APOD: Circumpolar Star Trails (2019 Jan 18)

[APOD Robot]

Circumpolar Star Trails

Explanation: As Earth spins on its axis, the stars appear to rotate around an observatory in this well-composed image from the Canary Island of Tenerife. Of course, the colorful concentric arcs traced out by the stars are really centered on the planet's North Celestial Pole. Convenient for northern hemisphere astro-imagers and celestial navigators alike, bright star Polaris is near the pole and positioned in this scene to be behind the telescope dome. Made with a camera fixed to a tripod, the series of over 200 stacked digital exposures spanned about 4 hours. The observatory was not operating on that clear, dark night, but that's not surprising. The dome houses the Teide Observatory's large THEMIS Solar Telescope.

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[Ann]

Hmmm. I'm a little disappointed.

The importance of star trail images is that they demonstrate that the Earth rotates about its axis. (Alternatively, of course, it might make some people believe that the Universe is rotating around us.)

(Yes, I know, some of these star trail images are pretty good at demonstrating the fact that stars are colored. Personally I prefer to find out about the color of stars by other means.)

Click to view full size image

North Celestial Pole. Note Polaris. Photographer unknown, sorry.

Click to view full size image

South Celestial Pole.
A. Duro/ESO - http://www.eso.org/public/images/potw1631a/

Otherwise, the only thing I really like about Celestial Pole star trails is that you can tell at a glance if you are looking at the North or the South Celestial Pole. That's because Polaris, the North Star, paints a brilliantly bright little arc right next to the "bulls-eye" of the star trails if we are looking at the North Celestial Pole.

So blotting out the Celestial Pole itself in an image like this robs us of the chance of telling right away which Celestial Pole we are looking at!

When I saw today's APOD, my thoughts went like this: Well, that's the South Celestial Pole, because Polares isn't there... wait a minute... surely all the other stars near the Pole can't be gone, too?

Ann

[bmesser]

The observatory was not operating on that clear, dark night, but that's not surprising...
This begs the question - why ever wasn't the observatory operating on a clear, dark night?

[bmesser]

I get it now!

[Guest]

Click to view full size image

From website: http://analemma.pl/teide-obsevatory
One single photo. Exposition 14 days. Solar trails visible on the left.

[heehaw]

My goodness, for a moment I feared that a black hole had swallowed Polaris!

[Tszabeau]

One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

[Chris Peterson]

One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

Actually, what it demonstrates is what happens when you use a crappy image editing program to make the lower resolution image. Properly done, you can produce the somewhat smaller image (and it's not that much smaller) without all the moire and aliasing effects.

[Fred the Cat]

As our solar system rotates around the Milky Way, I can hardly wait to see what it looks like in 115 million years on the far side of the galaxy.

By then I might be able to start to understand Chris.

[Ann]

One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

Actually, what it demonstrates is what happens when you use a crappy image editing program to make the lower resolution image. Properly done, you can produce the somewhat smaller image (and it's not that much smaller) without all the moire and aliasing effects.

As our solar system rotates around the Milky Way, I can hardly wait to see what it looks like in 115 million years on the far side of the galaxy.

By then I might be able to start to understand Chris.

And I thought it was just my ESL bewilderment.

Ann

[Cousin Ricky]

Why is the dome on top of such a tall tower?

[Cousin Ricky]

By then I might be able to start to understand Chris.

Ironically, the illustration of the audio wave aliasing is not visually anti-aliased.

[neufer]

Why is the dome on top of such a tall tower?

Solar telescopes need to as elevated as possible from the warm ground to minimize turbulent distortions caused by convection.

At night inversions hold turbulent distortions due to convection to a minimum so elevated telescopes are really more to keep the astronomers warm.

[Chris Peterson]

Why is the dome on top of such a tall tower?

Solar telescopes need to as elevated as possible from the warm ground to minimize turbulent distortions caused by convection.

At night inversions hold turbulent distortions due to convection to a minimum so elevated telescopes are really more to keep the astronomers warm.

In addition, solar telescopes are usually very long focal length and point straight up, with a tracking mirror of some type at the top to follow the Sun. So they often get part of their length above ground in a tower, and part below ground in a deep shaft.

(Years ago I worked at Big Bear Solar Observatory, which is set out in a lake to avoid the surface inversion layer effects, and doesn't require such a high tower.)

[neufer]

Why is the dome on top of such a tall tower?

Solar telescopes need to as elevated as possible from the warm ground to minimize turbulent distortions caused by convection.

At night inversions hold turbulent distortions due to convection to a minimum so elevated telescopes are really more to keep the astronomers warm.

In addition, solar telescopes are usually very long focal length and point straight up, with a tracking mirror of some type at the top to follow the Sun. So they often get part of their length above ground in a tower, and part below ground in a deep shaft.

(Years ago I worked at Big Bear Solar Observatory, which is set out in a lake to avoid the surface inversion layer effects, and doesn't require such a high tower.)

Yeah, but you did needed to learn Scuba diving.

[geckzilla]

One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

Actually, what it demonstrates is what happens when you use a crappy image editing program to make the lower resolution image. Properly done, you can produce the somewhat smaller image (and it's not that much smaller) without all the moire and aliasing effects.

Not necessarily a crappy image editing program, but perhaps a user who didn't know or think to change the interpolation from bilinear.

[Chris Peterson]

One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

Actually, what it demonstrates is what happens when you use a crappy image editing program to make the lower resolution image. Properly done, you can produce the somewhat smaller image (and it's not that much smaller) without all the moire and aliasing effects.

Not necessarily a crappy image editing program, but perhaps a user who didn't know or think to change the interpolation from bilinear.

Or that.

[Fred the Cat]

Just reading about Polaris in Sky & Telescope. I didn’t realize it was a Cepheid; changing brightness every 4 days. Odd too because it emits X-rays and is inconstant for stars in its class.

A target for Chandra to investigate?

[neufer]

Just reading about Polaris in Sky & Telescope. I didn’t realize it was a Cepheid; changing brightness every 4 days.

<<Many recent papers calculate the distance to Polaris at about 433 light-years (133 parsecs), in agreement with parallax measurements from the Hipparcos astrometry satellite. Older distance estimates were often slightly less, and recent research based on high resolution spectral analysis suggests it may be up to 110 light years closer (323 ly/99 pc). Polaris is the closest Cepheid variable to Earth so its physical parameters are of critical importance to the whole astronomical distance scale. It is also the only one with a dynamically measured mass. Prior to Gaia, Polaris was the only Cepheid variable for which direct distance data existed, which had a ripple effect on distance measurements that use this "ruler".>

Despite the advantages of Hipparcos astrometry, the uncertainty in its Polaris data has been pointed out and some researchers have questioned the accuracy of Hipparcos when measuring binary Cepheids like Polaris. The next major step in high precision parallax measurements comes from Gaia, a space astrometry mission launched in 2013 and intended to measure stellar parallax to within 25 microarcseconds (μas). Although it was originally planned to limit Gaia's observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen µas". Gaia Data Release 2 does not include a parallax for Polaris, but a distance inferred from it is 136.6±0.5 pc for Polaris B, somewhat further than most previous estimates and several times more accurate.>>