Starship Asterisk*

Tszabeau wrote: ↑Fri Jan 18, 2019 1:06 pm One thing this APOD illustrates extremely well is the difference between the hi-res and low-res versions.

Chris Peterson wrote: ↑Fri Jan 18, 2019 3:23 pm

Tszabeau wrote: ↑Fri Jan 18, 2019 1:06 pm 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 wrote: ↑Fri Jan 18, 2019 3:40 pm 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.

Fred the Cat wrote: ↑Fri Jan 18, 2019 3:40 pm By then I might be able to start to understand Chris.

Cousin Ricky wrote: ↑Fri Jan 18, 2019 4:39 pm
Why is the dome on top of such a tall tower?

neufer wrote: ↑Fri Jan 18, 2019 5:00 pm

Cousin Ricky wrote: ↑Fri Jan 18, 2019 4:39 pm
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 wrote: ↑Fri Jan 18, 2019 5:21 pm

neufer wrote: ↑Fri Jan 18, 2019 5:00 pm

Cousin Ricky wrote: ↑Fri Jan 18, 2019 4:39 pm
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.)

Chris Peterson wrote: ↑Fri Jan 18, 2019 3:23 pm

Tszabeau wrote: ↑Fri Jan 18, 2019 1:06 pm 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.

geckzilla wrote: ↑Sat Jan 19, 2019 5:14 am

Chris Peterson wrote: ↑Fri Jan 18, 2019 3:23 pm

Tszabeau wrote: ↑Fri Jan 18, 2019 1:06 pm 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.

Fred the Cat wrote: ↑Sun Jan 20, 2019 5:21 pm
Just reading about Polaris in Sky & Telescope. I didn’t realize it was a Cepheid; changing brightness every 4 days.

https://en.wikipedia.org/wiki/Polaris wrote:
<<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.>>