View Single Post
Old 09-11-2018, 09:59 AM
mental4astro's Avatar
mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,235
Peter, the Dawes' Limit ONLY works between two point sources of light, like a star, which are diffraction patterns, an Airy Disk, and in particular the Dawes' Limit is between two Airy Disks and how diffraction patterns interact.

For an extended object, this all changes, and the resolving power of say a knife edge line is less than 1/10th that of the Dawes' Limit.

This is why the Cassini Division was discovered with a 2.5" scope in 1695!, which is impossible if the Dawes' Limit applied to extended objects. And the Encke Division is visible in a 7" scope - I've seen it in a 7" Mak. The Encke Division is 0.05" - WAY smaller than the Dawes' Limit for a 7" scope!!!

If you still have any doubts, have a look at really high quality images of Saturn using 7" or 8" scopes, and the Encke Division is clearly visible in these. If the Dawes' Limit applied to extended objects, the Encke Division would not be visible.

And if you still have any doubts, if you have seen the Cassini Division in an 80mm scope, YOUR SCOPE JUST EXCEEDED ITS DAWES' RESOLUTION LIMIT!!! and you've whitnessed this yourself! The Cassini Division is not a challenge for the modest Skywatcher ED80. Yet somehow no one questions this!

This is a common misnomer about the "resolution limit" of scopes. The information regarding it is both only half explained, and then poorly understood by most people.

The actual application of how the Dawes' Limit and Raleigh Limit are acquired are like I said, for a pair of diffraction patterns. When it comes to extended objects, this diffraction patter is totally disrupted, and the TRUE resolution limit of a scope is significantly smaller than the Dawes' Limit.

I've only come to find out this in the last few weeks myself! I had a suspicion something wasn't quite "kosher" when I started seeing very fine details on the Moon, and on doing some trig calculations I was staggered to see that these structures were much finer than the quoted resolution limit for 7" and 8" scopes. But it is no mistake as I've come to find out

I explain it a bit more in this post: and in the rest of that thread it is talked about some more.

Peter, I suggest you reconsider your thinking about not following through with this project


Last edited by mental4astro; 09-11-2018 at 10:23 AM.
Reply With Quote