Flat field with Megerz 90 - anyone achieved it?

[rogerg]

G'day all,

I'd like to hear from anyone who's achieved a flat field with their Megrez 90 APO.

Without flattener the field is pretty good, but stars elongated somewhat on the edges. With WO FFIII the field is still pretty good, but instead of elongated towards the centre the stars are slightly (less so) elongated in a circular pattern around the centre.

I'll avoid going in to details of the alternative such as WO FFII, WO FFIV, etc none of which seem to easily solve the problem.

I'm using a Canon DSLR (350D).

If anyone has the silver bullet (be it spacers, a different focal reducer, or whatever), I would very much like to hear from you.

Thanks,
Roger.

[multiweb]

Elongated stars outwards you're under-correcting. Elongated sideways like what field rotations would do you're over correcting. I have seen similar patterns when spacing my MPCC in different positions. You need to play between the two to find the sweet spot. Might be a compromise.

[rogerg]

Any suggestion on how exactly I can space a Canon 350D from the focal reducer? There's only the t-ring inbetween, so a bit tricky (unlike a focuser draw tube where you can slide a camera in/out).

Thanks,
Roger.

[MrB]

I bought the latest FFIV, WO's third attempt at the Flat4, but there's one small problem.... there's not enough in-focus!
Need another 15 to 20mm in-focus, which is ofcourse impossible.

It was marketed on their website as working on the Megrez90, but any reference to the Meg90 has since been removed.
I can't believe someone at WO never actually tested the FFIV on a Meg90 :shrug:

I'm thinking of trying Stellarvue's SFF7-2 or maybe Astro-Tech's $150 one, both are just flatteners, not reducers.
Jerry Lodriguss has reviewed them here: http://www.astropix.com/HTML/I_ASTROP/EQ_TESTS/SV70ED.HTM