Optics & Observing Aids

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 complete. What is perhaps an…

Saturn and Antares

What was that flashing light in the sky?

I'm new to astronomy (1½ months) and I live in New Jersey. Last night, July 31st, I saw a bright planet (I assume Jupiter) in the southwestern sky, and just below it what looked like an airplane with a flashing red tail marker — but it never moved. When I got home I looked at…

What’s the difference between nebula filters and light-pollution filters?

What’s the difference between nebula filters and light-pollution filters? Nebula filters block out most of the visible spectrum while transmitting the handful of finely tuned colors that emanate from the most luminous constituents of cosmic gas clouds: hydrogen atoms and oxygen ions. Light-pollution filters, by contrast, are designed to block the colors emitted by typical…

When will (or did) the galactic equator cross the ecliptic very close to the latter’s northern and southern extremes?

As a teenager, when examining an equinox-1950 star atlas, I noticed that the galactic equator crossed the ecliptic very close to the latter’s northern and southern extremes (that is, the solstices at right ascension 6h and 18h). On equinox-2000 charts they are even closer. I’d love to know when the exact coincidence will (or did)…

SLRs: film (left) and digital (right)

Why do people doing CCD imaging often stack five 1-minute exposures instead of taking just one 5-minute exposure?

Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure? Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture. So-called image stacking is the easiest…