Piggybacking is a very tough test of lens quality. The stars are point sources, and any blur is immediately noticeable. Every star in the picture has to be in focus all at once; there is no out-of-focus background. And we process the picture to increase contrast, which emphasizes any vignetting (darkening at the edges).
Having said that, I should add that the situation with DSLRs is not quite the same as with film. High-speed film is itself very blurry; light diffuses sideways through it, especially the light from bright stars. When you put a very sharp lens on a DSLR and this blurring is absent, the stars all look alike and you can no longer tell which ones are brighter.
For that reason, less-than-perfect lenses are not always unwelcome with DSLRs. A small amount of uncorrected spherical or chromatic aberration, to put a faint halo around the brighter stars, is not necessarily a bad thing. What is most important is uniformity across the field. The stars near the edges should look like the stars near the center.
All good lenses show a small amount of vignetting when used wide-open; the alternative is to make a lens with inadequate glare stops. Vignetting can be corrected when the image is processed (p. 188), so it is not a fatal flaw. Another way to reduce vignetting is to close the lens down one or two stops from maximum aperture.
Distortion (barrel or pincushion) is important only if you are making star maps, measuring positions, or combining images of the same object taken with different lenses. Zoom lenses almost always suffer noticeable distortion, as you can demonstrate by taking a picture of a brick wall; non-zoom lenses almost never do.
One lens attribute that does not matter for astronomy - except in a backhanded way - is bokeh (Japanese for "blur").1 Bokeh refers to the way the lens renders out-of-focus portions of the picture, such as the distant background of a portrait.
The physical basis of "good bokeh" is spherical aberration. Years ago, it was discovered that uncorrected spherical aberration made a lens more tolerant of focusing errors and even increased apparent depth of field. But in astronomy, there is no out-of-focus background. The spherical aberration that contributes
1 Also transliterated boke. Another Japanese word with the same pronunciation but a different etymology means "idiot."
Distance from center of field (mm)
Figure 7.2. MTF curve from a good lens (left) and from one that warrants concern (right). Star fields are very demanding targets.
Figure 7.3. Orientation of targets for sagittal and meridional MTF testing.
to "good bokeh" could help the bright stars stand out in a star field; apart from that, it is just a defect.
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