User's Guide to the Hamilton Spectrograph

Table of Contents

Quick Reference
CCD Characteristics
Grating Tilt & Dewar Height
Filter Wheel & Shutter
Calibration Sources
Photon Integrator
Image Rotator
Iodine Cell & Slit Room Controller
Guide Camera & Filter Wheel
Data Taking System
Hamilton Motor Controller
Hamilton Focus
More Info:
Spectral Format
Navigating the Spectrum
Table of Orders
Setup Procedures
Observing Hints

Data Archive
Mt. Hamilton Homepage

Longslit Mode: Spatially Resolved Spectroscopy


In 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 Filters

Longslit 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 Rotator

Longslit 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.

Setting up for Longslit Observing

Apart from the addition of the image rotator (see above), the main differences bwtween setting up for conventional echelle work and longslit observations most of the setup are in the use of filter wheel and the filter wheel and slit assembly.

Removing the aperture plate and adjusting the slit width is done in hammotor_gui. From the Aperture section select "Out." This will cause the aperture plate to move clear of the underlying slit. From the Slit section of the Hamilton Motor controller enter the slit width, in microns. 640 microns -- a common choice for Hamilton observing -- projects to about 2 pixels on the detector and about 1.2 arcseconds on the sky at the 3-meter plate scale (CAT plate scale is 5 times smaller, so the on sky projection is about 6 arc seconds).

The narrow-band filter(s) is loaded into the filter wheel in the camera room. Changing filters is described in another section of this manual. If you have never changed a Hamilton filter, we recommend that you ask a support astronomer or other member of the telescope staff for guidance.

Calibrations for Longslit Observing

Calibrations are similar to those for conventional echelle observing, but exposure times will be different. Because of the narrowed wavelength range, flat fields do not require the combination of broad-band filters usually needed with conventional observing (see calibrations). The types and number of calibration frames will depend on the particular requirements of the observing program.

Guiding a Longslit Exposure

Extended sources (unless, like some galaxies with condensed cores, they have some point-like structure) are difficult to autoguide. Even if a star is visible in the field, off-axis guiding is not normally possible at coudé, because of field rotation. However, if the image rotator is in use, and field rotation is therefore counteracted, and off-axis guiding is permitted (further details on autoguiding with the image rotator are given in the image rotator section). If the source is extended and no guide star is available in the field, the observer must hand guide as best he or she can.