OOUK 12" Dobsonian - Mind the Gap!

[Relpet]

In a recent posting I described how I had bought second-hand an Orion Optics UK 12” Dobsonian.  The mirrors were fine, the tube showed signs of age, the finder – if it was original – was a disgrace and held by two sloppy screws and some double-sided tape.  The focuser was a clunky rack and pinion, single speed but clearly original.  The mount was largely OK but the braking system was scarcely usable with not a single original bolt or starred hand wheel remaining.  Nevertheless it was a good price and some quite valuable extras were thrown in.

I already had a 9 x 50 RACI finder ready to replace the disgrace and that is now installed.  Magic!

I’ve ordered replacement starred hand wheels for the mount, which should arrive tomorrow. 

The postman delivered this morning a second-hand dual-speed, low-profile Crayford focuser originally marketed by OVL.  Herein lies the rub.

Because the mounting holes in the OTA are not compatible between the focusers I shall have to drill four new holes and seal the old ones.  That task will be approached with great caution, as you might imagine.  Any advice on sealing the redundant holes will be appreciated but the big question is how to seal the gap between the mounting plate of the Crayford and the curve of the OTA.  The focuser is designed to fit tubes with apertures between 9” and 15” but, inevitably, there will be a gap between the reinforced ridges where the screw holes are located and the surface of the OTA.

 

I could fill this gap with the black foam the focuser was packed in but that will degrade over time.  Any suggestions concerning an appropriate gap filler will be greatly appreciated.

[Davey-T]

3D printer ?

Dave

[Relpet]

Yeah, thanks Dave.  You don't happen to have one?

Peter

[Stu]

Does it actually need sealing? These must be pretty commonly used and it will be interesting to hear what other do. I think I left mine open....

[Relpet]

3 minutes ago, Stu said:

Does it actually need sealing? These must be pretty commonly used and it will be interesting to hear what other do. I think I left mine open....

It's a good point., Stu.  If it was really necessary some bright spark would be marketing a range of sizes in moulded plastic.  I just feel that any opening, however tiny, is an invitation for inanimate dust to seek out and animate creatures just looking for a pristine, polished surface to use as a loo while they do their make-up.

[dyfiastro]

How about making a gasket from Camping mat,  High Density foam or an old neoprene mouse mat?
Cut to a rough size so once bolted down onto should self seal and the remainder can be cut off.

If you use camping mat then you can use the rest to make a dew shield at the same time.

[John]

I've seen such gaps left open quite often with no obvious detriment to the performance of the scope. Some neoprene foam cut to size would do the trick if you feel you want to fill the gaps.

You might need an extension tube to get eyepieces to focus - the OO design places the focal plane of the scope well outside the tube wall and that looks quite a low profile focuser.

I had to drill 2 holes in the tube of my 12" OOUK tube to fit a Moonlite focuser. The aluminum tube is easy to drill. The old holes were covered by the base of the new focuser so I did not need to fill them.

 

[Relpet]

22 minutes ago, John said:

You might need an extension tube to get eyepieces to focus - the OO design places the focal plane of the scope well outside the tube wall and that looks quite a low profile focuser.

I had to drill 2 holes in the tube of my 12" OOUK tube to fit a Moonlite focuser. The aluminum tube is easy to drill. The old holes were covered by the base of the new focuser so I did not need to fill them.

 

Thanks, John.  There is a built-in extension tube which people say is fine for observing but can be sloppy for camera work (not my scene). It extends the basic 28mm to over 40mm I believe.  I think it's quite likely the old holes will be covered to judge by the measurements so I may just tape them over.  Steady hands tomorrow!

[Relpet]

14 hours ago, John said:

 

You might need an extension tube to get eyepieces to focus - the OO design places the focal plane of the scope well outside the tube wall and that looks quite a low profile focuser.

 

Looks Like I bought the wrong thing, John.  OVL describe it as suitable for Newtonians - but evidently not this one.  I cannot get any eyepiece into focus from a 10mm unbranded plossl to a 27mm flat field Orbinar.  You suggested an extension tube.  I have several in France but none here in Surrey I can play with.  What size would you recommend please?  Failing that it looks like the original rack and pinion will have to go back on.  It's a real shame because I was almost hugging myself with glee after making such a good job of the swap-over.

[Moonshane]

I previously used a 50mm extension tube and all but my 16" do (I made a new cell and this pushed the primary up the cell by 50mm or so (on purpose).

[Ricochet]

Can you carefully withdraw the eyepiece from the focuser until it comes into focus and then measure the gap? That will give you an idea of what size extension piece you need. With any luck it will match up with one of the click lock extensions. 

Alternatively, if you put the original back on you can find the focus point and measure the distance from the tube and then subtract the height of the new focuser.

[Moonshane]

if you mainly use 1.25" eyepieces, and you have a barlow you could for now unscrew the element from the end and use it as an extension tube? Or maybe add some Baader fine tuning rings if you have them to the 2-1.25" adapter.

[Relpet]

17 minutes ago, Moonshane said:

if you mainly use 1.25" eyepieces, and you have a barlow you could for now unscrew the element from the end and use it as an extension tube? Or maybe add some Baader fine tuning rings if you have them to the 2-1.25" adapter.

Yes, I do most of my observing in France where in the barn of our second home I have a collection of  telescopes, 1.25" EPs, 2" EPs, Barlows of different strengths and sizes, extension rings, two sets of Baader tuning rings la-de-dah.  I'm only going down this second-hand road as a kind of emergency operation for when it's all happening at the end of July as this year we are compelled by family circumstances to stay here until September.  So the only EPs I have are a few 1.25 but I do have an unbranded 1.25 Barlow so I'll give that a try as soon as the weather looks less threatening.

[John]

1 hour ago, Relpet said:

Looks Like I bought the wrong thing, John.  OVL describe it as suitable for Newtonians - but evidently not this one.  I cannot get any eyepiece into focus from a 10mm unbranded plossl to a 27mm flat field Orbinar.  You suggested an extension tube.  I have several in France but none here in Surrey I can play with.  What size would you recommend please?  Failing that it looks like the original rack and pinion will have to go back on.  It's a real shame because I was almost hugging myself with glee after making such a good job of the swap-over.

A 50mm would be worth trying I think.

[Relpet]

Thanks, John.  I'm just trawling through the various options online.

[scarp15]

When initially a few years ago I purchased a second hand OOUK VX14 Dob, I did not like the gap created by a baader steel track focuser, as fitted by the former owner. Therefore I used a sealant adhesive which is solid when dry, has worked perfectly well.

    

[Relpet]

5 minutes ago, scarp15 said:

When initially a few years ago I purchased a second hand OOUK VX14 Dob, I did not like the gap created by a baader steel track focuser, as fitted by the former owner. Therefore I used a sealant adhesive which is solid when dry, has worked perfectly well.

  

Thanks, Iain,  I've stuffed the gaps with pieces of a foam mat for the moment but as you may have read, it may have to come off again.  So sealant may have to wait!

[Relpet]

On 19/07/2018 at 16:57, John said:

A 50mm would be worth trying I think.

Finally I got the new focuser fixed properly.  After a false start I had to buy a ring gasket from RS Components to reduce the aperture from 78mm to 60mm to get the draw tube exactly above the secondary.  Tube has more holes in it than a piece of Gruyere cheese now but all redundant holes, mercifully, covered by the focuser mounting plate.

A 50mm tube enabled me to focus all but the highest mag EPs so I've ordered an 80mm self-centring job to replace the 50mm (which will no doubt find a use some time).  The 80 mm should give me maximum flexibility but having got more familiar with the old R & P the last week or two I wonder at times if this wasn't all labour and expense in vain!  But thanks, as ever, for your advice.

Peter