Conical Baffle Fabrication
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. However, to avoid excessive central obscuration, a conical
baffle is necessary in addition to the usual primary and secondary
baffles. This conical baffle is suspended approximately midway
between the primary and secondary mirrors. It is formed of graphite
fiber reinforced plastic because of the high stiffness to weight
ratio of this material. The geometry of the part must be
well-controlled. Consequently, it is formed on a machined aluminum
mandrel.
The conical baffle was inspected on February 28, 1997 as it was
being fabricated at Quality Composites, Inc. (QCI, Sandy UT).
Paul Smith (QCI) cuts unidirection graphite prepreg (fibers
impregnated with dry epoxy) with a utility knife and straightedge
(left image). The yellow plastic film is wrapped around the part and
evacuated prior to moving it into a large oven. This process compacts
the fibers and cures the epoxy.
Matthew Weinberger (l, QCI) and Paul Smith (r) carefully add a
prepreg section to the baffle being formed on the aluminum mandrel
(right image). The heat gun (blue) is used to warm the epoxy to
increase the adhesion of the prepreg to the part. This is the second
layer of twelve. The central stiffening rib and the smaller baffle
ribs on the inside of the part have already been formed and cured.
This closeup shows the beginning of the third layer of
unidirectional prepreg. The contrast between the second layer (l),
oriented at 90°, i.e. perpendicular to the cone axis and the
third layer (r), oriented at 0° or parallel to the cone axis, is
apparent.
Prior to fabricating the part, QCI made test sections (left
image). Different means of forming the inside ribs and the central
stiffening rib were examined.
The central stiffening rib is a box structure with the inside
filled with plastic foam (center image). Ribs on the inside surface
of the baffle interrupt near grazing-incidence scattering. These ribs
are visible on the surface of the test section in the background.
Acrylic fibers are bonded to the outside surface of the baffle and
serve a purpose similar to the inside ribs (right image). The bonding
technique and strength of the resulting bond were tested.
Date created: 03/03/97
Last modified: 03/21/97
Copyright © 1997, Walter A. Siegmund
Walter A. Siegmund
siegmund@astro.washington.edu