negative relay lens for a Newt?

Hi,

Thanks very much for the detailed information! I can see the problem
using the barlow lens in this manner (and was why, in part I thought I
might strike lucky with using that 1.08x field flattener I mentioned).


It might be just as simple to use an afocal set up, if I can get a
decent 2" eyepiece. Though again one has the same issue about the
camera hanging off the tube.

Failing that it will force a major change to my existing telescope,
i.e. modifying the mirror cell (which uses an end ring that slips over
the outside of the tube), new focusser (perhaps) and new secondary and
mount.

Thanks again,
Dave Randell

wrote:
There was a S&T article about relay lens a while ago (2-3 years at
most). The relay itself is a pair of achromatic doublets kept at a
given distance from the original focal point and having a prescribed
separation. Although the solution is far from perfect (it could
easily
increase field curvature and usually there's a cost in terms of SA -
increased scattering) it might do what you want, that is moving the
focal point outward so that you have more backfocus to work with your
SLR. If you dig deep into Google saa archive you should find at least
a
thread on this topic with some worth contributions by experts in the
field (notably R.C.).

Thanks for the pointers, I'll seek these out. Opting for a simple
afocal set up is also another option.

BTW, using negative or positive projection, while
potentially solving your problem, will slow down your scope big time
and I'm not sure that's what you want to do.

Yes I was aware of that, and was why I thought that 1.08x meniscus
photographic flattener might be enough to do this without slowing the
scope down too much.

Shifting the primary
mirror forward must be done with great care as you might end up
vignetting it, thus giving up aperture as a trade off for increased
backfocus.

Yes good point. But in general when most people contemplate this route
they typically use a another larger secondary.

Thanks,
Dave Randell

PS. Have you managed to image Gamma Virginis with the 10" yet?

Have you considered switching to a low profile focuser? Check for
example the Clement Focuser or the FeatherTouch focuser.

Roger Persson

On my GSO R&P 2" focuser I can unscrew the outermost part. I think
there is a camera adapter you can attach here instead. M60-M42?

Roger Persson

wrote:

....

PS. Have you managed to image Gamma Virginis with the 10" yet?

Not yet. Crummy weather this side of the pond.

Andrea T.

wrote:

wrote:

...


PS. Have you managed to image Gamma Virginis with the 10" yet?



Not yet. Crummy weather this side of the pond.

Great weather here in the eastern Meditteranean but no scope (for
now)! :-)

Anthony.


Andrea T.

David Randell wrote:
wrote in message
roups.com...
I'll definitely need another secondary mirror to give me
a larger fully illuminated field than I have at present . . .

If you do elect to move the primary mirror, Mel Bartel's diagnol
calculator may be of help in determing the size of the replacement
secondary mirror needed to have full illumination.

Mel Bartel's diagnol calcultor
http://www.efn.org/~mbartels/tm/diagonal.htm