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