RE: filters

["Gamble, David" <dgamble@anglogold.com.au>]

I have found out a bit more about tunable filters. They are currently in use
on the AAT - there is a good description at: 

http://www.aao.gov.au/local/www/jbh/ttf/

-----Original Message-----
From: aah@nofs.navy.mil [mailto:aah@nofs.navy.mil]
Sent: Friday, August 04, 2000 1:13 PM
To: tass@listserv.wwa.com
Subject: Re: filters


I remember these tunable filters; we were considering
something like this for an instrument for the Large
Binocular Telescope when I was working for OSU.  There
are several drawbacks:
  (1) polarized light.  You lose 50percent efficiency
      immediately.
  (2) liquid crystal.  You had to heat the crystals
      in the winter to prevent freezing, and cool
      in the summer.
  (3) The transmission is a cos^2 function, which
      means you had to add red/blue blockers to
      limit the bandpass.
  (4) The bandpass does not have the same shape as
      the standard Johnson-Cousins filters.
The concept is certainly interesting; we use
Continuously Variable Filters (CVFs) in the infrared
quite often.  I think this particular filter is best
used in the lab where you can control conditions and
provide plenty of light.
Arne


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