Re: [ATM] Batterson address change

[mdholm@telerama.com]

>On the other hand, by monitoring the image, active devices can
>control the figure on a mirror.  (This was proposed to fix the
>Hubble, but the range of the actuators was not great enough).  This
>technique is, I believe, used to correct for bad seeing (and other
>things) in certain large active-optics ground-based telescopes.

No, this would have used a priori knowledge too, the knowledge gained when they
went back, examined their test rig and found the cause of the error.

When the active optics boys do it, they use a reference star they know to be a
single star, or manufacture a substitute using a laser.

As I remember reading it, the field width that can be corrected using current
active optics is small, even by the standards of very large telescopes that have
small field widths anyhow.  The reason is fairly fundamental, when you look at a
different angle, you look through a different air path.  You can only correct
for one air path at a time.

I may be misremembering, but I don't think the Hubble has the kind of actuators
needed to do this kind of shape tweaking.  It certainly wasn't built with active
optics in mind.  1.  The technology was just being developed for earthbound
applications at the time Hubble was launched.  2. One of the main reasons for
launching Hubble up in space, was to get away from one of the main reasons for
active optics, atmospheric distortion.  Hubble's optics don't even have gravity
to worry about.  A three point cell is all they would need (once in orbit), and
the support points wouldn't even have to be where Plop says they should be ;)

Mark Holm
mdholm@telerama.com
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