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50x Apperature Rule

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Posted by Don McCrady on April 29, 2002 17:39:54 UTC

That's the rule of thumb I've used as well to evaluate a telescope's theoretical performance, but sometimes this doesn't agree with what the manufacturer or seller states.

For instance, I'm interested in a Discovery 12.5" PDHQ scope, f/5, focal length 1587mm. The 50x/" rule of thumb would give a highest useful magnification of 625x. However, if you go to and look up the details on that scope, you'll see that it indicates the highest useful mangification of only 317x, about half the 50x/" rule.

My not-too-informed conjecture is that with a focal length of only 1587, you'd need a very small focal-length eyepiece (about 2.5mm) to get that 625x magnfication. The 317x figure could be achieved with a 5mm eyepiece.

I guess a good Barlow is pretty much a necessity for fast scopes like this.



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