Flat field eyepieces

[gooseholla]

What are the benefits of flat field eyepieces, and what would they be used for? deep sky, panoramic views, planets? A mixture?

Thanks

John

[cs1cjc]

If you have older eyes unable to adapt to field curvature then in principle you should see a sharper image at the edge, but there are many other factors affecting image sharpness. I do know some eyepiece/telescope combinations produce a field which is too curved for my old eye.

[Capricorn]

From a conversation with Alan at StL a flat field eyepiece is intended for the fast scopes, these are relatively short focus and so the image formed is a curve not flat.

Flat field eyepiece are to compensate for this curvature. They would comensate for the curved image field and produce for you a more pleasing image (flatter).

Whether or not they would operate well in a longer focal ratio scope is another question. The eyepiece would compensate for a curved image that isn't there.

And all this depends on what I was told being correct.

[cs1cjc]

Telescopes have very different curvatures of field, see:

Image field curvature

For Newtonians the radius is about the focal length, away from the eye;

for a normal refractor it is about 1/3 of the focal length again away from the eye;

Other telescopes have different properties. The radius of the curvature of field for my three telescopes varies from 140mm to 1610mm - no eyepiece is going to compensate for all. In fact eyepiece field curvatures are typically much higher than this (radius much smaller) and often towards the eye, see:

Telescope eyepiece 2

I think that flat field probably means flat field, but I have no direct experience.

[cotterless45]

I use an 8mm flatfield in f5 and f6 Dobs. It really does give focus right across the field, would recommend not only for lunar but dso use, Nick.

[gooseholla]

Thanks all for your replies.

Planning a few eyepieces to add over the coming year to my kit of plossls. BSTs are for sure, couple of hyperions for 8 inch newt. Given me a bit to think about with flat fields also.