Increasing Back Focus Distance

[andyc277]

I have a SW 200P, my wife finds that it strains her eyes somewhat and yesterday we tried out a Binoviewer that seems to alleviate the problem.  It does however throw up another issue, due to the very short back focus distance of the 200p focusing has become an issue. From what I have been told and some reading I believe I need to increase the back focus distance, we tried initially to use a Barlow lens (removed from the body and screwed on to the Binoviewer) but to no avail. 

Does anybody have a solution? Would a low profile focuser work?

[rikroe666]

I hope someone has an answer to this, i wanted to try some binoviewers with my 200p-ds but i have read that focus is a problem with  newts.

There's a couple of answers on here that might help?

http://stargazerslounge.com/topic/64480-binoviewers-advice-needed/

[John]

When I tried binoviewers with my 12" dobsonian it was inwards focuser travel that I was short of. Using the 1.6x barlow nosepiece just about allowed me to achieve focus with some eyepieces. A low profile focuser might help but moving the primary mirror a little way up the tube is another way to to it as that pushes the focal point of the scope further outwards from the tube thus allowing the binoviewers to reach it. 

[JED-E3]

Hi, have you tried using a low profile 2" to 1.25" adapter? 

My skywatcher 200p dobsonian will reach focus with the WO binoviewer and 1.6x barlow, but it needs a low(er) profile adapter.

James.

[andyc277]

We tried bodging the 2" to 1.25" as we didn't have a proper one to hand and it didn't seem to work. We used a x2 Barlow lens so maybe a 1.6 would have worked.

[RichM63]

Andy, you don't say what EPs you are using giving the problem.

The supplied SW EPs whilst adequate don't give much eye relief.

However, for example, the 2" 32mm in my signature pretty much allows me to stand way off from the scope and almost observe in passing as it were.

One other thing, check the length of your 2" adapter tube if you are using it.

The one for my 200P is 47mm whereas the one for the 150P is 40mm.

I had a heck of a time trying to focus the 200P until I realized this, I even thought that the 32mm wouldn't work in both scopes at one point.

Unless I'm mistaken a 2x barlow lens screwed directly into an EP will give 1.6x.

Best,

Rich

[andyc277]

I was using 2 SW  Super 25mm. we didn't use the 2" adapter just had the 2" to 1.25" adapter that connects to the focuser. Thanks for the info on the Barlow