Common corrector mount measurements
Sloan Digital Sky Survey Telescope Technical Note
19970728
Edward J. Mannery and Walter
Siegmund
Contents
Introduction
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.
Common corrector mount measurements
Flatness after 2nd anneal
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".
Roundness after 2nd anneal
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.
Summary
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
siegmund@astro.washington.edu