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Hi John,
I'm no guru, but speak from experience having gone down this road with correctors and SX ccds.
Using any reducer/ corrector that needs a specific distance can be a little tedious at times to say the least!
You're saying that there is still distortion in the corners, if it is an even amount to each corner, then it will definitely be your spacing distance.
Just measure from your CCD window to the reducer at the specs (without the added 4.1mm) and see how things look.
If it is better in the corners than your previous spacing with the 4.1mm added, obviously you'd be working around this. Probably a little less spacing will help. I believe that there is generally a +5/-5 tolerance to most reducer/ correctors.
If it is worse, then unfortunately, you'll have to keep persevering with adding a little more at a time. I found just adding in made up washers around 2mm to the train helped me get to an exact spacing.
If the degree of distortion to the corners is not even, then I'd be going back to the basics.
#1.Check your collimation first off.
#2.Check your imaging train for sag, poor connections.
If the above things are good, then I'd be checking the following.
#3. If you're using an SXVF/R- m25c or m26c, (as stated aps size) these being the newer camera enclosures, have the adjustable face plates.. I'd be checking that the CCD chip plane is orthogonal with the incoming light cone. (if you're using a DSLR, then don't worry about this..)
Hope this helps maybe a little. Any way, it's good to have someone to use as a sounding board after battling an issue on your own.
Rich
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