Mak Focussing

[Alien 13]

I realy like my baby Mak with a DSLR connected but have been wondering about the best focus position and if the fl changes with mirror position?

In normal use I screw a T adapter directly onto the visual back but I can also focus just as well with an additional 2 inch extension added, these two focus positions will mean that the mirror is further up the tube when used with the extension compared to the other setup. 

The question is does this change in positioning effect the systems focal length and/or the imaging circle?

Thanks in advance.

Alan

[Peter Drew]

Yes it does. The more extension created behind the primary mirror the higher the effective focal length when refocused. How much depends on several factors.