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foam-pad dew shield

DIY: Astronomy Projects & Guidance

Dealing With Dew: Dew Heaters, Dew Shields and More

How to keep your optics dry and clear even on the dampest, dewiest nights.

Removing Mirror

DIY: Astronomy Projects & Guidance

How to Care for Your Telescope Optics the Right Way

The mirror in your telescope will probably work fine with a bit of dust on it, but if it's really dirty, you may want to clean it — carefully!

Roof-prism binoculars

Hobby-based Q&A

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…

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…

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…

Laser beam from Subaru Telescope

Astronomy Questions & Answers

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…

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…

solar prominence

Hobby-based Q&A

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…

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 -…

Astronomy Questions & Answers

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…

Tim Hunter and his telescope

Hobby-based Q&A

How Can Amateurs Find 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,…

SLRs: film (left) and digital (right)

Hobby-based Q&A

Why do people doing CCD imaging often stack five 1-minute exposures instead of taking just one 5-minute exposure?

Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure? Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture. So-called…

Hobby-based Q&A

Can you get sharper images by stopping down your scope with a cardboard mask?

Old astronomy books often say you can get sharper images on a night of poor seeing by stopping down your telescope with a cardboard mask so it has a smaller aperture. New books don’t mention this. It this a forgotten secret? In my experience, if stopping down a telescope sharpens…

Sec

Hobby-based Q&A

Can you run programs from past issues of S&T, written in BASIC, on a Windows XP computer?

Can I run programs from past issues of S&T, written in BASIC, on my Windows XP computer? All the BASIC programs that have appeared in S&T since 1984 are collected on our Web site at SkyandTelescope.com/resources/software. All you need to run them is a BASIC interpreter or compiler. Interpreters let…

Newtonian

Hobby-based Q&A

Can you adjust a f/7.7 spherical mirror to act like a f/8.0 parabolidal mirror by racking the focus farther out?

My 114-millimeter (4.5-inch) Newtonian reflector came with an f/7.7 spherical mirror. I can purchase a 114-mm f/8.0 paraboloidal mirror. Can I simply adjust for the 34 mm of added focal length by racking the focuser farther out, or should I extend the main tube? Either way, is the upgrade worth…

Eyepiece Barrels

Hobby-based Q&A

Where did the 1 1/4-inch standard size for eyepiece barrels come from?

Where did the 1 1/4-inch standard size for eyepiece barrels come from?   This barrel size was adopted shortly before 1890 by John A. Brashear in Pittsburgh, Pennsylvania, says Bart Fried of the Antique Telescope Society. Then W. & D. Mogey of Plainfield, New Jersey, and other firms followed suit.…

Saturn and Antares

Hobby-based Q&A

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…

primary mirror

Astronomy Questions & Answers

Would a large concave mirror suffice for low-power views of extended deep-sky objects?

A large concave mirror, even of low quality, has lots of light-collecting power. Would such a mirror suffice for low-power views of extended deep-sky objects, even if it didn't show stars as neat dots? For example, a 20-inch plastic mirror might be fairly inexpensive. My 7th-grade science teacher once let…

Astronomy Questions & Answers

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…

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