Altair Astro Telescope Backfocus

[purgitoria]

I recently purchased from Altair Astro both the Wave Series 102mm f715mm EDT APO Triplet and the GSO 8" F8 RC. I own already a 12" Meade LX200 ACF and have been using that for a long time without issue but just wanted to get somethong for wide field imaging (also keep one telescope setup here in Scotland and the other in France) but i seem to have no end of issue with the backfocus on both telescopes and was wondering if i am just being silly or if anyone else has any experience with these optics (probably the former).

So far i have found that i am unable to achieve focus on either telescope without fitting an extention tube of around 80mm and if i want to use a 2X barlow then it is almost impossible to get focus without having several extention tubes in the assembly (had such a great view of Jupiter the other night but could not cobble together enough extention tubes to get focus on my camera for imaging). I know that the barlow lens will double the distance i need behind to be able to achieve this focus but i just seems unusual to me to require so many extention tubes. I incidentally also have all of the supplied 3" extention tubes fitted to the 8" RC in front of the focuser at all times even if i am using my SBIG CCD with attached off axis guider and integrated filter wheel and still have the focuer almost fully extended.

If anyone has any suggestions of advice i would be more than greatful.

[fatwoul]

The Teleskop Express page for the RC8 merely states that it has "generous back focus allowing to attach many accessories like e.g. off-axis-guiders, filter wheels etc." And does mention requiring needing a 50mm extension for an SLR. Most SLRs have a backfocus of 40mm (ish), implying you could easily expect 80mm or more of extension tubes to do the job.

Skywatcher refractors and Celestron SCTs are supplied with diagonals, I guess because it is assumed that people will want a more convenient viewing angle for eyepieces, but that similarly means lots of racking out for imaging.

So for visual use I'd say use a diagonal to fold up some of that distance. For using your imaging train, I dunno. Seems like you've got a lot of stuff there which should be filling up that backfocus nicely, but seems not to be. I understand why you'd want to get it closer to the scope - all that weight hanging off at a distance could indeed cause orthogonality problems.

You could try a Televue barlow, as I understand they work a little differently, and that the distances they sit from the optics and camera don't have quite the same effect as a "normal" barlow, but you'd want to try before you buy as I'm sure you're aware how expensive they are.

[purgitoria]

Thanks for the respnse. I do have a 2" diagonal on order for visual use which i may get away with using also for imaging as i just modded one of the new Microsoft LifeCam Studio HD webcams to try out for photography within the solar system.

I did look at the TeleVue barlows but could not find any difference in the description on their website, could it have been the TeleView Powermates that you were thinking of? Would be interesting to try it out if i had any dealer within a sensible distance of where i live unless there is anyone else on here that has experience of them.

[laser_jock99]

Seems to be standard with a lot 'fracs that you need extension tubes to get focus with a plain DSLR or CCD camera. The logic being that the extra 'room' might be required to put filter wheels, off axsis guiders and other crud in between!