Quote:
Originally Posted by fauxpas
I saw a tutorial stating (generally) the maximum magnification of a scope was twice the aperture in mm... so a 130mm aperture would be a max of 260x.
Then I also heard that with amateur scopes, because of the atmosphere, there's a maximum practical magnification of around 250x.
Is this all hogwash?
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Tony
A very rough rule of thumb for maximum "theoretical" magnification is 50x per inch of aperture, which is not far of the 2x per mm of aperture. So a 130mm scope which is 5" gives 250x. The critical word is
theoretical!
It assume very good to perfect optics, perfect alignment and collimation and above all, perfect seeing (a perfectly still atmosphere). A better bet with smaller scopes is more like 10x to 15x per inch (50x to 75x in a 5"/130mm scope) On some occasions it can be pushed beyond this, but I never managed satisfactory results, I never managed more than 90x in my 130mm scope, or at least the results were very ordinary!
With your second point, yes in most every use, 250x is pushing it even with a big scope. I do most of my observing in my 20" at 150x or 195x and find that does the job 9 times out of 10. Occasionally I slip in a 10mm EP for 254x but usually I am disappointed.
Having said that, when conditions are good, nothing beat cranking up the power on Eta Carina, Jupiter or the Trapezium!!
Malcolm