Longest focal length EP for binoviewing?

Started by Eddh, 02/02/2004 10:09PM
Posted 02/02/2004 10:09PM Opening Post
What is the lowest power/widest Fov EP to use for binoviewing? I presently use a pair of Meade 32mm Plossl on the TV binoviewer. Is there anything wider then that? I am all ears. Thanks.
Posted 02/03/2004 04:21AM #1
Eddie:

First, the 1-1/4" diameter of the eyepiece barrel is going to be your limiting factor. To keep it simple, we'll ignore any influence the binoviewer may have. The choke point in 1-1/4" eyepieces is around 32mm focal length. Beyond this the apparent field starts to narrow as you'll see in examples below.

The best way to get you an answer is with some math. I'll use my scope and a few eyepieces as examples.

My scope is a 4" f/6 refractor with a 600mm focal length. To get your real field divide your eyepiece's apparent field by the magnification it give in your scope. Do the same math with your scope's data for your personal results.

EXAMPLES:

32mm plossl w/52d apparent field 52d / 19X = 2.7d

25mm W70 w/70d apparent field 70d / 24X = 2.9d

40mm plossl w/44d apparent field 44d / 15X = 2.9d

32mm Konig w/52d apparent field 52d / 19X = 2.7d

As you can see the two eyepieces with the same apparent field - the Konig and the plossl, give the same real field, the design has no impact on the results.

You can also see the lower power 40mm eyepiece gives a wider true field but the 44d apparent field can look restricted to your eye.

Suprisingly, the higher powered 25mm eyepiece, because of its wider apparent field, offers the same real field as the 40mm, yet does so at a higher power and with a more pleasing, spacious apparent field.

Hope this helps.

Tom M

Clear, sunny skies.
Tom Masterson
Coronado-Lunt-Daystar Solar Filters forum Moderator