Why do fast scopes need high quality eyepieces?

[Andymarrison]

Could any of you guys explain to a newbie like me why a fast scope demands a higher quality eyepiece than a scope with a higher focal ratio, which is something I've read a number of times? What is it about a fast scope that puts extra demands on the optics of the eyepiece?

[themos]

Fast scopes bend light a lot so the eyepiece has to work harder to bend it again, in other words it must be of superior quality to do this correctly.

[NGC 1502]

I'm not an optics expert, but basically a 'fast' scope, that is one with a low focal ratio like F4 or F5 produces a shorter and steeper light cone.

This means that the light passes through the eyepiece at a steeper angle than with a slow scope, and that steep angle is more likely to smear out the image, especially towards the edge of the field of view.

Eyepiece manufacturers can design multi - element eyepieces to help correct this. That's why such eyepieces cost more, and are larger and heavier.

Regards, Ed.

[newman]

Any sort of optical defect and aberration will be easier to hide with a more elongated light cone, where the angle of refraction isn't that high. Let's take chromatic aberration for example. It happens on refractors, especially achromats, because each wavelength refracts at a slightly different angle when it hits the lens. Different spectrums then focus at different points, resulting in a purple / green halo around brighter objects, etc. Now, the steeper the light cone, the bigger difference between the focal points of different spectrums, resulting in a bigger offset / larger halos when you observe. With a long focal length / slower scope the angle of refraction isn't high anyway, and the differences are less noticeable. This is why a lot of old refractors were built at pretty long focal lengths.

Of course, CA is just one example - the "steepness" of the light cone has an effect on other types of optical aberrations as well.

[Andymarrison]

Chaps - thanks very much for these very clear and helpful explanations! Does anyone know of a good book on the principles of astronomical optics? I'd quite like to get a better understanding of all this stuff.

[newman]

They don't strictly deal with just optics, but more general knowledge any amateur astronomer needs - Nightwatch and Backyard Astronomer's Guide by Terence Dickinson are both excellent books.

[John]

Chaps - thanks very much for these very clear and helpful explanations! Does anyone know of a good book on the principles of astronomical optics? I'd quite like to get a better understanding of all this stuff.

There are some articles here that are worth reading:

Optical Theory

[Andymarrison]

Once again, thanks to everyone for the suggestions for further reading. Time to dust-off the old grey matter...!

[AlBoning]

I have a different perspective. It's not so much that a high quality eyepiece is required to handle the light from a low focal ratio objective, but that older designs (consequently cheaper and perceived of lower quality when if fact they are not) were optimized for the (larger focal ratio) scopes available at the time (when f/6 and slower were considered fast) of their design and it is only the newer designs (Wouldn't it be nice if someone came out with Plossls for f/5 and less?) that take into account the current trend of fast focal ratio scopes. So Plossls, Erfles, Orthoscopics (and others) are all designs that pre-date the current trend and just aren't up to it. Of course, being of a different perspective I think the solution is having a scope with a reasonably large focal ratio, say f/8 or larger and than it's no problem. Add to this the current rush to wide field and you add a lot of glass and a consequential increase in cost of modern eyepieces. In my experience the problem of edge of field aberrations tends to be reserved for long (say 20+ mm) focal length eyepieces in relatively fast (less than f/5) scopes.

[John]

....Wouldn't it be nice if someone came out with Plossls for f/5 and less?....

Someone did - Al Nagler in 1984. Tele Vue Plossls are designed and tested to work well down to F/4, like all their eyepieces