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.

Jim Melka

How can a telescope have an f/ratio of 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...

Pi

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...

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...

The radiant of the Perseid Meteor Shower is high in the sky by 11 p.m. for observers at mid-northern latitudes.Sky & Telescope diagram

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,...

Mars Oct. 21-22

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...

Bubble-Nebula_Ha2-OIII-Sync_PS2-V1-Sideways-4STM--Scaled

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

I have an ongoing dispute with everybody. I say an OIII nebula filter cannot be “oh-three,” since OIII 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 oxygen atom,...

Gaussian function

What does “error” mean?

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...

Venus 7 days from inferior conjunction, imaged by Damian Peach.

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...

Take a walk on the wild side

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)....

The Pillars of Creation in Messier 16, the Eagle Nebula, may be the Hubble Space Telescope''s best-known (and most beautiful) photo.

How can I see more colors through my telescope?

Nebulae and galaxies invariably look like shapeless, colorless blobs in my 6-inch telescope, a far cry from their spectacular appearance in photographs. If I buy a 12- or 14-inch scope, will I see a dramatic improvement? Would that it were so! A larger telescope will better reveal the shapes of nebulae and galaxies, and...