Re: ATM 4" newtonian

[The Silent Observer <silent1@ix.netcom.com>]

zoso@sccoast.net wrote:
> 
> 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?

Shouldn't be in a problem in this size, except in finding a short piece
of 6" diameter PVC pipe.

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

Most small scopes do fine with no offset, though with an f/4 in this
size, your correct offset would be a couple millimeters.

> 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.

You have to have space ahead of the diagonal for the spider and diagonal
mount, and a small amount of space behind the mirror surface to allow
for the cell -- in this case, you'll need a tube a little longer than
your focal length, most likely; say about 18" long total.  Best method
is to set up the diagonal as close to the forward end of an overlength
tube as possible, adjust the primary cell location so you get the
focuser travel allowances you want/need, and then cut off the back end
of the tube to the proper length so the cell is entirely inside, but
little tube left over.

It's also not a bad idea to allow some extra tube ahead of the diagonal
-- you'll get some of that anyway, since you'll have a minimum of 3-4
inches for you diagonal mount and spider, but some extra length helps
reduce stray light -- but you can always add an extension later if you
find that's a problem, and if compactness is a major issue, that may be
preferred in any case.  For this kind of tube, a dew shield is easy --
you'll just need a PVC union that slips over the tube at the front end,
and it'll be long enough for what you need, but still come off easily
for storage or transport.  BTW, you can also use PVC caps as dust caps,
slipped over the tube the same way, and guaranteed to fit.  B)

> 4. with a small primary mirror, would there be serious problems with
> glueing the mirror to a a metal plate for a mirror cell?
> 
> this is my first attempt at building so any help would be appreciated.

I definitely wouldn't recommend full surface gluing, but you could most
likely get away with gluing the mirror to a simple metal plate cell with
three spots of RTV silicone.  Be sure the glass is clean, and lightly
sand the metal where the silicone will contact, then clean it with 99%
isopropyl or hardware store acetone to remove any oily residue, and let
dry before you glue -- I speak from experience, failure to take these
steps can lead to your mirror coming off your cell (fortunately, mine
wasn't damaged when this happened).  Drilling a few small holes in the
plate and squeezing a little RTV through the holes also increases
holding power, but likely isn't necessary with this size mirror.

Be sure to put pennies or nickels between the cell and the mirror while
the RTV cures, so you have some free space, and then remember to remove
the coins after the RTV is set; that will give the RTV enough thickness
to take up the different expansion rates of the metal and glass and
reduce the possibility of a mirror that warps or pinches when it's too
warm or too cold out.

-- 
Some of their knowledge is corrupt, and inaccurate, being gleaned from
all manner of sources.  But like us, they are wizards too.
                                                    -- Jaldis the Blind

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
Rocket Pages                http://silent1.home.netcom.com/launches.htm
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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.