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Equipment Discussions >> ATM, Optics and DIY Forum

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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5568415 - 12/12/12 10:52 PM

Tim -

I have my primaries in a cell that is contained in the OTA on three threaded rods. With this arrangement, I can move the primary. I don't focus it by moving the primary, but I can position the primary so that I can control where within the focuser drawtube travel the focal plane is.

Tme main purpose of the rods is that two of them extend forward and I have knobs on them so I can collimate the primary while looking through the eyepiece at a defocused star.

So, if you have difficulty making a robust moving primary focusing system, There is another alternative that will at least give you control of the location of the focal plane thus allow you to get your focal plane close to the tube and not necessitate making big holes and cut outs at the front end of your beautiful tube.

Dick Parker


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: Dick Parker]
      #5568997 - 12/13/12 10:48 AM

Yep, that's the kind of thing I'm thinking.

I'm betting that it'll be easy to collimate and keep there with such a long focal length. I don't even expect there will be much, if any, image shift focusing this way. But in the end, even if I decide I don't like the moving mirror, I can just rebuild the cell and make a sled focuser at that point.

-Tim.


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Norm Folkers
member


Reged: 03/26/08

Re: Delmarvascopics 8" f/9 OTA build new [Re: amicus sidera]
      #5570155 - 12/13/12 11:38 PM

Hi,
I'm a latecomer to reading this post. But just in case no-one has suggested it. I would like to suggest that you consider using laminate flooring to construct a multi sided tube. The laminate comes in different thincknesses. Given it's strength and rigidity, I would use the thinest possible. The laminates click together along their long edge as well as at their ends. So you can make wider panels very easily and the joint can be glued or eopxied to achieve a really good seam. You can even click together sections to make them longer, glued of course. These laminates are already pretty much impervious to water so it would not be difficult to improve on that with paint or varnish, whatever. Once you have glued up sufficient sections for width and lenght, you can mitre cut them in preparation for glueing into a tube. A square tube need only be cut to length, and at 90 deg at that.
The only drawback to this material is that it is heavier than plywood, but I believe it is also a lot more dimensionally stable.
Open stock of laminate flooring is often available at you local harware store of choice. You only need to buy as many sections as you require for making the sections. For a 6" primary (f8 or slightly less) four pieces would likely suffice to make a square tube. Note that this stuff is hard on blades, so use a carbide table saw or buy extra blades.
Hope this suggestion gives some of you ideas to take my suggestion to the next level.
BTW, very nice workmanship on the tube.
Norm


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: Norm Folkers]
      #5586082 - 12/23/12 06:18 PM

Hi Norm.

I thought about making a tube out of laminate flooring, but I think I like the uniqueness of wood better. A solid wood tube, made with thin stock and modern glues and polyurethane is amazingly light and strong. I'm pretty sure that this tube is lighter than fiberglass, and stiffer.

Today I put another coat of poly on the tube by rubbing it on. I definitely think that thin ing the poly and wiping it on with a cloth is the way to go from now on. So much neater than brushing and spraying!

Okay, I've *BLEEP*-footed around quite enough! Time to design my rotating ring mounts, end rings, diagonal support, cell, and focus mechanism (sled or moving mirror)!

Tim


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tim53
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5586083 - 12/23/12 06:19 PM

Auto editors crack me up sometimes!

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5586100 - 12/23/12 06:29 PM Attachment (39 downloads)

Took a pic:

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5588624 - 12/25/12 03:36 PM

So yesterday I made one prototype tube ring out of 8 layers of drywall shims wrapped around the tube and glued together. This to see how thick I need to cut strips of birch or maple to make the real thing.

Next, I made a steam box to use with my rockler steam generator. All I need now is to cut strips to try bending. One other thing I might try if I don't like the steam bending results might be a bending iron like guitar builders use. My son doesn't have one of these... ...yet!

Tim

Edited by tim53 (12/25/12 03:37 PM)


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5589475 - 12/26/12 10:54 AM Attachment (22 downloads)

I know that doesn't sound like much, so here's a pic of the prototype ring as I was laying it up by gluing successive layers of drywall shims into a ring on the tube. If the real thing comes out exactly this size, I'll need to provide a means of clamping or screwing it onto the tube so it won't slip off. Particularly the rings I use to hold the rotating ring assembly to the tube.

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5589484 - 12/26/12 10:56 AM Attachment (23 downloads)

Here's what I was after - the ID of the ring has to be able to accomodate the OD of the tube, of course. But the OD of the ring I want to be comparable to the OD of the inner half of the lazy suzan bearings I'm using for rotating rings:

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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5589493 - 12/26/12 11:06 AM Attachment (19 downloads)

And the idea will be to drill through those 3 holes in the lazy susan inner ring to mount it to the tube ring, and drill through the 6 holes in the outer ring (with plastic "feet" pressed in them now) to mount the spreader rods between the rotating rings, and the saddle plate for the tube.

With 8 layers of drywall shims and glue between them, the prototype is surprisingly stiff. So much so that I'm tempted to soak it in epoxy just to see if this might be a reasonable way to make custom tube end rings without needing a router or milling machine. It's not very attractive, of course, and drywall shims aren't inherently strong by nature (they need to be torn off quickly in shimming drywall, after all). But if epoxy would make it strong, it could always be painted or even veneered.

So, I bought this steam bending kit from Rockler several months ago for this exact project. ...and then it occurred to me that what I'm trying to accomplish might actually have been easier with a bending iron, like used by guitar builders to make guitar bodies. But I already paid for the steam bending kit...

Initially I thought that maybe I could forego making a steam box out of 1x stock and simply use something like 4" PVC and end caps to make the steam box. Only downside I could think of is that a section of pipe with end caps on it might resemble a "device" that could attract unwanted attention! In the end, though, 4" PVC would have been a lot more expensive than the 12' or so of 1x4 and 1x6 pine boards I already had on hand, even if I had to buy new to get them. And it looks like a box, not a bomb!


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5589512 - 12/26/12 11:15 AM Attachment (20 downloads)

From the other end. The blue capped thingy is the steam generator. Just fill with water and turn it on. I've even drilled a hole in the top of the box for a meat thermometer for monitoring the temperature inside the box. They say it takes about an hour in the box per 1" of stock to make the wood flexible enough for bending. I'm not sure yet whether I'm going to try to make one of these rings in one swell foop or with multiple layers laminated together (like the shims, but not so many). Guess I was planning on experimenting to see how flexible various thicknesses become after steaming. I may have to make an MDF jig to bend the strips around an clamp into place while they cool and dry, but I am hoping that they'll be flexible enough that I can just wrap them around the tube and use a band clamp to hold them.

A single-layer ring will of course need to be glued together at the ends somehow or a clamping mechanism devised like a conventional clamping ring.

Years ago, I made tube rings for a Meade 4" mirror lens spotting scope (looks like a 2045 SCT but it's not) out of embroidery hoops. They worked great. The ID of the inner hoop was almost exactly the OD of the spotting scope. What I did was to glue the inner ring to the outer, and then cut through the inner where the clamp gap is. Then I lined the inner ring with felt so it wouldn't scratch the tube. I used the original screw clamps for the embroidery hoop to serve as ring clamps. Worked perfectly.

Here's the steam box from the other end:


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5589914 - 12/26/12 03:49 PM

Well, I can't work fast enough to prevent breaking 1/4" thick pieces. I think it's too cold outside, the parts get cold right after i pull them out of the box.

-Tim.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5590502 - 12/26/12 11:21 PM

Boiling works too. I've put wood in a water-filled steel pipe (yes, open at the top...) and built a fire under to boil it. Comes out like a big noodle.

Or is that TOOOO Cro-Magnon? Urgh!


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: ccaissie]
      #5590583 - 12/27/12 12:15 AM

I might try something like that at some point. I had to put everything away because I was getting too frustrated.

-Tim.


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herrointment
Post Laureate
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Reged: 03/12/11

Loc: North of Hwy. 64
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5591522 - 12/27/12 05:36 PM

Lots of vids on YouTube......I know that you have to move fast, prepare for a bit of spring-back and you can put the form and project into the oven to speed up the drying times.

I'll put my money on you getting the results you are looking for.


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: herrointment]
      #5591965 - 12/27/12 11:18 PM

Yeah, you're probably right. I did find some youtubes that suggested I should be able to get the steam box up to 200F. Mine wouldn't go any higher than 170.

I'll probably give it another go tomorrow, maybe throw a blanket over the box to help insulate it.

Took today "off" because it's our 37th anniversary!

-Tim.


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5596325 - 12/30/12 03:08 PM

I wonder if I can post a pic from my phone?

Apparently not. There isn't even a resize option

Tim


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5596698 - 12/30/12 06:36 PM Attachment (20 downloads)

So, took a break to much some chips and email myself some pics.

I decided to make plywood rings after all. I still have a fair amount of scrap 3/4" chinese plywood laying around from the shelving unit I built in the attic, so I figured I'd use it. The lazy suzan bearings I'm using for rotating rings are 11 3/4" OD, and about 9 3/4" ID, whereas the tube is about 9 5/8" OD. So I need a pair of rings to go on either end of the rotating ring assembly to hold them tight to the tube and help stiffen them. I also decided this time to leave the ball bearings in the lazy suzans and add a braking system to adjust the friction so the tube doesn't rotate too freely. Decided the easiest way to do that would be to make the ring on the sky end have a knob or two to tighten against the outer lazy suzan ring (the inner one is fixed to the tube via the plywood ring). I'll probably make my own screw and knob using delrin rod for the screw so it can serve as the clutch surface.

Anyway, so one ring needed to be 11 3/4" OD to allow for the friction knobs to be added. The rest are going to be 11 1/4" OD so they don't stick out so far from the tube and look "funny", but mainly because that's how wide the numerous scraps are left over from making the shelves! I probably have enough of the remaining sheet of plywood to make them all 11 3/4" OD, but I need to use up scraps anyway.

I need 5 rings in all:

The two on either side of the rotating ring assembly, one on each end of the tube, and one on each end of the mirror cell section - that I need because the tube is only 66 inches long and this is an f/9.

I usually just plan these things in my head as I go, but sometimes I scribble up a sketch. I couldn't find any paper, so I used a piece of the plywood for this sketch:


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tim53
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5596704 - 12/30/12 06:39 PM Attachment (15 downloads)

Kind of hard for mere mortals to read and understand, huh? I decided to go with the sled focuser after all, since moving the primary would require the control rod to pass down the inside of the tube, since the rotating rings are on the outside and close to the tube.

Instead of using a router to cut the rings, I decided to turn them on my Harbor Freight 12" wood lathe. I think they actually call it a 12 3/4" lathe, but I doubt you could swing something that big over the bed. The head swivels to turn bigger bowls, but I won't need that much room, so I'm not swiveling!

Here, I'm attaching a roughed out blank to the faceplate of the lathe.


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tim53
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Delmarvascopics 8" f/9 OTA build new [Re: tim53]
      #5596705 - 12/30/12 06:40 PM Attachment (19 downloads)

And here it is mounted in the lathe:

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