Hobby-based Q&A

Browse the stargazing questions and answers below to delve into the hobby of astronomy. Learn about different types of equipment and what they can do, and discover the limits (read: challenges) of observing the night sky with instruments and the unaided eye.

The Q&As presented here cover a wide range of questions posed over the years by our readers, and we’ve responded with detailed and well-researched answers. What’s the difference between a 6-inch and a 12-inch scope? Can we see the shadows of Saturn’s moons as we can Jupiter’s? And how can you turn your stargazing hobby into an astronomy career? Read on to learn the answers, and if there’s a question that hasn’t been asked, ask it yourself by sending a note to info@skyandtelescope.com.

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...

Is it possible to use a 12-inch reflector at f/42?

I was amazed at Jim Melka’s beautiful picture of Mars on page 136 of the January 2006 issue but puzzled by the caption, which said that he used a 12-inch reflector at f/42. How is this possible? Knowing that a telescope’s f/ratio is its focal length divided by its aperture, you’re probably imagining poor...

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...

What does the term “25 lignes” mean?

Paging through a reproduction of the 1909 Sears, Roebuck and Co. catalog, I found a telescope with an aperture of 25 lignes. Are you familiar with that term? In short, no. We’re also surprised to learn that such an archaic measurement was attached to a “modern” telescope. A ligne is one-twelfth of the old...

How well defined is a meteor-shower radiant?

How well defined is a meteor-shower radiant? Is it a point on the sky or a several-degree-wide spot? Radiants are spots, not points. A meteor shower’s radiant is the location on the sky where all the meteors would appear to come from if we could see them approaching in the very far distance. However,...

What is the faintest object imaged by ground-based telescopes?

I know that the Hubble Ultra Deep Field imaging campaign reached a limiting magnitude of 31, but what is the faintest object imaged by ground-based telescopes? Furthermore, how is it that an amateur astronomer was able to reach magnitude 24 with a 16-inch telescope, when even Hubble has gone no deeper than 31st magnitude?...

Is there a good test for optical quality?

In late October 2005 I hoped to see major surface markings on Mars, but my 10-inch scope showed only a uniformly yellow ball. Do I have an inferior mirror? The optics were well collimated. Is there a good test for optical quality? Starting in mid-October, dust storms on Mars spread a wash of yellow...

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...

Why do you need a hydrogen-alpha filter to see solar prominences?

If I can see solar prominences with the naked eye during a total eclipse, why do I need a hydrogen-alpha filter to see them at other times? During a total eclipse the Moon completely blocks the Sun’s photosphere, or visible “surface,” allowing prominences (and the corona) to shine in all their glory. Without the...

When you state that a telescope drive has a periodic error of 10 arcseconds, does that mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds?

In your product reviews, when you state that a telescope drive has a periodic error of, say, 10 arcseconds, do you mean that it has a tracking accuracy of ±10 arcseconds or ±5 arcseconds? All gear systems have inherent mechanical errors that limit the accuracy with which a telescope drive can track the sky’s...

Is it possible to see the crescent of Venus?

Is it possible, with better than normal eyesight, to see the crescent of Venus? That question has been controversial, but in fact some people can. The rough rule of thumb is that someone with excellent vision can just resolve two image elements 60 arcseconds (60") apart. At times, this is enough resolution to make...

How do you convert celestial coordinates for equinox 1950.0 to 2000.0?

I use Burnham’s Celestial Handbook, which gives objects’ celestial coordinates for equinox 1950.0. How do I convert these to 2000.0, the current standard? First off, the difference isn’t great. Fifty years of precession change an object’s right ascension and declination by a total of 0.7° at most (if it’s near the ecliptic; less elsewhere)....

When the Moon misses the Sun by the greatest amount north (or south), could the lunar crescent be seen in a telescope at new Moon?

Is the Moon’s orbit inclined sufficiently that, when it misses the Sun by the greatest amount north (or south), the lunar crescent could be seen in a telescope at new Moon? Probably not. The inclination of the Moon’s orbit to the ecliptic varies from 5.0° to 5.3°. French astronomer André Danjon (1890–1967) showed that...