios:

• Given the errors in the initial pointings of objects with HST, even with good coordinates, an unintended source may end up in either aperture, BOA or PSA. With good coordinates, objects beyond 5 arcsec should not pose a risk.
• Even without errors, a bright object could unintentionally end up in the other COS science aperture. This is true no matter which COS aperture is designated for use because light from both apertures reaches the detector for both the FUV and NUV. The most significant risk occurs when the BOA is in use because an over-bright source could unintentionally end up in the PSA.
• Finally, a nearby source that is very bright could throw enough light into the PSA to cause problems. Here we adopt the same criterion as used for STIS. The region of concern is an annulus that extends from 5 to 15 arcsec from the center of the PSA. Any object falling in this annulus may not produce a global count rate per second in excess of 1 × 10⁵ per segment for the FUV or 2 × 10⁵ per stripe for the NUV, nor a local count rate over 200 per resel (FUV) or 400 per pixel (NUV). These limiting count rates are those estimated with the ETC as though the source were in the center of the aperture.

To guard against the risks imposed by these scenarios, observers are required to use the tools in APT to certify that no potentially UV-bright objects lie within a zone that could cause problems. In some cases it may be necessary to choose a specific ORIENT for the observation to ensure that nearby bright objects cannot fall in a COS aperture.