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Off axis apertures - when, why, how?

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I keep seeing talk of using an off axis aperture on reflector telescopes to improve the view in some cases (such as solar viewing).

Would someone kindly explain:

1) When would such a thing be useful?

2) Why is it useful (what are the effect)?

3) How does it work? Why exactly does making the aperture smaller improve the image?


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hi Tim

the theory is that (especially for larger apertures (maybe 12" and above) the seeing can be worse than for smaller apertures. so:

1) useful for - bright objects for which aperture is not needed, planetary detail where more contrast helps, reducing the size of the e.g. solar filter needed to observe.

2) useful as it a) creates an unobstructed APO scope (like a refractor) :) effectively increases the focal ratio (FR = focal length (which is unaltered) divided by aperture) so in theory, more contrast and detail

3) it works by reducing the size of the window of sky you are observing. I am no expert (as will probably be shown by the follow ups to my answer) but the atmosphere consists of 'cells' or pockets of air. I think these are approximately 20cm across Astronomical seeing - Wikipedia, the free encyclopedia so therefore in theory a scope 10-20cm across will give the best views on average as the cells move around in the air. sometimes the seeing is more steady and then the higher resolution of the larger scope will give better detail, especially at higher magnifications but on average it will not. bigger aperture always wins on faint stuff as generally when the air is transparent it's less steady and this is the time for lower power faint stuff and more aperture.

BUT do be aware that less aperture = less resolution so there's more to think about than seeing conditions etc.

I am going to make an off axis cover for my 16" f4 which will equate to a 7" f9 'APO'. will be interesting to see how that performs.

Hope this is clear but probably not!

Edited by Moonshane
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