190 results
Newtonian anatomy

Astronomy Questions & Answers

Is software available to analyze optical designs of telescopes?

Does software exist, preferably downloadable from the Internet, that I can use to analyze original optical designs of telescopes? Yes. The easiest way to find what’s out there is with a fine-tuned Google search. Right now, if you enter “optical design software” as the search terms, you get a few…

Astronomy Questions & Answers

What size first telescope mirror should I make?

What size is best for someone’s first attempt at making a telescope mirror? Most books on telescope making recommend starting with either a 6- or an 8-inch mirror, and that’s good advice. Either size is suitable for a first-timer, but a 6-inch will probably take a little less time to…

Hobby-based Q&A

When's the next blue Moon?

When's the next blue Moon? To read the full story with a schedule that shows all blue Moons through 2020, see our article "What is a Blue Moon?" Here's the abbreviated version: In 1999 Sky & Telescope admitted to its “blue Moon blooper,” an error that had crept onto the…


Hobby-based Q&A

How can eyepieces offer a telescope's widest true field?

Some eyepieces are being advertised as offering the widest possible true field in any telescope. How is this determined? The amount of sky shown by an eyepiece is governed by the diameter of its field stop, the ring that defines the edge of the visible field. On a given telescope,…

Astronomy Questions & Answers

When is the earliest sunrise of the year?

The earliest sunrise of the year doesn't always occur on June 20th (the solstice), which has the longest day. Why? The earliest sunrise actually depends partly on latitude. In 2004, for latitude 40° north, it was June 13th. For 50° north, the earliest sunrise was on June 16th. At 30°…

Hobby-based Q&A

Focal Ratio vs. Aperture: What Makes an Object Look Brighter Through a Telescope?

How can you make a star or deep-sky object, such as galaxies or nebulae, look brighter? The total brightness of a star or other object is determined by the telescope’s aperture, also called light grasp. The focal ratio (also called f/number, that is, the focal length divided by the aperture)…

The center of the stellar merry-go-round

Astronomy Questions & Answers

What's the distance to Polaris?

I have three astronomy programs on my computer that give three different distances for Polaris. I just obtained a fourth program, and it says the distance is unknown. What’s the problem? Those new to astronomy expect the distances of bright stars to be very accurately known, but many still aren’t.…

Astronomy Questions & Answers

If stars appear as points, why are some stars big and some small in different images?

If stars are unresolved points, even in the largest telescopes, why do bright ones look bigger than faint ones? Our eyes are amazing instruments, but they’re limited by physics and biology. When starlight enters an optical system with a circular aperture, such as a telescope or a human eye, the…

Mercury near the Sun's limb

Astronomy Questions & Answers

How can I polar align in the daytime?

I want to see a planetary transit. How can I polar align my telescope in the daytime? One good way is to use the Sun. Carefully level your mount with a bubble level and set the polar axis to the latitude of your site. Hang a weighted string from the…

Hobby-based Q&A

How do I clean eyepiece lenses?

Very carefully — and not often! Here are some tips to help you keep your optics clean.

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