Best wood for a wooden box-tube?

[25585]

The appeal of a square box tube, in one length, is ease in construction, mounting the mirror cels, opening lid for access to inside, being able to switch to different FL optics if desired and the general simplicity of carpentry. Right angles rule!

Can anyone with carpentry know-how, suggest a good wood, for a 5 foot long box-tube, that will not warp unbraced? Mirror size 12 inches upwards, so minimum 13 x 13 inches inside.

[Astrobits]

Plywood.

Solid wood expands sideways with the humidity and is likely to warp as well. Warping can be avoided in solid wood by laminating several strips together with the orientation of the grain of each piece opposite to it's neighbours. It still expands and contracts sideways though.

Whatever you use, treat the outside similarly to the inside of the "tube" otherwise the sheet will bend with differential response to the humidity.

Nigel

[ronin]

Ply and at 5ft I guess bracing will be required as otehrwise the thickness and therefore weight will be a bit much. I need to build one for a Tal 100RS which should be about 1.2 meter. Intention is 6mm ply and 2 mid section braces - but the braceing can be utalised to ret the OTA on.

[Mr niall]

Definitely plywood, never warps. Brilliant stuff!

[Luna-tic]

3 hours ago, Mr niall said:

Definitely plywood, never warps. Brilliant stuff!

Plywood, for sure, but it will also warp if left unbraced over a long run. Take a look at single sheets at the lumber yards if they have been sitting long with no weight on them.

If you're going to make a 12" Newt, even at f/5 it's gonna be long. The thicker the plywood, and the more plies in the sheet, the more stable it will be in terms of not warping or twisting under its own weight. Contraction/expansion won't be an issue with plywood like it is with solids. The thicker the ply, also, the heavier it will be, to the point you'll struggle to set the thing up or move it. External bracing of the tube will be mandatory, which will add to the weight. As the tube gets heavier, even more weight will be needed at the rear to balance it in the mount.

At 12" aperture, I'd recommend building a trussed Newt/Dob. The primary mirror cell and diagonal mirror/focuser cell, make hexagonal or octagonal, and fabricate brackets for the trusses (use 1-1/2 or 2" diameter aluminum tubing to slide into and tighten in the proper orientation to properly secure the two cells together in correct alignment. Even using 1/2" ply , the mirror cell is going to be very heavy. Using a trussed design gives you direct access to the primary mirror without having to remove it from the rear; you can make a mirror cover that  can be placed over the primary just forward of it, and the whole thing can be disassembled if necessary for maintenance or moving. Collimation wouldn't be any easier or more difficult with a truss design over a solid tube if properly constructed.

Our club has several home-built Dobs, the larger ones use round paper tube (concrete forms) just to manage the weight, but one guy built a 16" f/5 from wood, trussed it and it is a piece of art. Next time I'm at the observatory I'll get a few pictures. Moving it, and the even bigger 25" f/5 we have, takes a minimum of two people to do safely. Our 25" can be disassembled and packed into a trailer inside of twenty minutes, reassembled and collimated inside of 30 minutes. At f/5, it's a bit over 10.5 feet long. The 16" f/5 is 7 feet long, and even trussed, two people can barely lift it using long poles slid through the base. A 12" f/5 would be 5 feet long. An all-wood 14" diameter tube, empty, made from 1/2" ply and properly braced, would probably weigh on the order of 50 lb. Add your mirror cell, mirror, grab handles, diagonal assembly, focusing tube and focuser, pivoting mechanism for elevation on the mount, rear counterweight, you'll be moving 85 pounds or more. The Dobsonian base would be that much more, at least, to support the tube's weight properly. It adds up quickly and multiplies even more rapidly with small increments in the mirror size.

Of course, if you plan to install this in a fixed position in a small observatory where it can be protected from getting wet and it won't have to be moved, make it solid.

[Moonshane]

If I were making a 12" scope from scratch out of wood, I'd make one based on the truss design. It would be far simpler than any sort of solid tube.  

You could certainly create a lighter solid tube structure with ribs and thinner ply but a truss would be easier. Also, think about weight and what you'll mount it on.

[Mr niall]

1 hour ago, Luna-tic said:

Plywood, for sure, but it will also warp if left unbraced over a long run. Take a look at single sheets at the lumber yards if they have been sitting long with no weight on them.

If you're going to make a 12" Newt, even at f/5 it's gonna be long. The thicker the plywood, and the more plies in the sheet, the more stable it will be in terms of not warping or twisting under its own weight. Contraction/expansion won't be an issue with plywood like it is with solids. The thicker the ply, also, the heavier it will be, to the point you'll struggle to set the thing up or move it. External bracing of the tube will be mandatory, which will add to the weight. As the tube gets heavier, even more weight will be needed at the rear to balance it in the mount.

At 12" aperture, I'd recommend building a trussed Newt/Dob. The primary mirror cell and diagonal mirror/focuser cell, make hexagonal or octagonal, and fabricate brackets for the trusses (use 1-1/2 or 2" diameter aluminum tubing to slide into and tighten in the proper orientation to properly secure the two cells together in correct alignment. Even using 1/2" ply , the mirror cell is going to be very heavy. Using a trussed design gives you direct access to the primary mirror without having to remove it from the rear; you can make a mirror cover that  can be placed over the primary just forward of it, and the whole thing can be disassembled if necessary for maintenance or moving. Collimation wouldn't be any easier or more difficult with a truss design over a solid tube if properly constructed.

Our club has several home-built Dobs, the larger ones use round paper tube (concrete forms) just to manage the weight, but one guy built a 16" f/5 from wood, trussed it and it is a piece of art. Next time I'm at the observatory I'll get a few pictures. Moving it, and the even bigger 25" f/5 we have, takes a minimum of two people to do safely. Our 25" can be disassembled and packed into a trailer inside of twenty minutes, reassembled and collimated inside of 30 minutes. At f/5, it's a bit over 10.5 feet long. The 16" f/5 is 7 feet long, and even trussed, two people can barely lift it using long poles slid through the base. A 12" f/5 would be 5 feet long. An all-wood 14" diameter tube, empty, made from 1/2" ply and properly braced, would probably weigh on the order of 50 lb. Add your mirror cell, mirror, grab handles, diagonal assembly, focusing tube and focuser, pivoting mechanism for elevation on the mount, rear counterweight, you'll be moving 85 pounds or more. The Dobsonian base would be that much more, at least, to support the tube's weight properly. It adds up quickly and multiplies even more rapidly with small increments in the mirror size.

Of course, if you plan to install this in a fixed position in a small observatory where it can be protected from getting wet and it won't have to be moved, make it solid.

You just can’t get those concrete tubes over here, I’ve looked everywhere!

[RT65CB-SWL]

Marine plywood... Surely it would be better against the damp and wet than normal 'vanilla' plywood.

Slightly off topic. I got on well the teenage son of a friend, (lost contact with them now). He built two pair of loudspeakers for his DJ system. One pair was 'vanilla' ply... the other was marine ply. We both agreed the marine ply sounded better! 

[Luna-tic]

7 minutes ago, Philip R said:

Marine plywood... Surely it would be better against the damp and wet than normal 'vanilla' plywood.

Slightly off topic. I got on well the teenage son of a friend, (lost contact with them now). He built two pair of loudspeakers for his DJ system. One pair was 'vanilla' ply... the other was marine ply. We both agreed the marine ply sounded better! 

Acoustic resonance isn't a factor in telescope building, I don't think. Actually, particle board is even better, acoustically, than plywood.

Marine ply is usually thick individual plies, as well as a thick overall dimension. It's going to be heavy.

If I were going to build a solid tube, purposed as a Dobsonian telescope, I'd use a birch veneered, hobby-grade or cabinet-grade, 1/4 or even 3/16" thick. Hobby and furniture grade usually is at least 5-ply, and sometimes 7-ply. The birch veneer sands very smooth and takes stain evenly, and has a wonderful fine grain. Using that thin a plywood will require an exoskeleton framework of longitudinal and circumferential "stringers" to stiffen it, but the overall structure would be light and strong. The rear cell and mirror supports I would make from 1/2" and use a honeycomb or diamond arrangement of bracing on its bottom surface. Stays light, but very strong and stable. The base mount I'd make from 3/4, for ultimate strength and to weigh more for stability.

But, if I were going to build my own Dob from any type of wood, it would be a truss design.