A Way to Accurately Focus your OAG

[michael8554]

If you have an OAG you may well have experienced difficulty in focusing the guide camera.
On my TS unit the periscope will not move smoothly once the lockscrew is loosened, so making small incremental changes is well-nigh impossible.
A non-rotating helical focuser would be ideal, but there's no room on my setup for even the slimmest of units.
So here's the best method I could come up with.

Once you have your imaging kit - camera, Focal Reducer, Coma Corrector, Filter wheel etc -  connected to the OAG with the correct spacing, roughly measure from the centre of the prism to the imaging camera sensor.
In the case of a Canon DSLR that's 44mm plus the distance from the lens mount to the prism, which you can measure more easily with the OAG detached.
Take the periscope out of the OAG and attach your guidecam to the periscope at roughly the same prism centre to sensor distance as the DSLR. Use spacers and the focal length adjustment on the periscope to get a setup with some range of adjustment left.
Re-attach the periscope and guidecam to the OAG and attach to the imaging camera.
Set the prism to be on the edge of the imaging camera sensor long side.
Attach the OAG to the telescope and focus the imaging camera on a star using a Bahtinov Mask.
Now adjust the guidecam focus as best you can for the lowest HFD reading on the PHD2 Star Profile window.
Then to fine-focus, you need a Digital Caliper as in the image - sorry for the focus, DSLR held with one hand.

Measure the current distance between the focus screw (screw 1) and the position screw (screw 2) .
Adjust the caliper to 0.5mm more, lock off and set the focus to this new reading.
Note HFD reading.
If it's worse, take 1mm off the caliper, and set the focus to this reading.
Note HFD.
Keep going with 0.5mm changes until you start to get worse HFD.
Go back to best setting and try finer changes until you hit the sweet spot.
I found best HFD gave an elongated star, and a slighly worse HFD gave a more rounded star.
The PHD2 developers say PHD2 will handle a poor star shape, but I opted for the rounder star.

Michael