Fousing a skywather Evostar 120 without the diagonal

[rusirius]

A while ago I used my wifes Skywatcher Evostar 120 refractor to do some imaging using a Canon 350D SLR camera. I wanted to remove the diagonal leaving only the camera in the light path. When I tried to do this though, I found that the I could not focus the image. Initially I thought it was because I was using the Big Barlow. However after a while with a bit of experimenatation I determined that even without the barlow, the image was only just starting to come into focus when the tube on the focuser was extended at full length. I changed the camera for an eyepiece and got the same problem. I tried a couple more with different FOVs and barlow/EP combinations but no joy. I put the diagonal back in and hey presto - I could focus again.

Is this problem normal and how do I solve it? Would some extension tubes suffice and if so, how long would the tubes need to be?

Or, is it better to simply image through the diagonal?

[Parallax]

Yes, you are correct - an extension tube is required.

If the lens element is able to be removed from the barlow, then you can use that as an extension tube.

Steve

[jgs001]

I've got this on both my fracs, extension tubes solved it on both. Putting an SLR on a diagonal is very likely to end in tears, or at least, that's how it seems to me ...

[rusirius]

Thanks. That confirms that extension tubes are the answer. I just need to figure out how long. How long a tube does one require? Is 2in enough or is longer reommended?

jgs001, I had also wondered about that. Having the diagonal in place not only adds an optic to the light path they tend to have a little give no matter how well you tighten them and hence add a little instability. Of course the extra length on the focuser tube might make it more likely to catch the tripod leg. Of course if one is careful then perhaps the unobstructed light path and improved stability hopefully outweight that.

[jgs001]

On my Konus I added a 30mm tube, and the C80ED a 50mm tube, although that wasn't necessary, it was just the shortest 2" tube I could find . Try the barlow without the lens element, that will give you an idea of how much you need, and where the draw tube ends up, then you can go from there.

[mr saddo]

Glad you mentioned that i was thinking about that on my evostar.

[alan potts]

It annoyed me too to find that you spend a great deal of money on a scope and then find it don;t work without a lot of add ons. TS in Germany sell all these and for what they are thay are not cheap..The other thing I found was that I bought a LX 200 first off and a full set of Series 5000 SWA ep's. When you use the 40mm the scope has very bad balance, solution, balance system, another 250 quid. They never tell you this in the adverts.

Alan.