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12" f4 Orion Optics OTA on home made dob base - my latest project


Moonshane

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I have wanted a 12" f4 scope for ages but they don't come up for sale very often. This one was bought off another SGL member at PSP in October 2012 and came with a Baader Steeltrack focuser (very nice indeed) and at a good price. The optics have no certification but I was not too worried about that as it's likely to be better than 1/4 wave.

The idea of this scope is to act as a grab and go at home for those nights when it's not worth dragging out the 16" f4 and I fancy more aperture than the 6" f11. This scope having a focal length of 1200mm is actually easier to handle and set up than either of these two scopes. I envisage cooling to complete equilibrium will be about an hour which is the same as my 6" f11 and less than my 16" f4. As I bought this scope to gain a wider field of view than my other scopes (it will provide about 1.7 degrees with the 26mm Nagler) I am likely to use it mainly as a wide field, low power scope so complete cooling will not really be necessary before I start observing. When testing it, I did manage to have a quick peek at the Pleiades and the whole of the main cluster was just squeezed into the field.

In due course, I'll be making a travel / truss-based system for the mirrors and focuser so I can take it camping, but for now I wanted to make a mount that will allow use in the meantime. A dob mount it is.

I made the base out of 18mm (and a bit of 12mm) birch ply. This material is a little more expensive than hardwood ply but really is very solid and stable with very few voids. I think there were two of less than 1cm in one layer in this whole project. Using templates I had previously made I used cut-outs to create grabbing places for carrying and to reduce weight. It also allows wind to blow through and therefore reduces the risk of the thing blowing over - recently this has been a real risk!

I used stainless steel fixings throughout and teflon pads / formica sheet for the bearing surfaces. The colour is my preferred black which was a water based stain (two coats) and then three coats of clear OSMO Polyx hard wax oil. This really gives a nice hardwearing finish and allows the wood to still show through a little at least.

One good thing about such a short focal length is that I can use a standard garden chair and view from horizon to zenith from that seat. Someone smaller might have to stand but it works really well on the whole. The speedy focal ratio does mean that a coma corrector is required to make the most of the wide fields but luckily I already have a paracorr.

The cell is a bespoke unit made out of marine grade aluminium. The result is a much better cell with a hole in the centre for a fan (to be fitted).

All in all I am delighted with this 'new' scope and it works very well indeed. It may need new coatings in the future but there's life in the old dob yet.

BTW sorry about the dead chicken on the lawn. For those of a nervous disposition, it's only a dog toy :grin:

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Future enhancements (pretty standard for my scopes) planned are :

  • fit a fan
  • flock the tube
  • fit a 'dob knob'
  • make a dew shield
  • make an aperture mask (probably with a 120mm aperture to create a 120mm f10 'dobfractor')
  • possibly paint the tube rings and end rings black (not sure about this though)
  • wooden block to extend the focus point out by 35mm or so - currently the paracorr has to be pulled out by this distance to achieve focus - use of a 35mm extension tube would not be feasible as they have a ledge 35mm in and therefore I would not be able to insert the parcacorr all the way
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ha ha thought you might like it mate. even though our OTAs are pretty much identical other than length, there's always little differences - I had two trunnion blocks (not like yours) and had to make a couple from a wide dovetail that cam with the scope - they work quite well but were slightly different in thickness so I had to do some shimming with washers to get the angles close the right at least. OOUK tube rings would be so much better if they designed then with a flat on each side but then I suppose they couldn't sell the trunnion blocks at £10 + VAT each (they may well have done up by now). thinking about it, a wide dovetail might be a good option although I could possibly make some in plywood.

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My OO primary is engraved on the back with the original specs and serial number and also has later stickers on the side noting the Hilux re-coating. I've also got the original Zygo print out for the mirror, which matches the serial number:

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Shane you really are a dab hand at this, and as usual a fantastic result...well done mate. The viewing height looks very pleasing...but you may have to stoop when its at its zenith!!...superb!

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My OO primary is engraved on the back with the original specs and serial number and also has later stickers on the side noting the Hilux re-coating. I've also got the original Zygo print out for the mirror, which matches the serial number:

cheers John - I'll have a butcher's at the back shortly. next time I have the mirror out (i.e. when I do some of the jobs above) I'll have a further look. maybe with a serial no. OOUK could confirm the figure but I suspect when this was done they didn't have their interferometer; he joined them later :grin:

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Looks like your in Dob heaven there Shane, lovely scope, what a choice to veiw with, small planetary, and two big beasities, ok one big beastie, and one HUGE beastie. One thinks of warm summer nights and some nice chill out time in the garden. Maybe after a BBQ

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defo Neil - in fact that's usually the only time my wife will join me generally when it's warm enough to see Saturn due south in the evenings! it normally involves red wine, a candle inside a big red vase and if I'm lucky, some Barry White music. no wonder the neighours complain..... :grin: no BBQ though - I worry about ashes!

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Make it into a self contained trunk. Where you can flip the mirror box over and store all parts in the base. Well maybe not the tubes, but even that I am sure you can do. Then it should give me ideas for my 4.5" suitcase scope I have been thinking about for awhile now. :)

Very nice purchase

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Count me in for one of your bases if you do go into production as I can't knock a nail in straight. The only exception being if it is more expensive than a metal OO.

In the meantime, could you post manufacturing drawings of the dead chicken as the hound savaged my display when it was featured.

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My OO primary is engraved on the back with the original specs and serial number and also has later stickers on the side noting the Hilux re-coating. I've also got the original Zygo print out for the mirror, which matches the serial number:

not taken my mirror out but it literally just has 'F4' engraved on the back face (I knew that :grin: ).

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Really lovely Shane.

What material do you use between the circular parts on the OTA and the base? I see 2 contact patches on each side about an inch. Is that just a teflon pad? Do you coat the surface of the circular face at all? Is it entirely reliant on the weight of the OTA to provide the appropriate friction or do you have some hidden tenioning system? :) I see OOUK have a tension brake as an option on their bases which bolts on the outside edge and effectivly seems to run a teflon furniture slide type object against the outer face of the circular part.

Also do you need to add weight to the base at all? I'd imagine that the scope with all it's accessories would weigh about 12-15kg or more. The base of just wood looks to weight about 5kg, maybe as much as 8kg. Is it not top heavy at all?

I know you told me you dont use lazy susans in the RA. Do you have a number of teflon pads in there as well?

I'm tempted to have a go myself if it isnt obvious :) My brother is a highly skilled joiner and has access to a professional workshop and materials. Tempted to try and make a similar base for my current 10" as i'm not a big fan of the skywatcher bases.

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Really lovely Shane.

What material do you use between the circular parts on the OTA and the base? I see 2 contact patches on each side about an inch. Is that just a teflon pad? yes, spot on it's 3mm x 18mm x 25mm teflon. Do you coat the surface of the circular face at all? yes, with laminate Is it entirely reliant on the weight of the OTA to provide the appropriate friction yes, and the tube rings of course help balance it or do you have some hidden tenioning system? no :) I see OOUK have a tension brake as an option on their bases which bolts on the outside edge and effectivly seems to run a teflon furniture slide type object against the outer face of the circular part. you could do this but if balanced properly, the scope, with the larger bearings, will be balanced with no sag from horizon to zenith. the only possible exception is that if using a longer tube (say f5.3 12") you might find that you need a small maybe 2lb counterweight for your heaviest eyepieces.

Also do you need to add weight to the base at all? not at all. the COG is not that high (it's the centre of the side bearings) and the main weight it at the bottom (the primary) so it's very stable. the key is that the azimuth teflon pads are above the three feet (three is important) so all the weight bears down onto the three feet through the pads. I'd imagine that the scope with all it's accessories would weigh about 12-15kg or more. The base of just wood looks to weight about 5kg, maybe as much as 8kg. Is it not top heavy at all?

I know you told me you dont use lazy susans in the RA. Do you have a number of teflon pads in there as well? yes, see above, plus I usually put one in the middle around the bolt

I'm tempted to have a go myself if it isnt obvious :) My brother is a highly skilled joiner and has access to a professional workshop and materials. Tempted to try and make a similar base for my current 10" as i'm not a big fan of the skywatcher bases.

hi mate

if you have access to a joiner then that would be a good way to go as it's relatively straight forward to make. answers above. if making one for a 10" scope, I'd personally make it big enough for a 12". that way you can just add a packer to the alt bearings and remove this if you ever decide on a 12" scope. there's a lot of work in them but it's very satisfying.

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That's really helpful, thanks mate. I'll have a stab at the 10" base across the summer and if I screw it up I always have the SW base to fall back to.

In the meantime I think I'm going to start posting wanted ads for a VX14 OTA, in a month or so. And when/if that comes perhaps we'll have a chat ;) I'd love to have one of your bases on a decent OTA.

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