Planetary Eyepieces (Part 1)

Started by Martian, 03/08/2003 10:31AM
Posted 03/08/2003 10:31AM Opening Post
I am probably opening up a can of worms by posting this message, but I will put in my two cents worth.

I must first state that there does not exist a perfect planetary eyepiece. Various designs are endorsed as being the ultimate design for observing the planets but this depends upon many factors. The primary factors are the following;

1) Sharpness
2) Contrast
3) Color correction
4) Eye relief

Sharpness:
One often hears of the "on-axis" or "off-axis" definition, or sharpness, of an eyepiece design. The planetary observer is primarily interested in the on-axis definition of an eyepiece design as the target (e.g. planet) is normally placed in the center of the field of view. Admittedly a well corrected eyepiece design will have good to excellent off-axis definition as well. Popular eyepiece designs such as the Abbe Orthoscopic (named after it's German designer Ernst Abbe in 1880) and Plossl (G.S. Plossl of Austria in 1860) enjoy excellent on-axis, as well as off-axis, definition. This is due in part to the fact that these designs maximize their corrections over a smaller apparent field of view (45-50 degrees) compared to a wide-field design (e.g. Nagler) whose off-axis definition falls off sharply over the outer 50% of the apparent field of view.

Contrast:
The perceived difference in brightness between two adjacent objects. This is accomplished by various factors such as the figure of the eyepiece lens (including polish) and light transmission (affected by the number of elements (lenses) and enhanced by the application of coatings (e.g. magnesium floride). Multicoatings on an eyepiece lens cannot make up for inferior glass/figure/polish. The simpler element (e.g. 4 elements, as in the Orthoscopic and Plossl) eyepiece design tends to produce higher contrast images at the focal plane compared to the multi-element designs (e.g. >6-7 elements, such as the Nagler and Meade Ultra-wide).

Color Correction:
The ability of a lens to bring together the various wavelengths of light to a common (focal) point. Properly matched lenses will bring the various wavelengths to a focal point to a degree that will not be perceived by the observer extending beyond the object in the field of view. Various designs (especially using different lens material) accomplish this rather well. The popular Orthoscopic and Plossl designs provide good to excellent color correction whereas some wide-field designs (e.g. Televue Radian) exhibit lateral color towards the periphery of the field (especially noted when observing the Moon). This may not affect the choice of a wide-field design but is a factor to consider.

Continued in Part 2


Posted 03/08/2003 01:28PM #1
Carlos,

How are you defining off-axis definition (no pun intended)? If you mean the spot size increase due to astigmatism I find it hard to agree with your figure (the outer 50% of the AFOV) regarding the Nagler design.

Dave Mitsky

Chance favors the prepared mind.

De gustibus non est disputandum.