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…

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…

Keck I

Astronomy Questions & Answers

How will you know when you telescope mirrors need re-aluminizing?

I have read that telescope mirrors require periodic re-aluminizing. How will I know when mine needs this? While it’s true that the aluminized surface of a telescope mirror will deteriorate over time, there is no hard and fast rule about how long this will take. Some coatings last only a…

Mirror Cleaning

Hobby-based Q&A

What should I do if my telescope mirror is dusty?

I took my new scope out last night and pointed a flashlight down the tube. My mirror looks really dusty. What should I do? The answer is simple: Don’t shine a light down the tube at night! Seriously, it only makes the dust look a lot worse than it is.…

Astronomy Questions & Answers

What’s the difference 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…

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