215 replies to this topic

[jragsdale]

As I hinted in other threads, I ended up biting the bullet and bought this refractor from a seller in Northern Utah. I had a friend who was driving by with a trailer coincidentally and he picked it up for me (along with a big Cave mount and 8" old classic Cassegrain). 

 

This scope was listed before on CN, as shown in the photo below. A lot of changes have happened to the scope since then. The main issue is the lens has a gargantuan clam, about 3", so I can mask this lens down to ~7" until I can source an affordable replacement lens. But also the tube has a lot of damage, which appears to be a wood fiber type product? It doesn't look like fiberglass, nor does it look like sonotube? So not sure there. But the front half of the tube is perforated to decrease weight. And in further weight balance modification, the metal rail baffles were removed, cut and 53lbs of concrete was added to the backend. This does make the scope very rear heavy, as the backend was already the same weight as the lens and cell, so the scope was center balanced before. But it also made the scope very heavy and dangerous to mount and unmount. I may consider adding a proper counterweight system to the rear in the future to allow the scope to ride higher up in the saddle. I'll post more photos below.

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Edited by jragsdale, 05 October 2022 - 09:27 AM.

[jragsdale]

Here's a closeup of cross sections of the tube. Thoughts on material?

 

12 3/4" OD
12 5/16" ID
Wall thickness: 7/32"

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[jragsdale]

Concrete plug attached to the rear backplate. 53lbs of concrete.

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[weis14]

Interesting find!  I'm not sure what the material is.  Could it be some sort of plywood?  Maybe its something normally used for a furniture application.  

[jragsdale]

Baffles along with rear port. I will have it all welded back together to restore original functionality of the baffles.

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[apfever]

That looks like a paper wound tube, like SonoTube concrete form. Is it paper or wood?  It might be high density or Medium Density Fiberboard (MDF). I wasn't expecting that. How rigid or springy is the section with all the holes, and are the baffles metal?  The baffles will go a long way toward keeping the tube rigid if the baffles are snug. You could replace the entire tube very cheaply with a concrete form tube from a hardware store or concrete supply. The inside of most tube forms have a waxy coating to make stripping the form off concrete easier. The outside is usually porous. Sometimes the inside waxy layer is easy to peel off. These can be soaked with a thinned solvent based finish to increase stiffness and outdoor durability. Thinning helps since you need to get the finish to soak in as much as possible and keep it wet while absorbing the first application. Once dry, it will seal and second coats won't penetrate deeper. 

 

You know we are waiting for pictures and details of the Gigas clam objective.

Edited by apfever, 05 October 2022 - 09:05 AM.

[brlasy1]

Here's a closeup of cross sections of the tube. Thoughts on material?

 

12 3/4" OD
12 5/16" ID
Wall thickness: 7/32"

Looks like Masonite.

[jragsdale]

You know we are waiting for pictures and details of the Gigas clam objective.

Avert your eyes for those that are squeamish.

 

"One Clam to rule them all"

 

"All your clam are belong to us"

 

"My clam is bigger than your objective."

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Edited by jragsdale, 05 October 2022 - 09:26 AM.

[jragsdale]

Is it paper or wood?  It might be high density or Medium Density Fiberboard (MDF). I wasn't expecting that. How rigid or springy is the section with all the holes, and are the baffles metal? 

Seems more paper than wood. It's about 30lbs still for the tube. Seems pretty rigid to me, surprisingly rigid. Baffles are metal and are a tight fit. They don't fit right now because of the holes cut push the material in a bit. But I bet 20 minutes with a sander on the inside of the tube holes would allow the baffles to fit again.

[Dave Trott]

Avert your eyes for those that are squeamish.

 

"One Clam to rule them all"

 

"All your clam are belong to us"

 

"My clam is bigger than your objective."

Just paint the clam with flat black and try the lens. It may work fine for you.

[jragsdale]

Just paint the clam with flat black and try the lens. It may work fine for you.

That's definitely one of the first orders of business. Should I try to black it out in a smooth round manner, even it it goes onto some of the good part of the glass? Or just black out the affected area?

[clamchip]

Yes, that's a good size clam, a big stewing clam.

Around here we have lots of clams.

What you've got there is a Geoduck:

https://en.wikipedia.org/wiki/Geoduck

I would try the clam full aperture, you may be

surprised how little it can affect the view - sometimes.

If you get stray light or ghosts try painting it on its sky

side to prevent light from reaching it.

Yours's unfortunately is large enough to change the

figure, a star test will notify you of that.

Or a bench test, if you can DPAC the lens.

See here's a big clam you can live with:

Robert

 

Edited by clamchip, 05 October 2022 - 10:14 AM.

[ccwemyss]

Before painting it, I would try making a mask out of black construction paper, with a flap that can be taped to the edge of the cell. That way, you can hinge it back and forth to compare with/without views and star tests, to see the difference it will make. 

 

Chip W. 

[Kasmos]

As mentioned in post 7 the tube looks like Masonite to me.

 

I also like Chip's idea.

 

Leave it to a guy named Chip to come up with a solution.

 

On smaller chips would an offset circle mask work or is it better to just paint them?

[apfever]

"My clam is bigger than your objective."

   a rare opportunity. 

 

 

Before painting it, I would try making a mask out of black construction paper, with a flap that can be taped to the edge of the cell. That way, you can hinge it back and forth to compare with/without views and star tests, to see the difference it will make. 

 

Chip W. 

This is a great idea. Make it a two person operation. The lens person can flip the mask on and off and say "Which is better, a or b, 1 or 2".  Make different shaped mask for fun. Heck, just use a long side of the black paper to block off straight sections of the lens that cover the clam. See what happens.  

I can reach all the way to the secondary of my big dob and never notice my arm.  On windy days I can grab a spider vane in the middle to dampen vibrations and never notice my arm or grip around the vane. 

 

I try to round out large clam covers. I'll flair a little as I get to the edge of the lens, instead of diving straight in. Think in terms of  eye liner done in a gothic style. I don't know if this helps with a potential diffraction line or not, but I do big clams like this. A final mask in front of the clam is a good idea since you don't know where light might go when it hits the crickley wigglies. I'd still paint the clam too. 

 

The picture is a nasty clam I did on a few surfaces. The arrows show how I flaired to the edge in eye liner style.  Just don't put a sharp point in your mask, round it out around the tip of that tidal wave area.

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Edited by apfever, 05 October 2022 - 05:31 PM.

[CHASLX200]

That clam would make Clamchip proud. Looks like the tube is made from the same stuff as my old 8" F/8 Edmund tube.

[jragsdale]

That clam would make Clamchip proud. Looks like the tube is made from the same stuff as my old 8" F/8 Edmund tube.

Any hunch on what material it was? I stopped by Lowes today and checked out the sonotube selection (Quikkrete brand) and they seemed pretty flimsy in comparison of what this is. Maybe this has been lacquered and almost looks like thick paint or gel coat on the outside.

[brlasy1]

Any hunch on what material it was? I stopped by Lowes today and checked out the sonotube selection (Quikkrete brand) and they seemed pretty flimsy in comparison of what this is. Maybe this has been lacquered and almost looks like thick paint or gel coat on the outside.

Go look at Masonite.

[jragsdale]

Go look at Masonite.

There are similarities but masonite is too brittle and as far as I can see, isn't used for making any tubing.

[tim53]

Avert your eyes for those that are squeamish.

 

"One Clam to rule them all"

 

"All your clam are belong to us"

 

"My clam is bigger than your objective."

Gadzooks!  That's horrendous! I gotta say, I'd be tempted to try to make a replacement of that element.

 

My 6" f/15 Jaegers has a couple clams that were felt markered out by the previous owner.  They look kind of bad, but dang, they're not that bad!  And the lens is a performer!

 

[CHASLX200]

Any hunch on what material it was? I stopped by Lowes today and checked out the sonotube selection (Quikkrete brand) and they seemed pretty flimsy in comparison of what this is. Maybe this has been lacquered and almost looks like thick paint or gel coat on the outside.

The Sono tube today is thin and cheap at the big box stores. Back in the late 1980's i bought a long piece to redo the Edmund tube.  It was much thicker also.

 Phenolic was the stuff i think for some tubes back in the day. I made a tube and the pier out of the Sonotube i bought back in 1988.

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[Senex Bibax]

Tube looks like the same material that the DDR used to make the bodies for Trabant autos

[brlasy1]

There are similarities but masonite is too brittle and as far as I can see, isn't used for making any tubing.

 

 

 

Masonite can be steamed and curled like thin plywood. 

 

Steam softens it and it can be curled around a form, then secured with belts or nylon straps, and left to dry.

 

My mom used to make all manner of curved shapes out of Masonite sheets for her artwork.  

[eric_zeiner]

Just paint the clam with flat black and try the lens. It may work fine for you.

Yes, I was going to suggest the same thing, I think you will be quite surprised and it sure beats having to stop down the objective.

[eric_zeiner]

Yes, I was going to suggest the same thing, I think you will be quite surprised and it sure beats having to stop down the objective.

As a case in practice, I once owned a Z10 Dob and the secondary fell off it's holder landing on the primary leaving a couple heavy duty divots in it.  A wonderful member of CN, (I can't remember who) suggested the same treatment for my mirror and it worked out perfectly.  I noticed no drop in performance and no light scatter either.  I wish you the best.