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woodblock

problem with 2 inch eyepiece

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Hi,

I recently bought a 32mm 2 inch skywatcher panaview eyepiece to go with my 14inch skywatcher dobsonian. Tonight was the first opportunity I've had to use it but there's a problem.  The focuser won't screw back far enough to bring it into focus. If I hold the eyepiece away from the telescope by about half an inch it will focus.

The telescope came with two 1.25 inch Ploesel eyepieces which are fine. It also had an adapter to adapt from 2 inch to 1.25 inch. If I take that out the new eyepiece fits perfectly into what I'm sure is the correct position which has two small knurled screws to hold it in place.

Cheers

Steve

  

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You should be using a 2" eyepiece adapter which goes into the Skywatcher focuser drawtube. The Skywatcher drawtube is not actually 2", it's a little larger so Skywatcher supply a 2" adapter which also moves the eyepiece outwards by around 40mm - 50mm so that it reaches focus.

The 2" adapter looks like this:

post-118-0-17010200-1453239355.jpg

Apologies if you are already using one of these !

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Someone should write a book called "The Idiots Guide to Telescope Adaptors". There seems to be dozens of them and the descriptions are not clear. Anyway, I've ordered one. I hope it's the right one but I wouldn't be surprised if it wasn't.

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The book would never end. It's like every time you put one foot in front of the other in astronomy there's a different adaptor for it. But it does keep those nice little surprise presents coming through the door every day lol. :)

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Brilliant Dave! Before I was merely baffled now I'm  totally nonplussed.

I like the idea of an artificial star for telescope collimation - it could solve our energy problems for ever.

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My 300P Skywatcher requires a Extention tube to gain focus, i use a Baader Click-lock 47mm extension.

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Odd that the focuser is not actually 2" and would require a 2" adapter.  Is there some logic to that?    

My Orion has a 1.25" adapter in the 2" focuser.  No 2" adapter needed.  I just assumed that was the standard approach.   Did not expect this.

It is a 2" focuser and no adapter is needed for the diameter, eyepieces lock fine, but most of my skywatcher OTAs are designed so you have enough back-focus to get even a DSLR in focus (possibly with a reducer as well). Since an eyepiece requires less back-focus than a DSLR, a 2" extension like the one in the pic above is also included so that they can reach focus. It is strange that the OP's did not come with the 2" extension, only the 1.25" adapter, is it possible that it was overlooked? In any case more back focus is a good thing, i.e. it is not a problem to require a bit of extension, the opposite issue is the hard one.

Edited by ecuador
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When I last had a Skywatcher newtonian I measured the inside diameter of the bezel that is fitted to the focuser drawtube and found that, on my instrument at least, it was just a little bit larger than 2". Sure, I could get a 2" eyepiece to fit into it and the 2 set screws would hold it in place. Apart from the issue of the eyepiece not reaching focus though I discovered another one - because the bezel was slightly oversize, the eyepiece was not quite held in the centre of the optical axis of the focuser. The 2 set screws pushed the eyepiece against the wall of the bezel opposite them.

It was just a couple of mm out but, as we know, you want all the optical elements of a scope to have aligned optical axis for the optimum performance.

Why Synta (who make Skywatcher, Orion and Celestron scopes) adopted this design I don't know and maybe it's changed in the latest scopes ?

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Hi Steve,


 


There are at least three types of (single speed) Sky-Watcher focusers fitted to SW Dobsonian scopes in the market place so depending on when you bought yours and whether it was new then or second hand it should have come with various extension tubes.


My first SW Dob was an early model and had a rack and pinion type focuser with two adapters for the two sizes of eyepieces as shown:


1.25" adapter msg-30550-0-26522800-1409856350.jpg and 2" adapter msg-30550-0-06763900-1409856395.jpg .


 


A subsequent SW Dob was fitted with a Crayford type single speed focuser which took exactly the same adapters. However my latest SW Dob, fitted with the latest single speed Crayford focuser came with two different eyepiece adapters with a different size of conical flange at the base. These conical flanges which fit them into the focuser tube are actually 2" wide whereas the older ones were 2 1/4". With the new adapters the 1.25" adapter will actually stack into the eyepiece end of the 2" adapter whereas the older type won't do this. I've read a few stories on SGL where people have been mightily confused when they've stacked the two adapters together and can't get anything to focus.


 


The new style of 2" adapter looks like this with the narrower conical flange.


 msg-30550-0-43867800-1409856649.jpg .


My guess is you should have had one of these supplied with your scope, and if you bought it second hand, the previous owner may have mislaid it and forgotten to give you it.


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