Again Wrong Working Distance from Sensor - Messier M8 Lagoon Nebula

Seeing was very good on August 2nd with low Moon impact till UT24h. So ideal to make pictures of deepsky object. Based on my earlier learnings I changed the position of the flattener in order to increase the working distance between camera sensor and flattener. Unfortunately I did not measure the final distance. The result showed still aberration at the edged. Not so big as with the flattener just in front of the sensor; but still, it clearly can be seen.

So, I did some detailed measuring and found out the picture was taken with the flattener at a distance of 151mm. The pictures taken earlier had a working distance of 117.8mm; wich is below 5% of the ideal working distance according the technical data.

Focal plane of my camera is shown below :

The recommended distances from the M48x0.75 thread to the corrector via technical data is shown below. In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be.

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm for my TLAPO 80/480 f/6
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length as from 800 mm: 108 mm

An underrun or an overrun of the distance of up to 5% has no visible effect on the sharpness in the field of sensors with formats up to APS-C. With larger sensors, the tolerance is reduced to 1-2%.