Can't focus with my Barlow in place

[JulianFR]

When I was out with my SW 120 refractor (the short one) recently I tried it for the first time with my Revelation 2.5 Barlow ... And was surprised to find that I could not then get the BST eyepieces to focus at all... Everything was completely blurred no matter how much I turned the focus wheels.

I tried the Barlow in front of, and coming out of, the RA corrector (I don't know if there was any point in trying it both ways).

Any thoughts about what is wrong and what I need to do (or, more likely, buy!) ?

[Ant]

The correct order of attachement is as follows.

Scope --> Diagonal --> Barlow --> EP.

A barlow moves the focus position further out. A reducer moves the focus point further in.

What you should see is that as you slowly move the EP out (using the focuser knobs) is that the out of focus blob gets smaller and smaller. It may be that you do not have enough out focus. Undo the little thumb screws that hold barlow in the diagonal and slide the barlow/EP out - this may give you enough to reach focus as a temporary measure.

You should have one of the cheapy barlows that come with the scopes, if you unscrew the lens section (usually comes off in a self contained part) you are left with a cheap 1.25" extension tube. Use this between barlow and diagonal.

Scope --> Diagonal --> Extension tube --> Barlow --> EP.

You should now be able to reach focus

Ant

[webboid]

I have the same problem with my ST102. There is not enough inward focus for that particular barlow with any of my eyepieces. 

Fortunately it works on my other scopes so I have gone back to using the skywatcher barlow on the ST102.

[m37]

I had the 102 and got an engineer friend to take a bit off the tube and drill new holes for the focus assembly. Allowed me to practise imaging with a filter wheel and OAG. The cut needs to be spot on though.

[JulianFR]

Thanks everyone. Sounds like my Revelation Barlow won't work in any circumstances ... Though I will try fiddling with its position in the focuser. My 120 didn't come with a cheap Barlow.

It's a pity but I don't use the Barlow that much so I guess it's not the end of the world. I'll use the Barlow with the Dob.

[JulianFR]

Actually, reading your post again, Ant, you mention having an extension tube (taken cannibalised from a Barlow). Could I just buy an extension tube specially for the purpose?