Closing flextube DOB OTA to shorten focal length

[Space Cowboy]

This is probably a silly question and shows my technical naivety but has anyone tried closing the OTA (shortening) on a flextube DOB to reduce its focal length? I briefly tried this a long time ago but could not achieve focus with a webcam but would it work with a DSLR?

[Stu]

This is probably a silly question and shows my technical naivety but has anyone tried closing the OTA (shortening) on a flextube DOB to reduce its focal length? I briefly tried this a long time ago but could not achieve focus with a webcam but would it work with a DSLR?

Shortening the OTA by leaving the truss not quite fully extended does not change the focal length of the scope (that is defined by the figure of the mirror).

It might, however, help you if you are struggling with infocus with a DSLR so it's worth a go. You have to consider what it does to the light cone and whether your secondary is still large enough to collect all the light, but provided you are only shortening slightly you may be ok.

I've never tried this so others may have real life experience

[Space Cowboy]

Thanks I don't have any problem acheiving focus with the OTA fully extended. I didn't realise that shortening the OTA had no effect on focal length which negates my brain wave completely!