Observing Too-Bright Objects with STIS

As described in MAMA Bright-Object Limits, the STIS MAMA detectors can be damaged at high local and global count rates. The MAMA detectors also suffer uncorrectable non-linearity at similar count rates (see MAMA Non-linearity). There are therefore configuration- specific count-rate limits for all observations that use the MAMA detectors; sources brighter than allowed by the limits cannot be observed in that configuration.

The STIS CCDs are not subject to the same bright-object constraints, as the CCD cannot be damaged by observations of bright sources. At high accumulated count/pixel levels, however, the CCD saturates and counting becomes nonlinear. As described previously (see CCD Saturation), CCD saturation can be avoided by keeping exposure times short when observing bright targets. The minimum exposure time for CCD observations (0.1 sec) dictates the maximum source brightness which can be observed without saturating.

The only way to use STIS to observe a source that is too bright is to use a configuration which reduces the flux from the target, bringing it into the observable regime. The options available to achieve this reduction are:

• Use a smaller slit to reduce the transmitted light for spectroscopic observations (see "Apertures" on page 325-you will find there the percent flux transmitted through each slit as a function of wavelength).
• Select a more appropriate grating or filter configuration. The solution may be a configuration with higher resolving power if it is the local limit which is being violated, or a configuration that covers a smaller spectral range if the global limit is being violated. In more extreme cases, you may be forced to choose a grating (filter) that covers an entirely different region of the spectrum. Note that if you are observing in first order in the near-UV, you can consider using the CCD near-UV first-order spectroscopic modes G230LB and G230MB (see Cross-Over Regions).
• Use a neutral-density-filtered full aperture. The neutral-density filters are described in the section Neutral-Density Filters; they produce attenuations ranging by factors from 10^-1 to 10^-6. Note, however, that the ND filters are located in the slit wheel. Thus, all supported ND full-filtered exposures will be slitless; i.e., you cannot use a slit and an ND full filter together. Similarly, you cannot use a ND full filter and a spectral filter in imaging mode. Also note that some ND filters come in quadrants, all of which are simultaneously imaged.
• Use one of the echelle or long calibration slits which contain neutral-density filters. Available-but-unsupported slits are 31X0.05NDA (with ND=0.4), 31X0.05NDB (with ND=0.8), and 31X0.05NDC (with ND=1.2). Supported slits are 0.2X0.05ND (with ND=2.0) and 0.3X0.05ND (with ND=3.0), where if ND=x, the flux is attenuated by 10^-x. For all the ND slits, the throughputs as a function of wavelength are posted on the STIS WWW orbital performance page.