|STIS Instrument Handbook for Cycle 24|
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. 25 arcseconds is 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.
• Offset of G140L and G140M Spectra to Avoid the Repeller Wire: First-order MAMA spectra in these modes are shifted by 3 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.