A recent thread on upgrades in eyepieces led me to reexamine something I wrote years ago about contrast in eyepieces.
One of the obvious improvements people seek when they buy a new eyepiece (but don't always receive) is improved contrast in the eyepiece.
But what is contrast in an eyepiece?
Forgive the long post--I'll break it up into two posts. I would be interested in hearing critiques so I can improve what I wrote.
CONTRAST IN EYEPIECES
What do we mean when we say one eyepiece produces better contrast than another? Is it just one characteristic of an eyepiece, like apparent field, or is it the result of the interaction of many parameters?
Let's discuss the term contrast first and then talk about how contrast in eyepieces is created and enhanced.
Contrast is the differentiation between light and dark areas. Better contrast entails the ability to distinguish brightness levels that are closer to identical.
In the case of telescope eyepieces, it also refers to the ability of an eyepiece to not scatter any light from brighter areas into darker areas; maintaining, and not reducing, the differences in brightness between adjoining areas.
That would mean that fainter features in nebulae and galaxies might be seen because of better contrast with a darker background, and it might mean that small lunar or planetary features might be seen because of less bleeding of bright light into darker areas, reducing contrast between features.
The resolution of an eyepiece will always be limited by the aperture of the telescope, but we don't want the eyepiece to reduce the ability to see details by reducing contrast in the image.
I come down on the side of those who point out that contrast in eyepieces is NOT just one factor, but the synergistic interaction of many factors.
We can discuss apparent field, edge of field distortion, light transmission, and the like, and each is a measurable characteristic of the eyepiece. Contrast, however, is not, in the sense in which it is used by amateur astronomers, a single measurable thing, but many. Yet, like pornography, we all know it when we see it.
So how is contrast created, and how can we improve it and why should we value improved contrast?
Let's discuss some of the factors influencing contrast and our ability to see small differences therein (and not all of the following are always thought of as "contrast" parameters):
1. Improved polish will reduce light scatter and increase light concentration in the correct places. This may be one of the most important factors of all. Making the lens surfaces smoother means less light is scattered by small irregularities in the surface of the lenses. This entails good polishing and QC on the part of the manufacturers. And partially explains why high-priced eyepieces are high-priced.
2. Improved manufacturing tolerances. The eyepiece has to be as close as possible to the optical design or negative effects can occur, like Spherical aberration or astigmatism. This factor rolls assembly quality and adherence to proper curves and coatings into one factor. It means the lenses cannot be tilted relative to one another and the glass and coating materials have to adhere closely to the optical design.
3. Improved choice of the correct coatings for the glass types used will improve transmission and reduce scatter. If the coatings are properly chosen for the glass types, more light will get through, and less will be reflected internally. Reduction of internal reflections is enhanced by higher transmission coatings. Proper choice of glass types will insure this applies to all visible wavelengths.
4. Improvement from the lack of internal reflections in the eyepiece from lens edges, spacers, filter threads, etc. The best eyepieces have NO visible reflections from any internals. Blackening lens edges, internal spacers, the barrel interior, filter threads, even the external body of the eyepiece near the eye, will result in less scattered or off-axis light making it to your eye. Though most eyepieces fail in this, ideally the brightest star in the sky could be just outside the field of view and you would never know it. [In fact, other things could give this away that have nothing to do with the eyepiece, but you get the point].
5. Improved wavefront accuracy AFTER passage through the lenses. Though some cancellation of errors can occur with multiple surfaces, it seems obvious that reducing surfaces, with all other factors equal, is more likely to result in less damage to the wavefront. We talk at length about the quality of the mirrors and lenses in our telescopes, but rarely talk about the optical figure of the eyepieces we view through. Yet, there are more surfaces in our eyepieces than in our telescope mirrors or lenses. Yet the quality of these lenses determines the quality of the images we see. So does their number. This is where, in theory, a smaller number of lens surfaces will be superior to a larger number. But specific examples may not verify that contention. Some multi-element eyepieces may have superior polish and better images than some low-element count eyepieces. Generalizing might not work, though, in theory, fewer elements could yield a better wavefront.
6. Improve the quality of the design. If the eyepiece has chromatic aberration or aberrations induced by the f/ratio of the scope, then the eyepiece will have reduced image quality. Eyepieces are designed to amplify the images created by the telescope. If the design cannot handle the wide light cone of a short f/ratio telescope without creating noticeable aberrations or does not focus all colors of light into the Airy disc of the telescope, then the design is not appropriate for all telescopes. In the history of telescopes, eyepieces have been getting better and better in this regards, with a lot of the high-end eyepieces good in scopes as short as f/4 and below. But there are still many earlier types of eyepieces being used that, while excellent in long focal ratio telescopes, cannot handle the shorter, wider, light cones of many of today's telescopes. Since no manufacturer wants to limit his market, many eyepieces suitable only for f/6 and longer are today being purchased for f/3 to f/5 scopes, where they will perform poorly. But some modern eyepieces are truly dramatic improvements over common eyepieces of yesteryear.
Edited by Starman1, 10 August 2020 - 05:22 PM.