Slitless First-Order Spectroscopy

The vast majority of STIS first-order grating mode observations use a long slit. The use of a long slit ensures a clean separation of emission lines arising from different spatial features. However, all of STIS's first-order gratings (see Table 4.1) can also be used slitless to obtain emission-line images. Figure 12.1 below shows a schematic example of a slitless spectrogram. Figure 4.8 shows one such observed image of SN1987A.

Figure 12.1: Schematic Slitless Spectrogram of Planetary Nebula

When STIS is used slitless (or with a wide slit), the image you obtain will be the sum of a series of monochromatic images of the field of view at a single wavelength, where prior to summing the monochromatic images are shifted relative to one another by the resolution (Å/pixel) of the grating. The range of wavelengths covered in the series of monochromatic images is dictated by the spectral range of the grating. The result is that there is not a one-to-one mapping of pixel location to wavelength in your image or of pixel location to spatial location on the sky. Depending on the structure of your source and the grating you use, it may be easy to deconvolve the spatial and spectral information, or it may be very difficult.

Slitless spectroscopy can be employed either for prime or parallel STIS observing. If you are designing a slitless spectroscopic observation there are a few important points to keep in mind:

• The more complex the emission-line, velocity, and spatial structure of your target field, the more difficult it will be to deconvolve the spatial and spectral information. It is important to match the grating you choose to the structure of your source. Gratings which produce images of multiple, kinematically resolved emission lines will be the most challenging to deconvolve. At the other extreme, a grating which covers only a single strong emission line at a resolution where the lines are kinematically unresolved will produce a clean image of the source in the single emission line (see Figure 12.1, above). You may also wish to specify the orientation for slitless spectroscopic observations to ensure that the most complex source structure is oriented perpendicular to the dispersion axis (see "Fixing Orientation on the Sky" on page 238).
• Since each point in the sky emits geocoronal light, the background due to the geocoronal emission lines (Lyman- 1216, O I 1304 and occasionally on the day side O I 1356 and O II 2470; see Geocoronal Emission and Shadow) will be observed at all pixels in the image when a slitless spectrum is obtained which covers these wavelengths. This background must be taken into account in your signal-to-noise calculations. Note that when a spectroscopic exposure is obtained with a slit, these sky emission lines are localized in the resulting image to the pixels at the corresponding wavelengths. For this reason, you may wish to consider using one of the two longpass ultraviolet blocking filters (see Longpass-Filtered MAMA Imaging- F25SRF2 and F25QTZ), instead of a clear aperture when performing ultraviolet slitless spectroscopy.
• Slitless spectroscopic data will not be fully calibrated by the STScI STIS pipeline. Slitless spectroscopic data will be passed through the first phase of calibration and a flat-fielded calibrated image will be produced; however, the pipeline will not attempt to spectroscopically calibrate the data. This process must be interactively done by the observer since, as described above, ambiguous overlap of spatial and spectral information will occur. The slitless task in the stsdas.contrib package (originally contributed by ST-ECF) is available to aid in performing this post-pipeline processing.

In order to properly calibrate slitless data it is necessary to know the position of each source along the dispersion direction. To do this it is usually necessary to take in the same visit a STIS image of the field for which you are obtaining slitless spectroscopy. Because there is lack of repeatability in the Mode Select Mechanism (which contains the mirrors and gratings) an additional special calibration may be needed in order to fix the absolute offset between the images and the spectrograms. Without this there will be an uncertainty of roughly ~5 pixels between the projection of the image and the projection of the spectrogram on the detector, due to small uncertainties in the placement of the mirror and the grating by the Mode Select Mechanism (MSM).

The standard STIS ACQ procedure automatically measures the offset between where the stellar image and an image of a standard slit appear on the CCD detector and uses this to place the target accurately in the desired slit. So if the ACQ target appears in the field of view, there is no need for an additional image to calibrate the MSM offset, although a separate full field image may still be needed to measure the relative positions of other sources with respect to the ACQ target. In cases where no STIS ACQ exposure is done, an image of the field should be taken either immediately following or immediately proceeding a lamp image taken through a narrow slit. For the CCD, a 1 second tungsten lamp exposure with the 52X0.1 slit will do nicely. This will allow the MSM offset for that image to be determined. It is important that no MSM motion (mirror or grating change) occur between the sky image and the lamp image of the slit. The MSM offset for the spectrographic exposure itself can be measured from the standard wavecal exposure. If extremely precise alignment between the spectrum and the field image in the cross dispersion direction is also required for source identification this procedure may require some modifications, and observers should consult their contact scientist or the STScI help desk at help@stsci.edu.

Finally we note that to achieve an accurate wavelength calibration for targets observed in slitless mode when those targets are well displaced from the nominal AXIS1 center, the dispersion coefficients at the off-nominal centerings must be independently calibrated. A STIS calibration program is currently underway to determine those calibrations for the CCD modes which are most commonly used for slitless spectroscopic observations (proposal 7654 for the CCD with the G430L and G750L gratings; proposal 8434 for the MAMAs with the G140L and G230L gratings and the prism). In the absence of such explicit calibration observations for slitless spectroscopic observations taken in other settings, the wavelength calibration must be based on the existing more limited ground based calibration data on the incidence-angle offset corrections and will be slightly less certain. We recommend that observers consult their CS or the help desk at help@stsci.edu if they are concerned about the calibration of observations taken of targets which are expected to be off-center in the dispersion direction (i.e., not centered in the slit) for observations in slits wider than or equal to 2 arcseconds.