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LonGSp: Optical Design

Optical Design

The optical scheme of the instrument is sketched in Figure 1 , it is designed to match the F/20 focal ratio of the TIRGO telescope. We have, following the optical path, the window of the dewar, the order sorting filter (not in figure), a field lens, in Calcium Floride, and the slit, in the focal plane of the telescope. The field lens put the image of pupil into the secondary mirror of an inverted cassegrain (with focal length of 1400 mm) that produces a parallel beam of 70 mm diameter. This beam is reflected into grating by a flat mirror. The grating has 150 grooves/mm, is arranged in littrow configuration and has a blaze wavelength of 2 µm. The modified Pfund camera, after the grating, (with focal length of 225 mm) collect the dispersed beams into the detector. The field of view of each pixel is 1.73 arcsec (the pixel size is 40 µm). The field covered on the sky by the slit is about 70 arcsec.

The resolving power is (for first order of grating) of about 600 in the center of J band and 950 in the center of the K band, using a slit of two pixel (3.46 arcsec). The back of grating is a plane mirror so that, when the grating is rotated of 180 degrees, the instrument works as a camera, with a field of wiev of about 1.5 arcmin of diameter, in the band defined by the filters. This facility can be useful for tests, maintenance and in centering weak sources on the slit.

The filters wheel (positioned before the focal plane of telescope) can hold a total of eight one-inch filters that are used to sorting order of grating. In table 1 is reported the set presently mounted. The silits wheel (put on focal plane of telescope) has also eight positions each one holding a field stop (set is reported in table 2) and a field lens. All the mirrors are gold coated to have a good efficiency over a wide spectral range, the optics is usable at least as far as 5 µm.

The optical components are cooled at about 80 K through thermal contact with a liquid nitrogen vessel at atmospheric pressure as described in the cryo-mechanics section, The mounting of optical elements is designed to take account of the dimensional changes between mirrors (in pyrex) and supports (in aluminium) generated by temperature differences.

The analysis of the optical system was obtained using the package Zemax (of Focus Software Inc., Tucson - AZ). The image quality is very good compared with the pixel size; fig.2 shows, for example, the spot diagrams of point-like sources put in three points of slits (upper boundary, centre and bottom boundary) and for three wavelengths in H band: 1.575, 1.650 and 1.725 micron (that are the central and the extreme wavelengths falling on the used area of detector), with grating working in first order; the squares represent the pixel size (40 micron).

Last updated 10 May 96

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