12.8 Special Considerations for Extended Targets

Special care should be taken when determining the placement of a long slit on an extended source. Observers should take into account the following:

• Slit lengths using the MAMA in First-Order Spectroscopy: Although the full 52 arcsecond length of the long slits is imaged on to the CCD, light from only a portion of these slits reaches the MAMA detector. Twenty-five arcseconds are available for G230L and G140L, and 28 arcseconds can be used for G230M and G140M. See Table 4.1 and Section 4.2.2 for details of these apertures.
• Fiducial Bars in the Long Slits: The STIS long slits have fiducial bars which will block light from two regions along the length of the slit. Section 13.2.6 and Figure 13.1 describe the bars and their location.
• Pseudo-Aperture Positions: Choosing one of the pseudo-apertures described in Section 4.2.3 will result in the target being off-center in the aperture.
• Offset of G140L and G140M Spectra to Avoid the Repeller Wire: First-order MAMA spectra in these modes are shifted by ~3.25 arcseconds to avoid the shadow of the repeller wire (Section 7.6). These shifts are done by adjusting the grating angle and so the spectrum is shifted on the detector, even though the target remains at the requested location in the aperture.
• Monthly Offsets of First-Order MAMA Spectra: Monthly offsets of up to about +/- 1 arcsec are applied to both FUV and NUV first-order spectra (Section 7.6).