Primary and Secondary Baffle Measurements
Sloan Digital Sky Survey Telescope Technical Note
19970703
Walter
Siegmund and Matthew Buffaloe
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. 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.
Secondary Baffle
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.
Primary Baffle
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.
Conclusions
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
siegmund@astro.washington.edu