Re: [APML]: lets talk more newts

[Wil Milan <wmilan@airdigital.com>]

Bobby,

An 8" f/4 Newtonian would have quite severe coma by itself, but with the
proper correcting optics the coma can be largely eliminated. Tele Vue
and Lumicon sell coma correctors, both optimized for f/4.5 systems, the
latter intended for use in the Newtonian EasyGuider. Peter's concern
about vignetting is something to consider, though my experience using
the Newtonian EasyGuider in an f/4 system resulted in very little
visible vignetting.

The other (and better) way to correct the coma in a fast Newtonian is to
use a full-aperture corrector ahead of the primary. Maksutov-Newtonians
and Schmidt-Newtonians can be excellent astrophoto scopes, but they're
typically a lot more expensive than just dropping in a Tele Vue or
Lumicon corrector into a conventional Newtonian.

An exception to the above is the line of Schmidt-Newtonians made by
Meade in the late 1980s. They made two models, an 8" f/4 (model SN8) and
the 6" f/5 (model SN6). The mounts they came with were awful, pure junk,
but the scopes were quite well made and the optics of the SN8 I owned
were fairly good, visually and photographically. These
Schmidt-Newtonians (mine, at least) were not as well corrected nor as
pin-sharp as Peter Ceravolo's Mak-Newts, but if budget is a
consideration then at $500 or so for an OTA they are very much worth
considering. My SN8 still had a bit of residual coma (about like an f/6
or f/7 uncorrected Newtonian) but overall the photos were quite good,
and because of how it was built (epoxied secondary without a spider,
etc.) it was rigid enough for guiding with an external guide scope. (I
used mine with an off-axis guider and an external guide scope and found
no difference in results.)

If you do want to pursue an SN8 you'll find them rather rare on the
market. These scopes were one of Meade's better ideas, but they never
made very many of them and today they are hard to find for sale. I did
see a few months ago that Focus Optics was advertising some new SN6s,
apparently new units which had lain hidden for years somewhere -- worth
considering if the 760mm focal length would work for you.

Wil M.
--
"When I consider your heavens, the work of your fingers,
the moon and the stars which You have set in place,
what is Man that you are mindful of him,
or the son of Man that you care for him?"  -- Psalm 8



Bobby Middleton wrote:

>
> I would like to pose another question [actually several :-)]
> about Newtonians before the group while we are on the subject.
> Fast newtonians are known to have coma, which I understand
> to be blurred and elongated stars around the edge of the negative.
> I presume it would also affect visual use. Would an 8" f/4
> be pretty severe in this respect, and will it be correctable
> with a good coma corrector? By correctable, can one
> expect complete sharpness across the entire negative
> for 35mm film?
> Lumicon makes a coma corrector that is designed for a f4.5
> system. Anybody using it with good results? What other
> correctors are on the market?
> Thanks for the info as always,
> Bobby Middleton
>
> astropics at:  http://www.webwide.net/~bobbym/astro1.htm