Optics & Observing Aids

Amateurs Finding Asteroids

In a 1996 article titled “Hunting Asteroids,” you said a dedicated amateur could discover an asteroid on almost any night using a CCD-equipped 8-inch telescope. Is that still true today? It was easier for backyard observers to discover asteroids a few years ago. Today, massive professional surveys such as LINEAR, NEAT,and LONEOS are sweeping...

Can an O III nebula filter be called “oh-three”?

I have an ongoing dispute with everybody. I say an O III nebula filter cannot be “oh-three,” since O III stands for doubly ionized oxygen atoms. I call it an “oh-two” filter. Who’s right? Sorry Philip, you lose. Not only is “oh-three” the universal usage; it makes sense. Well, sort of. A neutral, non-ionized...

Can 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...

Cleaning eyepiece lenses

How do I clean eyepiece lenses safely? Very carefully — and not often. If you rub fine optics, you risk rubbing microscopic bits of gritty dust against them. These leave tiny scratches (“sleeks”) that will scatter light and degrade contrast. A few sleeks won’t matter, but many of them eventually will, so your goal...

Comparing Plössl Eyepieces

I bought exactly the same Plössl eyepiece that a buddy has, but mine doesn’t work as well as his. Did I get a lemon? That’s possible, but very unlikely. Our guess is that your telescope has a short focal ratio, perhaps f/5 or less ,while your friend may have an f/10. Plössl eyepieces are...

Differences 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...

Eyepieces offer 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, you can compute the...

Graph error bars

You often print graphs in which data points have error bars that are too short. About a third of the error bars don’t overlap the line of best fit. So which is wrong? Neither! When it comes to describing statistical uncertainty, such as drawing an error bar, the usual measure is plus or minus...

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...

How many digits are satisfactory in the measurement of pi?

In the 3rd century BC, Archimedes proved that the ratio of a circle’s circumference to its diameter is less than 3 1/7 but larger than 3 10/71. That’s about 3.141. Later mathematicians have computed what we now call p (pi) to greater and greater accuracy — but how many digits are “enough”? The answer...

Is it possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear when Ganymede is at greatest elongation from Jupiter?

I’ve heard it might be possible to detect Jupiter’s satellites with the unaided eye if Callisto and Ganymede appear together when Ganymede is at greatest elongation from Jupiter. Will this happen anytime soon? Jupiter is now setting soon after sunset. But three times in 2008, Texans (and North Americans generally) will have a chance...

Making a first telescope mirror

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...

Original 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 Internet search using Google.com. If you enter “optical design software” as the search terms, you get a few million Web pages. But...

Polar align in 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...

Prevent your 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...