I don't see it so much at night, but was just looking at the sun thru the 5.5mm and the 14mm Meade UWF EP.
It's very spherical and I see alot of distortion. Just curious what other's who own these eye pieces think about this.
I know you can twist them to get different eye relief's, but the best that I've found is when they are almost flush with the top.
Even then, you have to put your eye right up against it. They aren't bad at night though.
Any eyepiece wider than about 40° has distortion.
What differs is what kind of distortion, and the amount.
The wider the apparent field, the more distortion the eyepiece has, and distortion is distortion.
The most common form of visible distortion in an eyepiece created for astronomical use is rectilinear distortion.
If a straight line crosses the field, it goes from ")" to "|" to "(" as it crosses if the distortion is positive, called pincushion.
If a straight line crosses the field, it goes from "(" to "|" to ")" as it crosses if the distortion is negative, called barrel distortion.
Pincushion is the least visually objectionable and is usually the form you see in astronomical eyepieces.
This form of distortion can be suppressed, but if it is, another form of distortion becomes larger and more obvious: angular magnification distortion.
If a feature crosses the field and goes from A to A to A, that is considered rolling ball, or globe distortion.
If a feature crosses the field and goes from A to A to A, that is the opposite sign of the more common form of AMD.
Both are forms of AMD, or angular magnification distortion.
Daytime use eyepieces often have reduced RD so straight lines stay straight, but, as a result, billboard lettering will shrink as it nears the edge of the field.
Nighttime eyepieces often have reduced AMD so double star separations and craters and planets stay the same size as they near the edge.
Now, back to the eyepieces used on the Moon or sun:
With AMD, the disc nearer the edge will shrink in size, distorting the Moon/Sun into a slightly egg-shaped disc with the smaller end pointed out..
With RD in the most common form, the disc will stretch radially as it nears the edge, turning the round disc into a slightly oval shape.
As they say, distortion is distortion.
How do you avoid it? Simple, use narrow field eyepieces. Plössls make excellent sun and moon eyepieces.
Otherwise, don't look at the sun or moon right near the edge of the field.
Now, moving the eyeguards in or out does not change the eye relief of the eyepiece, it merely moves the pupil of your eye relative to the exit pupil of the eyepiece.
The distance from the top lens to the exit pupil is the eye relief of the eyepiece, which is fixed.
Whether day or night, you want the pupil of your eye positioned at the exit pupil. It's harder in the daytime because your pupil is smaller, but it's the same process:
start back from the eyepiece and slowly move in toward the eyepiece. At the point you can *just* see the entire field with peripheral vision, stop. That is the exit pupil.
If you can't hold your head that far out, raise the eyeguard until holding your head there is instinctive. If you go too close to the eyepiece, the iris of your eye will start
causing blackouts in the image.