FW: [ATM] Spectroscopy and fiber optic info needed

["mlbrown@everstrive.com" <mlbrown@everstrive.com>]

Original Message:
-----------------
From:  jsgrmfg@gracemfg.com
Date: Mon, 19 Jan 2004 15:02:59 -0600
To: atm@atmlist.net
Subject: [ATM] Spectroscopy and fiber optic info needed


   It seems to me that the neatest solution to these problems is to put
the
spectrograph in an insulated box on the observatory floor and feed it
light through a fiber optic cable.  Alas, I know next to nothing about
fiber
optics.  A few hours of Googling the subject has taught me that optical
fibers
can be made from several different materials and in many diameters,
that cables may be single or multiple stranded, that multiple fibers may be
arranged in various ways, including a disk at one end and a straight line
at
the other, and that cable length should be minimized to avoid excessive
light loss.

    What I have not yet found is fiber optic information that addresses the
special needs of the astronomical spectroscopist.  Do any of the Wizards
of the ATM list have knowledge and experience in this area that they can
share with me?

    For design purposes I need to know such things as:  The best fiber
material(s)?
Single or multiple fiber cable?  Best fiber diameter?  Does the light
emerging
from the cable have the same optical characteristics (e.g., focal ratio) as
the
entering image or is the light scrambled by the fiber?  Has someone
invented a
clever way to know when the image of a star or other object is well
centered
on the fiber or fibers?

    Thanks,

Jim Sturtevant


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Jim,
 Your first problem will be in getting the light into the fiber.  You'll
need a lens for the fiber which will efficiently couple the light into the
pipe.  Think in terms of trying to use a garden hose to spray water into
the top of a distant 2 liter soda bottle.  Without a funnel, only a little
water will get in.  With the right funnel, a lot will go in.  The coupling
lens is that funnel.  In general such coupling lenses are extremely short
focal lengths and are placed with micron level accuracy with respect to the
end of the fiber.  You can purchase collimator lens+fiber assemblies
commercially.  Google on "fiber collimator".  Collimated correctly, your
fiber will show a high angular sensitivity, which, I'd think, is what you
want to be sure you are looking at just one star.

The fiber itself will not affect frequency, but will affect polarization
rather randomly.  I don't think that will affect the spectroscopy, but I
don't know a lot about that subject.

The light coming out of the other end of the fiber will spread out at a
wide angle.  Depending on your detector, you may or may not want a 2nd lens.

=Matt



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