On 24 Jul 2003 12:23:57 -0700, (Russell Martin) ...reflected:

Alan W. Craft wrote in message . ..
I'm considering a Parks classic Newtonian,
and to mount on a Vixen GP-DX equatorial
mount. While their 8" f3.5 seems to be just
a wee bit too fast, and their 8" f6 a tad too
slow for DSO's and the like(not to mention
the tube length), I've looked into the
possibility of an 8" f5 custom-made by
Parks and sold via Scope City...

"Thank you for contacting Scope City.
I have reviewed your request for
information on the Parks Optical tube
for telescopes. Based on the information
provided ( 8 inch - f/5 - F=1000, Newtonian)
you would need a 9 3/8th ID 9 3/4 OD
diameter tube, all we need to know from
you is the length you would like to purchase.
You can see a complete listing of all the
tubes Parks offers. We have many in
stock, custom order usually take a
few months."

"...the length I would like to purchase."?

Wouldn't the focal ratio determine that,
or am I missing something?

Substantially, yes, but I suppose you might want a bit longer tube
to cut down on stray light or drop the primary down a bit for some
close-to-the-tube focuser/smaller diagonal combination.

Are you suggesting a "low-profile" or helical focusser?

Though, I didn't understand what you meant when you wrote,
"drop the primary down a bit."

In any case, I would want the telescope to be arranged
and constructed just like the readily-available ones, but an f5
instead of an f6.

Also, please comment on the focal
ratio I've indicated...advantages...
disadvantages?

All opinions, good or bad, are most
welcome.

Alan

Like some others have said, there's nothing really wrong with an f/6
for most DSOs, IMO. FWIW.

I have this "brain cramp" that keeps telling me that you
need low, and even very low, magnification to see most
DSO's, but that's not necessarily true, is it?

Of course, you wouldn't want to use a 5mm on a galaxy.

Regards,
Russell

Alan