if it's give (only)1.25x wouldn't be a extension tube work on a sct optics for the same result? I forget the "true" F? for the given BFL of a C14 but perhaps a slider can be made to experiment for a small mods of the FL. It may alter the spherical correction of the system tho... so is the Barlow if it's not used with a designed distance me think.
Not sure I quite understand what you're saying Mick - an extension tube on the end of the scope itself as I understand your comment wouldn't achieve anything except make me have to throw the focal point back by bringing the primary closer to the secondary.....I understand about SA when shifting primary-secondary distance (as in extending the focus out from the barlow) but I've never thought about whether bringing the 2 mirrors closer actually effectively lengthens the native f/l..?
My inderstanding of optics most definitely isn't world-class
but I suppose the secondary is actually a "barlow" of sorts...
***stopped typing at this point to do some research (thanks for that!) and "yes" changing their relative distance does change the effective f/l.....I seem to remember however that nearer together is worse in some respects for SA and other distortions.....and perhaps the barlow isn't creating as much distortion when the image plane is nearer as opposed to further out from it.....
At any rate in the end I'm a practical person and if I don't see any obvious effects (and I don't) then I'm happy to go the way I have...
But thanks for your comments Mick - food for thought...it'd be interesting to experiment as see what sort of scales can be achieved the way you suggest, and I'll probably do so when I find the time..!
ps: I use an i7 quad-core laptop in the field but I really don't know/think if that sort of "performer" is necessary for the ASI120MM.....but not having any specific knowledge/advice I refrained from responding to your thread question.