[codemonkey (Lee)]

I tinkered around for ages trying to get the spacing between my sensor and flattener correct. I finally got it to a point that I was happy with (see the "good" image). Measured curvature over the entire field was about 11.7% as measured over a number of subs, with CCD Inspector (trial has since run out).

Now, without changing the spacing between the flattener and sensor, my image quality is terrible (see the "bad" image), and I'm at a loss as to why.

The observant may notice the following differences:

Good example: 5s single sub, L filter.
Bad example: 300s sub, OIII filter.

Note that I've also eyeballed 5s subs with the L filter and the bad example is representative of current state: there's very obvious elongation across almost the entire field. I just didn't save any of those test images so I can't give exact numbers.

The only things that I can think of that have changed are: I've increased the spacing between the OTA and the flattener by placing my OAG in between the flattener and the focuser tube. Flattener to sensor distance has remained the same.

I've rotated the flattener + camera etc chain relative to the OTA. The flattener to sensor is still in the same orientation.

Anyone have any thoughts/suggestions on this? I'm stumped.