81–100 of 188 results
Roof-prism binoculars

Astronomy Questions & Answers

Why do the best roof-prism binoculars need a phase-correction coating?

Why do the best roof-prism binoculars need a phase-correction coating? How does it work, and why is it not used in Porro-prism binoculars? A roof prism splits the light cone from an objective lens into two separate pathways, involving different internal reflections, before recombining them into an erect image at…

Closeup of 8-mile crater

Hobby-based Q&A

Why do photos of lunar or Martian craters sometimes appear inside-out?

Frequently when looking at photos of lunar or Martian landscapes, I see craters as blisters. This can usually be rectified by turning the picture upside down; then craters reveal themselves as indentations rather than bulges. Do I need spectacles? No; this common illusion is all in the brain. Whether you…

Astronomy Questions & Answers

Can Venus ever be far enough from the Sun to appear east of the meridian?

During the Northern Hemisphere winter solstice, when the Sun is farthest south and sets in the southwest, can Venus ever be far enough from the Sun to appear east of the meridian? Yes, but only when viewed from near-arctic latitudes. In late 2008, for example, Venus is the Evening Star…

2003 UB313 art

Astronomy Questions & Answers

Why are Kuiper Belt discoveries made at aphelion?

Why is it that so many of the recent Kuiper Belt discoveries — including the “10th planet,” 2003 UB313 — have been made while the objects are at aphelion? Timing and luck. Objects that orbit the Sun spend most of their time near the farthest point of their orbits, where…

Astronomy Questions & Answers

How many stars are in the Milky Way?

What is the best educated guess for the number of stars in the Milky Way? I’ve seen figures ranging from “some 20 billion” to “just under 600 billion.” The first seems too low, whereas the latter too high. The short answer is, nobody knows. But your reasoning is sound: 20…

Pi

Hobby-based Q&A

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…

Astronomy Questions & Answers

What does "lignes" mean for aperture of antique telescope?

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…

Hobby-based Q&A

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…

Hobby-based Q&A

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…

Laser beam from Subaru Telescope

Hobby-based Q&A

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…

Westerlund 2 as viewed by Hubble's WFC3

Astronomy Questions & Answers

What does “true color” mean in a deep-space photograph?

What does “true color” mean in a deep-space photograph? How would a galaxy or nebula actually look to the naked eye from close up? “True color” is how an object would look if you were nearby or were viewing it in a telescope under a dark sky, and if its…

Mars Oct. 21-22

Astronomy Questions & Answers

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…

Astronomy Questions & Answers

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…

Totality

Astronomy Questions & Answers

When will the last total solar eclipse occur?

With the Moon slowly receding from Earth, there will come a time when it no longer has sufficient angular size to cover the Sun completely. When will the last total solar eclipse occur? When the Moon’s at its closest and biggest and it can no longer cover the Sun at…

solar prominence

Sun

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…

Astronomy Questions & Answers

If asteroid 99942 Apophis ever strikes Earth, how big would the crater be?

If the near-Earth asteroid 99942 Apophis ever strikes Earth, how big would the crater be? Apophis (formerly 2004 MN4) ranks at the top of the worrisome-asteroid list. On April 13, 2029 (yes, that will be a Friday the 13th), it will pass only 30,000 kilometers (18,600 miles) from Earth’s surface…

Astronomy Questions & Answers

Will we ever see a mirror image of the conjunction of Venus and crescent Moon, as depicted on the Turkish flag?

On the evening of April 19th 2007, I witnessed a spectacular conjunction of Venus and the screscent Moon with earthshine. The cusps of the crescent pointed left (south) as the Moon set in the west. Will we ever see a mirror image of this event as depicted on the Turkish…

Science-based Q&A

How large an asteroid could a person jump off?

How large an asteroid could a person jump off? This interesting question goes back to 1952 or even earlier. It turns out that the asteroid must be no bigger than 3.9 v(ρe / ρ) kilometers in diameter, where ρ is the asteroid’s density and ρe is that of Earth in…

Schmidt-Cassegrain

Hobby-based Q&A

Why do Schmidt-Cassegrain telescopes have focal ratios of f/10?

Why do most if not all of the Schmidt-Cassegrain telescope I see advertised have focal ratios of f/10? During the past 20 years or so, amateurs have favored shorter tubes for both refractors and reflectors, even if it meant paying for more costly optics. Portability is the chief motivation -…

Gaussian function

Hobby-based Q&A

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…

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