Trouble focusing large eyepieces

[jamieren]

I've been fairly pleased with the Antares 10 mm speers - waler eyepiece I picked up from allstar telescopes (kudos to Russ), but have had a concern with it. When I focus it I have to adjust the focuser all the way in. This gets me just enough for a relatively sharp focus (I think there might be a fraction of a mm on the other side left, but it's hard to tell it's so close!). A nearby friend has a 5 - 8mm adjustable eyepiece of the same make, and I've noticed that to focus it would mean having to go beyond the range of the focuser I have. That is to say I bottom it out and the views are still out of focus. The reason for my concern is that I've been thinking about investing in the 4.9 & 17mm speers listed on allstar's website and I really don't want to invest $200 on eyepieces that are impossible to focus! Currently I use an Antares 12" dob with crayford focuser.

Any ideas?

[mdstuart]

Well a barlow lens plus any lens on my scope means you have to focus outwards so that may be a work around. Also on my scope the longer the lens focal length the further out I need to get focus so I suspect you may be alright with the 17mm but not the 4.9mm so by a barlow and use your 10mm to give 5mm?.

Ultimately most suppliers I am sure would allow you to return a lens if it does not work with your scope but check first.

Seems odd though your issue. Try to speak to someone else with an Antares 12" dob..

[John]

The Speers-Waler's are famous (infamous ?) for needing more inward focusser travel that most other eyepieces. This has been addressed to some extent in the newer series II's but is still something you need to watch out for.

In a dob, which usually have more limited focus travel compared to, say, a SCT, this can cause problems. Solutions could include using the lowest possible profile 2" to 1.25" adaptor (some are lower profile than others), fitting a lower profile focusser or even moving the main mirror up the tube a little bit. Speers-Walers are not alone in this though - I found the Celestron Ultima 35mm similarly afflicted - but not the other Ultimas which reach focus at a more normal point.

I think that Antares need to be a little more open about this issue as it can cause headaches in a number of scopes.

John