Field curvature due to poor collination?

[x6gas]

I'm new to this imaging lark and have only just bought a 'scope after years away from astronomy. So obviously I've only had a few sessions because of all this cloud, but I have a question.

The first time I tried imaging (prime focus, Nikon D40 DSLR) I had no idea what I was doing, the scope wasn't terribly well aligned, but I was quite pleased with the results. The second time I did some imaging, I got noticable field curvature. I understand that field curvature arises because the image is focussed at the centre of the sensor but not at the edges due to the longer light path to get to the edge of the sensor.

The differences (that I am aware of) in the second session:

1) the 'scope was better aligned

2) I used a Nikon D40x rather than a D40

3) I had mucked about with the collimation of the 'scope and unbeknownst to me the secondary was not quite perfectly centered on the spiders (about 1.5mm out though I think this was the case for the first session too) and was slightly too far up the OTA

The sensor on the D40x is allegedly slightly higher but slightly less wide than on the D40 but I can't see how this would make any difference. Obviously pixels are smaller on the D40x. The poor alignment first time out meant some star trailing but there was no discernable field curvature.

So that leaves me with the collimation. I've search on here and online and can't find any reference to poor collimation causing field curvature. Anyone know if it can or have any other theories?

I've correctly collimated my 'scope now (chalk up another one to Astro_Baby ) and will check next time the sky is clear, but I'd welcome your thoughts to save me time when the clouds finally disappear.

Hope I don't have to shell out on a field flattener (for all sorts of reasons...)

[x6gas]

Managed to get a few hours imaging in last night (hurrah for clear skies!)

Just processing now but no field curature apparent so I am assuming it was something to do with the collimation... that or effect or effects unknown... but I think Occam's razor applies!