Hot answers tagged

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 ...
user avatar
  • 16.8k
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 ...
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 ...
user avatar
  • 4,065
13 votes
Accepted

Theoretically, what is the biggest optical telescope that may exist?

It's complicated. Until late-20th century, we've tried to make bigger and bigger monolithic telescopes. That worked pretty well up to the 5 meter parabolic mirror on Mount Palomar in California in ...
user avatar
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.
user avatar
  • 9,933
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 ...
user avatar
  • 16.6k
11 votes
Accepted

Why does a mirror bent 'like a potato chip' allow space telescopes to be smaller and have a wider field of view?

Freeform optics are a response to the specific challenge of cramming a telescope in a very limited space. A traditional instrument would have all optics symmetrical and aligned on the same axis. It ...
user avatar
11 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. ...
user avatar
  • 91.3k
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. ...
user avatar
  • 91.3k
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 ...
user avatar
  • 7,160
10 votes

How do telescopes "zoom" and change angle of view?

Telescopes tend to have a fixed focal length. What changes is the size of the sensor in the instrument used. If a small sensor is used, then a smaller section of the field of view is exposed, ...
user avatar
  • 3,214
10 votes

Why can't the surfaces of stars be observed?

There are many different ways to get spatial information about the surface of a star besides direct imaging. Direct imaging is difficult because the angular resolution available goes as $\lambda/D$. ...
user avatar
  • 118k
9 votes
Accepted

Astronomical telescope making

The first question anyone asks about a telescope is "what is the magnification?" It is almost always not the most important thing. Any telescope can magnify a million times, given a short enough ...
user avatar
9 votes
Accepted

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 ...
user avatar
8 votes
Accepted

Is it possible to steer the sight of a Liquid Mirror Telescope using a plain mirror scheme? If yes, why hasn't it been done?

Designs where the curved primary mirror is fixed, and steering is achieved via a moving flat mirror (siderostat) in front of it (or a set of flats) have been done before. One example is the Pfund ...
user avatar
8 votes
Accepted

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 ...
user avatar
  • 2,857
8 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-...
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 ...
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-...
user avatar
  • 118k
8 votes
Accepted

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 ...
user avatar
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 ...
user avatar
  • 309
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 ...
user avatar
  • 7,250
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 ...
user avatar
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. ...
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). ...
user avatar
  • 200
6 votes
Accepted

Why aren't secondary mirrors offset to get rid of diffraction spikes due to the support vanes?

There is a 1.6 meter off-axis telescope for solar observation at Big Bear Observatory called the New Solar Telescope (NST). Off-axis is particularly helpful in solar observations because it reduces ...
user avatar
  • 2,857
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 ...
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 ...
user avatar
  • 2,377
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 ...
user avatar
6 votes
Accepted

Did Arecibo's secondary optics compensate aberrations when viewing farther away from vertical?

While the statement in the block quote about the sphere is correct as far as it goes, was the shape of the correcting optics (secondary, etc) above also independent of where you look? Or ideally would ...
user avatar

Only top scored, non community-wiki answers of a minimum length are eligible