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Square dobsonian

Okeydokey

By Okeydokey
in DIY Astronomer

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Okeydokey

Okeydokey

Posted
Posted

I have started looking at the idea of building a dob for the back garden.  It seems obvious to me but I am sure there is a perfectly rational answer, but... do dob tubes have to be round.  I appreciate the parabolic mirror bit needing to be round.  But I have just aquired a really good quality wooden packing case that is about right in my poor calculations for a 6 inch dob.  I guess all focusers are built for tubes so a mod would be needed for that, quite within my means but as for the rest of the set up, have I missed something?

I can't currently find a supplier of mirror elements that are economically sensible/viable, do they exist or should I buy a defunt dob and make amends?

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tich

tich

Posted
Posted

I've seen some home made square dobsonians for sale so I guess the tubes don't have to be round. If I'm right in thinking one of the first scopes that Pete Lawrence from the sky at night made was with some sort a square metal cage as the tube.

I found these 2 different examples while looking on the net.

3207481167_83f6e00b4d.jpg

10zoll.jpg

 

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Moonshane

Moonshane

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as long as your 'tube' does not ingress into the mirror area (i.e. make sure it's got an inch or so around the mirror at the bottom and dimensions top and bottom are the same) then it does not really matter what shape it is. you can fix a spider/focuser to a square box in the same way as for a tube. a round tube is just the most efficient shape for materials/weight.

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ronin

ronin

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No, can be any reasonable shape that allows the OTA to sit in the rockers and does not cut into the light path. Seen some nice looking Hexagonal and Octagonal ones. Used to have a link to some of a sort of "organic" shape but the link is locked inside a dead PC.

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callump

callump

Posted
Posted

Definitely no reason why the tube should not be square - quite common really in DIY dobsonian scopes.

There is less (no) need for the tube to rotate in a dob, as the eyepiece is usually in a reasonable position (except at low altitude) - unlike on a german eq, when you often need to be a contortionist to get to the eyepiece (with a newt).

Callum

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  • 2 weeks later...
Spiral

Spiral

Posted
Posted

Would anyone who has made one of these care to say how much it cost & where they sourced the reflectors etc.

Are the results good enough to justify the effort ?

Are there any plans or guides for making such scopes that anyone would recomend ? (books, PDFs, youtubes anything ) (The mechanical & fabrication aspects not a problem but I know very little about optics )

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JamesF

JamesF

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The Stellafane site is probably the place to start.

Then there's a book by Kriege and Berry called "The Dobsonian Telescope", though that does tend to concentrate on larger dobs.

In short, it is very possible to make quite decent quality dobs oneself given the will and the patience.

James

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Spiral

Spiral

Posted
Posted

The Stellafane site is probably the place to start.

Then there's a book by Kriege and Berry called "The Dobsonian Telescope", though that does tend to concentrate on larger dobs.

In short, it is very possible to make quite decent quality dobs oneself given the will and the patience.

James

Thanks a lot for the link, its a mine of information hi5.gif

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swamp thing

swamp thing

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I suspect that making a square tube is far simpler than a round one (and its attachments etc) for the average DIY person.  The only downside may be that you cannot rotate the tube to place the eyepiece in a favourable position.

Interesting thought Roger. It. Would only require a small amount of rotation to make Dob viewing perfect. Probably 90deg would be enough on any Dob.
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Spiral

Spiral

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I was horrified to see the prices for pyrex blanks, (maybe it was just the German site I was on ?)(time for an idiot newbie question) seeing as the mirror is only very slightly concave for a longer focal length, would it not be possible to use ordinary thick glass or an actual mirror with thick glass (thus saving another job) ?

Or is standard glass too green ?

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Church

Church

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Keep in mind I'm a noob, and don't know what I'm talking about from first hand knowledge... But I've read that one of the main reasons for using Pyrex is that it doesn't get all screwed up from temperature changes the way regular plate glass would. I read some stories about homemade telescopes that used standard plate glass, not even of the green variety, either, that took hours to adjust to temperature changes after the sun went down.

But once again, I don't know for sure. This topic is just interesting to me, so I've been reading about homemade mirrors a lot lately.

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earth titan

earth titan

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I was horrified to see the prices for pyrex blanks, (maybe it was just the German site I was on ?)(time for an idiot newbie question) seeing as the mirror is only very slightly concave for a longer focal length, would it not be possible to use ordinary thick glass or an actual mirror with thick glass (thus saving another job) ?

Or is standard glass too green ?

It's not about the colour cos it gets coated with silvering before use. Pyrex is normally considered to be one of the most thermally stable - hence it's suitability for mirrors.
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Spiral

Spiral

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It's not about the colour cos it gets coated with silvering before use. Pyrex is normally considered to be one of the most thermally stable - hence it's suitability for mirrors.

 Ah ! I had presumed that the mirror coating would be on the "back" or flat side like a normal common or garden mirror not on the concave side, or am I getting the wrong end of the wrong stick ?

Temperature would not be a big problem for me as I would be using it in my garden,(so it could go out hours before use ) and most of the time the temperature difference between night & day isn't more than 10°c over the whole 24 hour cycle anyway.

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Schorhr

Schorhr

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Posted

A while ago I have built a small newtonian out of a shaving/cosmetic mirror from the drug store, it worked to about 10x, with a dentist/mouth mirror as secondary. There where two weaker images that came from the glass as the mirrors are back coated. Also reflections. Contrast was pretty low due to the poor mirror quality,it was amazing that it worked at all.

I got 5 more mirrors for 1€ at eBay but those did not work at all ;-)

So the point of this little experiment:

The coating belongs ontop of the glas, and if I ever want to shave again, I need to travel to the moon while someone on earth walkie-talkies instructions to me.

The end.

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