Revelation Focuser extension help please!

[jamespels]

Hi, I have finally got round to trying out a second hand SW250P I bought last spring (yes, I know - I have had a lot on...) and am a bit hacked off to find I cannot get it into focus.

It has been modded with a Revelation 2" Crayford focuser that comes with a 50mm extension tube and 2" to 1.25" reducer.

Without the extension, using my 1.25" EPs, I can achieve focus by holding the EP a centimetre or so outside the reducer. If I only push the reducer and EP in each the bare minimum for the compression rings to hold, it will achieve focus but the whole thing is very insecure and wobbles all over the place - not good.

With the extension, I get a reasonable airy ring as I wind the focus in (while centred on a star of course!) but it will not go quite far enough in to achieve focus.

I have had a good look at the focuser and, fully extended, the draw tube inner end is almost level with the mounting wheels and the outer end had a nice machined lip so there is no extra movement to be gained in either direction. I have looked at the primary and could possibly locate it the or for mils further back to bringthe focus closer tothe secondary but nowhere near the 10-15mm I think I need.

The travel on the draw tube is 30mm and the extension is 50mm, suggesting I need another extension of around 20mm to have a decent amount of room to play with either side of focus.

Questions:

1. Does this make sense or is there something really obvious I am missing?

2. If this is correct, where would I buy an extention of this size with a decent compression ring fitting? I have checked with our dear sponsors and they do not seem to have anything less than 35mm. I suspect it would need to be a 1.25" extension as a 2" one will necessarily be a certain minimum length to accommodate the EP.

3. Am I heading into the realm of custom-made draw tubes? If so, any ideas?

Thanks in advance!!!

[jamespels]

Btw, there is an obvious question here about how it worked when I bought it. Unfortunately i have never seen it work; I bought the scope unseen from a chap in Scotland whose reputation did at least stack up. It was picked up by a work colleague so I never actually met the seller.

I am now seriously regretting selling my Tal to fund this - frankly the overall build quality is so far off the Tal it is almost a joke. In fairness to SW, this is one of the old blue tube scopes and I believe their quality has improved since.

[knobby]

It may well have focused using different EPs, you could try moving the primary up or down the tube ? But then you'll need to collimate it

Edit ... Or try a 35mm extension instead of the 50mm

http://www.telescopehouse.com/acatalog/Revelation-Self-Center-35mm-extension-tube-2-.html

[Moonshane]

Hi James

it sounds like the 50mm is to much and you need a 35mm extension. If you have time to come to stockport I could have a look. I have various options to hand

[jamespels]

Not sure where my maths was last night! Ïf 30mm out is not quite far enough but 50 is to far, then ending it back in and putting a 35mm extension on out should be fine...

Thanks for the link, knobby - looks like a sensible price, too.

Shane, I am planning to head up to North Staffs Astro Soc on Sat (three years in and I really need to join a society, I think! ); if the weather is not up to it, I will give you a shout - thanks for that!

[jamespels]

For the sake of £16, I have bought one of these from Telescope House - I can ebay it or put it for sale on here or at PSP if it doesn't work (if that happens, the scope will probably go with it - should NEVER have sold the TAL!!!!)

knobby - I did have a look at moving the primary a bit further down the tube but there is only about 3mm or so to play with so not enough to do the job. I have even looked at moving the focuser further up the tube as there is some space to play with in the secondary but moving the focuser is too much work, tbh.

At the price, the 35mm extension tube is a no-brainer and should work fine.

Shane: will drop you a line if this fails...

Thanks both,

James.

[Moonshane]

no worries James.

I just bought one of these and they are really excellent. The nosepiece is flat too with no undercut so you can use it like a variable extension, especially if you add a baader fine tuning ring. once you find a point where it allow all your eyepieces to focus, just leave it in situ.

[jamespels]

Hi both, the new extension arrived Sat; got our with it for a couple of hours last night and it works a treat. Thanks for the advice - much appreciated!

[Moonshane]

glad it all worked out James. hope to see you whirring around the skies at PSP! (as if you are, I will be too  ).