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focal lengths and fast scopes.

Started by gyrocopter24, 10/06/2010 01:30PM
Posted 10/06/2010 01:30PM Opening Post
Ive notice that some people report that this eyepeice or that eyepiece does not function well in a fast scope (f/5 or lower). Why is that. I have a zhumell 10" f4.92. Are there certain brands or fl that I shouldnt waste money on and will a paracorr solve all the ills in regards to an eyepiece not functioning in a fast scope?

-Terry
Posted 10/06/2010 06:21PM #1
Terry swan said:

Ive notice that some people report that this eyepeice or that eyepiece does not function well in a fast scope (f/5 or lower). Why is that. I have a zhumell 10" f4.92. Are there certain brands or fl that I shouldnt waste money on and will a paracorr solve all the ills in regards to an eyepiece not functioning in a fast scope?

-Terry

Terry:

This is how I think of it.

A telescope focuses light from a distant object to a focal plane forming a virtual image at that focal plane. That's what the telescope does. Now the eyepiece, it's job is relatively simple, it just magnifies that image by allowing you to place your eye close to the image and focus on it.

Now the light that forms that virtual image comes from all parts of the objective and converges on the focal plane and past the focal plane it is diverging. The greater the angle of divergence, the more difficult it is for the eyepiece make sense of it all.

That angle of divergence depends on the focal ratio of the scope, the faster the scope, the greater the angle. In an F/5 scope the included angle of the light cone is about 11.4 degrees, in an F/10 scope it is about 5.7degrees.

So what happens is that most simple eyepieces are quite sharp in the center of the field of view of any scope of any focal ratio but as you move off-axis in the faster, it will begin to show various aberrations, most commonly astigmatism, stars are no longer nice and round.

Eyepieces like the TeleVue Naglers are designed to address this problem and are rated to work down to F/4. There are a variety of other designs out there that offer varying degrees of off-axis performance depending on the scope and the focal ratio.

Widefield designs are most affected by fast scopes because they have the most angular field to correct. So, Plossls and such work reasonably well at F/5 but a careful eye will detect aberrations.

Now the Paracorr, it addresses a different problem, coma. Coma is an off-axis aberration that is inherent in a parabolic optic. Coma is a point with a fan shaped tail that points away from the center of the field of view. It is highly dependent upon the focal ratio, the coma free field is related to the 3rd power of the focal ratio, F/5 has about the half the coma of an F/4. Many observers are OK with the level of coma at F/5.

A Paracorr corrects the coma of the mirror but does little correct the off-axis aberrations in the eyepiece that result from the wide angle of the light cone.

The simple way to correct for the eyepiece aberrations is to use a barlow, an F/5 scope plus a 2x Barlow changes the angle of the light cone from F/5 to F/10 so that the eyepiece has an easier time of it. Of course this has certain disadvantages, that 32mm eyepiece with it's nice widefield of view has just turned into a 16mm eyepiece.

Eyepieces like the Naglers and since the invention of the Naglers about 30 years ago, many other eyepiece, incorporate a barlow like front section that allow them to handle those fast focal ratios with significantly less off-axis aberrations. Such eyepieces are called Negative-Positive because they have that Barlow-like Telenegative front section...

But the main thing you should do...

Forget everything I have written, save it somewhere and just get out there and look through your telescope and enjoy it. It's nice when the stars are nice pinpoints right out to the edge but you are young and enthusiastic and such things should be easily ignored.

Someday, after you have been looking through your scope for some years and you start to wonder why one eyepiece happens to be sharper over near the edge of the view than another, look through the astromart archives and find this post...

Till then, enjoy with enthusiasm the many joys of looking at the night sky through a wonderful telescope with some quite amazing eyepieces.

Jon