View Full Version : Optical Tube coupling (suggestion)

04-26-2013, 09:29 PM
Looking for some ideas on how to couple two 9" O.D. aluminum telescope tubes together. This is for an 8" f/18 Achromat telescope. I was "thinking" something could be machined out of aluminum, delrin or (something ... dunno) ... to serve as a baffle (the purple/magenta circle in the drawing) properly located of course and "also" to couple the two tubes together? Each 9" tube could slip "over" the baffle coupler sleeve and held in place with screws. I wonder how "long" the slip part of the coupling needs to insert into the tubes. Any ideas?

http://i688.photobucket.com/albums/vv241/TheOriginalSquattingDog/4-26-20135-16-00PM.jpg (http://s688.photobucket.com/user/TheOriginalSquattingDog/media/4-26-20135-16-00PM.jpg.html)


04-27-2013, 01:52 AM
The real issue is using plastic. Thermal expansion is bad enough with aluminum but every inch of plastic is about like a foot of aluminum. Then the problem is finding a piece of aluminum in that size. The best bet is to simply slit a piece of the same tubing so it can be closed up enough to fit the ID. Baffles are easy, cut them out of whatever like cardboard or even thin birch plywood. Wood also has the advantage of not holding much heat.

At least with a long focal length the tolerance for change of length is much better than it is for short focal scopes. Where the heck are you going to put a 10 1/2 foot telescope?

04-27-2013, 06:08 PM

The only reason I mentioned baffles was to incorporate one inside the coupling (killing two birds with one stone so to speak). I wonder if wood is an option, then turning it to the proper I.D. and maybe put some metal threaded inserts into the wood for fastening.

I'll put it the same place my 8" f/15 is (BTW, it's F/18 above)

http://i688.photobucket.com/albums/vv241/TheOriginalSquattingDog/1a.jpg (http://s688.photobucket.com/user/TheOriginalSquattingDog/media/1a.jpg.html)

http://i688.photobucket.com/albums/vv241/TheOriginalSquattingDog/BafflesFront.jpg (http://s688.photobucket.com/user/TheOriginalSquattingDog/media/BafflesFront.jpg.html)


04-27-2013, 06:27 PM
Both of my telescopes are open OTA so tube currents aren't a problem. Do find problems with convection cells in the OTA of the existing scope? It has occurred to me that putting heat sink fins on an aluminum newt reflector OTA might be an advantage as it would cool much more quickly as the temperature drops. My 10" f5 is extremely sensitive to correct focusing. The sweet spot is only a couple of thou deep. If I was to do it again I would use carbon/epoxy rods for the frame as I did on the 6" scope. It doesn't change length at all with temperature. It would be a bit expensive though for the required rods for the 10".

04-27-2013, 07:52 PM
I'm wondering how suitable fiberglass construction would be in comparison to aluminum, or other tube-making materials? An adapter or coupling could certainly be made from fiberglass/epoxy or even glass/polyester ( the typical fiberglass resin). You can buy fiberglass rods as kite material, roadside reflector posts, sometimes as fencing posts, etc. A lot cheaper than carbon fiber-

04-28-2013, 11:14 PM
Concerning the length of engagement of the coupling, your best bet is to put a shoulder on the insert and butt the ends of the tubes against the shoulder for alignment. Then the tube engagement could be as little as 3/8 inch. If the tubes are aluminum, the coupler might as well be aluminum as well for uniform thermal expansion. If you are not breaking the joint down periodically, then you also might as well assemble with loctite or epoxy. Assembly with screws will give the joint opportunity to work loose over time.

04-29-2013, 10:51 AM
I'm wondering how suitable fiberglass construction would be in comparison to aluminum, or other tube-making materials?

Crude rule of thumb engineering guess is most metals have an expansion coeff around two digits of millionths per degree and most glass/rock/ore type stuff (like fiberglass) is about one digit. I looked it up and fiberglass is about 3 times better than aluminum. The cool part about CF is its kinda like invar and some specific types are even better than invar basically zero coeff.

There are financial concerns too, in that I wouldn't make a $500 mirror and a $200 eyepiece and a $500 equatorial mount unusable just to save $100 on the tube. On the other hand if you're just screwing around and have a $25 homemade mirror (A small blank is like $15, even today) and a $20 eyepiece and a homemade alt-az mount there's little point in spending $500 on a tube.

You're dealing with thousandths of an inch over distances of feet. Once you solve the problem of droopiness (assuming you actually do) and vibration it turns out that millionths per degree really do matter. A ten degree swing over a 100 inch tube is going to be right at the border of noticeable. Then again that temp swing probably only happens once per night so it depends how much money you want to spend to avoid wiggling the focus knob. Then again if you spend an extra $500 to get an extra two inches of mirror aperture, it would be silly not to do a first class job on the tube.

Over the years I've never owned a scope as nice as these guys are talking about but I've owned a couple on and off over the years and people who don't know about scopes like to talk about optics but what really makes a scope usable is the mount and to a lesser extent the tube. What I mean by the tube is all I have right now is a tiny 80 mm ETX but the whole works is so small I can carry it in one trip with one hand and one shoulder so I get a lot more use out of it than if I'd have to hook the trailer up to the car just to transport a (admittedly cool) giant scope. Yeah yeah its just a little 3 inch alt-az goto scope, I know, but its just so incredibly portable...

04-29-2013, 12:36 PM
The only telescope that is any good at all is the one you actually use.