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The making of a 10" Dob

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#1 dave brock

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Posted 19 March 2016 - 08:15 PM

During the summer holidays (NZ) in January I realised I was spending too much time sitting in front of the computer so I decided I needed a new project to keep active. I had been given a fairly average 10" F/6 plate glass mirror a few months earlier so decided to refigure it. Here's how it looked before.

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Undercorrected and long edge zone.

 

Fixed the edge first

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then finished the correction.

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The mirror turned out a lot smoother than the last two pics show but I realised as I was packing it up to send to the coater that I hadn't photographed it and was hampered by air currents when I quickly took a few shots. I need to photograph in mid morning to get stable air. Nothing like a Foucault pic to show smudges as well.

 

I then remembered I had a section of 12" Formatube and some offcuts of ply stored away at work so decided to make a complete scope. 

 I started by peeling off the outer layer which includes a layer of tar.

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To be continued

Dave


Edited by dave brock, 19 March 2016 - 10:52 PM.

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#2 dave brock

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Posted 19 March 2016 - 08:31 PM

Then removed the waxy layer from inside.

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Then coated the tube inside and out with West System epoxy to seal

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There was some damage to the end of the tube from standing and being shifted around for a number of years that couldn't be cut off because the tube was only just long enough so I made a disc and cramped the end to that.

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#3 dave brock

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Posted 19 March 2016 - 08:39 PM

Peeling the outside revealed a spiral groove which needed to be filled. Added microbubbles to the epoxy for that then sanded the outside but left the inside rough.

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Next, made the cradle. The tube is going to be rotatable btw.

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#4 dave brock

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Posted 19 March 2016 - 08:55 PM

Made two rings, one for each end of the cradle, laminated one side to run against Teflon plugs that were drilled into the ends of the cradle.
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Cut a hole in the top of the cradle to be able to place a close fitting cover over the primary mirror. Made up a box as a router guide.
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The inside of the cradle ends have felt glued for the tube to turn on.

Edited by dave brock, 19 March 2016 - 08:56 PM.

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#5 dave brock

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Posted 19 March 2016 - 09:10 PM

Made the altitude bearings by first cutting a template for the cut-outs to router a template for the bearings themselves.

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I made them that shape because I wanted to have the Teflon pads wider apart than normal to make balancing easier.

 

Made the cell. Standard 6 point floatation. Mirror will be silicone to the 6 points. Hole for fan not cut when this photo taken, nor the 6 pads in place.new pics 55.jpg


Edited by dave brock, 19 March 2016 - 09:12 PM.

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#6 dave brock

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Posted 19 March 2016 - 09:35 PM

Made the finder from 1/2 a binocular and brackets from plywood. Used threaded inserts for the adjusting bolts to thread into.

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Made Crayford focuser. The driveshaft runs on Teflon plugs tensioned with M4 grub screws.

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#7 dave brock

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Posted 19 March 2016 - 09:45 PM

Made secondary holder but forgot to photograph as I was doing it unfortunately but pretty standard affair really.
Attached everything so I could get a balance so I could work out where to cut the tube in half.
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Added more epoxy to the cut edges and sanded then made the door for the cradle

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#8 dave brock

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Posted 19 March 2016 - 09:56 PM

Made the pieces to hold the suitcase latches that clip the tubes together.

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Painted the tube inside and out then attached the aluminium strip and latches. Painted the inside of the cradle and then bolted it in place via the rings.

 I forgot, the plywood parts were coated in 7-8 coats of Danish oil as they were finished. 

 

new pics 81.jpg


Edited by dave brock, 19 March 2016 - 09:56 PM.

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#9 dave brock

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Posted 19 March 2016 - 10:05 PM

Made the rocker box. Found that the low cost Pine ply I was using was not as stiff as the Meranti ply that I used on my 20" so some extra stiffeners were used that did the job quite nicely.

 

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Routed a plate into the rocker bottom for the azimuth bolt and drilled a hole in the bolt for encoder (haven't done the alt yet I've just realised)

 

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#10 dave brock

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Posted 19 March 2016 - 10:11 PM

The finished scope.

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In the corners of the door in the last pic are two little magnets that close onto metal washers screwed to the inside of the cradle.

I realise I haven't put on a handle at the top of the tube as well.


Edited by dave brock, 19 March 2016 - 10:18 PM.

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#11 dave brock

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Posted 19 March 2016 - 10:40 PM

Last three.

 

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The weights are not there because of a mistake but because with a rotating tube if it's not balanced radially, with heavy eyepieces, barlow, paracorr etc. you can have a situation where the tube will drop down when pointed low but also raise up when pointed high..

 

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You can see the black flocking paper opposite the focuser in the middle pic.

 

 

Dave


Edited by dave brock, 19 March 2016 - 10:46 PM.

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#12 sopticals

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Posted 20 March 2016 - 01:28 AM

Nice job Dave. The formica laminate strip on the altitude bearing works nicely.(same as you supplied for my big dob). Really a work of art. 

 

Stephen.(45deg.S.)



#13 Messyone

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Posted 20 March 2016 - 01:39 AM

You make it look so simple...Really nice Dave.

 

How do you decide on the height of the rocker box? I ask as my next project after my 2" pillow block is to make a Dob base for my 10" Newt OTA.

 

Matt



#14 dave brock

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Posted 20 March 2016 - 01:48 AM

Hi Matt.
I make the tube first so the balance point is known but in this case I made the rocker taller than expected then cut it down last thing before gluing together.

Dave

#15 Chucky

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Posted 20 March 2016 - 06:36 AM

Nice job Dave.  As owning a 10 inch homemade F6 myself, I really enjoyed your updated photos and comments.

 

What you going to do about lack of a tube handle?  That's a most important piece on my scope.

 

Again, great job.  Hope you have many happy moments using it under the stars.


Edited by Chucky, 20 March 2016 - 06:40 AM.


#16 m. allan noah

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Posted 20 March 2016 - 07:55 AM

Looks great, Dave! Are you concerned about the mating surfaces of the joint in the tube becoming damaged over time? Is that joint repeatable enough to not require collimation after every setup?

 

allan



#17 dave brock

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Posted 21 March 2016 - 01:49 AM

Chucky,

I'll probably just use a cheap black plastic D handle as they're very lightweight.

 

Allan.

I'm not concerned about the joint. The tube is quite stiff and strong when it has epoxy soaked into it. Half of the force goes into the latches themselves as well. I've already made a number of scopes of this design up to 16" and there haven't been any problems. Collimation holds very well also and often doesn't need any adjustment. One half of the latch has a tongue that fits into the other half which locates the two tubes together.This particular 10" will only come apart when I transport it in my car and mostly it's my 20" that goes to dark sites. I can actually lift the whole scope as one, including rocker, to carry it short distances.

 

Dave



#18 Chopin

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Posted 22 March 2016 - 12:26 AM

Dave, very nice work! The door on the cradle is ingenious. I most of all love that focuser, a real work of art!!! 



#19 Astro Daddy

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Posted 29 March 2018 - 05:55 AM

Made the pieces to hold the suitcase latches that clip the tubes together.

attachicon.gif new pics 72.jpg

 

Painted the tube inside and out then attached the aluminium strip and latches. Painted the inside of the cradle and then bolted it in place via the rings.

 I forgot, the plywood parts were coated in 7-8 coats of Danish oil as they were finished. 

 

attachicon.gif new pics 81.jpg

Dave

I am interested in doing the same thing in making a 2 part scope. Is the only thing keeping the two halves of the tube aligned the aluminum band? I must say you are really a craftsman very nice build.

John



#20 dave brock

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Posted 29 March 2018 - 07:12 AM

While the band does aid in the tube alignment, it's main purpose is as a weatherstrip to stop dew/condensation running into the join and dripping on the primary. It's also the reason for having the mirror cover hole (in the tube) on the underside when the scope is in normal use. I rotate the tube 180° to place/remove the primary cover.



#21 Garyth64

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Posted 29 March 2018 - 08:14 AM

I like everything in this thread.  You did a great job on your telescope!  It is amazing what can be done with just plywood. 

 

You did not rush thru it's construction and took your time.

 

The final product is the reward of paying attention to detail, design, and construction. 

 

So how does it perform?

 

I tried to do make a dob out of my 10" f/7.  I took my time and planned it all out.  I carefully cut out pieces of plywood that was needed, and laminated the exposed wood with formica.  It was hideous!   After seeing what you have done, I'm thinking of trying it again.


Edited by Garyth64, 29 March 2018 - 08:27 AM.


#22 dave brock

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Posted 29 March 2018 - 03:07 PM



So how does it perform?


I'm very happy with it's performance. Balance is good. Very good images and contrast.
I did add a fan controller. With the primary being plate glass, there is a small correction change with the fan running full blast. Turned to a slow speed it's a lot better.
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#23 dave brock

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Posted 29 March 2018 - 03:12 PM

Be careful in the hot sun or in a hot parked car if you formica with contact glue a dark colour. It will get quite warm and may start lifting.

#24 roscoe  Happy Birthday!

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Posted 30 March 2018 - 09:06 AM

I started back upward hitting the 'like' button, but turned out easier to just offer a sincere compliment on all your work!!

 

That spray booth looks like it has some serious miles on it!!

 

Russ



#25 dave brock

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Posted 30 March 2018 - 05:02 PM

Thanks Russ.

Yes the "spray booth" is just plastic sheet (that gets replaced 1-2 times a year) with a large extracting fan above.

The stools occasionally get a scrubdown to remove the loose overspray. Once the solvent evaporates, contact glue is not 

really any more flammable than other items in the laminating factory.

 

Dave




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