[ATM] Another way to do things

["Vladimir Galogaza" <vladimir.galogaza@zg.htnet.hr>]

http://einstein.stanford.edu/  is about Einstein telescope or Gravity probe
B experiment.

Contains extensive description of present state of the experiment including
some interesting details like this:

"Our science telescope is a variation of a Cassegrain reflecting type, in
which a series of three mirrors folds the incoming beam of light from the
guide star (IM Pegasi or HR 8703) into a single spot, which is then divided
into four quadrants by the the IDA, centered on top of the clear front end
cap of the telescope. This design enables us to achieve a focal length of
3.8 meters (12.5 feet) in a telescope that is only 35.6 centimeters (14
inches) long, with a primary mirror 14.2 centimeters (5.6 inches) in
diameter at the bottom of the telescope.

 Because the line of sight from the telescope to the guide star provides the
reference against which gyroscope spin axis drift is measured, image
stability was a critical requirement in its construction. To accomplish
this, the telescope had to be able to remain focused during testing from
room temperature to cryogenic temperatures-a range of 300 degrees Celsius
(572 degrees Fahrenheit). Astronomical telescopes tend to lose focus with
only slight changes in ambient temperature (and early GP-B telescope
prototypes were no exception), so this temperature insensitivity requirement
was a formidable challenge.



The solution was to fabricate all of the optical parts of the telescope-the
barrel, the mirrors, the front-end cap, and the IDA-from a single boule (a
cylindrical lump of material for synthetic gems, made by the Verneuil
process) of extremely high quality fused silica ( also known as amorphous
quartz).  Fused silica is chemically identical to quartz, but the molecules
are not aligned, making it mechanically homogeneous. In addition to its
uniform density, fused silica was chosen because it remains stable at
cryogenic temperatures, and it has no magnetic properties, and it is has
excellent optical properties.

Adjacent parts of the telescope were cut from adjacent sections of this
quartz boule, ensuring structural homogeneity and a common coefficient of
expansion/contraction throughout the telescope. This same boule of fused
quartz was also used to form the quartz block, which is bonded to the base
of the telescope. The bond between the telescope and quartz block needed to
work in the vacuum of space, not create any magnetic disturbances, and be
capable of going from room temperature to cryogenic temperatures without
cracking. No glue in existence has all of these properties, so GP-B
scientist, Jason Gwo, developed a new optical bonding technique, patented
under the name OptoBondT, specifically for this purpose.

Unlike normal astronomical telescopes which focus an image of a celestial
body on an eyepiece for viewing, the purpose of the GP-B science telescope
is to keep the entire GP-B spacecraft pointed at the exact center of the
guide star. To accomplish this ultra-precise pointing task, the GP-B
telescope has a slightly different optical path from a traditional
Cassegrain telescope, and it includes a special optical package (the IDA)
that divides the incoming light beam into four quadrants (two in the X plane
and two in the Y plane) and then sends the two half half beams for each
plane to special electronics packages that compare them.

Vladimir.





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