Re: Celestron Fastar - pros and cons

Jack wrote:
Last night I read about Celestron's Fastar assembly for some of their
8 and 14 inch S-C telescopes.

It changes the the focal ratio from f/11 to f/2, thus increasing the
field of view five-fold and reducing the exposure times during
imaging.

From the above, it sounds like a great combination to hunt for minor
planets!

What are the disadvantages of this setup?

One problem is that because it involves mounting your camera to a
corrector lens assembly that is on the front of the telescope, it
creates a risk of accidentally breaking the corrector plate.

Another is simply that the camera itself should be round, and small,
because it is going to be the new central obstruction, slightly larger
than the mirror.

Another is that, since it replaces the secondary, you won't see
anything by looking into the telescope at the back end any more...
and, again, since it's on the front of the telescope, off-axis guiding
is not possible (the corrector doesn't leave enough distance to the
camera to insert an off-axis guider or a diagonal anyways, but that
isn't a problem because in front of the telescope is no place for such
things). So you will only be able to guide your 'scope using the
finder. (Of course, you can always attach a bigger finder - also known
as a guide scope.)

As a result of the disadvantages noted above, the Fastar lens assembly
has been _discontinued_ by Celestron. Starizona makes its own
versions, apparently, but they're both fancier and more expensive than
Celestron's, I believe.

So much for the disadvantages. The advantages, as you've noted, are
shorter exposure times and a wider field of view; thus, I would almost
tend to say that if you have a suitably round-profiled CCD camera, the
Fastar lens assembly is almost as obvious a thing to get for a
telescope that can use one - as a Barlow lens. Which is why I think
it's a pity that it has been discontinued.

John Savard

.

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