Backfocus: is this too long or too short?

[vineyard]

Hello,

Here is a quick w-i-p.  Only 34 mins of data.  If I pixel peep in all 4 corners, the stars have oblique tails all pointing in from each corner towards the centre.  Does that mean that the sensor is a shade too close?  (I set it at 56mm which is what the reducer-flattener spec says, but on further thought, there are a couple of filters in the train, so I should increase that by 2mm right?)

Also how much tilt is too much tilt?😂  Actually a genuine Q (analysis image from ASTAP also attached).

Thank you!

Vin

[newbie alert]

I'd say you was very close.. So I'd buy a set of baader shims, they come in .3, .5 and 1mm