37 votes

Shouldn't this cause a fire?

It could start a fire if the screen is at the focal point of the optical system. That is how you light fires with a magnifying glass. Here, the blackboard is likely away from the focal point, so you ...
usernumber's user avatar
  • 17.5k
35 votes
Accepted

Shouldn't this cause a fire?

For a magnifying lens or mirror to be able to ignite something with light from the Sun, its surface area must be large relative to the square of the focal length. Solar energy will be spread ...
supercat's user avatar
  • 466
24 votes
Accepted

Why different specifications for telescopes and binoculars?

With a binocular, all its optical components are fixed - the user can't change them. What's important for the user to know is the size of the front lens, which determines the brightness (and in ...
Dr Chuck's user avatar
  • 4,304
13 votes
Accepted

Why does the Moon appear to be flat?

It is an optical illusion. We perceive nearby objects in 3d because we have two eyes. As we see objects from two different viewpoints, our brain can put the images together to make a 3d image. ...
James K's user avatar
  • 121k
12 votes

Has anyone ever tried to make a simple telescope using ice?

I'm not sure it counts as "simple" but there is the ice cube neutrino observatory whose detector consists of a cubic kilometer of very clear ice a mile or so down in the antarctic icecap.
Steve Linton's user avatar
  • 10.3k
12 votes

Shouldn't this cause a fire?

The key quantity, as others have noted, is the ratio of the objective lens area to the area of the Sun image. Suppose you use a magnifying glass of 70 mm diameter and 180 mm focal length. The Sun's ...
Mike G's user avatar
  • 18.7k
12 votes
Accepted

Did this satellite streak past the Hubble Space Telescope so close that it was out of focus? If so, how close was it?

As you note, that particular image has been identified as due an "out of focus" satellite (the other images presumably show more distant satellites in e.g. geosynchronous orbits). The Nature ...
Peter Erwin's user avatar
  • 16.7k
11 votes

Why different specifications for telescopes and binoculars?

Binoculars tend to be mostly used for daytime observing (of birds, ships etc) Telescopes are mostly used for astronomical observing. The users of the two types of equipment want different information. ...
James K's user avatar
  • 121k
11 votes

Is it possible to block the surrounding light in a solar eclipse if we made the moon bigger or closer to the earth?

Let's clarify what "ring of light" you want to cover. In the original photo, the bright spot is the photosphere ("P" in the annotated copy or the original image). The photosphere ...
JohnHoltz's user avatar
  • 7,982
10 votes
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Split telescope into two eyes

Absolutely. You are looking for a binocular eyepiece, or a binocular viewer. Most of them require you to insert two matching ocular eyepieces, so it can be fairly expensive. Most major telescope ...
Dan Hanson's user avatar
  • 1,161
10 votes

For lenses, what is the practical difference between plano- and bi-?

I don't know if this is a trivially obvious question for astronomers... Astronomers (amateur, pro, or otherwise) are a diverse bunch; some are intimately involved in telescope and instrument design, ...
uhoh's user avatar
  • 31.1k
9 votes

How does making a refracting telescope very long reduce the chromatic aberration of an uncorrected lens?

The actual math is a bit complicated, but there's a simple intuitive explanation. Longitudinal chromatic aberration happens because, when you cut a convergent lens in two, and you look at the cross-...
Florin Andrei's user avatar
9 votes

Why does one object (star?) in this JWST image have two sets of six+plus+two diffraction spikes but another, similar object nearby have only one?

if you think that's weird, you should check out this press-release image from ESA -- showing a mosaic of HST and JWST images of a region near the North Ecliptic Pole as part of the PEARLS program -- ...
Peter Erwin's user avatar
  • 16.7k
8 votes
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What is the cause of all of these sharp, concentric rings around bright stars in this HST image?

The diffraction pattern at the focal plane created by a circular aperture is called an Airy Disk or Airy Pattern. Both the outer opening and the inner hole plus secondary contribute to the exact ...
eshaya's user avatar
  • 3,726
8 votes

Why does the Moon appear to be flat?

In addition to James K's perception methods, one can make a stereo pair of images by taking photos at different times in the Moon's libration as explained in Stereoscopic Pictures of the Moon. ...
Keith McClary's user avatar
8 votes
Accepted

Why are Shack-Hartmann sensors so expensive (4k+ USD)?

I have been deeply involved in both Shack-Hartmann and lateral-shear polarization interferometers. Now I want something simple and slow for hobby projects and had the same question. I don’t think such ...
Jon Twichell's user avatar
8 votes

Does this CHEOPS first light image imply bad astronomy?

As the article you reference makes clear, the defocusing is deliberate. It spreads the light of bright stars (the main targets for CHEOPS) over more pixels and hence mitigates saturation and non-...
ProfRob's user avatar
  • 152k
8 votes
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Have there been studies of "old photons" to see just how constant things like Planck constant has been?

You can not check if a dimensional constant has changed because you can always reverse that change by a smart change of coordinates (system of units). Despite that, since the current Physics assumes ...
Catarina Alves's user avatar
8 votes

Why does this large Newtonian telescope's front cover have two or three holes in it?

What are these? These are used to reduce the aperture of the instrument. While you see three in this image, likely only one is removable. The instrument appears to be a Newtonian reflector. Which ...
Tim Campbell's user avatar
  • 1,561
8 votes

Shouldn't this cause a fire?

It all depends on how concentrated the energy is. Sun light travels through a lens like this: (left to right) The closer to the focal point the surface of the board is, the more the light that hits ...
Caston's user avatar
  • 329
8 votes
Accepted

Do telescopes exist that reflect the incoming light more than three times along their length?

If your willing to accept more than 2 discontinuous mirrors, the Three Mirror Anastigmat has 4 passes along some/most of the overall tube length. An early working prototype example (which I've ...
astrosnapper's user avatar
  • 8,357
7 votes
Accepted

Are there any mirrors in space?

Very likely not. If it's a gravitational process like galaxy lensing, then a "mirror" would require a very large deviation. That's extremely unlikely with this process. With other processes, e.g. ...
Florin Andrei's user avatar
7 votes

Constructing a periscope/telescope - trouble with lenses

You're close! What you built is a back-to-back set of Galilean telescopes (with an overall magnification of 1:1). The reason you're getting a smaller circle, but with a life-size image, is that the ...
nflemming2004's user avatar
7 votes

How are "parallel fields" implemented on the Hubble Space Telescope?

As the question Instrument aperture sizes on Hubble Telescope shows, the focal plane area is large enough to focus on several instruments at the same time (but with each capturing a different area). ...
amI's user avatar
  • 200
7 votes
Accepted

Can the interferometer called "Gravity" measure "a few centimeters on the Moon"?

A 130m baseline operating at 2 microns gives a theoretical resolution of $2\times 10^{-6}/130$ radians. At a distance of 400,000 km this translates to 6m. My guess is that Genzel is referring to the ...
ProfRob's user avatar
  • 152k
7 votes

What is it about the layout of the three gratings in a Bahtinov mask that makes its indicate focus in this way?

The movement of the diffraction spikes is similar to the phase detection autofocus that's been used in SLR cameras for decades. Before the days of autofocus, many SLR cameras had a split-image ...
Jerry Coffin's user avatar
6 votes

What is the faintest magnitude a telescope can see?

This is a term known at the limiting magnitude. This term refers simply to the faintest apparent magnitude your instrument can detect. Wikipedia has an article describing limiting magnitude. I pulled ...
El Bromista's user avatar
6 votes

Dish antenna as parabolic mirror for OPTICAL telescope?

I would have added this as a comment (not enough rep yet, I'm afraid)... To elaborate on Andy's answer, the first reason is that the surface of the satellite dishes are too coarse to form any kind of ...
nflemming2004's user avatar
6 votes

Dish antenna as parabolic mirror for OPTICAL telescope?

It's not possible I'm afraid. Optical wavelengths (light) are typically of a wavelength under a micron, and an optical surface needs to be accurate to this level or better to be useful. Radio ...
Andy's user avatar
  • 2,467
6 votes

Magnification of a telescope

The idea of magnification is not relevant to astro-photography, what is relevant is the image scale. The image scale depends on the focal length of the objective and the size of the sensor. The ...
Conrad Turner's user avatar

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