This is what is possible with an 80ED with the right modifications.
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http://users.bigpond.net.au/avandonk/Combine1ps01.jpg
The small pic below is a single unprocessed frame. Notice very little vignetting. This is due to a crayford focuser with a clear aperture of 50mm. The original had a baffle near the front of the focus tube only 42mm in diameter which was causing vignetting.
The almost perfect flatness and stars at the edges and corners as good as stars at the centre is due to a Hutech focal reducer/field flattener (
7887) with the thin spacer ring installed.
Details. Canon 5DH, 7X5min ISO 1600, 80ED at F 6.4, LPR filter,Hutech FR/FF
7887, stacked with Registar, adjusted for curves and levels in Photoshop. No gradient adjustment!
This little scope is now performing like a very expensive APO. This should also work for a 100ED! Next project?
Bert