Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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Today I was paging through McMaster-Carr plastics and was a bit overwhelmed by the number of choices I might have for an upper cage material.
What is the favored material these days for an Obsession-style upper cage? Is Kydex still King?
Note that I do have a large quantity of flocking paper, so I really don't need a material with a matte finish. Really, I could use anything that is somewhat rigid and not too heavy.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 12938
Loc: Amargosa Valley, NV, USA
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I used two cardboard writing tablet backs taped together and painted. I attach it with velcro so it can be removed and installed easily. Works great!
Edited by llanitedave (09/04/09 02:14 PM)
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Zoomit
sage
Reged: 12/04/06
Posts: 282
Loc: Tehachapi, CA
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Me too! I'm renovating my TeleKit UTA and am in search of replacement material. Isn't the major downside of Kydex is the potential to deform when heated by the sun?
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Brandon
Newts: 18" f/4.6; 7.4" f/6.4
EPs: TV 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x, Paracorr
Bino: 15x70, 12x50, 6x30
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polaraligned
sage
Reged: 12/26/08
Posts: 227
Loc: P. R. of New Jersey
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I thought the biggest complaint about Kydex was that it expanded and contracted too much. ABS is a better choice if I recall correctly.
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artmustel
super member
Reged: 05/30/07
Posts: 128
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I used Hobby Lobby Fun Foam, flat black, only $0.79 a sheet (it took like 3 sheets for my 12.5" Obsession style dob UTA). Excellent, cheap, no complaints about it. Who needs kydex???
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jayscheuerle
Post Laureate
Reged: 01/16/06
Posts: 4070
Loc: S. Philadelphia, PA
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Kydex warps in the sun and is unnecessarily heavy.
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Fight indignorance!
The Green Goblin - 12" of dobsonian excellence!
The PortaBowl-a $100 4.5" f/8 ball-scope YOU can build!
Eero2-a 6" f/5 ball-scope you probably can't. 
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3530
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kydex melts in the hot car, abs cracks in the cold
aluminum is nice and lightweight, but can dent if struck sharply, formica is a little heavier but lots of cool colors
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 965
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I got lucky once, and bought many 8 1/2 x 11 transparent Lexan sheets. 1mm thick. I do not know their original application. Note that they are the same size as a standard sheet of typewriter or copier paper. They have solved many problems, but I have not tried it for this thread's
application. I do not have an open tube Newtonian at this time. Would such material help here? It is certainly tough, and can be cut with metal snips or the heavy duty scissors at Harbor Freight.
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Zoomit
sage
Reged: 12/04/06
Posts: 282
Loc: Tehachapi, CA
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clean Lexan--that's interesting. You could roughen up the inside and paint it black. The outside might have a glossy black appearance. That might look good!
Here's some 1/16" thick stuff available from McMaster-Carr (I love them!) Polycarbinate
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Brandon
Newts: 18" f/4.6; 7.4" f/6.4
EPs: TV 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x, Paracorr
Bino: 15x70, 12x50, 6x30
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Stefan Rostyne
Pooh-Bah
Reged: 10/19/04
Posts: 1009
Loc: Assenede, Belgium
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I use thin wood veneer, and seal it with a few coats of epoxy to make it durable. I used to paint the inner side with a mix of schoolboard black paint and sawdust. Recently, I use flocking paper.
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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Thanks for all of the ideas! It looks like a good list:
ABS - Low density (.037 lbs/sq. ft), but can crack in cold.
Fun Foam - I need to get a look at this.
Metal - McMaster-Carr has several different metal options. Perforated, Mesh, and shim stock. I was wondering how well .005" stainless steel would hold up? Would it tear easily? A bit pricey, but interesting.
Lexan - Interesting, but the options I saw were all clear. So, without further treatment one would be looking at the adhesive backside of flocking paper. Density about .0435.
Formica - Lots of color options, easily available. A bit heavy also.
Wood Veneer - On the Aircraft Spruce website I found mahogany veneer, 1/16" for under $30. It will require more treatment, but this could be the winner. I need to think on it over the weekend.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5024
Loc: Mobile, Alabama, USA
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Very nice, I like how you did your light baffle. But I'm sure you paid a few bucks for the veneer. I made my own light baffle from beveled slats of thin, lightweight plywood and used painter's tape to hold them together. Glue was inserted between the slats, and the assembly was wrapped around a former made from the cutouts left behind when I cut out the rings for the upper cage. After sanding I painted the outside with glossy black spray paint, the inside with flat black paint mixed with fine sawdust. Yes it's heavier than many would care for, but I can attach a finder scope to it at a convenient location, which is exactly what I intend to do. The narrow board will hold a Telrad, and when the telescopes is finished the whole upper cage will go into a wooden case I'm planning to build for it.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5024
Loc: Mobile, Alabama, USA
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This is how the baffle started out.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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Zoomit
sage
Reged: 12/04/06
Posts: 282
Loc: Tehachapi, CA
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Quote:
Fun Foam - I need to get a look at this.
A few weeks ago, I bought an assortment of Fibre-Craft 12"x18" sheets of craft foam. I wanted the black, and got 2 sheets, but had to buy the 10 other sheets as well. See here to buy individual sheets. It appears relatively durable, but soft of course and not structural in any way. The surface would take a flat black paint really well, I'd think, as it has a fine cell texture.
I just measured my assortment, without packaging. It weighed 15 oz on my kitchen scale. That's 18 sq ft, or 0.052 lbs/sq ft. I haven't found it in larger sizes, which I want to use to minimize joints and since my UTA needs 13.5" high sheets.
McMaster-Carr has this stuff that might be worth a try.
Tear-Resistant Polyethylene Foam, Plain Back--Firm, Thin, 1/16"x36"x54" Only $8 for large sheet too.
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Brandon
Newts: 18" f/4.6; 7.4" f/6.4
EPs: TV 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x, Paracorr
Bino: 15x70, 12x50, 6x30
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Zoomit
sage
Reged: 12/04/06
Posts: 282
Loc: Tehachapi, CA
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My math says that Polyethylene foam is 0.021 lbs/sq ft. Certainly lightweight but durable in this application? Don't know.
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Brandon
Newts: 18" f/4.6; 7.4" f/6.4
EPs: TV 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x, Paracorr
Bino: 15x70, 12x50, 6x30
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artmustel
super member
Reged: 05/30/07
Posts: 128
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The fun foam is an excellent substitute, very cheap and easy to find. I sprayed the interior with flat black spray, although it is not needed actually. Of course it is soft, not structural...but you won't need that since the rings and trusses give a very solid structure. Another benefit: it is almost weightless, easy to attach with contact cement, and you can cut or shape it with scissors.
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rboe
Reged: 03/16/02
Posts: 45312
Loc: Phx, AZ
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I went the WilsonArt route (similar to Formica). The inside is pretty rough and takes the flat black very well - and a ton of colours and textures to choose from.
It will crack, a bit more difficult to work with and it's not cheap. Nor is it really expensive. 
Epoxy needs to be varnished with something that has UV inhibitors to protect it from the sunlight or it will degrade. But over wood like that it is drop dead gorgeous. Wow!
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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dave brock
super member
Reged: 06/06/08
Posts: 162
Loc: Hamilton, New Zealand
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I went the Formica route as well. Didn't find the weight a problem and easily worked (I do work with it for a living however).
Dave
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20" homebuilt truss dob
6" watson refractor
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1619
Loc: Under the SE Michigan lightdom...
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Well, in defense of Kydex my scope was fitted with some left over Kydex from my friends scope build. I left the Kydex in the cold garage overnight(it was February IIRC) and brought out the warm UTA frame from the house and used the directions in the Kreig and Berry book. After three years not a bit of a problem with it and it looks as good as the day I cut it, if it gets warm it expands a bit, but since I put it on cold it just gets tighter. It's shiny on the outside and dull on the inside. If I ever do have to replace it I have some Ebony star laminate left over from another project so even then it won't cost me anything and think it would look pretty good, too.
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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Nice looking scope Jim! Is your focuser board Zebra wood?
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1619
Loc: Under the SE Michigan lightdom...
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Thanks Jeff,
Yep, Zebrawood for the focuser board, finder board, and a friend of mine turned a matching knob for the mirror box cover.
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3530
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everybody always loves your focuserboard! (myself included)
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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Quote:
I use thin wood veneer, and seal it with a few coats of epoxy to make it durable. I used to paint the inner side with a mix of schoolboard black paint and sawdust. Recently, I use flocking paper.
Well, wood is the winner. I was very impressed with Stefan's scope. Aircraft Spruce has a 1/16" thickness mahogany 2' x 4' sheet for about $25. It's actually about twice as much as I need, but nice to have a little extra just in case. The weight should be just around one pound before I apply flocking paper.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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I'm kind of kicking myself now about the focuser board. The scope I am replacing has a mahogany focuser board (photo attached) and altitude bearing guides. I also started this project with one, but it was warped. Instead of getting it planed, I just replaced with baltic birch.
The Zebrawood is really striking. I've also been interested in PurpleHeart and Bloodwood. Well, I guess that just gives me a project for the winter!
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1619
Loc: Under the SE Michigan lightdom...
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Quote:
I've also been interested in PurpleHeart and Bloodwood.
I was looking at those, too. The guy at woodcraft told me the zebra wood I had was less oily/resiny than PH and BW, so I used what I had. I haven't noticed any deterioration of it in the three years I've had it. I did spray on many coats of spar urethane.
Back OT: After thinking about it I just might change my Kydex to Ebony Star Formica this winter. I'll see how it looks next time I get the UTA out of it's case.
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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rboe
Reged: 03/16/02
Posts: 45312
Loc: Phx, AZ
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Both purpleheart and bloodwood are very heavy. I've worked with purpleheart and avoid it like the plague. Very hard, dusty and doesn't smell good.
Bloodwood usually has built in stresses that release when you cut it so it's always a surprise. Veneering would be a very nice option in this case.
You'll have to check out woodworkers source next time you're passing through - lot's of nice woods to choose from.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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I used a grey underpad foam for rugs.
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10" f/5 dob unf.
7x50 Tasco binos
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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Aircraft Spruce delivered the 1/16" mahogany plywood promptly and it is being prepared now. I think this will be a good material for ATM use. Firstly, it looks GREAT. Aircraft Spruce calls it African Mahogany. I was not aware of this so I suspected another species such as Luan. Close inspection against Honduran Mahogany I have on hand shows no obvious differences. When cut and sanded, it has the typical smell of real mahogany. If it is not the real deal, it's pretty darn close.
The center ply apparently is poplar. The face plies are extremely thin. I gave it a light sanding with 400 sand paper and some artifacts (from the core?) became visible. Further finishing has covered this, but it probably could have went straight to finish w/out sanding. I am preparing it as I would with regular mahogany:
1) Apply grain filler and stain 2:1 mix;
2) Four coats of Sam Maloof oil/polyurethane; and
3) Two coats of Sam Maloof oil/wax.
It is taking the finishes very well. It looks like a million bucks. But there are a few issues with it:
1) On a 24 x 48 sheet the grain runs the long direction. Since the circumference of my upper cage is 42", it means that if I use one continuous piece, the wood grain will be oriented 90 degrees to the wood in the rest of the scope. For matching grain orientation I had to cut the sheet in half, so my light shield will have a seam opposite the focuser.
2) The flexibility is less than Kydex or ABS. For a 12.5" scope it is workable. For a smaller scope, one may need to immerse the piece in water and then form it into the required cylinder.
3) It will not be the lightest material available. I don't have a specific density for this. However, using a figure from Mel Bartels ATM site for denser Baltic Birch, the 5.8 square feet I am using should weigh no more than 1.15 pounds. (Having used carbon fiber epoxy in my last scope I am a bit spoiled in this regard.) Of course, the inside surface will still need a blackening treatment which will add a bit more weight. My new design is a double-truss, so it is much less sensitive to upper end weight than the Obsession style Dob.
Attached is a photo of two panels, raw and stained/filled. No oil applied in this photo. I have cropped it down and selected a low JPEG quality to make the CN file size limitations, hopefully the grain detail still shows.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Don H
member
Reged: 12/28/06
Posts: 81
Loc: Midwest US
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Hey Dave,
How cold does it need to get for ABS to crack? I made a scope with it last year, and didn't have any problems yet. Maybe it has to be very thin (mine is about 1/8"). I think they use it on dashboards and other things where durability and extreme conditions are a consideration.
Thanks,
Don
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3530
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it never cracks when its new, it seems like a year or two, you see the cracks around the screw or nail heads.
we have discussed making each hole a slot so maybe it will not be as likely to crack as it expands and contracts.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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rboe
Reged: 03/16/02
Posts: 45312
Loc: Phx, AZ
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I've been taught, since A&P school, that you over size fastner holes by 1/8" to accommodate thermal expansion. Otherwise you can't avoid the cracks.
Really big fastners (beyond the scope are discussing here demand a bit more but I don't recall the cut off.
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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Dick Jacobson
super member
Reged: 12/22/06
Posts: 131
Loc: Plymouth, Minnesota, USA
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If anyone needs more ideas, I went to a fabric store and found something called "costume satin". Very dark black, non-reflective on one side, transmits very little light. I assume that it is used mainly for making witches' costumes.
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Equatorial Newtonians, 20" and 14"
"The purpose of life is to be defeated by ever greater things." - Rielke
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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OK, here is the "finished" result. No blackening has been applied to the inside yet. I could go Krylon UFB or ProtoStar flocking paper. Since this scope is a test bed of several ideas I wanted to try, there is a good chance of a rebuild and I don't want to use up my flocking paper. (Make that 99% chance of rebuild since I goofed on a few key measurements  )
The finished panels look very nice to my eye. There is a visible seam where the veneer was applied, but I can live with that. The panels had enough flex to fit inside the upper cage rings - barely. Attachment was made with 1/2" #6 screws.
The 2' x 4' sheet had the grain oriented lengthwise. Had I used it as a single wrap-around sheet installation would have been simple but the grain on the mahogany would have been oriented 90 degrees to the rest of the wood in the scope. So I slit the sheet to get two 2' x 2' panels with grain orientation matching the rest of the scope. The drawback to this is that the seam opposite the focuser board was under a great deal of stress, creating a 1/4" gap in the middle as both sheets bowed out. To remedy this I glued a narrow length of surplus mahogany ply to the joint and re-installed. The resulting seam is of course visible but acceptable.
The finished weight works out to about 1.1 ounce per square foot, very much in the ballpark with alternative materials. If I do rebuild the upper cage, I think I will use this material again.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1985
Loc: Prescott, AZ
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One more view with the telrad/light shield extension attached. It is held on my a single dowel pin and 8 magnets.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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rboe
Reged: 03/16/02
Posts: 45312
Loc: Phx, AZ
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Looking good, I need to see that in person some time. 
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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