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  #21  
Old 06-10-2021, 05:41 PM
WattleHill
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Rod, fantastic work mate - from the mirror to the beautiful craftsmanship of the tubes Well done!

Reminds me of the only 6" mirror I made back in the 1980s - plaster tool with tiles stuck on it with pitch! - A real labour of love so I appreciate the effort that goes into mirror making.
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  #22  
Old 07-10-2021, 06:59 AM
Rod
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Thanks for the kind comments Stephen.
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  #23  
Old 15-10-2021, 10:01 AM
ausastronomer (John Bambury)
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Quote:
Originally Posted by Rod View Post
Thanks for the nice comments. If you are looking for a good cheap ply for an octagonal tube, I’ve found Bunnings’ red oak 7 mm thick works well. The tube comes out a bit stronger than the birch ply I normally use. I think it may be because the individual plies are thicker. You need to check the sheets carefully. The veneer quality can vary as can sheet flatness.

I use to use polyurethane glue for these tubes but now I find Titebond 2 is just as strong and much cleaner to work with.

Rod
Hi Rod,

Nice looking scope and grinding machine. The mirror also looks like its coming along nicely.

I just wanted to point a couple of things out re your material choices that you may or may not be aware of.

On the basis that a telescope tube is used outdoors only and can get quite wet on the exterior due to dewing, the bunnings red oak plywood, isn't a good choice. This is an interior only grade plywood and it does not use exterior grade, water resistant glue to bond the laminations together. It also uses timber species on the inner plies that are less water resistant. That's why it's "cheap". The Baltic Birch plywood uses "A-Bond" glue which is highly water resistant and it also uses Baltic Birch on all plies, internal and the faces, which is a more water resistant timber species. Baltic Birch is one of the best plywoods for telescope construction because of these properties and is the reason its used by professional telescope builders like Peter Read at SDM and Dave Kreige at Obsession. You could use a few coats of marine varnish, or polyurethane on your tube inside and out and this may help its water resistance and longevity, but one ding that damages the integrity of the varnish and the water will get in and eventually the ply laminations will bow and separate.

The other thing is that while Titebond 2 is an excellent glue and specified for external use, it is classed as "Water Resistant". Titebond 3 is classed as "Waterproof". I have both Titebond 2 and Titebond 3 on hand and use both for different purposes. They are both excellent glues, but if I was choosing one to glue a telescope tube together, I would use Titebond 3. That having been said Titebond 2 should work fine in your case. If someone is starting from scratch and planning to buy some glue for this purpose, I would be buying Titebond 3, not Titebond 2.

Cheers
John B
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  #24  
Old 15-10-2021, 05:01 PM
Rod
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Hi John

Thanks for the feedback. I take your point re the red oak. Most of my scopes are birch ply so I’m familiar with it. I think for the smaller scopes the red oak might be ok but I get that you would need to monitor the integrity of the finish.

I have Titebond 2 and 3 too. I don’t like using Titebond 3 on light coloured woods as I’ve seen examples of it leaving a dark line at the seam - I’m probably overly obsessed with how the final product looks! As you say Titebond 2 is good enough.

Rod
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  #25  
Old 15-10-2021, 06:47 PM
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multiweb (Marc)
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It is a very fast scope. Are you visual solely or will you image with it? If you do what will you use as a coma corrector? Off the shelf or make a corrector?
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  #26  
Old 15-10-2021, 09:00 PM
Rod
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I’ll use it visually. I have one of these:

https://explorescientificusa.com/pro...coma-corrector

Rod
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  #27  
Old 16-10-2021, 03:34 AM
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mura_gadi (Steve)
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Nice scope... can't start to think how you would make such a fast small mirror. Great little desk top unit!

With the A-Bonded plywood I think you also get no gaps or holes when cut. The others will require filling. Flexiply looks really versatile for round tubes and has a high grade surface and A-bond glue. Also make sure it is hard wood core, not a softwood fill if your taking weight on it. A light colour for the wood is better over dark for dewing.

There are companies in Oz that will make plywood to spec, for glue finish, wood types and surface panel finish.

Last edited by mura_gadi; 16-10-2021 at 04:08 AM.
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  #28  
Old 16-10-2021, 08:39 AM
Rod
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Hi Steve

Just to clarify, this scope is not made of red oak. I bought the plywood from plyco. It’s exterior grade and there are no voids. Most of the ply I use in scopes is like this:

https://plyco.com.au/pages/birch-plywood-collection

I’ve made two round plywood tubes as described here:

https://www.iceinspace.com.au/forum/...d.php?t=174149

I like round tubes for larger scopes and octagons for 8 inch and under.

Rod.
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  #29  
Old 16-10-2021, 11:27 AM
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Sorry, was just trying to point out the hard core and abond glue feature on the plywood options as well.

Who's mirror cell design are you using on the scopes?


Thanks
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  #30  
Old 16-10-2021, 11:40 AM
Rod
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No worries. For this scope I’m trying Gary Seronik’s design:

https://garyseronik.com/a-simple-dou...our-reflector/

Rod.
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  #31  
Old 23-11-2021, 11:47 AM
Rod
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I have done a bit more work on this scope so here is an update. I added a wire spider, installed the altitude bearings and my friend Mark 3D printed a nice secondary holder for me. I’ve also lined the tube with black velvet. Temporarily I’ve mounted it on one of my other scope mounts.

Here is the wire spider being installed with a jig and the diagonal holder.
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  #32  
Old 23-11-2021, 12:05 PM
Rod
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Here is the OTA temporarily placed on one of my other scope mounts. My son is standing next to it for scale. Notice that the altitude bearing is located high up on the tube. The weight of the eyepiece and coma corrector on a short tube make vertically balancing the tube much more important than on a more conventional sized dob. This also means a normal dob mount won’t allow access to the zenith as you can see when the scope is fully elevated.
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  #33  
Old 21-03-2022, 08:37 AM
Rod
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Mount now complete

I thought I would finish off this discussion by showing the recently completed mount. Here are the parts.
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  #34  
Old 21-03-2022, 08:53 AM
Rod
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Here is the finished scope. Again I’ve placed a magnetic strip just underneath the rim of the base ring. This allows some adjustment for the pointer above the degree circle and eliminates the need for a metal strip on the surface. The rocker is a little wider than normal to accommodate the altitude bearings sitting higher than normal to compensate for the heavy eyepiece and coma corrector.

Rod.
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  #35  
Old 21-03-2022, 10:22 AM
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Tinderboxsky (Steve)
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Your finished scope looks fantastic, Rod. I like the detailing highlights.

Have you had a successful first light observing session?
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  #36  
Old 21-03-2022, 11:09 AM
Rod
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Thanks Steve

Yes I have had it at the ASVs dark sky site once although there was only 2 hours of truly dark sky and I’ve used it at home for tests. That was all on the temporary mount shown in earlier posts. After some testing I removed the moonlite focuser shown earlier. It was a little redundant as all the focusing is done by the coma corrector. It’s been replaced with a kineoptics helical focuser I already had. I used it to get the position of the corrector right and then left it alone. All the focusing is done with the corrector’s threaded top.

The scope works well optically. Nice wide fields as expected. The only issue I’ve had is with the Explore Scientific coma corrector. The threads on the focuser section of the corrector are to me looser than they should be. This causes my heavy eyepieces to lean a little and this showed a single small spike on stars. A friend had a look and found if we adjusted the three locking screws we could minimise it’s size. I’ve added a little grease to the threads which has also helped. This issue is only noticeable when using a 20mm 100 degree ES eyepiece.

I’ve been surprised at the quality of the views at higher magnification with a 5 mm APM eyepiece. The Moon is very detailed and just fits inside the field.

I’m going to Heathcote this Saturday so will hopefully get a few hours dark skies to test everything out further.

Rod
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  #37  
Old 06-02-2023, 11:37 AM
Rod
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I thought I would give an update on a recent change to this scope. Overall I have been very happy with it. The only nagging issue has been the spike in star images that would show up if the locking knob on the focuser was too tight. I also found that the knob that adjusted the helical focuser friction was pushing the corrector slightly off centre.

I think I have a good solution now. Since the focuser just holds the corrector in place and the coma corrector does the focusing, I drew up the contraption below in Tinkercad. The three adjustment knobs keep the coma corrector centred and the star images are now nice and round and much easier to keep that way.

Rod.
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