Quote:
Originally Posted by ausastronomer
Hi Markus,
It comes from the fact that despite what a couple of people on this forum will try to tell you, eyepiece designers rarely design eyepieces to work in certain telescope types. They design them to correct for certain aberrations to satisfy the design parameters and intended end use of the eyepiece.
The 7mm, 10mm, 14mm and 20mm Pentax XW's were designed for Spotting Scope use and that's the way they were marketed by Pentax for well over 15 years. On the Pentax website these 4 eyepieces were listed under "Spotting Scope Eyepieces" and all of the other focal lengths in the XW series were listed under "Astronomical Eyepieces"
Being intended for Spotting Scope use these 4 eyepieces are corrected for "Rectilinear Distortion" (Barrel and Pincussion) where you need straight lines to appear straight and not bent. Most Astronomical eyepieces are corrected for "Angular Magnification Distortion" and not for rectilinear distortion. Despite modern computer programs, glass types and advanced eyepiece design, it is not possible to correct for both at the same time. I spoke at length with Al Nagler on this very topic about 12 or more years ago. He advised that his eyepieces were generally corrected for "Angular Magnification Distortion" and not rectilinear distortion, with the Nagler T4's being the exception, as these were corrected for rectilinear distortion and suitable for daytime use in refractors.
Cheers
John B
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John,
Try the T4's in the daytime-- (look at any straight line) they have substantial rectilinear distortion, in the form of pincushion. Maybe they don't have as much AMD as the apparent fields would imply, but they are not corrected for rectilinear distortion.