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Dawes limit


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Fascinating article in the January 2014 Astronomy Now on the Dawes limit; resolving power of a scope to separate two close stars.

Dawes was born in 1799 and spent a lot of time resolving doubles.

He got a load of his mates to try and separate doubles with their own telescopes, and used the data to work out a formula of the resolving power of telescopes of given aperture. His formula is:

R = 116 / D

Where R is the Dawes limit, and D the aperture in mm [i'm not sure he would have used mm but hey]. The limit is measures in arcseconds.

Interestingly as optics have improved, with coatings and the like, it appears we can now separate closer objects with any given aperture.

The article concludes by asking readers to attempt to separate some suggested doubles and to submit their findings along with the info on their scope; repeating Dawes' experiment.

One of the more interesting articles in this months journal; i wasn't so interested in eight articles on how hydrogen formed in the big bang.


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I didn't renew my subscription to AN as I found less and less of the articles were about things I actually wanted to learn about. That sounds like a good one though TFS :)

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Thanks for sharing :) I probably would be interested in reading about quark/gluon soup ;)

Don't Heinz make that? sounds tasty :D

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If anyone has a copy of the list i'd be interested In having a go at the doubles with the frac but I don't subscribe to AN. Might have to find a copy.

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I've spent a lot of time viewing and imaging the brighter double stars. In practice, the Dawes limit is very difficult to achieve. When I get there or nearly there, it is usually with high magnification. I'm aiming to bag a few doubles with a DSLR and ST80 and Skymax 127.


I have to admit I don't buy any magazines these days thanks to the <expletives> that caused the recession and put my job in jeopardy but there's Astronomy Wise, which is free:




There is a certain writer who contributes but it's not an advert, as the writers don't get paid.


I prefer to read "how to" articles myself but do find the more theoretical articles interesting, too. It's a case of balance. I DO, however, think that most "beginner" astronomy  books have too much information about the Big Bang and not enough about what really matters, like trying to find Jupiter, etc, etc.

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The list:


epsilon Bootis, 2.85 arch seconds

zeta Bootis 0.7 arc seconds

eta Orionis 1.7 arc seconds

14 Orionis 0.7 arc seconds

xi Ursa Majoris 1.7 arc seconds

gamma Virginis (Porrima) 1.0 arc seconds


Encke Gap in Saturn's rings 0.7 arc seconds at opposition.


Submit findings to:


[email protected]



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