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Penguin Pete

DIY octagonal wooden 8" dob project

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I've posted these pictures elsewhere but this forum is probably more appropriate. It was supposed to be a quick & fun project but I had some nice walnut wood and so it seemed worth putting some extra effort in.

I still need to design the base so if anyone knows of a particularly nice looking design I'd love to see it. Many thanks in advance.

The photo of the moon & Saturn is a composite of two to bring them closer together and it wasn't properly collimated at that point (still isn't - I'll cross that bridge when I've made the base!).

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Pete, how about something like this? (asides from the octagonal tube rings..)

IMO this type of mount design would really suit your beautiful looking 8"! :) ?

Edited by Telrad

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Looks beautiful, majorly impressive :) Would love to do a project like this.

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Super telescope you have made there. Really very nice indeed, and baffled also.

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Excellent work Pete.

A bit spooky, as you've stolen my thunder a bit there!

I'm currently designing a hexagonal wooden tube for my 6" newt which will be constructed in the same way as your tube (only with 6 sides of course). I made one many years ago for my original 10" which turned out quite well and was great fun to make (see pics below). It was incredibly light being 3.6mm ply. The only regret was my decision to paint it green! I've still got the tube in the attic - I daren't part with it!

If plans turn out the way I hope, my 6" will be left as natural wood, though I'll be using ply rather than walnut!

Really impressed with the quality of your work there (I doubt mine will be anywhere near as good!). Whatever you decide for the base, I'm sure it will worth waiting for. Looking forward to seeing it.

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Pete, how about something like this? (asides from the octagonal tube rings..)

IMO this type of mount design would really suit your beautiful looking 8"! :) ?

Nice design Telrad. Is this your scope?

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My friend,that is one beautiful piece of handywork! There is nothing more stunning than a great piece of natural hardwood. Except maybe Jessica Beil.

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Pete,

I've just noticed that, in your design, the ring supports get wider (as in, smaller diameter of the circle cut-out) towards the primary end of the OTA. Please could you explain your logic behind this? I presume it's to provide greater structural support at the business end?

Also, is it possible for you to tell me the internal diameter across the flats? I'm guessing 10" or so.

Thanks

Edited by Astrokev

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That octagonal dob looks magnificent :) Looks like it goes back to the days of beautifully made instruments where great pride was taken in workmanship and finish. It's a real credit to you :) Images look good too B)

There are other nice looking wooden builds on here too :eek: I always think varnished wood looks so nice :mad:

Edited by Gina

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Nice design Telrad. Is this your scope?

Kev, unfortunately it's not mine. Wish it was though.. I like the simplicity of the base.

Your scope looks really good. :) I'm going to attempt to build a dob when I retire. Cheers

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Lovely telescopes gents....perhaps a few brass accent pieces for a cool 'steampunk' look???

:)

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I always think varnished wood looks so nice

Is that varnished, or oiled? Which ever, it looks really good.

Edited by focaldepth
Context

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Kev, unfortunately it's not mine. Wish it was though.. I like the simplicity of the base.

Your scope looks really good. :) I'm going to attempt to build a dob when I retire. Cheers

Cheers. This tube is my old 10" OTA and was made back in the very early 90's using nothing more elaborate than hand/coping saws and a B&D electric drill! Couldn't afford to buy a tube big enough back then, so built my own!

My current 10" uses a glassfibre tube. There's a picture of it somewhere on the forum.

My interest in PP's tube is that I'm currently designing a smaller hex tube, built along the same lines as my old 10" (and Pete's) to house an old set of 6" f8 mirrors. I want to make a portable Dob for imaging ISS solar/lunar transits. My 10" is unfortunately too big and heavy to transport.

Edited by Astrokev

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Many thanks for all the nice and encouraging comments, and to Telrad for the photos of a an interesting base design. Nice to see and quite a few interesting ideas there.

Astrokev, to avoid vignetting when using a wide angle eyepiece, the ring at the mouth needs to be wider than the one nearest the primary as the light enters the scope in a cone shape rather than parallel. I can't honestly remember how I calculated it - I can't run "Newt" on my Mac but it may have been with this program; Newtonian Baffle Calculator. Yes, the opening is 10" and the baffles give more strength at the primary end.

If I were to do it again I would put one more baffle between the primary and secondary as it needed careful assembly with such a long gap either side of the 4th ring. Finished strength isn't a problem though, and with the flocking the baffles are really more about ease of construction than cutting out stray light.

Focaldepth, it is oiled with "Osmo Poly X Oil" available in the UK from Brewers (a paint shop) and flooring suppliers etc. It contains some hard Carnaube wax and only needs two or maybe three thin coats. It dries with a lovely natural look to the wood and is very hard wearing.

Gina, thanks - I'll look through old threads more for other ideas - I didn't see many in my first quick look.

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You're welcome Pete. I can't wait to see your finished octagonal 8" Dob. It's going to be fantastic! :)

Edited by Telrad

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Thanks Pete for explaining the logic. I maybe ought to consider this if I can get head around the theory. As you can probably see in the images I posted, my first tube used the same size rings throughout, which may have been a mistake.

A final question if I may - for the longitudinal joints between each face, did you chamfer both edges to ~ 22.5 degrees and glue the joins? If so, how did you chamfer the edges to get accurate alignment? I think I'm going to struggle with this bit as I don't have a router. I could maybe make-up a jig to allow the edges to be chamferred with sandpaper. Since I'll be using thin ply (probably 4mm) it should be easy to profile by hand - it's getting consistency that will be a challenge. Wasn't a problem with my 10" tube, as I filled imperfections and painted the tube. I need to be more careful and accurate to achieve a natural wood finish.

Thanks again.

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Astrokev, I just put a 45° on one edge and let the other edge over hang. You only need to be careful with gluing the rings onto the first piece (lining them up against the 45° edge) and then to make sure it is on a dead flat surface before continuing with the next piece. If you look at one of the early photos I think you can see the piece overhanging ready to accept the next one which will be bevelled. Putting a 22.5° bevel on each edge is more like coopering and struck me as harder to do if using baffles.

As for the theory of tube diameter, I do remember being advised just to make the opening at the front of the telescope an inch wider (in radius) than the primary. Calculating more precisely probably just enables you to reduce the tube diameter a little if you are really trying to keep it to a minimum.

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Astrokev, I just put a 45° on one edge and let the other edge over hang. You only need to be careful with gluing the rings onto the first piece (lining them up against the 45° edge) and then to make sure it is on a dead flat surface before continuing with the next piece. If you look at one of the early photos I think you can see the piece overhanging ready to accept the next one which will be bevelled. Putting a 22.5° bevel on each edge is more like coopering and struck me as harder to do if using baffles.

As for the theory of tube diameter, I do remember being advised just to make the opening at the front of the telescope an inch wider (in radius) than the primary. Calculating more precisely probably just enables you to reduce the tube diameter a little if you are really trying to keep it to a minimum.

Thanks Pete. I did notice the overhang in one of your pics so thought this may have been the approach you took. Thanks for explaining further - very helpful.

I've played with the baffle programme, and confess that I was a bit, er, baffled :). Most of it made sense, but the devil was in the detail. I've decided to wing it and make the front ring as wide as I can and gradually narrow towards the primary. Gives more or less the dims that you quoted.

Thanks again for your help.

Edited by Astrokev

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I normally do not like diy tube type scopes, but yours is a thing of beauty !!!

I do have a love affair with walnut I must admit, my first job after leaving school was wardrobe stripping by hand with nitromors I saw plenty of stunning walnut pieces then.

Humms things to do when the obsy is finished.. i wonder what the largest mirror could be made in this style.

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Lovely wood work guys, it is a beauty to behold this kind of craft. The satisfaction of using and viewing the cosmos afterwards must just add the icing on the cake. :)

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Astrokev, I've looked back through my notes and found a simple formula for calculating the angle the light cone makes entering the scope: True field = apparent field/magnification. So, using your lowest power eyepiece, in my case a 33mm eyepiece with a field of 72° in a 1200mm scope gives 72/36 = 2°.

I now remember being baffled by that program too but did find the excel spreadsheet calculator and other info here very useful: Oldham Optical Telescope Design page

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Thanks Pete. I'll take a look.

EDIT - I've had a play and this is a really useful spreadsheet. Cheers.

Edited by Astrokev

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