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Al,
A couple of guys over here have used old barlow tubes as extension tubes for the SA100...
We haven't found any advantage in going beyond 80mm distance from the grating to the CCD chip. Remember the deviation angle is about 3.2 degrees so unless you add a Grism ( recommended!) you'll have off axis issues.
I've just been given a Baader Afocal camera adaptor which clamps on the focuser and allows you position the camera behind an eyepiece. I'm going to put the SA in a 25mm extension tube and use the DSLR positioned at the 3 degrees from the optical axis see what happens...
Only problem in the UK is that spectroscopes still dont penetrate clouds!!
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