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Mechanical Design
The Ultraviolet Imager mechanical design for the camera
incorporates a three point flexure mount between the baseplate
and the optical bench to prevent stress and deformation of the
optical layout. The three off axis mirror elements are each mounted
with three precision ground spacers to panels which form the
optical bench. The filter wheel and its precision stepper motor
(.063 degrees per step) are mounted on the optical bench with
a separate panel. Each filter is comprised of three reflective
and one transmissive filter. These four elements are mounted
in a very compact, self aligning filter box which is mounted
on the filter wheel. The camera mechanical structure is made
primarily of a magnesium alloy with a corrosion resistant, electrically
conductive surface treatment applied. The optical bench has an
additional coating of black paint (Z-306) to absorb stray light.
The thermal control of different elements
of the camera is critical to the operation of the UVI. The CCD
detector is required to run at temperatures of less than -55C
in order to reduce the thermal noise. Yet the optical bench is
required to remain within a 10C window in order to maintain alignment.
The CCD is cooled via a passive radiator which is located on
top of the camera. The radiator is made of 1 inch square tiles
epoxied on an aluminum plate which is thermally isolated form
the top housing of the camera. The tiles are made of fused silica
with a thin film undercoating of silver and an overcoating of
an electrically conductive transparent film (ITO). This combination
enables a high emissivity (which allows heat to radiated away
efficiently) and a high reflectivity (which reflects any light
from a hot source) and with an electrically conductive surface
(which prevents any charge build up which would affect other
instruments on the Polar spacecraft). A thermal link between
the CCD and radiator is made with two gold plated aluminum components.
In order to control the temperature on the optical bench, four
2.5 watt heaters are attached to four of the six panels. The
three mounting flexures are designed to minimize the amount of
heat lost to the baseplate from the optical bench. |
