[time-nuts] BYMLBI (Back Yard Medium LBI)

Chris Albertson albertson.chris at gmail.com
Fri Mar 18 22:37:45 UTC 2011


On Fri, Mar 18, 2011 at 12:29 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
>
>> From what I can learn about "seeing" the atmosphere instability is too
>> great to allow making measurements optically in the 1 ms area.
>
> There has been some interesting amateur work done on taking lots of
> short-exposure pictures and sifting through them to find the good ones.  (I
> can't find a good URL right now.)

Video is used for imaging planets.  They are bright because like earth
they are lit by the sun.   But measurement is different from imaging.
 For this job we don't need pictures.   The method using by FASST is
called "drift scan".  The images of the sky is allowed to move over a
large CCD sensor.  The telescope does not track the sky so the stars
images drift.    CCD sensors are read out one row at a time then the
entire images is shifted down and the next ros is read.   It a drift
scan camers the charge shifting in the CCD is synchronized to the
motion  of the image.   The result is a very. long image.    Think of
it as like a flat bad scanner ony the image moves not the sensor.
This allows for some longer exposures and also covers a lot of the
sky.  One BIG advantage of drift scan is that the telescope is bolted
down solid and never moves, so uncertainty in pointing is greatly
reduced.

The FASST camera continously  a large part of the sky as it drifts
overheadand they match that data to a catalog of stars and then know
the rate the sky is drifting and thenthe rate the Earth turns

I worked on a drift scan camera project a few years back.  We were
able to generate the wave forms to control the CCD in software.  The
Earth does not really turn all that fast  I had a prtottype camera
mounted on my Garage roof for a long time,  It used a chaep telephoto
lens (135mm f/2.8) as the telescopes and we were getting surprizing
accuracy for a "junk box" optical system.   An example of one of the
prototypes is the white tripple lens unit here
http://www.tass-survey.org/.     Software ran under a rel-time version
of Linux with timming by NTP but later re-calibratd by the stars
themselves.  Our system not not nearly as good as FASST, not even
close but used many of the same techniques



-- 
=====
Chris Albertson
Redondo Beach, California



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