Focusing issues with Barlow lens

[UniDonk]

Please could I ask for some ideas of what might be wrong and how to fix it? I am unable to bring objects into focus using my 2x Barlow lens.

I am using a heritage 130p dobsonian - it's my daughter's! I bought BST 8mm eyepiece and BST 2x Barlow lens. I wanted something better than the kit 10mm eyepiece and the Barlow lens was to help improve views of planets.

Initially, the combination worked well. Of late, though, I am unable to focus images when using the Barlow lens. This is not limited to the 8mm/Barlow combination: I have tried using the kit 10mm and 25mm eyepieces. This yields the same results.

I previously had to screw the focuser down almost as far as it would go in order to focus images when using the Barlow. Now, although the images come closer to sharp focus, it is almost as though the focuser won't screw down far enough! Images don't QUITE reach sharp focus.

I have no issues focusing using the lenses alone. 

I thought it might be a collimation issue:  I have neglected this important part of reflector husbandry! But I got a laser collimator (itself well collimated) on advice from FLO and have collimated the scope pretty well, I think. Both the primary and secondary mirrors were out of alignment and, while images of the sky are now much improved, the issue with focusing using the Barlow persists.

The Barlow itself has been kept boxed in its protective wrapping. It has not been dropped or damaged in any way.

So... what do you think is wrong? Any suggestions what to try next?

Many thanks in advance 👍

[Cornelius Varley]

12 minutes ago, UniDonk said:

I previously had to screw the focuser down almost as far as it would go in order to focus images when using the Barlow. Now, although the images come closer to sharp focus, it is almost as though the focuser won't screw down far enough! Images don't QUITE reach sharp focus.

I have no issues focusing using the lenses alone. 

I thought it might be a collimation issue:  I have neglected this important part of reflector husbandry! But I got a laser collimator (itself well collimated) on advice from FLO and have collimated the scope pretty well, I think. Both the primary and secondary mirrors were out of alignment and, while images of the sky are now much improved, the issue with focusing using the Barlow persists.

The problem could be the collimation. When you recollimated the telescope you might have moved the primary mirror further from the secondary. If this occurs the point of focus will also move closer to the secondary meaning that you have to adjust the focuser closer to the secondary to achieve focus and run out of inward focuser travel with the barlow. Recollimating and paying attention to moving the primary closer to the secondary will probably solve the problem.

[Pixies]

What happens if you don't  extend the scope to it's full length?

Perhaps when you were getting focus, it's wasn't fully extended?

[Louis D]

Try loosening all the primary collimation screws equal amounts to let the primary move forward toward the secondary until the Barlow reaches focus again.  Then, tweak the primary's collimation back to where it was before, only this time further upward in the tube.