Hi Bobby,
Quote:
Originally Posted by Bobbyoutback
Thanks for your input Steve , splitting close doubles could show the difference a bit clearer ? maybe 
Cheers
Bobby. |
In terms of splitting pairs (this also applies to all other visual applications), the difference between 150mm -v-152mm is about 2/3rds of 5/8ths of ...
The Dawes limit (approximation) difference between say 150mm and 165mm is 0.77 arc seconds -v- 0.70 arc seconds. In relatively small apertures, a small increase gives a pretty small gain. 150 -v- 152 is a minuscule difference.
Dawes limit approximation formula: R=11.6/D where D is the diameter of the aperture in centimetres.
So for example. with my 46cm f/4.9 Newtonian the Dawes limit is 0.25 arc-seconds. With the 63.5cm f/5 Newtonian, the Dawes limit is 0.18 arc seconds. Obviously enough, the seeing (virtually) never permits large apertures to approach their Dawes limit. It's a very, very rare night when the seeing is running at about 0.5 arc seconds or marginally better than that. 0.2 arc-seconds is probably something you might see in the Atacama Desert, maybe once every year or few.
I think I've seen seeing better (marginally) than 0.5 arc seconds three times in my observing career of nearly 50 years in NSW. It is a bit better here where I live now than Sydney (where I used to live) that normally has mediocre seeing and rarely good or very good. I think generally, the seeing in Broken Hill is going to be better than either where I live, or Sydney, but still only rarely approach 0.5 arc-seconds.
Best,
L.