Supported neutral-density-filtered aperture for slitless 1st order MAMA spectroscopy of targets too bright to be observed with a clear aperture. The F25NDQ filter is unique in that it is divided into four quadrants, each having a different neutral density factor. In clockwise order starting from the upper left these are
F25NDQ1,
F25NDQ2,
F25NDQ3, and
F25NDQ4, where the number appended to each name is the approximate dex ND factor. Approximate dimensions of each quadrant and the default location of an external target in each quadrant are given in
Table 13.34.
The default aperture locations where the target is placed in each quadrant are significantly offset from the center of the detector in both the spatial and the dispersion directions. For observations at these nominal aperture locations this will cause a significant shift in the wavelength coverage for any given grating setting. For example, when observing with G140L in
F25NDQ1, wavelengths shorter than about 1330 е will fall off the detector. Users may wish to use a
POS-TARG to bring the target back closer to the center line of the detector. However, if the target would violate BOP restrictions in any quadrant of the aperture, the restrictions regarding pointing close to objects violating safety limits discussed in
Chapter 7 and
STIS ISR 2000-01 will apply. This latter rule renders the
F25NDQ4 quadrant mostly useless, and the
F25NDQ3 quadrant is redundant with the full field
F25ND3 aperture, so in practice only the
F25NDQ1 and
F25NDQ2 quadrants are commonly used. Also the dividing lines between the quadrants are somewhat displaced from the center of the detector and are not quite parallel to the detector edges (see
Figure 13.83), so observers for whom the exact locations of the quadrant boundaries are important should consult the
Help Desk.
Note: Aperture throughputs values for NDQ1 and NDQ4 at λ<1330 Angstroms and for NDQ2 and NDQ3 at λ>2850 Angstroms
are extrapolated.
