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Old 07-10-2018, 02:26 PM
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mental4astro (Alexander)

mental4astro is offline
Join Date: Jun 2008
Location: sydney, australia
Posts: 4,334
Thanks Carlton and Brendon

RESOLUTION - What da???

I've been involved in astro for more than 35 years. Yet I've only recently come to understand one of its most oft mentioned but most poorly understood aspects - Resolution - and how there are two very different aspects to this with our scopes.

WARNING: Physics talk below!

When we read the specs table of our scope, frequently we will read a spec called "resolving power". In short it is the "smallest" size detail our scopes are supposed to be able to make out.

HOWEVER! This value is somewhat misleading! Our scope are actually capable of resolving much smaller details, close to 10X smaller than the quoted value!

There's two parts to this resolution thing. Let's start with the quoted figure and what it means. I promise to keep it simple.

Here's the thing with stars and our scopes. We may think that the stars we see through our scope are just pinpoints of light. They are actually a tiny, tiny disk (airy disk) with a series of faint rings (diffraction rings) that get fainter out from centre.

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You can actually see this Airy Disk and at least the first diffraction ring when you use high power on a very bright star and under good seeing conditions. So none of this is techno-mambo-jumbo! It's real and easy to see for yourself.*

So, being a tiny disk, the quoted "resolution" comes to be the smallest distance the a given aperture can provide a distinct separation - NOT a total separation.

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If you would like to follow this up with the maths that's involved with this, you can follow this excellent link that deals entirely with telescope optics:

Telescope Resolution

NOW, this resolution value is ONLY for two Airy disks.

When it comes to expended objects such as the Moon and planets, things are very different!

Because you are not dealing with point sources of light, the entire situation is different. The apparent diffraction patterns are totally disrupted. This in turn means that the actual resolving capacity of the optics is now greater than that provided by two airy disks.

The evidence for this is seen in those amazing photos of Saturn's Encke Division. It's angular size is 0.05 arcsec. This is more than 1/10th of the quoted resolving power of an 8" scope! Yet there are photographs of Saturn showing the Encke Division taken by 6" telescopes. I have also seen the Encke Division in a 7" and 8" scopes. I have also been able to resolve markings on the Moon that are smaller than 250m, and using simple trigonometry maths, these features come in at 0.15 arcsec.

So, this is why I am putting up these two particular challenges. These will show you finer detail than the quoted "resolving power" of your telescope.


Knowing this now, how does this affect you and your telescope?


The "quoted" resolution of your scope you now need to think of only in terms of two stars. This resolution value will only affect the smallest separation that can be resolved say for double stars. When it comes to the Moon and planets, the resolving capability of your scope will be better than 1/10th of this

If you do not know the resolving power of your scope, the following telescope resolution calculator will give you this. All you need to do is input your aperture:

Telescope resolution calculator

This link has two values, the Dawes Limit and the Rayleigh Limit. The Dawes Limit is the smaller of the two, and you will be able to resolve finer details than 1/10th of this with the Moon and planets


* One thing needs to be kept in the back of your mind when it comes to the Airy Disk. This disk is not the actual disk of any single star. All stars are actually impossible to resolve into there actual disk. Instead this a phenomenon to do with point sources of light meaning that ALL point sources of light will show an Airy Disk. So it does not matter if you look at Alpha Centauri (4.3 light years away) or the quasar 3C 273 in Virgo (which is 2.4 Giga light years from us, and visible in amateur scopes as it shines at magnitude 13!), both will show an Airy Disk.

Last edited by mental4astro; 10-10-2018 at 07:15 AM. Reason: Typo
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