2.5-m telescope baffle installation
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 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 doubles as a light baffle, the primary baffle that
extends from the primary center-hole toward the secondary (left), the
secondary baffle that extends from the perimeter of the secondary
toward the primary (right), and the conical baffle that is suspended
approximately halfway between the primary and secondary (middle).
The wind baffle is covered with panels that are 25% porous to the
wind. In cross-section, these panels are composed of interlocking
flattened "C" shapes. The primary and secondary baffles are
constructed of a series of aluminum alloy annuli. The baffle tips are
machined aluminum alloy truncated cones with interior vanes. 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 conical baffle is fabricated of graphite fiber
reinforced plastic.
French Leger is preparing to install the wind baffle on the
Thursday morning, May 21, 1998 (left image). The wind baffle panels
(foreground) have not yet been installed on the wind baffle frame
(background). The images on this page are linked to larger 30 to 50
Kbyte images. In the right image, the wind baffle panels have been
installed on the upper surface of the wind baffle. The haze in the
background is from range fires in Texas and Mexico.
Jon Davis and French Leger (l to r) pose with the wind baffle
frame immediately after it was assembled with the telescope. The wind
baffle panels have not yet been installed.
The secondary and conical baffles and the primary baffle tip (l to
r) are visible. The conical baffle is supported by 0.91 mm diameter
wires that attach at the middle of the baffle. Each pair of wires are
at the same 30° angle as the secondary vanes so their
diffraction is coincident with the secondary vanes, even for off-axis
sources. This is true for light incident on the primary mirror. For
light passing from the primary to the secondary that passes on the
outside of the conical baffle, diffraction from off-axis sources will
occur at a somewhat different angle. Vanes on both the inside and
outside of the conical baffle interrupt grazing scattered light
paths.
The conical baffle suspension wires terminate at these anchors. A
turnbuckle in line with each wire allows adjustment of the three
translational and two rotational degrees of freedom of the conical
baffle. The lever is held in the position shown by ball plungers that
limit the force that can be applied to the baffle by the wires to
about 250 N. If this force is exceeded, the lever swings out and the
wire end slips off the lever hook. Two protection wires at diagonal
locations prevent the baffle from detaching entirely from its
anchors. The levers allow the conical baffle to be removed without
changing the turnbuckle adjustments. The wires, turnbuckles and
anchors have not yet been painted black.
This is a view of the secondary from near the center of the focal
surface. The secondary is bordered by the interior of the secondary
baffle. Reflected in the secondary is the Tyvek cover over the
primary mirror. The thin black annulus is the conical baffle. It
obstructs the entrance pupil less than 3% at this field angle. The
thicker black annulus near the center is the primary baffle. At the
center is the reflection of the transfer jig and photographer
(Patrick Waddell) at the focal surface.
On Friday, May 22, 1998, after the installation of the wind baffle
and conical baffle, the collimation of the secondary mirror was
checked.
Date created: 05/24/97
Last modified: 05/24/97
Copyright © 1998, Walter A. Siegmund
Walter A. Siegmund
siegmund@astro.washington.edu