Flattener correcting the view field - Importance of working distance.

Flattener at a distance of 10mm from sensor showing clearly aberration at the edges.

I was not aware about the importance of distance from the flattener to the sensor. Below pictures show the difference. The picture with Messier M56 was taken with the flattener fixed at a distance of 10mm. This pictures shows clearly aberration at all edges.  The aberration is almost gone on the picture with M39. For this picture the flattener was located at a distance of 100mm.
According to the technical data the ideal working distance to the sensor is depended of the focal length of the refractor :

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ 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 ab 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%.

Flattener 100mm - Picture is sharp at the edges