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Thanks for all the comments. I never mind if anyone wants to play with my images Scott. I may (always) learn something. I dont want to turn this thread into cutting telescope tubes 101, but here are a few pictures I hope they are self explanatory.
You are correct PhotonCollector I decided to use the Nebula filter as the Moon was out. The Hutech focal reducer flattens the field beautifully but only has a reduction of 0.85.
The Astro Physics focal reducer also flattens the field beautifully but only at a reduction of 0.8. If you use the AP FR at about 0.7 or 0.6 it introduces field curvature and this is where the Hutech comes in. Place the Hutech FR/FF behind the AP FR the field is flat again. Careful adjustment of relative distances should end up giving me a focal reduction of about 0.6 (or better) and a flat field. I could go into the theory but its better to try it for real.
This turns the 100ED into a 540mm at F5.4, or thereabouts. I hope to get all the distances correct by experiment. Even if I don't get the result I am after at least I still have a working 100ED that performs like an 100mm APO.
My reason for doing all this is the 5DH's huge sensor shows up any optical shortcomings such as field curvature or vignetting and rather than spend a lot of time massaging images to overcome this, I want to stop it at the source as I intend to collect a lot of images.
I hope this makes sense.
Bert
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