Longslit Mode: Spatially Resolved Spectroscopy
TheoryIn the Hamilton's customary configuration, an entrance aperture limits the height (i.e., spatial dimension) of the spectral orders, preventing them from overlapping one another. While necessary for seperating the large number of orders that gives the Hamilton its broad wavelength coverage, this requirement precludes spatially resolved spectroscopy (i.e., limited by the height of the entrance aperture, all sources effectively become point sources). However, by using the so-called "longslit mode," extended sources can be spatially resolved, though at the price of wavelength coverage. In longslit mode, a narrow-band filter removes all but the one or two orders included in the filter's bandpass. Because the height of the entrance aperture (or slit) determines the height of the orders, elimnating most of the them eliminates most of the problem of overlap, and allows a full-length slit -- one along which the spatial details of the target are resolved -- to be used. The long slit is approximately 50 mm long (note: clear aperture still needs to be determined, so useable length will be somewhat less), which is about 95 arcsec with the 3-m plate scale and 470 arcsec long with the CAT.
Narrow-band FiltersLongslit mode requires a 2x2-inch (50x50-mm) narrow-band filter, centered on or near the wavelength to be observed. Ideally, a filter would be the breadth of single order and centered the wavelength(s) of interest. In practice a perfect match is unlikely, and one must accept some overlap from the adjacent orders. However, unless a strong spectral feature in a neighboring order happens to coincide and interfere with the feature of interest, some contamination from the adjacent orders can be tolerated. Narrow-band filters are identified by their central wavelength and the full width at half maximum of their bandpass, usually in Angstroms (e.g., 6563/75). Lick's filter collection is available to Hamilton users, but observers should contact a support astronomer to confirm that the needed filter(s) is available, and to help with locating and installing it.
Image RotatorLongslit observations usually require that the a particular axis of the target be aligned along the long (spatial) dimension of the slit, and further, that the target's orientation with respect to the slit remain fixed for the duration of the observation (the coudé field rotates as the telescope tracks, at the rate of 15 degrees per hour). Both these requirements are met by the image rotator.
The rotator may be used with either the 3-meter or the CAT, though its use with the latter requires the installation of additional mirrors. It consists of a clock-driven roof prism that counteracts the rotation of the coudé field. The image rotator is described in another section of this manual.