Space Telescope Science Institute   7.1.7 Charge Transfer Efficiency  7.2.2 Policy and Observers' Responsibility in Phase I and Phase II

7.2 SBC Bright Object Protection


7.2.1 How Do You Determine if You Violate a Bright Object Limit for SBC Exposures?


High global and local count rates can cause catastrophic damage to the SBC detector. Therefore, targets should be checked to verify that their fluxes do not exceed the defined SBC safety limits. As a first step, you can check your source V magnitude and peak flux against the bright-object screening magnitudes in Table 7.4 for your chosen observing configuration. In many cases, your source properties will be much fainter than these limits, and you need not worry further.

However, the magnitudes in this table are hard screening limits that correspond to the count rate limits in Table 7.4 and the output of the ACS Exposure Time Calculator. In earlier editions of this Handbook, these magnitudes were made fainter by arbitrary 1 or 2 magnitude pads, depending on the spectral-type range, which have now been removed. If your target is near these limits (within 2 magnitudes or a factor of 6.3 of the flux limits), then you need to carefully consider whether your source will be observable in the chosen configuration. Remember that the limits in these tables assume zero extinction. Thus, you will want to correct the limits appropriately for your source reddening due to interstellar dust.

You can use the information presented in Section 9.2 to calculate peak and global count rates. You can also use the ACS Exposure-Time Calculator to calculate the expected count rate from your source. The ETC has a host of template stellar spectrograms. If you have a spectrum of your source (e.g., from IUE, FOS, GHRS, or STIS) you can also use it directly in the calculator. As implied by the footnotes to Table 7.4, the model spectra in the ETC cannot be used for bright-object checking at the solar spectral type and later; the UV spectra of such stars are dominated by emission lines and continua not reproduced by the models. For these types, more realistic theoretical or observational input spectra (e.g., from the IUE or HST archives) must be used. The calculator will evaluate the global and per pixel count rates, and will warn you if your exposure exceeds the absolute bright-object limits. We recommend you use the ACS Exposure Time Calculator if you have any doubt that your exposure may exceed the bright-object MAMA limits.


 7.1.7 Charge Transfer Efficiency  7.2.2 Policy and Observers' Responsibility in Phase I and Phase II
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