Sloan Digital Sky Survey Telescope Technical Note 19970703
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. This rather fast, f/5, optical system requires well-optimized light baffles to minimize scattered light and to satisfy the constraints of mass, precision, stiffness, wind-loading, ventilation, and cost.
The light baffles consist of the wind baffle that surrounds the telescope and serves as a light baffle, the primary baffle that extends from the primary center-hole toward the secondary, the secondary baffle that extends from the perimeter of the secondary toward the primary, and the conical baffle that is suspended approximately halfway between the primary and secondary. The wind baffle and conical baffle were fabricated earlier and are not discussed below.
The primary and secondary baffles are constructed of a series of annuli. The baffle tip is a more complex machined rib construction. The porous construction of the baffles provides better air ventilation of the surfaces of the primary and secondary mirrors and the upper transmitting corrector than the traditional solid wall baffle. Also, it minimizes mass, wind loading and cost without sacrificing performance.
The primary and secondary baffles were inspected on July 2, 1997 after fabrication and assembly at Machinists, Inc. (Seattle, WA). The secondary rings (or annuli) are numbered 1 through 5. Ring 1 is the ring nearest the secondary. Ring 5 is the secondary baffle tip. The primary baffle rings are numbered 11 through 19. Ring 11 is nearest the primary mirror. Ring 19 is the primary baffle tip. Rings 6 through 10 do not exist.
Because of time constraints, not every dimension was measured. However, enough measurements were made, especially of the overall height of the two assemblies and the dimensions of the two baffle tips, that we have good confidence in the quality of the parts.
The assembled secondary baffle was weighed by lifting it with a spring scale supported by a hoist. It weighs 258 N (58 lbs). This includes the weight of the strap, about 5 N. It balances fairly well on its lifting pins and tips only about 10°.
The dimensions of the secondary baffle assembly were recorded at a number of equally spaced intervals around the assembly. Measurements were made in inches and subsequently converted to millimeters. The number of digits recorded in the individual measurement in inches gives an indication of the precision of the measurement. Below, only the mean and range of the measurements of each dimension are reported. Angles were measured counterclockwise viewed from above (refer to above image and diagram) from one of lifting blocks.
The assembled secondary baffle was measured on a surface plate to have an average overall height of 466.50 mm with a range of 0.56 mm peak-valley. The specifications were 467.33 and 4.06 mm respectively.
The outside diameter of the tip of ring 5 (the secondary baffle tip) was 1285.7 mm with a range of 0.8 mm p-v. The specifications were 1284.6 and 7.6 mm respectively.
Ring 5 is terminated with a small bulb. Its diameter was 1.67 mm with a range of 0.01 mm p-v. The specifications were 1.57 and 0.5 mm respectively. On the inside of ring 5, the bulb extends 0.38 mm inward from the adjacent surface with a range of 0.03 mm. The specifications were 0.32 and 0.13 mm respectively. The tip of the bulb has a very narrow flat surface that is perpendicular to the axis of the part. While this was not desired, its effect should be inconsequential.
The outside diameter of the base of ring 5 was 1273.4 mm with a range of 2.0 mm p-v. The specifications were 1272.1 and 8.1 mm respectively.
The inside diameter of ring 4 was 1183.0 mm with a range of 0.5 mm p-v. The specifications were 1182.9 and 0.8 mm respectively.
The outside diameter of ring 2 was 1265.0 mm with a range of 0.5 mm p-v. The specifications were 1264.1 and 0.8 mm respectively. The inside diameter of ring 2 was 1128.2 mm with a range of 0.4 mm p-v. The specifications were 1128.4 and 0.8 mm respectively. The outside diameter was up to 1.0 mm larger than specified, but it was felt that the tolerance was tighter than necessary. The inside diameter was within tolerance.
Ring 1 was measured to be out of plane by 1.04 mm p-v. Forcing this ring co-planar at four mounting locations will likely cause ring #5 to go out of round and should be avoided by shimming or machining the mounting spacers to the proper length.
The assembled primary baffle was weighed by lifting it with a spring scale supported by a hoist. It weighs 619 N (139 lbs). This includes the weight of the strap, about 5 N. It is not well-balanced on its lifting pins but can easily be held level by one person.
The dimensions of the primary baffle assembly were recorded at a number of equally spaced intervals around the assembly. Measurements were made in inches and subsequently converted to millimeters. The number of digits recorded in the individual measurement in inches gives an indication of the precision of the measurement. Below, only the mean and range of the measurements of each dimension are reported. Angles were measured counterclockwise viewed from above (refer to above image) from one of lifting blocks. Because the assembly has left-right symmetry, a "0" was stamped next to that lifting block.
The assembled primary baffle was measured on a surface plate to have an average overall height of 1264.4 mm with a range of 1.3 mm p-v. The specifications were 1267.3 and 3.8 mm respectively. The inside diameter of the tip of ring 19 (the primary baffle tip) was 857.7 mm with a range of 0.8 mm p-v. The specifications were 858.0 and 8.6 mm respectively. Since the ring was so round and the diameter so accurate, the error in the overall height is inconsequential.
Ring 19 is terminated with a small bulb. Its thickness was 2.09 mm with a range of 0.25 mm p-v. The specifications were 2.08 and 0.5 mm respectively.
The outside diameter of the base of ring 19 was 914.61 mm with a range of 0.94 mm p-v. The specifications were 915.0 and 8.1 mm respectively. The inside diameter of the base of ring 19 was 836.3 mm with a range of 0.9 mm p-v. The specifications were 836.2 and 6.6 mm respectively.
The outside diameter of ring 12 was 1158.8 mm with a range of 0.8 mm p-v. The specifications were 1158.5 and 0.8 mm respectively. The inside diameter of ring 12 was 748.7 mm with a range of 0.5 mm p-v. The specifications were 748.8 and 0.8 mm respectively.
Ring 11 was measured to be out of plane by 1.14 mm p-v. No specification was established for this parameter. Forcing this ring co-planar at four mounting locations will likely cause ring #19 to go out of round and should be avoided by shimming or machining the mounting spacers to the proper length.
The primary and secondary baffles satisfy or exceed the tolerances on the drawings with only a couple of minor exceptions. These were deemed inconsequential. The crucial baffle tips, rings 5 and 19, turned out very round. The peak-valley diameter variation was 0.8 mm for each. This is very much better than the 8 to 9 mm specified and is due to exceptional care taken by the vendor.
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Date created: 07/03/97 Last modified: 07/13/97 Copyright © 1997, Walter A. Siegmund Walter A. Siegmund
