Pentax Eyepiece Data

While reading thru posts on Pentax eyepieces I found the following link posted by Mike Hosea.

http://www.pentax.co.jp/japan/tech/xo-xw/gif/p6-syusa.gif

Mike, I was wondering if you could explain these plots, I couldn't seem to figure out what properties of the eyepieces are shown. What is is X-axis, Y-axis? What do the solid and dashed lines represent?

I stumbled on a pictoral summary of the lense arrangements of the XW line.
I find it interesting that each eyepiece does not appear to be just a scaled version of the others.

http://www.pentax.co.jp/japan/tech/xo-xw/61.html

Thanks for the help,
Steve Bolser

The x axis is the deviation from the paraxial focal length, and the y axis is the angular distance off-axis. So, in English, the curves tell you how you how the focuser will need to move in order to switch from focusing the center at infinity and focusing a point at a given distance off-axis at infinity, assuming your telescope has no curvature or astigmatism (ha!). There are two curves. One represents the meridional (also called tangential) focal surface, and the other represents the sagittal focal surface. You can read some about this at

http://www.microscopyu.com/tutorials/java/aberrations/astigmatism/index.html

and

http://www.vanwalree.com/optics/astigmatism.html

but basically you want these two curves to coincide as much as possible, and you want them to be close to the vertical center line. When they swing to the right as they rise, you will have positive field curvature (requiring in-travel to compensate for defocus at the edge), and if they swing to the left you will have negative field curvature (requiring either out-travel or accommodation by the eye to compensate for defocus at the edge of field). The farther apart they are, the more astigmatism you will see, rather than field curvature.

This is incredible data to get from a manufacturer, but unfortunately since it is so rare to have, we don't have much to compare it to.
--
Mike