STIS MAMA modes are subject to local and global bright object limits that can restrict the observation of bright objects. One way to work around these limits is to use a lower throughput aperture with a given grating setting to reduce the count rate to an acceptable level. However, the officially supported neutral density slits 0.2X0.05ND and 0.3X0.05ND have very large attenuation factors of about 100X and 1000X respectively. This leaves a very large gap in dynamic range, and as a result, observing a target that is just a little too bright for the smallest clear aperture can be very inefficient.
STIS has three additional neutral density apertures, the 31X0.05NDA, 31X0.05NDB, and 31X0.05NDC with intermediate attenuation factors of about 6X, 15X, and 30X with respect to a large clear aperture. However, these apertures are not officially supported, and there was little calibration data available, especially for the echelle modes.
To improve this situation, a Cycle 19 GO calibration program (12567: "Bridging STIS's Neutral Density Desert") included new echelle observations using these slits to determine whether use of the long slits in the echelle configuration mode would introduce significant additional cross-dispersion scattered light compared with the normal supported short echelle slits. This program found that extracted spectra taken with these long slits matches spectra taken with supported short echelle slits, indicating that such effects do not appear to be important for the calibration. In particular, observations of Vega using the E140H-1271 setting with the 31X0.05NDC aperture show resolution and spectral purity comparable to that obtained when using the 0.2X0.05ND aperture.
A number of issues make the absolute aperture throughput calibration for the 31x0.05NDA[B,C] aperture challenging despite the newly acquired calibration data. The narrow widths of these apertures make the throughput achieved in a particular observation very sensitive to the target centering and to small variations in the HST focus; these effects alone can cause throughput variations of up to 50%, especially at the shortest FUV wavelengths. The currently adopted calibration attempts to trace the upper envelope of the observed aperture throughputs. While this allows conservative estimates of bright object limits when planning new observations by scaling results from other modes, for the current pipeline calibration of individual observations absolute and relative flux errors of 50% or more are possible.
The Cycle 21 large program 13346 (Advanced Spectral Library II: Hot Stars), is in the process of acquiring a signficant number of spectra of bright standard stars. Many of these observations use these intermediate ND apertures. To ensure that these and other similar data are available to the STIS community, the COS/STIS team is working on the full pipeline implementation of these 31X0.05NDA[B,C] apertures in order to provide spectra uniformly calibrated by the On-the-fly-recalibration (OTFR) pipeline. It is expected that users will be able to retrieve fully calibrated 31X0.05NDA[B,C] data in the next couple of months.
It is recommended that when using these available long-slit apertures, a peakup be done using the same aperture as that used for the science observation. This differs from recommendations for the supported apertures, where a peakup in a smaller aperture may be used prior to an observation with any larger slit.
These apertures are not yet supported in the STIS exposure time calculator. To aid observers who may wish to consider use of these slits for their HST observations, here are a few examples of relative count rates for an unreddened Kurucz model of a B1V star (Teff = 25400 long = 3.9).
While the use of these apertures with the STIS MAMA gratings may become fully supported in a future cycle, for Cycle 22 they will remain "available-but-unsupported", and users must accept some additional risk for observations using them. See section 2.3 of the STIS Instrument Handbook, for a full discussion of these policies.