Top five longest back focus refractors ??

[coldlegs (Stephen)]

Need some help here folks.

Always wanted to by a refractor but have been reluctant due to the back focus problems that can occur when trying to add bits and pieces. I would particularly like to use an ONAG to guide through the scope with as I already have one so I'm looking at about 100-110mm of back focus needed.
What I would like to do is use my QHY9M (22.5mm diagonal) to LRGB image 1-3 degrees of view if possible but I'm not sure what the focal length would be needed. The cost needs to be less than $2k if possible. I know there's some really nice Taks out there that would do the job but they are way over my budget. Size? Well I guess 102mm would be the minimum but 80mm can do some very nice images as well.

so the question is...

What are the top five longest back focus short F/L refractors that could fit my requirements?

Cheers
Stephen

[troypiggo (Troy)]

I have a TMB92SS. It has a section at the back that you can remove for more back focus. Maybe 100mm long. More than enough on top of what the lovely 3" FeatherTouch can give.

[coldlegs (Stephen)]

Originally Posted by troypiggo

I have a TMB92SS. It has a section at the back that you can remove for more back focus. Maybe 100mm long. More than enough on top of what the lovely 3" FeatherTouch can give.

Now that's an interesting beast. It's cleverly organised to have lots of back focus which is what I need. That's now on my list of potentials.
Thanks Troy

Cheers
Stephen

[brian nordstrom (As avatar)]

Sellarvue 80mm 'Aplanet' , sadly no longer made , an awesome scope in every way .
Over 140mm.
Brian.

[troypiggo (Troy)]

Originally Posted by coldlegs

Now that's an interesting beast. It's cleverly organised to have lots of back focus which is what I need. That's now on my list of potentials.
Thanks Troy

Cheers
Stephen

I'm very happy with it. One word of warning, though. It's a triplet but does need a flattener for stars in corners. I tried about 4 different ones and by far the best was, surprisingly, the Orion Flattener. (Not reducer)

[The Mekon (John Briggs)]

Astro-tech 106 has too much for me. To use it visually straight through I have to add over 120mm of extensions. Cannot use a 1 1/4 diagonal, only 2" and then focuser is extended 65mm out of 80mm travel. Tube should have been made at least 50mm longer to make me happy.

[coldlegs (Stephen)]

Originally Posted by The Mekon

Astro-tech 106 has too much for me. To use it visually straight through I have to add over 120mm of extensions. Cannot use a 1 1/4 diagonal, only 2" and then focuser is extended 65mm out of 80mm travel. Tube should have been made at least 50mm longer to make me happy.

Astro-tech 106 is now on my list. I checked out the specs and it's quite an interesting beast and will easily fit my ONAG.
Thanks John
Cheers
Stephen