The basic characteristics of the instrument are set by the aperture, focal length and resolution of the image forming lens, the dispersion of the spectrometer, and the sensitivity of the detector. These specifications are summarized below.
Objective
of the image-forming optical system
is a 400mm f/2.8
Nikor ED (extremely low dispersion glass) lens. The lens is internally
focused, and maintains acceptable focus (2 pixels in the detector plane)
over a typical spectral coverage of one grating setting. The primary focal
plane scale image scale is 516"/mm.
Field of View
along the spatial dimension of a 25mm
square detector in the final spectral image plane the 400mm lens is
3.2°.
Angular resolution
as set by two 25 µm pixels in
the spectral image plane is 23" with the 400mm lens.
Entrance slit
masks the image of the sky along a
strip 25 µm wide and 25mm long. The slit width may be opened to 3mm
to acquire objective grating spectra of
dense star fields and monochromatic images of extended emission line
sources.
Collimator
is a 180mm focal length f/2.8 Nikor ED
lens.
Diffraction grating
is a 102×102 mm, 300
groove/mm, 7620 Å blaze, Milton Roy precision grating. The peak
efficiency is 92% diffracted into the blaze angle.
Spectrograph
camera lens is a 200mm f/2 Nikor ED
lens. Combined with the 180mm collimator, this lens magnifies the image
plane by 1.11 times. The use of a large, 100mm diameter, 200mm focal length,
f/2 lens reduces vignetting at the spectral or spatial edges of the field.
Filters
are inserted into the 200mm f/2 lens close
to the spectral image plane. The lens accommodates cut gelatin filters. We
use a Wratten 21 which transmits from 5500 Å to 1 µm and blocks
second order diffraction from the grating. To cover from 300 Å to 5500
Å the high pass Wratten 21 filter is removed and replaced with a low
pass filter. This allows one grating to be used close to its blaze for all
wavelengths from the atmospheric cutoff in the ultraviolet to the detection
limit of the CCD in the infrared.
Spectral resolution
as set by two pixels and a
dispersion of 3.92 Å/pixel in the spectral image plane is 7.8 Å.
The actual resolution is one pixel in the regions of best focus.
Spectral coverage
at 3.92 Å/pixel is
approximately 4000 Å at one setting of the grating in first order. The
instrument typically is set for 5500 Å to 9500 Å. In second
order, for the blue, the coverage is half this and the instrument would
span 3500 to 5500 Å in one exposure.
Detector
The spectral image is detected with an
Apogee CCD camera. It utilizes a SITE 1024×1024,
25 µm square pixel, backside illuminated, zero defect, scientific grade
CCD. The actual noise in the signal is determined by photon statistics
rather than the CCD in most
imaging situations with some background skylight present.