Quote:
Originally Posted by Shiraz
well, maybe give Marc's idea a try - there will probably be a primary orientation that minimises the problem.
another question, does the best focus off axis correspond to the sharpest image in the field centre (ie is there much field curvature)? If you can get a sharp central region and round stars across the field at focus, maybe just accept that there is a little astigmatism at the field edge.
|
I'm not sure yet; I've just got this corrector and there was definitely tilt such that the best focus in the top left corner was *not* the best focus in the center, but it wasn't the best on the right either... but there could be curvature in there as well, I can't say just yet.
Quote:
Originally Posted by multiweb
Forgot that one. The slightest pull on the secondary glass (glue/silicon dabs, etc...) will create astigmatism so something to check first is the mounting. If you haven't fiddled with the secondary then rotate the primary in increment of let's say 10 degrees and check for the star shape intra/extra focus until you find a sweet spot. You usually don't have to recollimate so it's fairly easy to do. Not very scientific I know but it worked for me.
|
Thanks mate. So just to confirm, as per Ray's message above, a poorly mounted secondary (and presumably primary) will result in *on-axis* astigmatism? If that's correct then I can rule that out, because on-axis is fine.
Looks like at least a week's worth of clouds now... and I'm probably going to organise a Lite Crawler tomorrow (pending the sale of the Avalon), which will require the return of the Moonlite CRL and thus prevent me from doing any further testing for the foreseeable future.