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optical software
group just two weeks to become operational
with the SPEOS software, and he expects
design times to be reduced by 50 per cent.
telescope design need not
be baffling
In any telescope, it’s important to keep stray
light, such as that coming from sources that
are located near, but not in, the desired field
of view, from reaching the mirrors and thus
the detection elements, whether the human
eye or an electronic sensor. Stray light is
particularly significant when considering
very bright sources, such as the sun, that can
be many orders of magnitude brighter than
the object of interest.
The primary tool for controlling stray light is
the baffle, which is a shade, blade or vane with
a non-reflective coating placed around or inside
a telescope. A well-designed baffle can drop
light intensity by several orders of magnitude,
but some stray light does reflect and/or scatter
and continue towards the detector. Therefore
many modern telescopes employ baffles
containing two or three vanes that block the
path of stray light to a high degree.
When designing baffles it is necessary to
the multitude of cockpit instruments must be legible in any ambient light conditions,
find the optimum shape, spacing, alignment from bright sunlight to darkness. (photo courtesy of Bombardier)
and coating. Interestingly, the baffles for the
original Hubble Space Telescope, which hazardous near-Earth asteroids. Its imaging
went into service 20 years ago, were initially system employs the world’s largest digital
designed without computer assistance. camera, with 3,200 Mpixels. To achieve its
Analysts using crude stray-light analysis very wide undistorted field of view requires
software then came up with a solution that three mirrors rather than the two used by
had three times fewer vanes in the baffles, yet most existing large telescopes as well as a
considerably improved stray-light reduction. very complicated set of baffles.
Today, the use of modern analysis software has ‘When stray light became an issue 30 years
become routine to optimise the performance ago, it took designers months to perform
of these incredibly expensive instruments. hand calculations to determine stray-light
In the design of the LSST (Large Synoptic rejection of an optical system for just a few
Survey Telescope), scientists at NOAO (the off axis angles,’ explains Mike Gauvin, VP of
National Optical Astronomy Observatory) sales and marketing at Photon Engineering.
have turned to the FRED software package ‘With the first software for stray-light
from Photon Engineering. Expected to analysis 20 years ago, engineers had to
start operation from its mountaintop site in describe the geometry with equations, and
Chile in 2015, the LSST is unique among
with experienced use, there is a high degree
it took a month to examine a simplified
large telescopes due to its very wide field of correlation between actual images taken version of the instrument composed of only
of view: 3.5° in diameter (by comparison,
by modern telescopes and simulation results
the most basic optics and baffles. Today,
both the Sun and Moon, as seen from the
produced by current state-of-the art analysis
with CAD-based software such as FRED,
Earth, subtend 0.5°.) Taking more than
software. Here, you can compare an actual
scientists can analyse the entire telescope,
image (top left) and freD simulated image
800 panoramic images each night, it can
(top right) of the apache point telescope
not just the optics, with millions of rays –
cover the sky twice each week. Hundreds looking 1° north of Jupiter, which is a source of and all in a few hours. Without such tools,
of images of each part of the sky will be
stray light, as well as an actual image (bottom
telescopes like the Hubble would not be able
used to construct a database, making it far
left) and freD simulated image (bottom right)
to discover the origins of the universe, find
easier to identify celestial bodies ranging
looking 2° north of Jupiter. in this comparison
black holes and take the stunning pictures
you can see why stray-light problems vary
from exploding supernovae to potentially
from different out-of-field sources.
of the cosmos that it has.’
scieNtific coMpUtiNG worlD february/march 2009
SCWfeb09 pp20-24 Optical.indd 22 4/2/09 10:47:27
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