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22" F3.38 DIY Scope Build


SimonfromSussex

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the sling provides support only in the vertical although as it is 'tight' against the primary it effectively holds it in position in the horizontal too. that said, if you knock your mirror box with the scope vertical (i.e. pointed at the zenith) then there's a chance the mirror could move slightly 'upwards'. not by much though as the three retaining posts (that the sling just holds it off) are there to maintain an approximate position. this is a classic Kriege and Berry arrangement in many ways.

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That's an excellent project  with fantastic fit & finish. What is the total weight of the Dobsonian?

Sam you don't lift classic dobs they're built for strength and rigidity. They are moved about with wheelbarrow handles. 

Therefore weight is immaterial. The bulk and size is the chief concern.

Ultra ligt designs one can lift but they aren't as strong and lack the rigidity of this design. 

These type classic scopes strong points are when you use them, not when you transport them.

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Cheers simon,

I understand the floating cell bit but what puzzles me is the wire sling bit, does this just loop around the edge of the mirror to act as a wire hammock for the mirror when horizontal ? If so I wondered how you managed to stop any movement in the mirror when moved from vertical to horizontal whatn theres nothing actually holding the mirror as such ?

Cheers

i think (stand to be corrected)car seat belts work well as theres no stretch?

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i think (stand to be corrected)car seat belts work well as theres no stretch?

They do Cal but.......

There's a new school of thinking, as they now believe that edge support should be in a specific place (COG of the mirror) rather than right across the edge (as in a seat belt type sling)/ Not sure how much difference it makes as I'm yet to make my own comparisons. Edge support is the latest debate amongst the premium scope builders in the US it would seem. With many arguing its case. 

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That's an excellent project with fantastic fit & finish. What is the total weight of the Dobsonian?

Weight is a big issue for a lot of us and one of the main reasons why many go down the route of lighter weight design.

Like a lot of us I live under light polluted skies and didn't want to break my back (or buy a trailer) to transport a big scope to a dark site as it wouldn't be worth the aperture if I couldn't but the trade off (as Steve? swampthing says) is the stability and rigidity suffer where the classic scope designs excel.

Weighing up what's right for the individual is a very personal choice and all the better if you can build it yourself or work closely with the person building one for you as Simon is doing to create your vision.

Alan crafted a superb canopus style scope and now a classic one of large aperture.

Great thread and well done up to now guys.

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They do Cal but.......

There's a new school of thinking, as they now believe that edge support should be in a specific place (COG of the mirror) rather than right across the edge (as in a seat belt type sling)/ Not sure how much difference it makes as I'm yet to make my own comparisons. Edge support is the latest debate amongst the premium scope builders in the US it would seem. With many arguing its case.

I've gone for the edge support at 4 points across the bottom on a floating triangular cell.

The first two points are 22.5degrees from the vertical with the next two at 67.5degrees both pivot on a point 45degrees to The vertical. In theory supporting the bottom 135degrees.

The points also have to take into account the sagitta on the mirror in my case 60mm deep mirror the point had to be 27mm from the rear of the mirror.

All sounds complicated and I can say its no where near as easy to make as the sling type support.

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I've gone for the edge support at 4 points across the bottom on a floating triangular cell.

The first two points are 22.5degrees from the vertical with the next two at 67.5degrees both pivot on a point 45degrees to The vertical. In theory supporting the bottom 135degrees.

The points also have to take into account the sagitta on the mirror in my case 60mm deep mirror the point had to be 27mm from the rear of the mirror.

All sounds complicated and I can say its no where near as easy to make as the sling type support.

Is your camera broken :D

any fear of some pictures of this build? ;)  :grin:

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Just one point about the clamps for the tubes - as the camps are completely separable - put a thin piece of fishing line as a safety to prevent one of the clamps accidentally dropping off onto the mirror.

The threaded rod is loctited into the knob, and then is highly over length too, passing right through the captive nut by some distance. The clamping section of the blocks have a spring between the fixed section. This means that the knob can be opened enough to slide the poles in without fiddling about trying to keep them open enough, or as you point out, risk them coming out completely as they have been undone too far. And the latest pics will show the light baffle arrangement, this will remove the risk entirely.

Sad day today, Si took the 'scope home :cry: :cry: ... :grin:  My work is done and it is over to Si for the really fun bit :wink: of sanding it all up and painting.Really enjoyed this build, learned a lot and got a buzz from the timescale. I am sure Si will be along later with more pics.....

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Is your camera broken :D

any fear of some pictures of this build? ;):grin:

He he I could say I'm not into astro photography which is true but does that count photo's of telescopes?

I will do a full build thread very soon as during my home hours I've not much time to sit at the computer and go through it all.

After next week it will be finished and I can put my feet up for a bit.

Posting a finished scope wouldn't be right if people dont see the journey to get there.

The scope will be going to Kelling with the owner.

Damian

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Well I must be one lucky fella, not only do I have my dream scope very nearly complete, I took it home yesterday and what do you know a clear sky to try it out!!

The moon came up and spoiled the party quite early and without a secondary light baffle and shroud the contrast was affected but all this considered it was still Wow!

The mirror is amazing, razor sharp and the build around it is rock solid so even with really fast optics there was no flex and there didn't appear to be any loss of collimation. 

As Steve said weight for me was less important and a rock solid scope more so given how fast it is. I will wheel it in and out so hopefully no issues for me there. As you say though Damian, some people need to lift these things and then you perhaps have to trade the other way. We made the bearings removable just for this reason, i.e. in case I ever have to lift it.

I need to work on balance, the top cage is ok with the 21mm ethos but with smaller eyepieces it's not heavy enough (as the maths predicted). I was just going to have a Telrad but I'll probably add a finder to help with this now plus maybe some extra weight down the tubes.

We started on the veil and it was like 3d. You almost wanted to reach in and grab it. 

Without a finder we had to limit ourselves to easy targets so the ring and dumbell were manageable and both were stunning. Certainly some blue coming through on the ring and the dumbell was so well defined.

Then the moon started to light the sky but even so M2 looked like M13 in my 16". Incredible amount of stars.

The highlight was M82. Even with a moon glow the detail in the galaxy was astounding. Almost like a picture. I cannot wait to try this when it's better placed and the moon is gone!!

Finally we looked at the moon and the mountain detail was really really good. Glad of a moon filter mind, don't think my eyes would take it without.

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On to painting now. I'm going to go with a flat black finish and black shroud. Boring I know but I want it simple and to be able to recognize any knocks that it might pick up so I can repair it quickly.

I might bling it a bit with some red on the bearings and top tubes.

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Shouldn't worry about balance yet Si

You've still got the shroud and light baffle to add yet. Mine is slightly bottom heavy until the shroud goes on.

Great that you got a chance to use it. M82 is kinda photographic at these apertures isn't it. The thing I love most with large aperture. The size things appear in the eyepiece, no more squinting at little tiny objects. It's like "pow" there's ya galaxy. Image scale rules :)

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Great first light report Simon and really pleased to hear all works great and your dream scope gives you the views you were expecting.

Well done and black will look fantastic on this.

22" is the new 12" can't wait to get mine started

And fingers crossed observe with you guys one night even if it means a trip down.

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Ive really enjoyed this thread too. It's probably been the shortest build time from mirror delivery to full scope that ive seen in a long time, and even more impressive that it has been with such a big scope.

What it does show very well is how simple, yet effective, the dobsonian design is. It still amazes me that wiith fairly common tools and materials such a wonderful scope can be in reach of amateurs.

Well done chaps.

Cheers

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