Barking up the wrong tree

"Jim" wrote in message
...
Mike Dworetsky wrote:

I would recommend using a scope designed for what you want to do, if
only
because adding extra optics to the light path adds risk of optical
aberrations. Also, a long-focus telescope is much easier to design and
make
to high image quality specs than a short-focus telescope (esp.
off-axis).

In such situations as you describe, I would use the shorter FL eyepiece
rather than use a Barlow. My experience of most Barlows is that they
degrade the image slightly, add a bit of chromatic aberration, etc.

Also, having given it a wee bit more thought, a Barlow doesn't double
the focal length of the 'scope - it doubles the focal length of the e/p,
yes? Hence it's not going to alter the F number at all. I'd need to whip
up an aperture mask to do that.

Thanks for the reply.

Jim
--

Actually, it effectively halves the FL of the eyepiece. (Twice the
magnification.)

--
Mike Dworetsky

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