Diffraction Limited Optics - Is this a good thing?

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What does it mean?

You may wonder why your telescope has 'Diffraction Limited Optics'. 
It sounds as though that's a bad thing, doesn't it?
Why should you accept 'limits'?

Well, the truth is that all optical systems have a theoretical limit to the detail thay can show.  A small telescope can only possibly show so much detail - A larger one will show more detail (Regardless of the magnification used).  If your telescope has inferior optics then it will show less detail than one with a good figure.
In the diagram above we can see that fine detail (in this case, a double star) is not shown in the smaller telescope sizes because they cannot focus the light into a small enough point.  You need larger telescopes to see finer detail.


Left: 'Spherical aberration limit' Newtonian view.................Right: 'Parabolic Diffraction Limited' Newtonian view.
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Left: 'Spherical aberration limit' Newtonian view.................Right: 'Parabolic Diffraction Limited' Newtonian view.
A Diffraction Limited Optical Mirror MUST be Parabolic.
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A Diffraction Limited Optical Mirror MUST be Parabolic.

Diffraction Limited

So, the limit for seeing fine detail in an image is dependent on the accuracy of the telescope mirror or lens.  A lens that is perfect, optically, is said to be 'Diffraction Limited'.  Because the only thing that is limiting it is optical physics.  That is, it's as good as it can be, given the size.

So diffraction limited optics are the best. 

Always look for diffraction limited optics when buying your telescope.  If it doesn't say in the blurb or specification page - ASK!
In a Newtonian Reflector, for example, the mirror MUST be parabolic to achieve diffraction limited images.  A spherical figured mirror is always out of focus at some point!
(See diagram:
Parabolic mirror focuses all the light at A. 
The Spherical mirror focuses some of the light between A and B, but not all of it at any point!).
Diffraction Limited 180mm Maksutov-Cassegrain.
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Diffraction Limited 180mm Maksutov-Cassegrain.

The Dawes Limit.

This is the formula for finding the resolution limit of your telescope (If it has diffraction limited optics).

This is also known as The Rayleigh Limit.

R = 4.56/D     D in inches, R in arcseconds
R = 11.6/D     D in centimeters, R in arcseconds
where   : 
D is the diameter of the main lens (aperture)
R is the resolving power of the instrument
You can use this information to work out if you can see the moons of Jupiter as discs or not, for example - A very useful formula.

This turned out to be a very good seeing night!
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This turned out to be a very good seeing night!

Don't accept Less!

Modern manufacturing techniques mean that it is easier than ever for manufacturers to produce very good optics.  A company like SkyWatcher, for example, produce excellent value, diffraction limited telescopes.  They include this information on all their specifications.  If you're looking at a telescope and it doesn't say' Diffraction Limited', you can bet that they aren't and you'll have inferior views.

The biggest obsticle to your viewing clarity is the atmosphere.  Whilst your telescope may be diffraction limited, it will also, always, be atmospheric seeing limited too.  It's not about clouds, it's about how stable the atmosphere is.  There are bad nights, good nights and very good nights.  It's all part of the game!


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Barry Cooper Cert. H.E. (Open)

IF IN DOUBT PLEASE ASK!
IF I CAN BE OF ANY HELP WITH YOUR SELECTION I AM MOST WILLING TO ANSWER QUESTIONS ON TELESCOPES BY E-BAY MESSAGE.
I will answer as soon as I can and it's no bother – Honestly!

I have nothing to do with SkyWatcher, neither do I sell telescopes professionally (I do sell on eBay from time to time as I upgrade). 
The information in this guide is my own opinion and can be taken as honest advice from someone with over forty years experience in astronomical observation.

More information? Google "supercooper telescope help".


SuperCooper_2008
Clear skies and good seeing... I hope this guide has been useful. :o)
All text and images © Barry Cooper 2008-16 unless otherwise credited.
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