Focus/eyepiece width

[Solanum]

I've noticed an odd effect with a couple of my eyepieces and hoped someone could tell me what is going on. Viewing Saturn with a 1.25" 25 mm GSO Plossl and 2" GSO 2x barlow, the image is pretty sharp (apart from the boiling air coming off the roof I'm viewing over that is), switch to 15mm and the image is sharp at the edges of the field of view (after focusing), but somewhat blurry in the middle, same with 9mm, switch to 6mm and image is fine, no blur in the centre.

I thought that I'd got fingerprints on the lenses, but they're clean. Refocusing doesn't help, can't get it any clearer. Is this something to do with the Barlow, or something else? Haven't checked whether the same occurs without the Barlow and the scope is back at Andrews having the glue cleaned off the primary.

As a side note I have to say that Lee Andrews was very good about it and isn't even charging me freight back to him (not that he should, but many warranties these days don't include freight back). So thumbs up there from me.

[iceman (Mike)]

hmm that sounds interesting, I'd try using the 6mm and 9mm without the barlow and see if you get the same effect.

Usually if the image is sharp in the middle and blurry at the edges, it's a result of either (a) field curvature or (b) coma or (c) bad correction at the edges for use in a fast focal ratio scope.

but i've you've tried refocusing I can't think of what it would be except something on the lens?