Sweet Spot

[FLO]

Here's something I copied and pasted into a Word file some time ago (I would like to credit the author but don't know who wrote it):

Let's just briefly talk about focal ratio because this is HUGELY important. Many beginners question whether or not they should have a 10" F-5, 8" F-6 or a 6" F-8 for example, thinking that the longer focal ratio will yield the tightest images. That is absolutely not true. Get the largest aperture you can, however, I don't suggest anything more than 14.5" for most applications but don't be blind about focal ratios.

What I'm gonna share with you is where the failure begins and is HUGELY underestimated. I have shared a few numbers From Nils Olof's collimation site. Imagine this in your mind. All the information from the primary will be reflected on to the secondary mirror. When you look down the focuser, you will see the mirror that looks like a circle from your perspective. In a fast optical system like an F-4, there will be a very tiny field of perfect optical definition at the center of that mirror. No matter what the aperture is, at F-4 it's only 1.4mm which is about half the width of a Q-tip stick. That means the stars should look like tiny points and anything beyond that, coma or distortion for a simpler word starts to kick in. If the optical system is F-6, the size of that little field of perfect optical definition increases to 4.8mm. That's less than the width of a typical Bic writing pen.

Can anyone shed some light on this? In particular, how is he arriving at the size of the 'field of perfect optical definition'?

Thanks

[GazOC]

I don't know the maths but (I'd guess) he's on about the size of the area that is "defraction limited" (warning: can of worms ), its known as the Rayleigh criteria and it has to do with the amount of coma (in waves) in an image.

The size of the 'sweet spot' increases exponentially with the f#.

EDIT: found this

http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/raylei.html

[FLO]

Thanks Gaz,

Interesting, but isn't that an indicator of maximum resolution, rather than the size of the sweet-spot?

There must be a book that covers this subject somewhere...

[GazOC]

It's the area on the mirror where the resolution is defraction limited. I looked for a book on basic optical theory a few years ago and couldn't find one, let me know if you have better luck - it's something I've been meaning to swot up on for a while.