John Donne wrote: ↑Wed Nov 03, 2021 10:03 pm
"Great optical performance in a tracking scope. In a non-tracking scope the angular magnification distortion makes the sky appear like it's moving over the surface of a ball or globe as the sky moves--the center of the field appears closer to you than the edge."
I have not experienced this on any of my refractors.
I am curious with which telescope have you experienced this strange distortion. ?
It would seem that it should be the same with a tracking or non tracking scope.
It's well known the Noblex/Docter has significant Angular magnification distortion (AMD) that results in things at the edge of the field being lower in magnification than the center. This type of distortion is not unique to that eyepiece, being found in the Nikon NAV-SW
eyepieces and the APM 12.5mm 84°. It results in the field appearing closer to you in the center than the edge, but you only notice is scanning across a field.
Generally, astronomical eyepieces will reduce that distortion to a minimum, which leaves in a lot of rectilinear distortion (RD), which causes straight lines to curve as they cross the field, like ) | (.
Rectilinear distortion is usually not noticeable in a widefield astronomical eyepiece, but is not desirable in a daytime spotting scope eyepiece, so RD is usually reduced in daylight use eyepieces, leaving in lots of AMD.
Note that both forms of distortion are larger the wider the apparent field of the eyepiece, so not usually seen in narrow field eyepieces.
If the focal length of the scope is short, a 12.5mm eyepiece isn't a very high power, so the field drifts across the eyepiece slowly, which is why you probably don't see the "rolling ball" or "globe" distortion in the Docter/Noblex eyepiece.
In my scope, which doesn't track, my focal length is close to 2 meters, so the field drifts across fast enough to notice the movement of the stars in the field and the nature of the distortion.
That all such optical solutions are compromises can be seen here in this discussion of distortion in binoculars:
The Docter is similar to the k=1 condition, where most astronomical eyepieces are closer to k=0
Here is a relevant discussion:
https://www.cloudynights.com/topic/1098 ... try1446673
Note: back then, I hadn't seen a Docter, or APM 12.5 or Nikon NAV-SW
eyepieces. But I hadn't used spotting scope eyepieces adapted for astronomy then.
In a friends scope, which tracked, the Docter was a very nice eyepiece. I immediately saw the Barrel distortion in my dob
, however (barrel distortion accompanies positive AMD).
The TeleVue Apollo 11 eyepiece (similar AF of 85° and similar focal length) is completely different, and scanning across fields didn't display any noticeable distortion, proving to me that reduced AMD, with RD, is a preferable solution to reduced RD, with AMD.