Re: [APML] Need Hhelp turning an 8" f6 reflector into a photographic scope.

["Bobby Middleton" <bobm@koyote.com>]

----- Original Message -----
From: Alan Brent Jordan
> I own a home built 8" F5.95 newtonian reflector with a very good old
> Parks mirror.  I would like to turn it into a photographic scope so I
> can shoot a wider range of targets than my 4" refractor allows.  I need
> a few questions answered so I can determine if this is a realistic thing
> to try.
>
> Currently, I am using a painted sonotube which is most likely not stiff
> enough for photography.  What should I be looking to use for a tube
> material, and does anyone know some sources?

Aluminum irrigatgion tubes, or go here to look at very nice Parks
fiberglass:
http://www.pocmtnop.com/west/parksoptical.html

>
> What size diagonal would I need for an 8" F6 scope in order to cover a
> 35mm frame?  Currently I use a 1.83 inch diagonal, which I suppose will
> be too small.
>

A 2.6" will be pretty much minimum. You'll get a 1.2" 100% illuminated
circle with a 1.55" profile 2" dia focuser, plus room for an OAG. If you go
with a larger secondary you would have to get a larger focuser than 2"
diameter to take advantage of the extra illumination.

> I currently own an older NGF2 2 inch focuser.  Would that cause
> vignetting on a 35mm frame?  What solutions are people using as a
> focusser for a newtonian, and how are you attaching a camera in a way
> that avoids vignetting?

I'm not sure which focuser that one is. If it is close to 1.5" minimum
height it should work if it is sturdy enough. Is it a JMI focuser? The JMI
focusers are good; If you go with a upper-end over-sized focuser like an AP
2.7 you will need custom adaptors. This will not be necessary if you are
willing to live with some slight vignetting. At f6 it won't be too bad with
a 2" focuser.

>
> Would I need a coma corrector shooting at F6?  If so, does anyone make a
> suitable one for that focal ratio?

I don't have an f/6 scope but I need a corrector terribly bad at f/4.25 with
my 8", but at f/7 with my 12.5" it is not necessary. There are no commercial
correctors or flatteners made for f/6. A custom built one will be mega$$.


> I am assuming that I can get by with my G11 as a mount.  I suppose I am
> going to forget about the idea if I have to purchase a heavier mount.

The G11 will do fine.
One other thought: you will want to make sure your spider is a good stiff
one. I recommend the Novak spiders/holders. Same for mirror cells.
Bobby Middleton




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