Quote:
Originally Posted by glend
There is a big difference in focal length between those two scopes! Short focal length scopes are much more tolerant of guiding irregularities than long focal length ones. And those budget RCs are notorious for collimation and focuser shifting problems. Of course the light is being blocked in the RC by that large central obstruction. Start with fundamentals, is the RC properly collimated with your image train installed on it? Honestly, i would not worry about a flattener on that RC just yet, they are supposed to be a flat field design, at least flat enough that its not the main cause of your problem.
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Agree - collimation worth checking first before worrying about buying extras!
Collimating an RC is not a fun task - but there are some great guides out there (I found the "hall of mirrors" technique invaluable for getting very close to collimation
http://www.davidcortner.com/slowblog/20110815c.php).
Just a side query to Glen - I thought that RCs were coma free, but they still have field curvature (which will manifest as radial differences in optimal focus position). Certainly mine needed a field flattener!