WIYN / Bench Upgrade: VPH 3300 l/mm gratings


Two high-density gratings are being made by CSL. The first (v3300a, CSL/WP3200) is a small, test grating, 3300 l/mm, made on float-glass. Based upon initial performance evaluation will finalize specification requirements for a 2nd, larger grating (CSL/WP4200, tentatively v3550a). Work in progres on CSL/WP4200 definition can be at this design page, and this optimization page.

For CSL/WP4200 we have finalized the substrate size and thickness. The size is 500 x 230mm with a 480 x 210mm clear aperture. To place a severe specification on the final wavefront (lambda/4) requires an appropriate glass thickness and flatness (lambda/10). The rule of thumb used by polishers is that the thickness should be at least equal to a tenth of the maximal inplane dimension (diagonal or diameter). For a 500 x 230 mm grating the thickness should then be 55mm. However, we are informed by CSL that their coating machine can admit substrates with a maximum of 33mm thickness. Reducing the thickness may reduce the blank stiffness and thus increase the final wavefront error. Charles Harmer's analysis is that a thickness of 30 mm is acceptable. The final substrates (2) purchased from Zygo have dimensions of 500 x 230 x 30 mm, and have been delivered on Jul 06 2004.


Calculations are based on rigorous coupled wave analysis (Barden, Bershady). Partial measurements for test-grating, WP3200, from CSL exist in August 08, 2003 report. Note the difference between Barden's and Bershady's model. This is due, presumably to differences in grating parameters d, n, dn. We want to modulate CSL's delivered parameters to be closer to the those use by Barden, and in fact with the optimization pushed slighlty to even higher angles.

RCWA predicted efficiency of 3300 l/mm VPH grating with unknown parameters (Barden)
RCWA predicted efficiency of 3300 l/mm VPH grating WP3200 parameters from CSL (Bershady):
d = 12 microns (effective)
dn = 0.048
n1=n3=1.43, n2=1.4
Horizontal, colored lines span sampled Bench band-pass at given grating angle at minimum efficiency.

Optomechanical Components for Testing on Current Bench

Design and drawings are courtesy of Bill Schoening. Components fabrication has not begun. See v740a page for further illustrations of optomechanics for low-density gratings.

Configuration for high-density grating (large angle). Rectangular housing (inner and outer) fasten to existing, encoded, grating turret (#1), currently used for reflection gratings.

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last updated: Sep 20, 2005 (mab@astro.wisc.edu)