Delmarvascopics 8" f/9 OTA build
Posted 23 June 2012 - 03:43 PM
I finished this mirror at the Delmarva mirror class in March 2011, got it coated by L&L about a year ago, and it's been sitting in my drawer for about a year now. I NEED to make another mirror, so I have to get this one done so I can make room for the next project!
The mirror started life as a Telescopics kit, with pyrex tapered blank and ceramic tool, that I bought about 39 years ago! I started grinding it, intending to make an f/9 from the get-go, since I already had the Optical Craftsmen 8" f/6 Discoverer that I'd bought the year before.
Anyway, finished the mirror, finally, at the mirror class last year. If I may say so myself, the mirror tested out on Dick Parker's autocollimation rig as damned bitchen', so I'm anxious to siphon some planetary photons down with it.
So, for a tube this time, this is what I was thinking: Something that would interchange with the tube on my Optical Craftsmen 8" f/6.3 Connoisseur mount. But I didn't want to have to pay arms, legs, and other perhaps more precious appendages for a fiberglass tube. So, I decided to try to make a round tube all by my ownsome.
Last weekend, I experimented with rolling up thin plywood sheets that came as packing material with a bunch of garden chairs we bought a few years back. This stuff is about 2mm thick, so after wetting and letting it sit in the sun for about a half hour, it wasn't that hard to roll up to fit inside a 5" cardboard shipping tube I had layin' around.
Here is the result after I took it out of the tube:
Posted 23 June 2012 - 03:45 PM
Posted 23 June 2012 - 03:49 PM
Next, I got all cocky and thought I could just as easily roll up a full 4x8 sheet of 2.7mm Home Despot plywood. Here's the first squeeze:
Posted 23 June 2012 - 03:58 PM
I figured i'd make innies and outies. the innies would be removed after the tube is done, as I'm not one of those dudes who likes baffles in my Newtonians. Most of the outies, I figure, will be round inside and square outside, to hold the outside shape as I staple the plywood to the innies. A few (four?) of the outies will be rings that I'll permanently attach to the tube. Two of these will be end rings and two will be the fixed halves of the rotating rings that will work with the mount's rings. I realize as I type this that these rings will have to be removable so they can be adjustable. It probably makes sense to make the end rings in a similar fasion, then. That way, I won't have to cut off the square form rings to free up the tube when I'm done.
I've only got two Rockler strap clamps here, but I bet I can get by with some cheaper hold down straps from Home Despot. We'll see.
I'm off! let's see how successful I am! I've only got ten bucks invested in the plywood, so if this doesn't work, I won't cry much or for long.
Posted 23 June 2012 - 04:48 PM
Posted 23 June 2012 - 04:55 PM
(in a tiny voice may I ask) Would going for a pre-made tube..maybe a Protostar...be out of the question???
(duck and cover)
Posted 23 June 2012 - 05:38 PM
Protostar, plywood columns, fiberglass = all expensive. If I succeed, and that's a BIG IF, I'll only have 10 or 20 bucks invested in this tube. Well, and a whole herd of time!
I thought I was a gonner when this happened, though:
Posted 23 June 2012 - 05:39 PM
Posted 23 June 2012 - 06:31 PM
So, I'll have to make something out of scrap, or start with the innies, which I could cut on the band saw.
Posted 24 June 2012 - 01:53 PM
Well, not something I didn't think might happen, but here's what I was trying to achieve:
This is the thin stuff I rolled up last weekend, sitting inside a couple of the 8 "outie" forms I made with scrap 18mm Home Despot Ecuadorian plywood left over from that shelf unit project I'm building upstairs. I have enough of this stuff to make a two-layer tube in 2-foot long segments to bold together. But I'll need an innie form to do it - otherwise the layers will not be tight.
Posted 24 June 2012 - 01:55 PM
Posted 24 June 2012 - 02:06 PM
This is all getting a bit too Rube Goldberg even for me!
So, I think I'm going to fall back on my tried and true method of making faceted tubes. I don't have a sheet of plywood to use, so I'm either going to have to go get one, or go ahead and use the beautiful 2x6 Cherry stick I bought last year for this project. It's only 5 and a half feet long, so I'll likely need to add a "dew/lightsheild to the end, and/or mount the mirror cell onto the end of the tube so the mirror surface isn't far inside the tube (great for cooling anyway) like I did for the 10" Delmarvascope:
...since the Fl for the 8" is 6 feet.
Posted 24 June 2012 - 04:53 PM
Well, I may have wasted the whole weekend on this stuff. I suppose if I were to use the thin plywood pieces that I have, I could make a full length tube by putting the sections together while I'm laying on the 2nd layer of plywood and overlapping them over the joints between two sections. Would have to paint it, of course.
But although I've spent little money on this, I've burned a ton of time. I should either go ahead and make a polygonal tube out of 1/4" plywood (fastest method of making a tube I've ever come up with), or just order a plywood column from Anderson, an aluminum tube from Hastings, or a fiberglass tube from Parks.
Like I said, the hardest part about a polygonal tube is adding rotating rings to it. For my Springfield, the Cass, and the Jaegers 6" f/15, this wasn't a problem because the tubes don't rotate.
Anyway, before the day is over, I'm going to put this project away and work on something else that needs to be done - clean up my workshop!
Posted 24 June 2012 - 05:49 PM
Precision Paper Tube Company
High-strength phenolic-impregnated tubes, perfect for telescopes - those made from Resinite RS-8717 would be the best, IMO; extremely strong, fairly lightweight and dimensionally stable - easily sawn and drilled, too. Edmund used almost identical tubing for all their reflectors from '78 onwards, and judging from the examples I've seen they've held up very well over the years
Posted 24 June 2012 - 05:52 PM
Rockler has veneers available in 2x8' sheets, but the grain appears to run across the width rather than parallel to the length
what makes you say that? I never saw the grain run the 2' and always run the 8'. was that they're allwood veneer which is two different woods?
I'd buy their 3'x8' paper back veneer and expoy several layers.
Posted 24 June 2012 - 05:59 PM
Oddly, the cheapest I've seen so far is the plywood columns for $218 for an 8' length of 10" OD and 5/16" wall.
Posted 24 June 2012 - 06:08 PM
Also, no need to line these babies with cork or other insulation... they're very thermally-stable.
Posted 24 June 2012 - 06:23 PM
Posted 25 June 2012 - 12:00 PM
Prices seem reasonable, too. But I think I'd want to use a low-viscosity epoxy to stiffen it and protect it some, like Abatron's liquid wood that I use for home repairs and stuff.
Posted 25 June 2012 - 12:10 PM
still need to paint the tube in a catalyzed poly, that stuff can make bullets bounce off of it.
Posted 25 June 2012 - 02:04 PM