Focuser drawtube?

Curious to hear comments. What would most consider to be the minimal focuser draw tube length for visual use(1.5", 2.0" or 2.5") that would accomodate most eyepieces. Curious to hear answeres covering both 1)with and 2)without inclusion of barlow/binoviewer My object here is in optimizing a design built around a small secondary, and avoiding having the drawtube entering light path with shorter FL eyepieces. Thoughts?
Joe

Joe,

I would advise studying the charts over at the JMI website. My JMI NGF-55D 2-speed has 1.2" of travel and a standard length of 3". This is about minimum length to give a range of focus and not intrude into the light path. JMI has several drawtube length options for $12 extra at time of order. I would advise ordering direct from JMI as your credit card will not be charged until the unit ships. I paid for mine up front through Astronomics and then waited a month until the next production run at JMI. There are so many options for JMI focusers that it's best to call unless you want to study charts until your hair hurts.

Also, you might consider a Baader 2" T2 nosepeice and a Baader Click-Lock EP Clamp. It gives you about 32mm of extra out focus and instant and secure EP changeouts. I'll never go back to thumbscrews! Astro-Physics has free shipping on Baader products and sometimes a bit lower price than other Baader dealers, which are few in the USA. I go to the low profile 1.25" JMI EP clamp if the Click-Lock isn't short enough. Many JMI focusers now come with T2 threads on the 1.25" reducer, so you don't need the $41 Baader T2 nosepeice to mount the Click-Lock EP clamp. The Click-Lock deserves a LOT more attention than it has gotten since its introduction, as the anachronistic thumbscrew EP clamps are finally getting phased out. Axial alignment is automatic with a Baader Click-Lock ($66 at A-P), it puts a death grip on the EP with little turning force, and I will figure one into the focuser sizing for my setup from here on out. It seems built to last a lifetime with very smooth operation. Jacking the EP up a bit to increase outfocus will not affect automatic axial alignment.

If I hadn't bought the Click-Lock, I would have needed the 1.5" extension tube to bring my EP's to focus. As it now stands, I can adjust in or out about 1.5" so I have nearly 4" of total travel with my setup. A 40mm Baader T2 extension tube is $25 from A-P, which kicked me out to around 5.5" of total travel available with a 1.2" travel focuser as I can set it up 4 different ways to achieve focus.

1.5" will usually work, but I normally buy the 2" so I can have a lot of out focus for the combination 1.25"-2" EPs like the 12mm Nagler or the 14mm UWA.