Focussing issues with barlow

[banders808]

Hi all. I've just tried out a (cheap) 2x Barlow on our scope (heritage 76 dob). I can't focus with it on distant objects. Trees that are about 200m away are OK but Moon and Venus won't focus. I've tried with both 25mm and 10mm eyepieces but to no avail. Is this due to it being a cheap Barlow? Or is there another issue?

Thanks in advance

[Geoff_L]

Simple answer is that you probably need to add an extension (like a barlow but with no lens) to move the eyepiece far enough out to achieve focus.

More complicated answer: a barlow increases the effective focal length of your 'scope. Your 2x barlow will double the effective focal length of your 'scope, which means that you'll need to position your eyepiece roughly twice as far out as you did without the barlow. In other words, your eyepiece probably needs to be further out than your focuser can achieve -- hence the need to add an extension.

Something similar to this from FLO will probably do the trick.

Edited to add: Before splashing out on an extension it might be worth trying moving the eyepiece out a short distance (so it's not quite so far into the barlow) and/or moving the barlow out of the focuser a short distance as that might give you the extra distance needed.

HTH & Clear skies

[John]

Barlow lenses can use up some focuser movement so you might have the opposite problem to the one described above - not enough inwards focuser movement to reach focus on the more distant objects.

Can you say which direction you run out of focuser movement - inwards or outwards ?

What is the strength of the barlow by the way - 2x, 3x, more ?

[banders808]

Thanks for those replies. I'm pretty sure its inward focus that I'm lacking. 

 

[John]

9 minutes ago, banders808 said:

Thanks for those replies. I'm pretty sure its inward focus that I'm lacking. 

 

Thats more difficult to sort out, unfortunately. What type of barlow lens are you using ?

[banders808]

Its a cheap one from UK Hobby store. 2xbasic Barlow. Didn't want to fork too much at this stage as I'm new to it all. 

 

[John]

I'm assuming that you can focus on astro targets using just the 25mm and 10mm eyepieces (no barlow lens). Is that correct ?

[banders808]

1 hour ago, John said:

I'm assuming that you can focus on astro targets using just the 25mm and 10mm eyepieces (no barlow lens). Is that correct ?

Yep. No issues with the eyepieces, its only when I use the Barlow.