I have been doing DSLR imaging so far with a few of my scopes: C8, SW ProED80 w/ FF/FR, 6" newt with coma corrector. With these FF/FR/CCs they all seem to be designed to work directly with a DSLR with a T-ring, so I have not had to do any thought about proper distance to the image plane etc.
Now I picked up a QHY163C which has a 17.5mm back focus (distance between image sensor and the front of t-2 (M42) connection. I want some advice on making sure I duplicate the correct back focus from the FF/FR/CCs and I wanted to know "How precise must this really be"?
DSLR has a back focus of: 44mm
A typical T-ring adds: 10mm (my guess with ruler)
So we want to replicate 54mm distance right?
Let's use the Pro ED80 FF/FR as a first example.
- First I'll subtract the QHY back focus from the target of 54mm (giving me a target of 36.5mm with extensions).
- To connect to this I need a M48 to M42 adapter (and the one I have now adds ~6mm)
- I can get a kit on Amazon and combine a 20mm + 10mm
- 20 + 10 + 6 = 36mm. (not quite 36.5)
How precise must one try to be?
Also, another question: By using a train of M42 size tubes, should I be concerned with vignetting on a 4/3" sensor diagonal?