10.2 Screening Limits

The global and local count-rate screening limits for each COS configuration are given in Table 10.1. The limits are independent of observing mode (TIME-TAG or ACCUM). Compliance with these limits must be checked for all proposed COS targets by means of the Exposure Time Calculator (ETC), which issues warnings if they are exceeded. In Phase I, all proposed targets must be screened against these limits. In Phase II, the results of more detailed target and field checks must be submitted with the observing proposal; details are provided in Section 10.6. Note that the local count-rate limits given in Table 1.2 and the local count-rate screening limits given in Table 10.1 are not applicable to the 1055 and 1096 cenwaves of the COS G130M grating. For those modes, the local limit is 0.2 count s^-1 pixel^-1 for both predictable and irregular sources.

It is also useful to note the distinction between the global count-rate screening limits and the data-rate limit on TIME-TAG exposures. While both limits may affect the feasibility of proposed observations, the global count-rate screening limits for each COS configuration are defined to protect the detectors and the data-rate limit on TIME-TAG exposures is defined to protect against the loss of data (as discussed in Section 5.2.1).  In the FUV, a steep spectrum could exceed the global count-rate screening limit of 15,000 count s^-1 per detector segment without exceeding the 30,000 count s^-1 data-rate limit. In the NUV, the global count-rate screening limit is considerably higher at 30,000 count s^-1 per stripe, however, a target that is perfectly safe to observe could be far too bright to observe in TIME-TAG mode if it exceeds the 30,000 count s^-1 data-rate limit.