# Tag Info

36

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 can see the shape of the eclipse (and you get a bigger image) without setting things on fire. Although this is fairly safe, there are a few things to pay ...

35

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 throughout the projected image, and the size of that image will be essentially proportional to the focal length, making its area proportional to the square of focal ...

24

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 theory sharpness) of the image, the magnification, and the field of view. These are all useful things to know. A telescope has an interchangable component, namely ...

13

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 the 1940s. It kind of worked, but just barely, for the 6 meter mirror on Caucasus in Russia in the 1970s. It did work, but that was a major achievement, for the ...

12

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.

12

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 apparent angular diameter is 32 arcmin or 9.3 mrad; the focused Sun image diameter is $$180~\text{mm} \times 0.0093 = 1.7~\text{mm}$$ As the lens area is 1750 ...

11

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. You may find large, mounted, astronomical binoculars described more like a telescope. Similarly, small "mononcular" may be described in the same terms as ...

10

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, resulting in a narrower field of view being imaged than the equipment is capable of. If a larger sensor is used, more of the field of view of the telescope is utilised. ...

10

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 $\lambda/D$. For a 8-m telescope and light at 500 nm, one can resolve $6\times10^{-8}$ radians (assuming the blurring of the atmosphere can be overcome by adaptive optics or ...

10

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. Objects that are more distant perceived as 3d if they are moving fast enough for us to see them change appearance. Again it is not in the eyes but in the brain that ...

9

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 would waste a lot of space within the cubesat. Also, traditional designs tend to be much longer than they are wider; they don't fit well in a cube; it is very ...

9

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 eyepiece - the problem is, how good the image is. For observing planets, the main thing is resolving power - the ability of the telescope to discern tiny details. ...

9

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 eyepiece manufacturers sell them. I'm not sure what the rules are for mentioning brand names, but a quick google for 'telescope binocular viewer' will turn up ...

8

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 telescope. The main design constraint that led to the invention of the Pfund telescope was the need to have a focal point which is fixed in space and easily ...

8

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 function. This is usually not easily observed with ground based telescopes because the seeing fluctuations due to air turbulence smears it out. These images use ...

8

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-linearity problems in the detectors. The first light images look very similar to simulated pre-flight images (e.g. Hoyer et al. 2020; Futyan et al. 2020). The first ...

8

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 the immutability of certain constants, you can verify this assumption by testing the change of an adimensional constant. One of the most common adimensional ...

8

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 the surface of the lens will be concentrated into a smaller area on the board. More light means more heat, which will start a fire. If the board is further ...

8

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 actually seen in person many years ago) was built at the University of Cambridge by Dr. Roderick Willstrop. The Institute of Astronomy has a page on the Three Mirror ...

7

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 light bundles in the middle are diverging faster, and so you lose the edges if the Galilean assemblies are too far apart. Military perisopes use optical relays ...

7

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. classic reflection, it seems even more unlikely. We don't know any natural processes that could craft mirrors at cosmic scale.

7

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-section, the edge of the lens looks a bit like a prism, doesn't it? (look at the diagram that you've posted above, the top of the lens) And it does exactly what ...

7

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). If two objects of interest are separated by a certain angle (the instruments are fixed within the focal plane), the telescope can be rotated so that two ...

7

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 a project is as daunting as might be imagined. One would like 12 bits in the camera, the Sony CMOS cameras are so good, the last bit is nearly noise free. The ...

6

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 image. Polished optical surfaces are smoothed to a polish (generally much smoother than one would achieve in polishing a car, though). Other problems come ...

6

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 wavelengths are typically 10-20cm or longer, and an adequate reflector can be made with surfaces accurate to a few centimetres (at a guess as I don't know the exact ...

6

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 Barlow effectively modifies the focal length, so if you have a $B\times$ Barlow and an objective of focal length $f_O$ the effective focal length is $f_e=B\times f_O$...

6

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 you like a different secondary shape depending on how far off-vertical you look? If so, is this what happens? I will answer under the assumption this question ...

6

You can probably get most if not all of your questions answered by perusing the main DESI web site, which I encourage you to check out. There is, for example, a nice video describing the assembly of the main focal plane elements (the fibers and the associated robot positioners) here. But in simple terms: the circular focal plane is divided into ten wedges (...

6

So it looks something like this? https://cloud.planetmaker.de/index.php/s/RkmkBNfb8YyDHSE That sounds like the circum-horizontal ring (yes a name as simple as that). It (probably) is caused by hexagonal ice crystals floating horizontally. Where it touches the 22° halo you might see sun dogs which can be slightly colourful brighter patches. See also https://...

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