8.2 Bright-Object Protection
8.2.1 Limiting magnitudes and bright object limits
Micro-channel plates are susceptible to degradation if exposed to bright sources of ultraviolet light. 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, 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 ~200 ct s-1 pix-1 (~1800 ct s-1 resel-1). The NUV global count rate limit can be increased slightly at the expense of Doppler compensation during the course of the exposure.
Table 2-2 contains approximate estimates of the bright limit fluxes for the medium- and low-resolution COS modes at several wavelengths using the count rate limits listed in Table 2-1. 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 2-2.
Table 8.1:
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Detector
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Mode
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Type of limit
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1Entries are counts per second.
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COS Count Rate Limits
Table 8.2:
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Detector
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Wavelength
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Global limit (FEFU)
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M gratings
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L grating
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M gratings
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L grating
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1Second value listed is the equivalent V magnitude of an O9V star.
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Local and global flux limits for COS
