Wikipedia:Reference desk/Archives/Science/2019 November 1

= November 1 =

Amateur solar telescopes
The common designs seem to focus on blocking most of the sunlight, so it isn't blindingly bright. But this seems to be a wasteful approach, to me. Most telescopes try to maximize light gathering power, but solar telescopes try to reduce it. Why not project the image onto a large enough area that it isn't blindingly bright, so as to not lose detail by filtering out light ? Wouldn't that make features like sunspots more visible ? Watching eclipses (partial or full) would be another application. SinisterLefty (talk) 06:30, 1 November 2019 (UTC)


 * Your suggestion only fits small-aperture telescopes, here's why: with a big telescope (my guess is that 200mm is too large) the heat that will be focused through the eyepiece will be enough to crack it, or even blow it to pieces.
 * I know for certain that with a 70mm aperture you suggestion is safe. I tried that. Directing the telescope to the sun was very frustrating, btw.
 * EDIT: I noticed your "so as to not lose detail". You do not lose detail if you make the holes in the opaque telescope cover at its opposite edges. The detail is determined by the effective aperture, and the effective aperture stays the same this way.אילן שמעוני (talk) 07:32, 1 November 2019 (UTC)


 * Not following your last comment. If only one photon comes from a particular feature on the Sun, that would have been collected by the telescope, and that photon is filtered out, then you lost that detail.


 * As for heat, here I would think filtering out the light would make things worse, if that filtered out light is turned into heat in the telescope. Rather than absorbing it, reflecting the excess light or refracting it, to a place designed to take the heat, would help. But if all the light passes through the lenses, very little should be turned into heat there. Keeping dust off the end of the eyepiece would be critical, though, as that would absorb light and heat up. SinisterLefty (talk) 15:42, 1 November 2019 (UTC)
 * The problem is the CONCENTRATION of heat in the eyepiece. The main mirror focus the heat from its entire area into a much smaller focal plain. Much like burning a paper with a magnifying glass - but with a larger scale. The cover you must use sits at the telescope front end, and so just gets warmed normally as if you laid the cover out in the sun. Only a tiny fraction of the radiation is allowed into the telescope itself.
 * As for your concern for image quality: Telescope angular resolution is proportional to its aperture. since the light from both holes focus to the same focal plain, it builds a single image - so you get the full width resolution.
 * I do not know of a specialized screening eyepiece except digital eyepieces, which are either low quality or VERY expensive (sometimes both). Projecting straight from the eyepiece is manageable enough, you just have to position the screen perpendicular to the common L shaped focuser. I am quite sure I'll find illustrations for this arrangement. אילן שמעוני (talk) 17:08, 1 November 2019 (UTC)

FOLLOW_UP: Is there equipment designed to project the telescope image onto a large screen ? I imagine you might need a long distance, without a special lens, to get the image to be large enough to not be blinding. Would the image also go out of focus over this distance ? SinisterLefty (talk) 16:03, 1 November 2019 (UTC)
 * The Helioscope has been around for quite a while. Jeremiah Horrocks used one for his observation of the 1639 transit of Venus and it was invented long before that. See also https://www.skyandtelescope.com/observing/observing-the-sun/ Richerman  (talk) 17:21, 1 November 2019 (UTC)
 * Take a peek at this short guide. Also, be aware that without very expansive H-Alpha filter you can not see solar eruptions. You can see granules as in the photo there, and sunspots - but, alas, there are currently no sunspots. אילן שמעוני (talk) 18:35, 1 November 2019 (UTC)
 * Further to the article externally linked by אילן שמעוני above, the method shown in the section titled 'Projection with a Telescope or Binoculars ' is easy to perform and known to all amateur astronomers. (If using binoculars, only one 'ocular' is used, with the other covered by its lens caps.)
 * The screen can be placed at any convenient distance from the eyepiece (a lightweight screen-holder may be constructed to attach to the scope), and the image is easily focussed using the eyepiece focusser: however, the further away the screen is, the larger but fainter the image will be.
 * I myself used as a teenaged amateur to project from the eyepiece of an stopped-down 8" Newtonian telescope, within my school's small observatory, an image of a foot diameter on to a hand-held screen (with a pre-drawn circle for the sun's outline), which was entirely bright enough for making observations and sketches, but in astronomy society meetings I have seen an image of perhaps 8 feet diameter projected onto a white wall in a darkened room.
 * Professional Solar telescope observatories were purpose built to eliminate extraneous light and maximise contrast and size when observing a raw image, though often they instead added a spectrograph to the optical train to observe and record the Solar spectrum. Nowadays, of course, professionals (and many amateurs) mostly use digital cameras and computer screens rather than projections and naked eyes. {The poster formerly known as 87.81.230.195} 2.122.179.237 (talk) 06:20, 2 November 2019 (UTC)

I have a couple cheap telescopes that I wouldn't mind damaging, in order to give this a test. I assume that the image of the Sun should always be pointed down, and nothing reflective should be down there, to avoid risk of eye damage. I also have some loose large lenses, such as one from an overhead projector, that may work. And I have a nice set of binoculars, but don't want to risk damaging those. Is a flat screen OK, or should it be parabolic ? SinisterLefty (talk) 06:34, 2 November 2019 (UTC)
 * I have never seen anything but a flat screen used. In theory this might introduce a small distortion, in that the regions further from the centre of the flat-screen image will be slightly stretched, but for a home set-up this would be imperceptible. Additionally, the centre and edge of the image will be at best focus at slightly different distances, so close examination of one or the other might require very slight refocussing of the eyepiece. This would only matter if the 'screen' was actually a photographic plate being used to capture a single complete image. {The poster formerly known as 87.81.230.195} 2.122.179.237 (talk) 19:54, 2 November 2019 (UTC)
 * It should be obvious that observing a common star that is only about 93 million miles away is dramatically different from all other aspects of amateur astronomy. Eclipse observation presents a great opportunity to observe the sun in "non boring" circumstances. I have had the privilege of observing an annular eclipse in Redding, California in 2012 and a total eclipse in Madras, Oregon in 2017. At such events, you can meet amateur astronomers with a high degree of expertise in the full range of observing techniques. In Oregon, we had two inexpensive solar telescopes, two DSLR cameras with telephoto lenses, and a four inch telescope. Rigging up the cameras and bigger telescope with homemade but well researched solar filters was fun. Roaming around seeing the incredible variety of amateur telescopes and discussing telescope techniques with amateur experts was memorable. Cullen328  Let's discuss it  07:13, 2 November 2019 (UTC)
 * User:SinisterLefty, what are the apertures (diameters) of the telescopes you have in mind? I asked several friends that are in my amature astronomy club. On used 114mm f10 without a problem. Another tried 6" (approx. 150mm) and said the focuser got very hot very quickly so he aborted. I heard that some people had eyepieces literally explode, which is dangerous beyond damage to the telescope.
 * All in all you got it right. You can use plain white paper, or a wall as a screen.
 * important: You may be into disappointment. with 70mm no granules where seen, only sunspots - but currently there are no sunspots! To view granules I think you'll need 6" or above. Many people will be disappointed by granules, though - they're not that spectacular, and the changes are too slow to perceive. You can forget about these spectacular solar prominences - that requires H-Alpha filter. אילן שמעוני (talk) 12:58, 2 November 2019 (UTC)
 * Although there are currently no spots, there was a cluster of faculae the last time I checked: however these are a rather subtle phenomenon for a first-time solar viewer. {The poster formerly known as 87.81.230.195} 2.122.179.237 (talk) 19:54, 2 November 2019 (UTC)

My cheap telescopes are 2.5 inches and 3 inches in diameter, which I assume will be unable to show any detail other than sunspots. On the plus side, heating shouldn't be much of an issue. Is there a good site with sunspot forecasts ? Or can they only say what the current conditions are ? SinisterLefty (talk) 21:33, 2 November 2019 (UTC)
 * Forecasts aren't very good. If you wanted to see say the most spotted sun of the next solar cycle you'd have to check hundreds of times to be sure you don't miss it. Some scientists are predicting that sunspots might be going downhill in the first third of the 21st century and will stay infrequent for up to several lifetimes so the next sunspot cycle might be the most impressive you'll ever see. The cycles are slowing down so late 2020s might be a good bet. Now is a deep solar minimum, infrequent, few and small sunspots. In 2019 75% of the time the sun had no sunspots of any size. Sagittarian Milky Way (talk) 02:14, 3 November 2019 (UTC)


 * OK, late 2020s it is, assuming both I and the Sun are still around then. SinisterLefty (talk) 16:12, 6 November 2019 (UTC)

Distance between and position of objects in the galaxy
So, one thing I have noticed when reading about other stars and exoplanets is that we already have estimated distance from Earth for most of them in terms of parsecs and lightyears. Does this imply that their positions relative to Earth never significantly change as they orbit around the Milky Way galactic center and that the speeds in which they move is not really different from the speed of the solar system? Is it actually possible to draw a map of our galaxy with fixed position for every significant object inside it and use it as a long-term reference? 70.95.44.93 (talk) 09:17, 1 November 2019 (UTC)
 * No. While many stars do roughly move along around the galactic center, the variation is huge. For example μ Columba AE Auriga are moving very fast at a path unrelated to orbiting the center. I seem to recall an application or maybe a site that shows how the night sky will change in the future. Whithin 10,000 years the change is significant.
 * EDIT: Didn't find app, but found a relevant vid. אילן שמעוני (talk) 10:27, 1 November 2019 (UTC)


 * And it may seem like the objects in the sky are moving slowly, but keeping in mind that the distances are huge, maybe thousands, millions, or billions of light years, it takes a long time to move a significant portion of those huge distances, even at extremely high speeds. SinisterLefty (talk) 15:48, 1 November 2019 (UTC)


 * The answer is complicated because many Star systems are "binary"(2), "trinary"(3) or even hole "clusters" of suns (1000), sometimes assumed with a dominating Black hole or alike "monster"-object in their center, so the movement of individual suns in that system may change very dynamically over short time.


 * Also realize that this field of research has actually just started and demands very, very expensive tools and instruments. The James Webb Space Telescope will be launched next in March 2021 and NASA has projected the costs at roughly over 10 Billion $ then. Just for this single Telescope! --Kharon (talk) 22:22, 1 November 2019 (UTC)


 * Meanwhile, Gaia has done a fine job of cartographing a significant part of the Milky Way, including measuring the velocities of millions of stars. The James Webb is completely unsuited for that sort of work. --Wrongfilter (talk) 22:46, 1 November 2019 (UTC)


 * אילן שמעוני in 5,000 years the night sky will change more due to precession of the earth axis, then due to stars proper motion. The whole cycle for precession takes 11,000 years. AboutFace 22 (talk) 01:45, 2 November 2019 (UTC)
 * It wouldn't. The poles will be elsewhere, but you would just have to take the current maps and adjust the coordinations grid. That's true for most visible naked-eye stars, there are some exceptions, but not because of percession. אילן שמעוני (talk) 12:46, 2 November 2019 (UTC)
 * 26000Sagittarian Milky Way (talk) 03:25, 2 November 2019 (UTC)
 * The Sun moves at 220 km/s around the center of the Milky Way, around 1/1400 the speed of light. That means it takes 1400 years to move a light year so if another star somehow stood still in the galaxy and the Sun moved directly towards it, it would take 1400 years to change the distance by a light year. And the relative speed between stars is usually much lower. If a star is measured to be n light years away then it doesn't matter much in what year it was reported, considering how briefly we have been able to make such measurements. But it does matter over many millenia as seen in File:Near-stars-past-future-en.svg. It takes the Sun 225 to 250 million years (a galactic year) to orbit around the Milky Way center. PrimeHunter (talk) 03:52, 2 November 2019 (UTC)


 * That's interesting. So the Sun has only circled the galactic center some 17-19 times. SinisterLefty (talk) 05:33, 2 November 2019 (UTC)


 * And it takes 171 to 190 years for the Sun to circle 1/360/60/60 times which is about 1 pixel through a telescope limited by the inability of humans to take 0.001 second retina snapshots to freeze twinkling blur. Sagittarian Milky Way (talk) 19:02, 2 November 2019 (UTC)

Standing wave, not
See. The article describes an actual standing wave, where two waves of the same period coming from opposite directions create a resultant wave that doesn't move horizontally, but changes greatly in magnitude, and features many equal peaks and troughs (see this video at 1:20: ). However, the pic at the top of the article is another phenomenon, where a wave doesn't change in either position or magnitude, and there is no interference between waves. This type generally has a large central peak/trough, with progressively smaller peaks and troughs following it, and seems to result from the geometry of the river bed just upstream. What is the real name for this phenomenon, and the physics behind it ? SinisterLefty (talk) 17:03, 1 November 2019 (UTC)


 * A Stationary hydraulic jump. Mikenorton (talk) 17:31, 1 November 2019 (UTC)


 * Thanks. SinisterLefty (talk) 18:32, 1 November 2019 (UTC)