Hamilton Spectrograph User's Guide

Individual Components

The parts of the spectrograph occupy two rooms in the basement of the 3-meter dome: the slit room and the camera room. The slit room lies at the base of the 3-meter's polar axle. It contains the slit, the decker and aperture plate, the acquisition/guide TV, the image rotator, the calibration sources, and the coude integrator.

The camera room lies behind the slit room, sloping downward along the line of the telescope's polar axle. The room contains the Hamilton's shutter, a filter wheel, the optical components of the spectrograph, and the CCD detector in its liquid nitrogen dewar.

These individual components are briefly described below. For technical information on the spectrograph and detectors, see Spectrograph Parameters and Detector Characteristics, respectively.

Slit

The polished steel slit jaws open and close unilaterally, the west side being fixed (this is important to remember, since opening or closing the slit will produce small shifts in the apparent line centers in the final image). The slit is remotely adjustable in 1-micron steps.

The slit projects on the sky at 530-microns/arcsecond at the 3-meter plate scale, and 106-microns/arcsecond for the CAT. The slit projects to the detector at 330-microns/15-micron pixel.

Decker and Aperture Plate

In order to maintain separation of spectral orders in the final image, an aperture plate or decker is inserted above the slit. Both serve to limit the height of the slit--and therefore the height of the spectral orders. Both are remotely adjustable for a variety of slit heights.

The aperture plate is a relatively recent alternative to the decker, and is generally preferred as the image reflected from its surface has a more uniform shape that results in better autoguiding. The aperture plate consists of a solid piece of polished steel with twenty-six rectangular holes, each of which serves as both decker and slit. (When using the aperture plate, the slit jaws are moved out of the way by opening them to their widest position. The desired aperture is then positioned over the wide-open slit.)

The apertures are in five groups of five each, plus a narrow aperture for focusing. Within each group are apertures with the same width at five different heights. Available widths are 320-, 480-, 640-, 800-, and 960-microns. Heights within each group are 2.0-, 2.5-, 3.0-, 5.0-, and 6.0-arcseconds (3-meter plate scale). The focusing aperture is 100-microns by 2.5-arcseconds.

The decker (now rarely used) is a step wedge with a variety of openings through which the slit is seen. Each opening corresponds to different slit height. In practice, deckers from about 2.0 to 3.0 arcseconds are used for observing, and from 5.0 to 6.0 arcseconds for flat-fielding.

The Hamilton is occasionally used in "longslit mode" in which the vertical (spatial) extent of the slit is not restricted by aperture or decker. Longslit operation permits spatial resolution of targets along the length of the slit, but because the resultant "tall" orders overlap one another, a narrow-band filter must be used, limiting the light reaching the detector to a bandpass approximately equal to the wavelength range of a single order. Coude field rotation also necessitates the use of the image rotator in longslit mode.

Acquisition/Guide TV

The acquisition/guide TV is located in the slit room to the right of the slit. The slit assembly is slightly tilted so that light spilling from around the slit is reflected to the TV camera. The camera can reach objects to about 19th visual magnitude--far fainter than the practical limit of the spectrograph. All TV functions are remotely controlled. See the Lick Autoguider Manual.

A filter wheel is mounted in front of the TV's lens, allowing the use of colored filters to compensate for the affects of atmospheric dispersion at large zenith distances, and for the use of neutral density filters in the case of a very bright star.

Image Rotator

An image rotator is available for counteracting the rotation of the coude field (15 degrees/hour as the telescope tracks), allowing extended objects or double stars to be aligned--and remain aligned --along the slit. However, because of the necessarily restricted slit height in an echelle system, resolution in the spatial direction is only an issue when the Hamilton is used in longslit mode (i.e. without a decker or aperture plate to limit the slit height--order separation being accomplished with a narrow band filter which passes only the light of a single order).

Calibration Sources

In general only two calibration sources are required for the Hamilton: a hollow cathode Thorium-Argon lamp for focusing and wavelength reference, and a continuum quartz lamp for flat-fielding. These lamps, along with several others, are mounted near the slit. All are remotely controlled.

The day sky can also be very helpful for finding one's place in the Hamilton spectral format. The CAT can be partially opened during the day, weather permitting, to provide a source of daylight.

Coude Integrator

The coude integrator monitors throughput at the slit. A reflective chopper behind the slit diverts a small part (less thgan 10%) of the beam to a photomultiplier. The integrator predates the Hamilton, being originally used as an exposure meter for photographic work. It is, however, occasionally used with the Hamiltion, and can, for example, provide a photon-weighted midpoint for an exposure.

Filter Wheel

Filters are held in a twelve-position, remotely-controlled filter wheel, just behind the slit assembly in the camera room. Users may install filters that have been mounted in standard Lick filter holders (designed to accommodate filters 2-inches square). If they do so, they must remember to record their changes in the Hamilton control program. If you do not know how to install a filter, consult a member of the staff.

While order-separating filters are generally not needed with the Hamilton, broad-band filters are often used for calibrations. Filters used commonly for flat-fielding and other calibration purposes are generally kept in the Hamilton filter wheel. Filter wheel contents can be displayed, and filters selected, from the Hamit, the spectrograph control program.

CCD's and Dewars

At present, two CCD's are available for use with the Hamilton. These are briefly described here. Click on the link to see detailed characteristics for the device.

Dewar #6 contains a thinned, back-illuminated Ford 2048 x 2048 CCD with 15 micron pixels. The chip has good QE and cosmetic characteristics but, due to charge diffusion produces a relatively broad PSF: about 2.1 pixels fwhm for a single emission line through a narrow slit.

Dewar #8 (technical specs not posted, inquire with mountain staff) contains a front-illuminated high-resitivity device developed at Lawrence Berkeley Labs [LBL]. It has 15-micron pixels, good QE, and good charge diffusion characteristics, producing a PSF of about 1.4 pixels fwhm. However, despite being housed in a specially shielded dewar, it accummulates cosmic ray and local particle hits at a much higher rate than the thinned device.