254mm F4.5 Compact Dob

[stringscope]

Just started building another new scope. I was very happy with the one I built for Phil Hart so I am now using a 10" F4.5 mirror I bought from 33South a couple of years ago as the basis for another one of very similar design.

Attached images are the 303mm former for the UTA built up from scrap timber and the glue up of the first 2 layers of 1.8mm plywood. A final third layer to go tomorrow night. The UTA will have an ID of about 303mm, an OD of about 315mm and is 385mm long.

Final image is of the 8" F6 I built for Phil. This one should look similar, just a bit on the "plump" side.

Cheers,

[iceman]

Looking very nice Ian, your work is truly art!

[h0ughy]

looks like your making a barrel for fine wine!! Its fantastic how you can exhibit your ATM skills Ian!!

[Satchmo]

Nice work Ian. Are the dark highlights at the edges of the mirror box stained and how did you do them ?

[rmcpb]

Lovely work there. Its always nice to see the different variations people come up with in ATM.

[stringscope]

Thankyou for your kind words guys.

h0ughy, This is the third UTA I have built using this technique, every time I build one of the formers they remond me of a winery basket press - we use to get a lovely basket press Shiraz from Rockford winery when we lived in the Barossa Valley ;) - sigh!!

On the 8" scope the mirror box is edged with Jarrah. I will see if I can find a more close up image in my files. By edging in this way all the ply end grain is hidden and the exposed edges now present a very hard wood that better resists damage.

Final main layer of ply tonight. Note the outer face has been protected from glue staining and scratching with masking tape.

Cheers,

Ooops, sorry I have just noticed I failed to rotate some of the images.

[stringscope]

Here is an image of the rear of the 8" scope that better shows the Jarrah edging. As the particular piece of Jarrah I used to cut the edging strips from was a fairly light red, I used Jarrah stain as well just to provide a better contrast to the Hoop Pine plywood. Was a real pain to do, needed very careful masking and application.

The circular plate resting on top of the mirror box is the rear dust cover.

For the 10" I will be using a different mirrror cell arrangement.

[matt]

You're a talented man, Mr Ogilvy:)

[anj026]

Very interesting Ian

Thank you for sharing your techniques and pictures.

[Satchmo]

Nice work Ian.So do the end grains of the sides touch at all or are they mounted to an internal wooden block.

Re the 10" is the rolled section overlapped much an what is the thickness of the ply. Is this the diagonal cage ?

[rmcpb]

Where do you get such thin ply?

[xstream]

Beautiful work as always, Ian.
You're a credit to the ATMing fraternity. :thumbsup:

[stringscope]

Thanks guys. The power of the web, we can share these ideas in an instant - I still find it amazing:eyepop:

I source the ply from a local timber supplier, Turners Timber in Fyswick. I believe they get it from BrimsPly in Sydney. They call it "bending ply" and is 1.8mm thick 3 ply Hoop Pine, comes in 4X8 size sheets. So with 3 layers of ply you get a light weight UTA with 9 laminations and less than 6mm thick :D.

Here is a link to Dave Gaults website where he describes the ply tube fabrication process much better than I could. This is where I got the idea from, thanks Dave :thumbsup:

http://users.tpg.com.au/users/daveg/Tube.html

I found a cutoff from the 8" mirror box and the attached images show how the Jarrah corner pieces fit. I also sanded one corner so you can see the colour difference. There are additional bracing pieces glued on the inside to provide additional rigidity and strength. I did make a couple of test pieces first. When subject to the breaking test, the glue joins remained intact and the wood gave way.

Mark, this is the Upper Tube Assembly - tube component, yes, the diagonal cage. Re overlap there are 3 thin layers of plywood and the joins are placed about 120 degrees apart.

[stringscope]

I guess I should have included some additional detail in my first post:

Objective is to build a fully featured 10" Dob that basicaly only takes up one passenger seat in a car for transport and a similarly small volume for storage.

This then determined the use of a truss tube design and most importantly the ability to "nest" the UTA inside the mirror box. The "nesting" requirement has driven the use of a formed ply tube for the UTA (to minimise the diameter) as opposed to the more traditional cage design.

Large diameter altitude bearings and an azimuth bearing diameter no larger than the mirror box width, result in a small/compact rocker box. The altitude trunions will be removable without tools if required.

This is only a small 10" scope and weight/size control is not as critical as for larger scopes so compared to a larger scope, an oversize mirror box is of no great concern.

I have attached some images of the 8 inch taken torwards the end of construction, before the altitude trunions were fitted, that show the concept.

[stringscope]

Third layer glued up OK so I glued an additional 40mm wide ply band around the base of the UTA tonight. This provides some additional thickness for the truss attach points. I will probably stain this like I did with the 8" scope.

I should also have mentioned the former was covered in Gladwrap before placing the first layer of plywood. This is to ensure the plywood tube doesn't become permanently glued to the former, nearly happened to me the first time I did this :P .

I am using moisture cure polyurathane glue BTW.

A couple of pics showing remasking and clamping for the 40mm strip.

[Satchmo]

Ian,

How do you get the tube off the former when it is finished. Is it supposed to slide off?

Dave Gault comments on how light the tube is ( say 30% lighter than formatube).

With the way the grains run can you roll the ply just as well width ways as lengthways on an 8' X 4' sheet ?

[stringscope]

Good question Mark. No way it will slide off.

I noted Dave's comments on removing the 3 disk former from inside the tube. As I am only making short tubes I can get away with 2 disks so I decided to make split formers. Removal is really simple. Undo the screws attaching the supporting disks and the former splits in half lengthways. Remove the lose half and then carefully pull away the half that is lightly pinned with 12mm brads. Attached pics probably explain this better.

When you attach the first ply layer, it has to be held against the former without using clamps so you can apply glue and then the second layer. The method Dave used and I have copied is to clamp it down and then use 12mm brad nails at about 25mm spacing to the butted edges. Seems to work OK.

I agree with Dave. Very light compared to other simple solutions. I am sure CF is lighter, I just haven't been brave enough to work with exotic materials :shrug:

This ply is much easier to bend around the long axis of the sheet. This means the largest diameter tube you can make without edge joining sheets is about 350mm. But you can make one 8ft long if you want:lol:

[CoombellKid]

I used the same 1.5mm Hoop pine ply for the SC wall on my 16" and made a
tube 440mm bending along the long axis. I'm buying another sheet to
make a hat box for the SC. So it can be done, you can also get it 0.7mm
3 ply aparently.

Lovely job on the 8" Ian :thumbsup:

regards,CS

[stringscope]

Thanks Rob. That is good to know you can bend it lengthways, where do you get the 0.7mm ply from BTW??

UTA has now come off the former. Focuser and truss attach points fitted. I am leaving the masking tape on for the moment to protect the ply surface. 2.6" secondary and 4 vane mount has been ordered from Protostar.

[PhilW]

I found some 3-ply that is just under 1mm thick (yes, 3-ply!) at Hearns Hobbies in Melbourne. Amazing stuff, I had no idea ply could be made that thin. I used it for the light baffle on the rebuilt UTA that I recently posted here (image re-posted). It came in a sheet that was about 150x30cm. Being so thin it bends very easily.

I also found a very nice sheet of 1.5mm 3-ply there, which I used for the non-load-bearing sides of the mirror box on my 14". It is an octagon; the sides, top & bottom are 8mm ply, & the rest are this thin stuff. The box is 20cm deep. Pic also attached. This would also be a good material for baffling a UTA.

I love your UTA Ian; it looks incredibly strong for the weight.

Phil

[CoombellKid]

Actually what happen was I went into town and inquired at a hardware store. I knew you could get 1.5mm and the hardware bloke had never
heard of 1.5mm ply and didn't quite believe me so he rang his supplier.
It was the supplier that said he had 3 ply down to .7mm which amazed
both of us. But I went with the 1.5mm as I thought it would be better
for a SC wall.

That was Thrifty Link Hardware in Casino, but I suspect they probably go
through Lismore Timber and Ply.

regards,CS

[stringscope]

UTA now has a couple of coats of polyurathane, the focuser and truss attach bolts fitted and put to one side.

Started work on the mirror box. Cut 4 sheets of 12mm ply, 12 strips of Jarrah and started gluing the whole lot together. Mirror box will have internal dimensions of 360mmX360mmX500mm.

[tnott]

Great project. Thinking about making a compact 8"-12" dob myself for when I don't have enough room on family trips etc. for the big one.

[stringscope]

Agreed :thumbsup: You can get a pretty good astro "fix" with an 8-12" scope, particularly from a dark site. No ladder required for a 12" F5 ;)

[stringscope]

Continued glueing the Jarrah edging onto the mirror box sides tonight.

[stringscope]

Mirror box sides trimmed, sanded and glued up.

[stringscope]

I have had a few distractions like .............. work and gardening :shrug:

Mirror box all glued up a couple of weeks ago and UTA fits as advertised.

I have now got around to starting work on the 6 point morror cell. All the major parts are cut out from 20mm square steel tube. Pivot arms from 15mm. Edge support points @ 45 degrees. Parts laid out to show arrangement. Planning to drill all holes and fasten parts to board tomorrow night in preparation for welding.

Cheers,

[stringscope]

Cell frame and support points complete.

Mirror retaining clips added.

45 degree edge supports and collimation rods to go.

[Starkler]

What a beautiful thang :thumbsup:

[stringscope]

Hi All,

What with work, Christmas and a medical issue I have not had mch spare time of late. However, I have now managed to get back to this job.

Focuser, spider and secondary now mounted in UTA.
Primary mirror cell finished and mounted on rear plate.
Primary mirror baffle fitted.
Focal plane position established giving 400mm gap between top of MB and bottom of UTA.

Cheers,

[GrahamL]

very nice Ian .. not far off now ?

[stringscope]

The mirror box has had a couple of coats of polyurathane, steering handle fitted to the UTA and tonight a trial fit of the truss assembly.

UTA has been shortened by 12mm so as to be flush with the top of the MB when nested inside the MB.

The truss should be 20mm longer than optimal length at this stage. I will shorten as required once I have double checked the focal plane position.

Cheers,

[stringscope]

Yep, needed to shorten the truss by 20mm as planned - better then having to try and lengthen it though ;). Mounted the finders and initial check on CofG position.

Clouds cleared for a little while tonight so I centre spotted the primary, spent some time on an accurate collimation and took the scope outside for a first light.

As expected Paracor is a must @ F4.5. Initial impresion of the mirror is a good one, nice "snap to focus" and no sign of astigmatism. Seeing wasn't the best so didn't get above about 145X. The mount wasn't up to tracking at higher mags either :P.

[tnott]

Very cute scope. I like the woodwork especially.:thumbsup: