Focussing issues

[beetlejuice]

Here's a prime focus shot taken recently using my C9.25 and Canon 400D. It's a single unstacked shot. While the craters at the terminator are pretty sharp, the focus falls off across the disk. I presume this is due to the slight field curvature of a SCT. I suppose the only way of getting it sharp across the entire disk is change to a refractor (!) or mosaic it. I'm pretty new to astrophotography lark so any suggestions greatly received!

[Merlin66]

No, the image looks very nice and is normal.

The dark shadows and the craters show up in good contrast because they are near the terminator - the point between day and night on the moon. The detail towards the edge is in bright sunshine and at the extreme edge the angle of the sunlight is so hgh very few shadows are formed, that makes it looked washed out.

Concentrate on the detail along the terminator as it moves during the month.

[beetlejuice]

Thanks Merlin: it's just the focus looks a bit flabby along the limb. It took me a lot of shots to get the contrast right as the glare from the limb caused the terminator to be underexposed in a lot of cases (thank goodness for digital). I suppose the slight fuzziness of the limb could be due to air turbulence.