FlexTube Dobs

[Johnboy]

Hi folks

I enjoy using my binoviewer with my sct but i'm thinking of changing my scope to a Skywatcher SkyLiner 350p FlexTube. As it's a dob I know there will be focus problems with the binoviewer but I was wondering if the trusses on Flextubes can be locked at any point therefore shortening the focal length which will allow focus? Or do the trusses only lock at the fully extended position?

Thanks in advance folks,

Best regards

John

[JamesF]

The 250 flextube has captive sprung balls to lock the trusses in position at full extension. Those wouldn't operate if it weren't fully extended. There are T-bolts as well however, and they might do the job as long as you can be sure the trusses are all extended the same amount, otherwise collimation is obviously going to be a pain.

How much more in-focus do you need? Would a low-profile focuser help?

James

[Johnboy]

Thanks for your reply James - it is most helpful. I used to own a 12" Lightbridge and I fitted a Moonlite low profile focusser but I still needed a corrector to reach focus with the binoviewer. The corrector was a multi mag from Siebert Optics which worked very well. Alas I sold it when I sold the Lightbridge. I really fancy another dob but didn't want to start again buying things over again. The flexTube seems like a good idea if I could just lock the trusses at a slightly shorter length.

Thanks again.

John

[JamesF]

Can't see why it wouldn't be possible to modify the trusses so they'd lock at a shorter length. You'd have to watch out for vignetting if you shortened them too much though.

An alternative might be to try to move the mirror up the tube. I believe you can get longer collimation bolts to do that.

James

[swamp thing]

Hi John

Not sure that any modifications are needed. Pingster seems to have got round the problem with both his 12" and 16" flextubes.

Check out this thread:

http://stargazerslou...be-need-advice/

[acey]

The flextube struts don't need to lock - you can tighten them down and they'll hold fine. Collimation is no more an issue than it always is when setting up the scope. On occasion I've accidentally used my 12" flextube without fully extending the tubes (which happened to be very stiff) - I only realised it when I found that some EPs wouldn't come to focus. You aren't changing the focal length by doing this (that's an invariant function of the primary) but you are moving the focal point outwards with respect to the focuser.

[Johnboy]

Thanks Steve - that's a great thread. Thanks for the pointer to it and tells me all I think I need to know.

Cheers Acey - fantastic info. If I go ahead and buy the FlexTube I'll be sure to check double check the stiff trusses are correctly extended.

Thanks again to everyone who has replied.

Regards

John