Entrance Apertures
The Hamilton was originally designed with conventional slit jaws,
surmounted by a stepped decker to limit slit height. (Because the
echelle format consists of many spectral orders, slightly separated by
cross dispersion, slit height must be limited to prevent orders
overlapping one another.) Slit and decker have been functionally
replaced with a selection of precise rectangular openings in a
polished steel aperture plate, which offers the observer a variety of
height/width combinations. (The slit is still present, but when the
aperture plate is selected in software, the jaws are automatically
opened to their maximum width so as to be entirely clear of the light
path.)
The apertures are arranged on the plate in five groups. Each group
comprises five apertures of a single width at different heights.
Apertures are selected in accordance with program requirements and
observing conditions. Choosing the best aperture for your needs is
discussed in the
Setup Procedures
section of this manual.
Occasionally, Hamilton observations require spatial resolution not
possible with the aperture plate's limitation on slit height. In such
cases the aperture plate is removed and the old slit jaws are used in a
techinique called
longslit
observation. Longslit observations trade wavelength coverage for
spatial resolution and have special setup and operating requirements.
The table below lists the available aperture widths, given in arcseconds
on the the sky at the 3-meter and CAT plate scales, in pixels as projected
on the detector (assuming 15-micron pixels), and their actual widths
in microns.
| Aperture Widths |
|---|
| apertures | B-F | G-K |
L-P | Q-U | V-Z |
| arcsecs, 3-m scale | ~ 0.6 |
~ 0.8 | ~ 1.2 | ~ 1.5 |
~ 1.8 |
| arcsecs, CAT scale | ~ 3.0 |
~ 4.0 | ~ 6.0 | ~ 7.5 |
~ 9.0 |
| pixels | ~ 0.5 |
~ 0.66 | ~ 1.0 | ~ 1.25 |
~ 1.5 |
| microns | 320 |
400 | 640 | 800 |
960 |
Each of the five widths is available in five heights: 2.0, 2.5, 3.0,
5.0, and 6.0-arcseconds at the 3-m plate scale, for a total of 25
apertures labeled B-Z. Aperture A is 100-microns wide by
2.5-arcseconds, and is used exclusively for focusing.
Aperture choice is controlled using the spectrograph motor control
software,
hammotor_gui (see Figure 1
for the aperture selection choices). The letter identifiers for the
apertures are not listed in the motor control interface, but go in
order from A 100:25 through Z 960:6.0. To do longslit observations
set the aperture to Out, and adjust the slit width to the desired
setting in microns using the Slit section of the hammotor GUI (shown
in Figure 2).
Figure 1: Aperture Plate Selection Menu
Figure 2: Slit Size
Support Astronomers (sa@ucolick.org)
Last modified: Sun Feb 13 16:49:16 PST 2011
