Re: ATM 4" newtonian

["Jan van Gastel" <jhm.van.gastel@mdw.vu.nl>]

Becky,

My answers are in your text.

From: <zoso@sccoast.net>
To: <atm@shore.net>
Sent: Monday, December 25, 2000 8:05 AM
Subject: ATM 4" newtonian


>
> i have a 4.25"f/4 mirror and a 1.5" secondary that i want to build a
small,
> compact newtonian scope.  i have the following questions if anyone can
help...
>
> 1. is there any problems in using pvc pipe for a telescope tube?

No problem. Exept maybe be that PVC has a rather high coefficient of
expansion. This could be a problem if you want to do long expoure time
photography with it. But maybe it is no problem with such a short tube.

> 2. is there any need for offset in the secondary using the listed optics?

Offset has only to be 2 mm (0,078 inch). It is better to offset then not to
offset, but There will be no problem if you don't.

> 3. what is the shortest length i can make the tube?...i have read that
> subtracting the distance of the secondary mirror to the focal plane from
> the focal length gives you the distance from primary to secondary mirror,
> but was unsure on this subject.

That is correct. So if the distance from secondary to focal plane is 4 inch,
the distance between primary and secondary has to be 17-4 = 11 inch. Add
something to that for the thickness of mirror and mirrorcell, lets say 2
inches, and some tube lenght above the focuser, lets say, 4 inches. Then the
tubelenght will be 4+11+2 = 17 inch.

> 4. with a small primary mirror, would there be serious problems with
> glueing the mirror to a a metal plate for a mirror cell?

I would use silicone RTV: three blobs of about 3 mm thickness and 0.5 inch
width, evenly spaced on a circle at 0.28 of the mirrordiameter (according to
PLOP, assuming a 1 inch thickness of the mirror).

> this is my first attempt at building so any help would be appreciated.
>
> thanks, becky


Jan
http://www.geocities.com/janvangastel/