Download Design and Fabrication of an Optical System for a Balloon

Design and Fabrication of an Optical System for a Balloon-borne Space
Telescope
Stephen Furst, Thomas Dow, Kenneth Garrard
Precision Engineering Center, NC State University, Raleigh NC
The NASA Balloon Experimental Twin Telescope for Infrared Interferometer (BETTII) concept1,
shown in Figure 1, consists of two identical telescopes mounted onto a space frame and carried
to an altitude of 130,000 ft. by a balloon. Each telescope has four optical elements – a primary
mirror, a turning flat, a secondary mirror, and a tertiary mirror. The balloon-based mission brings
with it many unique design challenges for designing and fabricating an optical system. During
the ascent into the high atmosphere, the telescope will experience temperature changes of 250
degrees F. To minimize the effect of these changes, the entire telescope is Aluminum 6061-T6
so that the optics will all expand and contract at the same rate during this thermal cycling. Also,
the telescope is tilted 13.3
degrees with respect to the
horizontal, so the mirrors
must be light-weighted on
the backside with a
honeycomb structure to
minimize
weight
and
combat gravity sag. This
light-weighting complicates
the design because the
features on the back of the
mirror can print through to
the optical surfaces during
machining.
Figure 1: BETTII space frame truss
Design and fabrication of a
pair of the telescope assemblies shown in Figure 2 is the subject of this paper. The primary mirror
is a segment of a 2.1 m parabolic mirror that is 651 mm off-axis with an aperture of 550 mm. This
mirror which has a mass of 5 kg, is mounted to the
truss and the 220 mm flat turning mirror at the right
is hung from it through the thin wall “trough.” The
other mirrors are complex, non-rotationally
symmetric designs but are smaller (< 40 mm) and
can be seen sticking through a slot at the bottom of
the trough.
The design challenge was to keep the overhanging
load from distorting the primary while robustly
supporting the turning mirror which has a mass of
2 kg and is 1m from it. This is accomplished by the
combination of mounting rings that are bolted to
each end of the trough and kinematically support
the two mirrors. Both of the telescope assemblies
are mounted to the steel/carbon fiber space frame
1
Figure 2: Telescope assembly
Balloon Experimental Twin Telescope for Infrared Interferometry, S. Rinehart, Proc. SPIE, 7734, 2004
by struts attached to the mounting ring. The struts have flexures that allow the aluminum mirrors
and trough to contract at a different rate than the space frame without imparting significant
moments into the optic mounts.
Figure 3 shows the design of the primary mirror with light-weighting ribs on the back and toroidal
features on the front that form half of the kinematic mount with the V-shaped groove on the
mounting ring. The pattern on the back has thicker ribs with a wide spacing to combat gravity sag
and thin ribs spaces 30 mm apart to reduce the amplitude of print-through during diamond turning.
Analysis shows that this design will sag by 426 nm PP when tilted 13.3 degrees, and print-through
is expected to produce features less than 120 nm PP.
Figure 3: Primary mirror back (left) with light-weighting ribs and front (middle) with kinematic
coupling, as well as deflection due to gravity sag (right)
The mirror is machined on a Nanoform 600 DTM coupled with the FLORA II fast tool servo built
at the PEC. The off-axis parabolic section is machined on axis by translating the shape to the
center of rotation, tilting it and finding the best fit rotationally symmetric surface with a sag of 13
mm for the DTM to follow. The 1 mm correction for the non-rotationally symmetric surface, which
is the desired off-axis parabola, is simultaneously added by the FTS. The result is an optical
surface with form error of < 200 nm RMS and surface finish of < 30 nm RMS.
The components in the mounting structure, shown in Figure 4, are machined from single pieces
of aluminum 6061-T6 and
heat-treated to relieve internal
stresses. The two sides of the
trough are joined in the middle
first, to produce a one-piece
base to which the rings can be
attached and then shimmed as
needed to align the mirrors.
The trough is simultaneously
light and stiff to reduce the
distortion from the overhanging
flat mirror while being relatively
easy to fabricate thanks to the
faceted design.
Figure 4: Mounting structure with thin-walled trough and
coupling rings