With a 30mm eye lens and zero distortion, the apparent field with a 17mm eye relief would be 82.8 degrees.
Since distortion is definitely present in all widefield eyepieces, I see no reason to disbelieve it could be as large as 85 degrees.
If it's 100 degrees, the eye relief is a lot shorter.
Now, as for evaluation, here are some points:
2. Chromatic Aberration-axial, lateral and fringe. Be sure to hold your eye properly. Sometimes you see some, but it's due to looking through the eyepiece at the wrong angle.
3. Field Curvature-negative and positive and scope interaction. Is the center in focus when the edge is and vice-versa. If not, this could be FC in the eyepiece, but if the scope is a refractor shorter than 2000mm in focal length, or a reflector shorter than 1000mm focal length, it could be the scope.
4. Angular Magnification Distortion (+/-). Look at a double star. Is the apparent separation the same at the center and edge? Then no AMD. If yes, how much is present?
5. Rectilinear Distortion a) pincushion b) barrel and relationship to astigmatism. Does the edge focus sharply? If not, and it isn't simple coma, then the eyepiece has astigmatism in whatever f/ratio scope the test scope is. Do straight lines curve in or out as they near the edge (they will, and it's probably better to have them bow in)
6. Spherical aberration. This is if and only if you know what's in the scope. If other eyepieces have taught you that, then you can take a shot at the eyepiece. Otherwise, forget this point.
7. Transmission anomalies by Frequency: coloration (tint) and overall transmission. Does the image appear bright or dim compared to other eyepieces, and is the Moon yellow or white or blue-white? Can you see the color difference between Saturn's rings and disc?
8. Light loss due to a) reflection b) absorption c)scatter d)internal vignetting. Is the edge dimmer than the center when you hold the eyepiece up to the daytime sky? Do you see a lot of light scatter at night when pointed near the Moon? Do bright stars outside the field create streaks inside the field? Is the edge brighter than the center?
9. Spherical Aberration of the Exit Pupil and relationship to eye relief. Can you hold the eye steady and see the field edge with peripheral vision at the same time you can see the center with direct vision?
10. Chromatic aberration of the exit pupil (Ring of fire). If used in the daytime, do you see only a tiny thin line of color at the edge, or is the outer 10% of the field tinted yellow or orange?
11. Coma in off-axis light. Ignore this one. You won't find it in this eyepiece.
12. Vignetting (loss of edge brightness) due to improper lens diameters, barrel diameter, normal design. Is the edge dimmer than the center when holding the eyepiece up to a bright sky? It may indicate a reduced illumination at the edge on purpose in the design.
13. Astigmatism a) tangential and sagittal b) tilted elements c) wedge d)relationship to focal ratio of scope e)relationship to astigmatism of objective. Can you see astigmatism at the edge when the scope is cooled to ambient temperature? Do stars focus into radial slots on one side of focus and circumferential slots on the other side of focus and then down to a tiny + sign in focus?
14. Wavefront aberration a) poor polish b) poor figure c) result of more surfaces in eyepiece. Is the center star sharp in focus, or somewhat soft compared to other eyepieces? If sharp, does that change as the star is moved away from the center? If so, where, and how much?
15. Light Scatter:
a. Surface scatter-roughness (creates a fog around bright stars. Careful: moisture on the glass or mirrors will do this.
b. Reflections: lens(edge and surface-polish and coatings), and barrel (ghosting). Do you see ghost images of planets or bright stars elsewhere in the field when viewing them?
c. Lateral rays (lens edge and barrel and low incidence scatter by coatings). Do you see reflections at the edge of the field? Does the edge get brighter as a bright star approaches it?
16. Design Flaws
a. Field stop not in focus. Look directly at the field edge. Is the stop in focus and sharp or somewhat vague?
b. Critical f/ratio too high (inadequate off-axis ray handling). Does it work great in an f/10 scope but poorly in an f/5 dob?
c. Improper internal ray handling, causing vignetting or reflections. Ignore this.
d. Wrong glass refractive index used. Ignore this one.
17. Thermal issues due to size, improper housing. As the eyepiece cools, does the image quality improve or get worse? Some large eyepieces have to be cooled just like an objective lens to yield the best image quality.
18. Blur circle and spot diagrams and focus of different colors. Is there any chromatic change in the centered star image as you move it in and out of focus in either direction?
You'd need a reflector or well-corrected triplet or quadruplet apo refractor to judge this one.
Just some of the things you'd look for in judging an eyepiece.
Oh, in case you wondered, #1 above was:
1. Contrast (how it’s many factors and why it will be difficult to measure—why AF relates to perceived contrast—how critical dark adaptation is—how cataracts affect--):
Contrast in an eyepiece is tied to the suppression of scattered light. To this end, blackening of the lens edges, full multi-coating, very dark internals in the barrel and blackened threads and bottom edge may all help. The best way to test this is to put a very bright star just outside the FOV. If spikes or ghosts of the star are visible in the field, the eyepiece gets an F. If a brightening of the field is noticed at the edge just inside the field from the bright star and it's quite bright at the edge, then the eyepiece gets a D. If you can tell which direction of the compass the bright star lies outside the field, but little evidence of the star's brightness is inside the field, the eyepiece gets a C. If you can tell there is a bright object outside the field of view, but it's uncertain what direction is lies, the eyepiece gets a B. If you literally cannot tell there is a bright star outside the field, the eyepiece gets an A. One caveat: newtonians with great contrast, used in dark skies, will show the spikes from a bright star in the FOV if the star is still in the field of the telescope, but just happens to be outside the FOV of that eyepiece. In that case, seeing the spikes might indicate terrific contrast in the eyepiece.
Narrow fields of view may allow a perceived improvement in contrast, but this is not certain.
It is critical to evaluate eyepiece contrast when fully dark adapted. Otherwise, a dark background may simply be an indication of a lack of dark adaptation. If the site is so bright you cannot dark adapt, I would argue you cannot accurately assess the contrast in the eyepiece.
Cataracts in the eye's lens scatter light and cause anomalous flares and glare, too, so obviously this would make it very difficult to assess the contrast in the eyepiece.
Many factors influence contrast. Some have already been mentioned. Also important are polish on the lens surfaces, good design, good adherence to design, induced aberrations from the f/ratio of the scope, chromatic aberration, and a host of other issues. Superior contrast requires a high level of quality on all aspects of design and manufacturing. Generally, if the eyepiece performs well in all the other factors, it'll do well in this one (as nebulous a term as contrast is, it's like pornography--we all know it when we see it, but it's really hard to define )
Edited by Starman1, 19 December 2014 - 01:46 AM.