The LRIS collimator is an off-axis paraboloid made of BK7 glass which has its optical axis coincident with the telescope and spectrograph optical axis. The field of view is therefore off-axis with respect to the collimator. This arrangement produces excellent images over the field being used, probably because the centers of curvature of the telescope focal plane and the paraboloid are nearly coincident.

The collimator was manufactured by Optical Surfaces Ltd, U.K.

The cell for the mirror was manufactured by the University College London.

Figure 1: Schematic of the LRIS collimator

Physical Specifications

True parabolic focal length 78.75 in.
Axial displacement 12.27 in
Finished diameter 21.40 in
Used mirror surface diameter 21.00 in.
Off-axis angle 8.91 deg.
Off-axis distance 1.57 in.
Material BK7
Edge thickness 2.20 in.
Accuracy blur circle diameter no more than 0,0030 in.
Surface quality 60/40 or better
Coating (original) overcoated silver
Coating (new) Silver on aluminum
Enclosed energy 95 % in 24 microns

Reflectivity Measurements

The reflectivity of the collimator is shown in the Figure 2. The collimator was last re-coated in 2001 with a special coating procedure developed at Lawrence Livermore National Laboratory (Norm Thomas, P.I.) in which a thin silver layer is deposited on top of Al. Thus, the UV is transmitted through the thin Ag layer below 400 nm wavelength, and is reflected from the Al layer underneath. This UV-shifted durable coating provides a valuable alternative to the aluminum coating for mirror coatings where high throughput and durability are important considerations. Aluminum also has a reflectance dip at 850 nm caused by inter-band transitions which is eliminated by placing the thin Ag layer on top.

Figure 2: Collimator Reflectance Curve (2 measurements)

Light beams on the collimator

Some of the f/15 light bundles at the corners of the field of view are shown in the Figure 3. It should be noted that there is a little vignetting at the outside corners of the field.

Figure 3: The beams at the corners of the focal plane rectangle superimposed on the collimator.

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