# Tag Info

### How exactly is UT1 measured/calculated?

("How exactly is UT1 measured/calculated?" -- with "interest in fine details about how UT1 time is measured.") "UT1" is a standard measure of time currently close to mean ...
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### Why does X-shooter use double passes through prisms for Echelle cross-dispersion instead of gratings?

Maybe this is a bit too late, but I think I can answer this question, from my experience with developing echelle spectrometers (not for astronomy though). As pointed out, both gratings and prisms can ...
Accepted

### 2D Galaxy Spectrum taken by long-slit-spectrograph

In the plot, the spectrum is clearly more bright on the left One possibility is that this was taken with a blue-sensitive CCD, and/or the diffraction grating has a higher efficiency in the blue. In ...
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### SKA 101: How will it "perform the most precise tests of Einstein's theories" and Australia & South Africa arrays be "made to work seamlessly together"

To answer the first part of your question: Einstein's theory included predictions about the nature of black holes and the delays they would cause to light travelling through space, so astronomers ...
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### How did asteroid (7482) 1994 PC1 get its "face"? Is it reconstructed from optical or radar imaging, or something else?

The face situation is getting curiouser and curiouser! At first, when I went to the actual page in NASA Eyes for the asteroid https://eyes.nasa.gov/apps/asteroids/#/asteroids/7482_1994_pc1 I could not ...
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### How did asteroid (7482) 1994 PC1 get its "face"? Is it reconstructed from optical or radar imaging, or something else?

These are images taken from Nasa's Eyes on Asteroids site. It lets you see the orbit and get information about the asteroid in a nice interactive way. For asteroids with a known shape and surface ...
• 103k
1 vote

### What parameters about galaxies can be measured using SAO DS9?

SAO DS9 is a powerful software tool for viewing and analyzing astronomical images, and there are many other parameters you can determine for galaxies using DS9. Here are some additional parameters you ...
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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 ...
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### Small bright constellation on the photo

The object inside the red circle is indeed The Pleiades. Note that it is not a constellation, it is an open star cluster located in the constellation Taurus. A good way to identify constellations in a ...
• 801
Accepted

### Small bright constellation on the photo

These are the Pleiades. I verified it comparing your photo with the Stellarium app.
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Accepted

### Right ascension of a star and local sidereal time

If your local sidereal time is 05h 55m 10.30536s, then Betelgeuse is on the meridian. The local sidereal time is always equal to the right ascension of a point on the meridian.
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Accepted

### Telescopes with equatorial mount and "catching" stars

The stars have an apparent movement in the sky like the Sun1: along their path on the sky they move 15° every hour (actually this is an effect of the rotation of the Earth's, a full rotation of 360° ...
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### Viewing black holes?

With the emergence of gravitational wave astronomy, it has become possible to obtain a direct view of black holes via their gravitational-wave emissions. Anything undergoing acceleration emits ...
• 353
You want to basically assume that the supernova is a "standard candle," i.e., if we know with confidence how bright it $\it{should}$ be, then we can estimate the distance after measuring it'...