terry56 you need to get it around 55 mm but that is because if you get it much more than 55 mm, you may not be able to achieve focus.
Yes, the spacing between the Petzval's flange and the camera flange is flexible because the focuser's draw tube is in between the rear most element of the telescope and the sensor.
It is not that the spacing is not important. The focuser is the one that determines correct spacing. When you are in focus, you have the correct "backfocus spacing".
With the RedCat51, you can use up to 59.7mm of spacing between its flange and the sensor ("draw tube" in the shortest position). I forget what the shortest backfocus is (I have not touched my RedCat in a year now), but it is that 60mm minus the maximum draw tube travel.
As such, you also need to focus a Petzval very carefully since any defocus will also bring up the coma at the edges of the frame. Just 1ºC temperature change could make an FSQ-85 (another Petzval) require refocusing.
However, if you ever place a reducer (not that William Optics makes one for the Cat51), you will then need to maintain accurate distance between that piece of glass and the sensor. For example, the FSQ-85 has a strict backfocus when you attach a flattener, a reducer, or tele-extender (Takahashi chose 56.2mm as the design distance; a number etched in my memory).
If there is a problem with off-centred halos, it may be that is more to do with tilt in the image plane?
That is also correct.
The RedCat51 is well known to have poor QA regarding tilt (mine had a tilt that can be seen even on a APS-C sized sensor). So much so that they quietly introduced a second version, about a year after the initial release, that included a tilt adjuster. You could (I don't know if you still can) buy the tilt adjuster independently.
Chen