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I am not an expert in this field but I have looked at the original shot and there is more than one problem.
Firstly the elongation of the stars radially out from the centre will be due to trying to focus a spherical image onto a plane surface. This is more noticable on large area viewing planes. This can be corrected by what we refer to as a FR or FF. However the degree of flattening and linearity depends on matching the FR/FF to the rest of the optics. Before this can be properly achieved you will need to get the rest of the optics properly collimated..
If you are using an OAG (as stated) and a moonlight focuser (as stated) the camera will be well behind the rear plane of the main mirror. To reach primary focus the main mirror will be well forward in its housing. This will put the corrector plate and secondary mirror at the limit of its best position. This means that the standard FR/FF may not be optimum for the set up.
Now in regard to focal plane tilt. Trevor bought a moonlight focuser that was in fact faulty (it was replaced without bother) but the spacer block was wedge shaped and the focuser distorted to try to get around this. The amount of wedge shape was only a few thou but was easily measureable with a micrometer. In the current case the OAG + FR/FF + Focuser will be quite a large overhang and could certainly aggravate any tendency to sag and tilt the focal plane.
So the procedure should be to get the collimation correct with all the attachments. Ensure that the focal plane at the chip is orthogonal to the optical path and then try to correct the field flattening.
Barry
PS all this may take some time to get right and will need some good viewing nights (often a rareity in certain parts of the country)
BG
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