Wispi Optical Specifications

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 24 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 may use a Wratten 21 which transmits from 5500 Å to 1 µm and blocks second order diffraction from the grating if needed. The grating may be used in second order from 3000 Å to 5500 Å by inserting a low pass filter. However, with the Kodak detector's low blue sensitivity, there is no significant problem in overlapping orders when the grating is set to cover 4000 to 8000 Å in a single exposure without a filter. In this configuration there is adequate sensitivity down to H-beta, and the peak response is at H-alpha.

Spectral resolution
as set by two pixels and a dispersion of 4 Å/pixel in the spectral image plane is 8 Å. The actual resolution is one pixel in the regions of best focus.

Spectral coverage
at 4 Å/pixel is approximately 4000 Å at one setting of the grating in first order. The instrument typically is set for 4000 Å to 8000 Å. In second order, for the blue, the coverage is half this and the instrument would span 3500 to 5500 Å in one exposure with an appropriate filter.

Detector
The spectral image is detected with an Apogee U6 CCD camera. It utilizes a Kodak 1024×1024, 24 µm square pixel, 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.