henry link
Well-known member
I am happy to report that the replacement scope for my friend’s Zeiss 85mm Diascope has arrived and it is completely free of the defects the first sample had. I measured it’s resolution to be 1.6 arc seconds, a good figure for a birding scope of this aperture, but not as close to diffraction limited as the 1.9 arc secconds I measured recently on a Swaro ATS-65HD.
The scope star tested fine in terms of defects, no sign of astigmatism, miscollimation or pinched optics. There were completely round, very strong diffraction rings on one side of focus and very weak ones (except for one very strong outer ring) on the other side, indicating a bit more spherical aberration than I saw in the Swaro 65, which is really to be expected from the lower focal ratio objective. Most importantly the image quality is exactly what it should be in this scope, quite sharp and high contrast at lower magnifications with just the relatively small amount of deterioration at high magnification that is to be expected from the SA seen in the star test and the measured resolution
I can’t think of a better example than this of why new scope buyers should immediately do the simple 60X star test. My friend was ready to accept a seriously defective scope simply because he didn’t have the optics experience necessary to recognize a flawed scope image when he saw one. In fact, most birders don’t have that kind of experience with telescopes, so there could be quite a few dud scopes in use.
(Dave, if you see this I was able to read the tiny numbers on the $1 bill at 128’ through the replacement scope at 60X.)
The scope star tested fine in terms of defects, no sign of astigmatism, miscollimation or pinched optics. There were completely round, very strong diffraction rings on one side of focus and very weak ones (except for one very strong outer ring) on the other side, indicating a bit more spherical aberration than I saw in the Swaro 65, which is really to be expected from the lower focal ratio objective. Most importantly the image quality is exactly what it should be in this scope, quite sharp and high contrast at lower magnifications with just the relatively small amount of deterioration at high magnification that is to be expected from the SA seen in the star test and the measured resolution
I can’t think of a better example than this of why new scope buyers should immediately do the simple 60X star test. My friend was ready to accept a seriously defective scope simply because he didn’t have the optics experience necessary to recognize a flawed scope image when he saw one. In fact, most birders don’t have that kind of experience with telescopes, so there could be quite a few dud scopes in use.
(Dave, if you see this I was able to read the tiny numbers on the $1 bill at 128’ through the replacement scope at 60X.)
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