Light Cone Dimensions

[anth]

Hello all

I was speaking to the guy from FLO today (very helpful) and he was saying that because I have a cpc925:icon_scratch: with F/10 i dont need to buy expensive eyepieces to get a good result asthe light cone thatthe scope recieves is not as acute as a fast scope. Now I can picture this but am a little unsure as how this would make it easier for a low budget eyepiece to givea good view. Any mathematical explanations would be appreciated. Thanks

[John]

Can't give a mathematical explanation but it is the steep light cone in fast scopes that makes it challenging (and expensive) to design and manufacture wide angle eyepieces that sharp sharp stars across the whole field of view.

A practical example was a decent quality Japanese ultra-wide angle eyepiece that I owned a couple of years ago. It was a 2" eyepiece with a 30mm focal length and an 84 degree field of view. It worked very well in my F/10 refractor but when I tried it in an F/5 refractor it was quite poor in the outer 50% of the field of view with astigmatic stars which were pretty distracting. That eyepiece cost around £80 used I seem to recall.

Today I have a Tele Vue Nagler 31mm which cost around 4x as much but the stars are sharp right across it's 82 degree field of view, even in my F/4.8 newtonian.

Perhaps someone with a better grasp of optical theory can explain the maths for you but I can say that what you have been told does hold true in practice.

[sixela]

Not a mathematical explanation but a geometrical one. On fast scopes, beyond the focal plane the light cones diverge again and widen fast. That means that they enter the eyepiece lenses through larger circles.

The trouble with simple eyepiece designs is this: take an off-axis object, take the central ray of its light bundle and see where it hits the lens.

Suppose you turn the eyepiece so that that point is on top of the middle of the lens. Now take two sections of the lens through that point, a vertical one in a plane perpendicular to your line of sight (as if viewing a sheet of paper) and one horizontal one perpendicular to the first one (i.e. towards you). The radius of curvature of that lens section isn't the same for the two sections, and that generates astigmatism (a vertical slice of a light bundle, i.e. towards the centre of the eyepiece, will be bent differently from a horizontal one, i.e. one square to it.)

The trouble with wide circles that mark where the light cone hits the lenses is that the difference in the curvatures described above will make a larger impact, because the difference between the location of the real lens edge and the ideal one that would not generate astigmatism for this bundle becomes larger the wider the bundle is.

As it's a geometrical argument, it's hard to describe without an illustration.

[anth]

I understand a little more now. I agrre a diagram would be helpful, or even a java applet! Many thanks guys.