"Large scopes are more adversely affected by seeing than small ones."

The idea behind this myth is that when it comes to planetary observing, there is little point in getting a large telescope unless the seeing conditions at your observing site are unusually good. Some actually claim that a small scope can outperform a large one on nights of mediocre seeing.

I call this assertion a myth for two reasons. First, my own observations do not bear it out. Second, no one has proposed a plausible mechanism for it. Telescopic resolution is limited by the weakest link in a chain made up of optical quality, atmospheric steadiness, telescope design, and the observer. No doubt, some small scopes give better views than some large ones, but this can easily be ascribed to factors having nothing to do with the atmosphere. In particular, poor collimation and poor thermal characteristics often plague large reflectors.

When I first heard that small scopes could beat out big ones, I checked it out for myself. At the time my principal telescope was an optically good 12½-inch f/5 reflector. I built a 5-inch-diameter off-axis mask that could quickly be placed at the front of its tube, making it into an unobstructed 5-inch reflector. Over the course of several years I took this mask with me to every observing session and compared full-aperture planetary views with those seen with the mask in place. I did this on nights of good seeing, okay seeing, and poor seeing. Not once did the reduced-aperture view show greater detail than the full-aperture view.

Admittedly, when the seeing (or atmospheric steadiness) was below average the off-axis mask produced an aesthetically pleasing view. But this did not translate into greater detail — though it's easy to understand how a casual glance would give this impression. Most often I would wind up using my telescope's full aperture because even on nights of substandard seeing, occasional brief moments of stillness would allow me to see details that simply could not be seen with the aperture mask in place.

The Tip of the Iceberg

The myths discussed here represent only the tip of an iceberg — many more are in circulation. Many of these myths rely on anecdotal evidence and invalid comparisons. When evaluating provocative claims, let's all do our best to combine well-established optical theory with unbiased first-hand observations. Ultimately, all astronomers will benefit.