Optics & Observing Aids

Mercury near the Sun's limb

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 mount (between the tripod legs)…

Mirror-making at Stellafane. Richard Koolish

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…

Baader filter.

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…

Dennis diCicco m

Should I wear eyeglasses while observing?

Should I wear eyeglasses while observing? If you are near-or farsighted, with no other eye problems, definitely take them off. You’ll have to tweak the focus, but you’ll see objects in the telescope just as clearly as if you had 20:20 vision. By taking off your glasses you’ll avoid the “tunnel vision” effect often caused…

Samples of lunar soil collected by Apollo 17 astronauts
          in 1972 continue to yield new findings; they contain an isotope of beryllium
          that is providing clues to the workings of the Sun's atmosphere. Eugene
          A. Cernan and Harrison H. Schmitt (pictured here) returned to Earth
          with more than 100 kilograms of lunar material.

Could you see astronauts on the Moon?

If there were astronauts on the Moon, could we see them? I get some variant of this question almost every time I show someone his or her first view of the Moon through a telescope, especially at high magnification. The breathtaking sight of lunar craters, mountains, and other features down to the limit of perception…

wired for warmth

How Can I Prevent My Finder From Dewing Over?

How do I keep my finder from dewing over when I observe? Your finder should have a light shield (“dewcap”), which you can cover between uses. Or you can purchase an anti-dewing device. This need not be expensive. You probably have a box full of them at home: A simple tissue stuffed into the light…

How is the date of Easter determined?

How is the date of Easter determined? The rule most people remember is that Easter falls on the first Sunday after the first full Moon following the March equinox. In practice, Roman Catholic and Protestant churches follow a method of calculation adopted with the Gregorian calendar reform of 1582. The ecclesiastical dates of the March…

Galileo Galilei sketched the Praesepe star cluster (now known as the Beehive Cluster) in 1610.Octavo Corp./Warnock Library

Were stars artistically depicted with diffraction spikes before the invention of the Newtonian reflector?

Were stars artistically depicted with diffraction spikes before the invention of the Newtonian reflector? If so, why? Stars were being drawn with points or spikes long before Isaac Newton announced his reflecting telescope in 1671. Just look at early works of art, flags, ancient coins, and the charts of the Pleiades and Praesepe star clusters…

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…