Old astronomy books often say you can get sharper images on a night of poor seeing by stopping down your telescope with a cardboard mask so it has a smaller aperture. New books don’t mention this. It this a forgotten secret?

In my experience, if stopping down a telescope sharpens the view under any circumstance, it means there’s something wrong with the scope. An aperture stop will mask off a turned-down edge on a reflector’s mirror or will hide the worst of its spherical aberration. Stopping down raises a telescope’s f/number, so an aberration-ridden eyepiece will work less poorly. A stop also masks off “tube currents” of warm air hugging the walls of a telescope’s tube. This was an often-unrecognized problem in large old scopes that didn’t leave enough air space between the tube walls and the light beam (S&T: January 2004, page 114). Raising the f/number also reduces the substantial chromatic aberration in large old refractors.

Stopping down a large scope can also serve to dim a too glary image of the Moon or a bright planet, but this is better done with an eyepiece filter.

If a large telescope is optically good, my experience is that it will never perform worse than a small scope no matter how awful the seeing. The worst that will happen is that they’ll perform about the same. Maybe the fact that newer books don’t recommend stopping down is a sign that telescopes are getting better.

— Alan MacRobert


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