Starship Asterisk*

alter-ego wrote: ↑Wed Jul 21, 2021 3:52 am

neufer wrote: ↑Tue Jul 20, 2021 3:28 am ...

Thermodynamics requires that no image of the Sun
can be hotter than the photosphere, itself: 5,772 K.

The Moon is 400,000 times dimmer than the Sun such that no visible light
image of the Moon can have a bolometric luminosity greater than 230 K.

...

Very nice!
A real nugget of knowledge for me. I've got a whole new appreciation for the 2nd Law of TD.

neufer wrote: ↑Tue Jul 20, 2021 3:28 am ...

Thermodynamics requires that no image of the Sun
can be hotter than the photosphere, itself: 5,772 K.

The Moon is 400,000 times dimmer than the Sun such that no visible light
image of the Moon can have a bolometric luminosity greater than 230 K.

...

Chris Peterson wrote: ↑Mon Jul 19, 2021 11:42 pm
An interesting consequence of this, and an interesting thought problem, is to consider why we don't blind ourselves when looking at the Moon with a large aperture. In particular, consider a telescope with, say, a 1 meter aperture and an eyepiece chosen to produce a magnification of one (e.g. a 5 m focal length EP on a telescope with a 5 m focal length). On first thought, we might consider that this would introduce more light to the eye by the ratio of the squares of the telescope aperture to the pupil diameter. So, 1000mm^2 / 6mm^2 = 28 thousand. That seems like it would be enough brighter to do some damage. But the formula you give for the exit aperture works both ways. If your telescope has a magnification of one, the entrance pupil of your eye maps to the entrance aperture of the telescope. Doesn't matter if it's a meter in diameter, only light entering the central 7mm makes it into your eye. So the image through this telescope is exactly what you'd see without the telescope (except for some reduction due to losses in the system).

<<Venus reaches its greatest magnitude of about −4.5 when it is an intermediate crescent shape at the point in its orbit, when it is 68 million km away from the Earth (a combination of its closeness and the fact that it is 28% illuminated).>>

http://palomarskies.blogspot.com/2009/01/60-years-ago-today-first-light-for-hale.html wrote:

<<60 years ago today [Monday, January 26, 2009]Edwin Hubble took the first astrophotos from Palomar's 200-inch Hale Telescope. The photo above comes from the LIFE photo archive hosted by Google. In the picture you can see Edwin Hubble inside the prime focus cage high above the 200-inch primary mirror. In those nights the astronomer had to ride inside the telescope to take images and spectra of the objects they wanted to study. Long nights and winter temperatures were known to take their their toll on astronomers who eventually took to wearing World War II surplus electrically-heated flight suits.>>

Chris Peterson wrote: ↑Mon Jul 19, 2021 11:42 pm
You could light a piece of paper on fire using the Moon and a large enough mirror... but in that case the optical system isn't a telescope, it's just a single objective.

https://en.wikipedia.org/wiki/Concentrated_solar_power wrote:
<<Carbon neutral synthetic fuels production using concentrated solar thermal energy at nearly 1500 °C temperature is feasible technically and viable commercially in near future with declining costs of CSP plants

There is evidence that current large area solar concentrating installations can melt birds that fly over them. Near the center of the array temperatures can reach 550 °C which, with the solar flux itself, is enough to incinerate birds while further away feathers are scorched leading to the eventual death of the bird. Workers at the Ivanpah solar power plant call these birds “streamers,” as they ignite in midair and plummet to the ground trailing smoke. During testing of the initial standby position for the heliostats, 115 birds were killed as they entered the concentrated solar flux. During the first 6 months of operations, a total of 321 birds were killed. After altering the standby procedure to focus no more than four heliostats on any one point, there have been no further bird fatalities.>>

neufer wrote: ↑Mon Jul 19, 2021 11:10 pm

http://www.ianmorison.com/rich-field-telescopes-and-wide-field-observing/ wrote:

Professor Morison's Astronomy Digest
Rich Field Telescopes and Wide Field Observing

<<As the name implies, a Rich Field Telescope is one that will show the observer the maximum possible number of stars within the field of view when looking, say, towards the Milky Way. Using such a telescope to sweep along the Milky Way on a dark moonless night, is one of the most beautiful sights that can be seen in the heavens.

It turns out that we tend to see the most stars when the exit pupil of the telescope/eyepiece combination equals that of the dark adapted eye. In young people this is approximately 7 mm but, sadly, as we age this slowly drops down towards 5 or 5.5 mm. The exit pupil is the telescope aperture divided by the magnification. Let us use a mid-range exit pupil diameter of 6 mm and calculate the appropriate magnifications for a range of telescopes. For a 70 mm aperture one would need a magnification of 11.6, for an 80 mm refractor x13.3, a 102mm (4 inch) telescope x17, a 150mm telescope x25, a 200 mm (8 inch) x33.3 and a 300 mm (12 inch) x50.>>

• 1) At lower magnification than the Goldilocks/optimal faint object magnification (= aperture/6mm) all the light collected by the telescope aperture does not make it into the pupil such tbat one cannot see the faintest/most distant stars.
  

  

  Schematic of Keplerian refracting telescope.

  ---

  2) At higher magnification than the Goldilocks/optimal faint object magnification (= aperture/6mm) the faintest/most distant stars are all visible; however, they are now spread out such that fewer are in one's field of view (so that the average star field is fainter). Similarly, other resolved continuous fields like large gaseous nebula will also appear fainter.
  
  The ideal magnification for small gaseous nebula is discussed by alter-ego:
  http://asterisk.apod.com/viewtopic.php? ... 83#p314983

http://www.ianmorison.com/rich-field-telescopes-and-wide-field-observing/ wrote:

Professor Morison's Astronomy Digest
Rich Field Telescopes and Wide Field Observing

<<As the name implies, a Rich Field Telescope is one that will show the observer the maximum possible number of stars within the field of view when looking, say, towards the Milky Way. Using such a telescope to sweep along the Milky Way on a dark moonless night, is one of the most beautiful sights that can be seen in the heavens.

It turns out that we tend to see the most stars when the exit pupil of the telescope/eyepiece combination equals that of the dark adapted eye. In young people this is approximately 7 mm but, sadly, as we age this slowly drops down towards 5 or 5.5 mm. The exit pupil is the telescope aperture divided by the magnification. Let us use a mid-range exit pupil diameter of 6 mm and calculate the appropriate magnifications for a range of telescopes. For a 70 mm aperture one would need a magnification of 11.6, for an 80 mm refractor x13.3, a 102mm (4 inch) telescope x17, a 150mm telescope x25, a 200 mm (8 inch) x33.3 and a 300 mm (12 inch) x50.>>

• 1) At lower magnification than the Goldilocks/optimal faint object magnification (= aperture/6mm) all the light collected by the telescope aperture does not make it into the pupil such tbat one cannot see the faintest/most distant stars.
  

  

  Schematic of Keplerian refracting telescope.

  ---

  2) At higher magnification than the Goldilocks/optimal faint object magnification (= aperture/6mm) the faintest/most distant stars are all visible; however, they are now spread out such that fewer are in one's field of view (so that the average star field is fainter). Similarly, other resolved continuous fields like large gaseous nebula will also appear fainter.
  
  The ideal magnification for small gaseous nebula is discussed by alter-ego:
  http://asterisk.apod.com/viewtopic.php? ... 83#p314983

orin stepanek wrote: ↑Mon Jul 19, 2021 11:03 am embarrassed-kitty-cat-hiding-behind-tree-ella-kaye.jpg
Kitty says peek-a-boo!

VictorBorun wrote: ↑Mon Jul 19, 2021 8:37 am Somehow I have always seen Antares as red.
I think naked-eye color perception is so poor that
orange is taken for primary red,
bluish is taken for white,
any dim light, be it a distinct star or the Milky Way, is taken for dim white,
any dark thing under a moonless sky, be it a green tree crown or a brown tree trunk, is taken for black

shaileshs wrote: ↑Mon Jul 19, 2021 5:24 am Beautiful view. Just wondering about all these blue stars seen adjacent to the plane of galaxy. Normally, with naked eyes or even the telescopic photos I've seen of our milky way plane, we see bluish hue background adjacent to the plane but the thousands of stars we see there are normally seen white bright. Here, there seem to be many blue isolated stars conspicuously..

Wikipedia wrote:

The Scorpius–Centaurus Association (sometimes called Sco–Cen or Sco OB2) is the nearest OB association to the Sun. This stellar association is composed of three subgroups (Upper Scorpius, Upper Centaurus–Lupus, and Lower Centaurus–Crux) and its distance is about 130 parsecs or 420 light-years.
...
The Sco–Cen subgroups range in age from 11 million years (Upper Scorpius)[3] to roughly 15 million years (Upper Centaurus–Lupus and Lower Centaurus–Crux). Many of the bright stars in the constellations Scorpius, Lupus, Centaurus, and Crux are members of the Sco–Cen association, including Antares (the most massive member of Upper Scorpius), and most of the stars in the Southern Cross.
...
The stellar members of the Sco–Cen association have convergent proper motions of approximately 0.02–0.04 arcseconds per year, indicative that the stars have nearly parallel velocity vectors, moving at about 20 km/s with respect to the Sun. The dispersion of the velocities within the subgroups are only of order 1–2 km/s,[7] and the group is most likely gravitationally unbound.