The Tube


The telescope tube should be about 2 inches wider in diameter than your primary. A 10" in diameter primary mirror requires a 12" in diameter tube. An 8" in primary mirror requires a 10" in diameter tube.
 
The tube can be fabricated from a variety of materials. One thing to consider is weight. How often and how far will you be
lugging this thing? To the back yard and then back in the house, or to the car, then across a field or up a mountain?

Many use the concrete form tube "Sonotube©", available from most Lowes© or other hardware stores.
While these are popular and light weight (beneficial if you are making a large tube), I don't like the idea of putting
expensive and delicate optics in a roll of cardboard. If you are set on using one of these, then at least reinforce it
with a few coats of epoxy or fiberglass resin. It will be much stronger but still keep it's light weight. Make sure you do this
outside or in the garage or you will risk burning out a few brain cells along with your marriage.

Some others use PVC pipe, probably good for 6" or smaller scopes, but I couldn't find a 10" diameter piece that had a consistßßant
shape or circumference through out it's entire length.

There are web sites that sell custom made tubes according to your measurements and choice of material, heavy cardboard,
solid or veneer woods, choice of finish, and for a very solid price.

I chose a piece of 10" dia. aluminum irrigation pipe. The wall is a little less than 1/8" thick. It weighs about 20-25 lbs. without the optics but I didn't see this as an issue since it can be maneuvered with one finger when mounted on the base. I don't have to worry about paint or rust, it's very sturdy and it always looks good. (My opinion and all the above are popular.)
loading image.....
loading image
Looking  down the front of the tube.
The secondary holder and the surface of the primary are visible.
Note the entire inside of the tube is blackened.
Looking from the rear of the tube.
The primary cell and it's collimating screws are visible,
along with it's reflection on the underside of the primary.


What ever material you choose, you will need to blacken the inside after drilling the holes, but before mounting the
mirrors to eliminate any reflected light from the tube itself or any exposed screws or shiny fasteners.
This also can be done several ways. A simple coat of flat black paint will do.
A slightly textured surface seems to help even more. You can apply a first coat of paint, and then while it is still wet sprinkle in
some sand, wait for it to dry and then seal with a second coat of paint.

I lined the inside of mine with a thick grade of black felt, it already had a texture and wasn't that difficult.
I cleaned the inside good, then sprayed a coat of adhesive and pressed on the felt. I did it in four sections so it would be
easier to manage.

Although not very visible, this is an important part as where the eyepiece will not only magnify what ever you are looking at,
but also any unwanted reflections from the inside.
Baffling
Once the tube's interior is finished and the mirrors installed, look down through the front of your scope. Can you see past the
primary and out the back of the tube as in the illustration above? If so, then this can be a source for stray light to find it's way in.
A simple fix is a circle of cardboard or thin plywood painted flat black. I looked around for a while and ended up using a black
plastic lid that's used to reseal an open can of paint. It fits snugly inside the rear of the tube and I cut out a round hole in the
center so it resembles a donut, this lets ventilation through but keeps the stray light out.

Finally, the most common and critical source for stray light is in through over the front of the tube opposite the focuser.
This can be easily fixed by making sure the front end of the tube extends far enough. A simple rule of thumb is to extend the
front of the tube past the focuser the same distance as the diameter of your tube. If you are using a 10" tube, then the distance
from the center of the hole for the focuser to the end of the tube should be 10".

top of page

Hosted by www.Geocities.ws

1