5.4 Neutral Density Filters

STIS has a complement of neutral-density (ND) filters, which can be used with the CCD, the NUV-MAMA, and the FUV-MAMA spanning from ND1 (attenuation of 10^–1) to ND5. The ND5 filter appears to have a UV throughput of at most 10^–6, substantially smaller than prelaunch estimates.

F25NDQ1, F25NDQ2, F25NDQ3, and F25NDQ4 are physically four separate quadrants of a single 25 × 25 arcsecond filter occupying a single position on the slit wheel. Each quadrant having a different neutral density factor and the number appended to each name is the approximate dex ND factor. The target is put into the appropriate quadrant when one of these apertures is requested. However, it should be noted that all four quadrants project onto the detector, and each quadrant plus a 5 arcsecond surrounding perimeter must be considered for bright object limits. Thus, although one may request placement of a bright object centered in F25NDQ4, no other objects in the surrounding region may violate the bright-object limits for the other quadrants. This includes a 5 arcsecond restrictive border around the interior of the selected quadrant, effectively reducing its size. In practice, for safety reasons any object that would produce a global count rate exceeding 1.5 × 10⁶ counts/s or a local count rate above 500 counts/s/pix in F25NDQ1, may not be observed using any of the other quadrants, since pointing problems could cause this bright source to end up in F25NDQ1. To ensure accurate placement of a bright object in the selected quadrant, a CCD target acquisition must also be specified. Also note that the boundaries between the four quadrants are not well centered, and as a result the sizes of the quadrants vary. Figure 13.83 shows a lamp image of this aperture, and Table 13.34 gives the approximate dimensions of each quadrant and the default locations where targets are placed.

F25ND3 and F25ND5 are each individual 25 × 25 arcsecond filtered apertures, occupying unique locations on the slit wheel. Detailed information for F25ND3 and F25ND5 can be found in F25ND3 - NUV-MAMA, F25ND3 - FUV-MAMA, F25ND5 - NUV-MAMA, and F25ND5 - FUV-MAMA.