12.2 Long-Slit Echelle Spectroscopy

The STIS echelle gratings (see Section 4.3.1) were designed to maximize the spectral range covered in a single echellogram. The orders are therefore closely spaced, and to avoid overlap between orders, short echelle slits must be used. Indeed, the majority of STIS echelle observations are of point sources and use these customized echelle slits (see Section 4.3.2). Nevertheless, at the price of confusion due to order overlap, the echelle gratings can be used with a long slit to obtain high-resolution spectroscopy of extended objects, or they can be used slitless with a full clear, filtered, or ND aperture. An example of a scientific application that would benefit from long-slit echelle spectroscopy might be observations designed to map the kinematics of planetary nebulae and stellar outflows around young stars. Observers contemplating such observations should be aware that the problems of order overlap, scattered light, and the broad wings of the PSF from the Optical Telescope Assembly will make accurate calibration and line-profile work extremely complex for extended sources with a continuum (see Section 13.7).  While the three 31X0.05ND(A-C) apertures provide a range of attenuation factors between those of the small clear apertures and the 0.2X0.05ND aperture (e.g., for maximizing the S/N for targets that are too bright for the small clear apertures), order overlap in those long slits will produce slight increases in the inter-order background even for point sources, which can complicate determination of the flux zero points.

The 6X0.2 slit (6 arcseconds in the spatial direction and 0.2 arcseconds wide in the dispersion direction) is supported for use with all four of the echelle gratings. However, observers should be aware of the ambiguous overlap in the resulting echellogram that makes the reduction of long-slit echelle data an inherently source-dependent and interactive process (see Figure 12.2).

Observers should also note that, for long-slit echelle spectra, images of monochromatic lines on the detector are rotated by an angle that differs significantly from the physical angle of the slit, and which varies from one echelle grating to another. When using the 6X0.2 aperture with an echelle grating, the same aperture is also used for the auto-wavecal, and this wavecal image can be used to measure the change of the wavelength scale as a function of position along the slit. However, long-slit wavecal lines will overlap multiple orders, causing calstis to calculate incorrect wavelength and spatial offsets. Users may wish to consider adding an additional GO wavecal exposure using a smaller aperture (e.g., 0.2X0.2) to more easily derive an accurate wavelength scale for their science image. When using any other long-slit aperture with the echelle gratings, the auto-wavecal will be done by default with a small aperture. In such cases, the user may wish to add an additional GO wavecal with the 6X0.2 aperture in order to measure the change in the projected slit angle.