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 x 25 arcsecond filter occupying a single position on the slit wheel. The target is centered in 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" 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" restrictive border around the interior of the selected quadrant, effectively reducing its size to 7.5x7.5". In addition, any object that would produce a global count rate exceeding 1.5x10⁶ counts s^-1 or a local count rate above 500 counts sec^-1 pixel^-1 in F25NDQ1 may not be observed using any of the other quadrants. To ensure accurate placement of a bright object in the selected quadrant, a CCD target acquisition must also be specified.

F25ND3 and F25ND5 are each individual 25 x 25 arcsecond filtered apertures, occupying unique locations on the slit wheel. Detailed information for F25ND3 and F25ND5 are given in "F25ND3-NUV-MAMA" on page 393, "F25ND3-FUV-MAMA" on page 420, "F25ND5-NUV-MAMA" on page 395, and "F25ND5-FUV-MAMA" on page 422.