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Old 31-01-2010, 05:09 PM
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Satchmo
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Join Date: Dec 2005
Location: Sydney
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Quote:
Originally Posted by ngcles View Post
If you'd like to become the most un-popular person at your next star-party, take a wander around the field at night and ask to look through various people's 'scopes. SNIP

Then tell them exactly what is wrong with their optics. Repeat with the next person ... and the next etc etc.

Pretty soon you'll be the most un-popular person at the party ...
Les, Yes this does look like a sure fire way to become unpopular..Nothing worse than somebody trying to tell you your mirror which you know to be excellent has problems

I would advise anybody new on this path to do lots of testing and understand the way telescopes behave out in the 'wild' , before telling anyone that their telescope is has genuine problems . Collimation , internal seeing , atmosphere and thermal inertia can all make a telecope look defective in terms of surface smoothness, and spherical aberration and astigmatism. I know some large dobsonians can show significant over and under correction while cooling depending on their design and some nights will never reach their equilibrium state. I've seen the same optics perform superbly on other nights.

Ronchi gratings at focus in the presence of inclement seeing make it particularly hard to judge any correction defects better than 1/2 to 1/4 wave for faster than F5 scopes, though will certainly weed out lemons. Even then the bowning of the lines can be very subtle, and very difficult to determine the differnce between a mediochre one and a superb one. Seeing and tube currents usually limits the ditance from focus to four to six bands bands. In the workshop with good seeing I find a single band is about right for seeing correction errors that seperate the average from the excellent.

There are ways to increase sensitivity like barlow lenses or higher frequency grating , but these can also amplify the swimming effect of seeing conditions . So tread very carefull before declaring a mirror either a gem or a lemon with a casual ronchi test under the real sky , it may take prolonged observation and further star testing on a few nights before making any meaningful declarations particularly with faster mirrors . At a casual glance most telescopes will show rudimentarily straight bands ...
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