Ratboy99 wrote:
On a star, the diffraction is thrown outside of the star image. On an
extended image diffraction smears the image itself.

Unless the binary is very tight, I don't see how diffraction will throw
light outside the secondary star--even mostly outside the secondary star.

I suspect the main reason why higher powers seem tolerable on binary
stars (and lunar observing) is because the input contrast is so high.
Even when you are low on the MTF, the output contrast is still high
enough for your eye to pick up on the detail.

In contrast, on DSOs, the contrast is low, so that being low on the
MTF means low output contrast as well. If the object is magnified too
much, the variations in brightness may be too difficult for the eye to
pick out.

Another aspect of binary observing that might be relevant is that it is
a relatively well-circumscribed observation. Primary color, primary
brightness, secondary color, secondary brightness, separation, and
position angle. That's about it. Whereas other sorts of observation
are much more varied, and benefit much more from greater contrast.

Brian Tung
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