Fancy a change from a dob ?

[Ajohn]

Posting in part because people might not be aware of this site and there is a lot of information about on it.

This is but one example - fork mounted equatorial newtonian, fork made of timber - hollow. I understand the same idea has been used up to 400mm and it would be easy to lighten it these days but sizes indicate Texereau style rigidity and even full thickness mirrors.

http://amas.free.fr/?page=instruments/pegase/pegase

Found by following links from this page

http://serge.bertorello.free.fr/index.html

There are some technical pages on various things that have been translated to English but there is far more in French. If feel that they are often a lot better than similar ideas and information from elsewhere.

John

-

[cassiewoofer]

I do like the look of those fork mounts, and have been considering how small/light I could weld one.... must get practicing my Alu welding!

[Ajohn]

I'm pretty sure they are made of timber. There are plans of some of them about on the site or off one of the links. One is timber with aluminium bearings.

This one for instance.  CTBX 15 seems to be 15mm marine ply

http://serge.bertorello.free.fr/details/sirius/sirius-fourche.html

I fabricated a dob tube in the same way they seem to in 1/4" exterior ply. Hexagonal though and I suspect octagonal would  be a better option. I just carefully set up a jig saw at the right angle and used a length of wood as a straight edge - checked by eye. It worked better than I expected. I just glued the edges together and expected to have to add some fillets but it worked fine without. Very well damped and no problems with tube currents.

I tried tig welding 1/4 plate once just fusing the edges using industrial gear - it's not easy. I used to hide  behind the welding curtains for the odd 1/4 hr when no one was looking at work. As I have had an interest in this area asking around some one told me that Machine Mart do a range of welders but the smaller one doesn't come with TIG gear but will work with it.

Meant to mention there is an excellent design for sector drive on there. It pulls steel tape round the sector, no problems with friction drives slipping etc.

John

-

[cassiewoofer]

I have one of their MIG welders and have done a bit of Aluminium welding. Just a bit (LOT) more scary than steel! I have the TE135 and have welded up a simple DOB base but will spend the next few weekends stiffening it and improving the smoothness of rotation by replacing the skate bearings on which it currently runs with small wheels and a clutch.

 Then maybe start welding an alu fork... or press on with grinding the 18",... just don't know what to prioritize!

Might just dump the Mrs!!!

[JamesF]

There's also a picture of the timber fork construction here: http://amas.free.fr/?page=instruments/pegase/pegase-fourche

I'm inclined to think however that a dob *is* a fork mount

James

[spaceboy]

Adding an EQP to a dob would be a better I think ??

[Peter Drew]

I,ve made several aluminium fork mounts for large telescopes on equatorials, 12mm framework clad in 1.5mm sheet, all fabricted by screwing together, no welding, very solid. For alt-az mounts I don't think the Dob configuration can be improved on. 

[Ajohn]

The big difference to a dob really is that they are all equ mounted and no flip problems like a german mount. I'd agree about the dob and alt az other than smaller scopes which can finish up too low so need a stand - for me anyway. There are some photo's taken using that style of mount about on the site. Seems it's sized to give 2 hrs of guiding and easy to reset. It looks like the steel tape and smooth sector to me. There are separate detail on that elsewhere on the sites.

Mirror 1st. That way you can design the sizes to suite. I'm not a fan of mig. I've hammered away at it and found that it wasn't really welded, thin sheet - some one else's to. Never had that happen with stick and have for instance welded the floor back in a 911   proving you can weld rust after a fashion. I'd guess stitching aluminium with a tig welder wouldn't be too bad with some practice, seam welding it via fusing it is tricky. I've never tried mig on aluminium and didn't even know it was possible. I have done a bit of stainless. Maybe I shouldn't sell my mig. I am intending to and probably wont change my mind.

I found the site I posted via this one some how

http://www.astrosurf.com/astroptics/index.html

John

-