field flattener for SCT?

i recently tried a a denk II with my celestron nexstar 8 GPS (SCT) using the star sweeper and 19 mm antares w70 eyepieces and noticed that i could not get stars at the periphery and stars in the center in focus at the same time.

after this i checked out the view using a 24 mm panoptic (monocular) and indeed noticed i could not focus edges and center at the same time (especially noticable viewing the moon or big clusters)

i'm ruined

is anyone else troubled by this?
i was wondering if one of the f/6.3 field corrector/flatteners would help.

could it be a collimation issue? it seems ok to me. also, i would think collimation would be more of a difference from one side of the field to another, rather than center to edge.

any suggestions would be appreciated.

thanks

bill

aww come on
someone leave the 2" thread for a minute and take a look here

any comments on non-flat SCT focal plane and wide field views

wups
just realized i never posted my follow up
field flattener did not correct the problem i was having.
got a set of bob's knobs and made an artificial star...
voila, moon is in focus across the entire field
in binoviewer with starsweeper and panoptic straight thru

i was surprised that it was a collimation issue

sorry for the lag in follow up

thanks for all your replies

bill