Need a short 2" extension tube

[Bart]

Hello all

i have a problem. My 10" Dob has a focuser that has an outward travel of about 35mm. My 20mm ES 100* reaches focus at about 25mm. The 14mm and 9mm 100* reach focus at about 40mm. This means I have not fully insert the latter into the focuser tube to get focus. A 35mm extension tube is grand then for the latter but no good for the 20mm!!  Is there a shorter length 2" extension tube, say 20mm?  Or will I need to add par focalising rings into the 14mm and 9mm, which I don't like. I like to fully insert these EPs into the focuser. Any suggestions, please?  Thanks

Barry

[Astrobits]

It's going to depend on your focuser. If it is larger than 2", has a push-fit reducer to hold 2" eyepieces then a short extension is possible. However if the focuser is 2" max then if you extend your eyepieces by 20mm there will still be some bit of the eyepiece in the focuser and no way to mount a conventional push-fit extension tube.

Screwed extension tubes should be no problem.

Maybe you need an engineer to look at your set-up to see if a dedicated solution is possible.

Nigel

[Bart]

Nigel

Thanks for that. Afterwards, a screwed extension did spring to mind, but I'm not sure if the focuser tube has threading at the top. I must look to see if that's a possibility.

thanks

Barry

[parallaxerr]

If your eyepieces are 2" and threaded for filters you could use these 

http://www.altairastro.com/m48-2-spacer-set-4x-units-5-10-20-30mm.html

I have used them on my Hyperion Zoom in order to reduce back focus (for mount clearance)

[Bart]

Thanks

does the addition of this mean the bottom of the EP meets an internal ridge and makes the EP sit out proud of the focuser? If I'm making myself clear? Cos I'm not sure there is an internal ridge in the focuser tube. Thanks though