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Barlow and focus

Posted: Sun Feb 11, 2018 1:04 pm
by denababy
Hello all I hope you are having a good day. This morning (Sunday 11th Feb) the Sun was out so I set up and put in a x2 Celestron Omni Barlow but I could not get enough inward travel to achieve focus. I have fitted a Feathertouch focuser and this was the first time using it. I took off the Barlow and focus was easy but the clouds rolled in so I never had chance to experiment. There was a comment on here that stated the focus distance on my Lunt 50 is 12.5, can anyone throw any light on this please. Once I get enough clear sky I should be able to experiment but in the mean time I don’t want to reinvent the wheel anyone been here and what did you do? My camera is a ZWO 120mm.

Re: Barlow and focus

Posted: Sun Feb 11, 2018 2:06 pm
by marktownley
Unscrew the nose of the barlow and put it on the end of the camera.

Re: Barlow and focus

Posted: Sun Feb 11, 2018 3:25 pm
by PDB
The 12.5 for the ASI is not the focus distance of your Lunt. It is the backfocus distance of the camera. This means from the front of the camera to the sensor the distance is 12.5 mm. (ZWO has a different front plate to make this even shorter but I think these are only usefull when you can't reach prime focus (and that seems to work).


and did some reading again on Barlows. It is complicated. Not all Barlows seem to be the same. It looks like most need more "inward" focusing, but some discussions say telecentrics need more outward focus.

Re: Barlow and focus

Posted: Sun Feb 11, 2018 11:36 pm
by MapleRidge
I have usually found that the barlow element need to have the focuser racked out to re-focus after fining focus with out it. Is it possible that the upgraded focuser takes up more 'space' which limits the amount of focus travel?

Just a thought...hope you get it sorted out.