Astronomy Hacks

Eliminate Diffraction Spikes and Increase Contrast

Build a $0 aperture mask to turn your $500 Dob into a $2,000 apo refractor.

Well, not really. But a simple, cheap hack can let your inexpensive Dob provide most of the benefits of an expensive apo refractor. Read on.

There’s an old saying among astronomers: Aperture Rules. And it’s true. A larger aperture gathers more light and provides higher resolution of fine detail. Assuming the larger scope has at least decent optics, you can simply see more with a larger scope than with a smaller one, period. That’s the reason Newtonian reflectors, particularly Dobsonians, are so popular. They provide a lot of decent-quality aperture for not much money.

But if you attend a large star party, you’ll see something puzzling. There’ll be a lot of people using Dobs up to 20” or larger. There’ll also be a lot of people using 8”, 10”, and 12” SCTs. But mixed in with these mid-size and large scopes, you’ll see a fair number of people using 3” to 5” premium apochromatic refractors, such as those made by Tele Vue, Takahashi, and TMB. And those refractors, small as they are, aren’t cheap. Even a small apochromatic refractor can easily cost $2,000 or more without a mount. The 5” models sell for $4,500 and up. So why would anyone pay so much for such a small scope? In a word, image quality. (Well, OK, two words.)

The big Newtonian reflectors and SCTs provide a lot of aperture, but they are obstructed scopes. They have a secondary mirror in the light path ...