Sloan Digital Sky Survey Telescope Technical Note 19970728
The Sloan Digital Sky Survey 2.5-m telescope has a uniquely large 3° field of view. The novel two-mirror optical design achieves zero distortion in the imaging mode using two transmitting correcting elements. The corrector nearest the secondary mirror, the common corrector (so named because it is common to both imaging and spectroscopic configuations) is located with respect to the telescope optics support structure by the common corrector mount.
The common corrector (blue) is located near the front surface of the primary mirror. The fused silica common corrector is 32.5 inches in diameter and is 0.47 inches thick. In combination with the camera corrector (just above the telescope focal surface at the bottom of the Figure), it greatly reduces telescope astigmatism and minimizes field distortion. The common corrector is supported by the common corrector mount (red). Also shown are the primary mirror (blue), the primary support structure (black) and the primary mirror lifting fixture (magenta). The instrument rotator that supports the common corrector mounting and the camera structure are omitted for clarity.
In the left image, Don York and Ed Mannery (l to r) examine the common corrector retaining ring. In the right image, the common corrector cover and the common corrector base are on the top shelf of the cart. On the bottom shelf is the common correcter mount.
The common corrector mount was inspected on the morning of 25 Jul 1997 at Machinists Incorporated. Early in the week the three major parts of the common corrector mount arrived back from the annealer where they had been annealed for a second time, this time at a lower temperature to minimize surface scaling.
Certain machined faces on all three parts are flat where they need to be flat to better than +/- 0.002 inches. The large cylinder was the best. It was measured to be flat to better than +/- .0005".
The register on the largest part (which engages the instrument rotator bearing bore) was measured outdoors at 8 am by Mannery with the part still in full shade. It measured 33.496, 33.494, and 33.497 inches across three more or less equally spaced diameters at a random orientation angle with respect to the part. At 10 am inside the shop three more diameters chosen in the same random fashion were measured by the shop foreman with results 33.500, 33.500, 33.500. The diameter growth of about 0.004" is consistent with the differenct in ambient temperatures of about 18 deg F.
The ID of the short cylinder end that mates with the lens ring measured between 33.066 and 33.053. This is round enough for our purposes. The present holes in the short cylinder will be enlarged by a precision indexed boring operation to ensure that they are located so as to accurately match the mating holes in the lens ring. They are currently undersized (shop error) and their locations are suspect.
Measurements of the OD of the lens ring ranges from 35.995 to 36.002. Again, this is adequately round for our purposes.
These parts left the shop for annealling distorted by amounts as large as 0.03 inch and returned from annealling as described above. Measurements before were made by shop persons and reported to John Galbreath. Measurements after annealling were made by Ed Mannery and Steve Caywood, the shop foreman. No additional machining seems to be called for.
Date created: 07/28/97 Last modified: 04/19/98 Copyright © 1997, 1998 Edward J. Mannery and Walter A. Siegmund
