How Do You Quantify Seeing and Transparency?

[stu]

I've noticed on the astrophotography thread that many people quantify seeing and transparency on 1-10 scales. How is this done?

[trinitree88]

I've noticed on the astrophotography thread that many people quantify seeing and transparency on 1-10 scales. How is this done?

Seeing and transparency are related but not intrinsically. The darker the sky, with less back-scatter from light pollution, the better the transparency...usually far from urban areas, and being at altitude doesn't hurt.
Seeing is a little trickier. Despite good transparency, atmospheric turbulence can cause images to flicker and sway from small changes in the refractive index due to density gradients of layers of air stratified in the atmosphere.
Generally speaking, the choice of observing sites involves solitary islands of landmass that project up into the atmosphere....Hawaii, isolated mountain peaks...then the air flow is fairly laminar...and the seeing is good, along with excellent transparency. Dome C will be tricky due to the cold.
Under conditions of superb seeing, and transparency...splitting close doubles is possible. The better the conditions...the closer the pair split. :shifty: Pete.

[stu]

Right, I know what they are, like I can tell if it's "good" or "bad," but I don't know how to say the seeing tonight was a 6/10 and transparency a 3/10.

[Dave Mitsky]

For quantifying seeing, the Pickering Seeing Scale is useful.

http://uk.geocities.com/dpeach_78/pickering.htm

Transparency is a bit more subjective. Visual limiting magnitude depends upon the visual acuity of the observer. In recent years, the Bortle Dark-Sky Scale has become popular for judging site darkness:

http://skytonight.com/resources/darksky/3304011.html

http://www.frostydrew.org/observatory/columns/essays/bortle.htm

Dave Mitsky

[stu]

Thanks. Apparently, the topic reply notification e-mails aren't coming through for me again, which is why I didn't realize you'd replied.

I've only seen Airy disks once or twice from my location ... I knew I have horrible seeing because of nearby mountains, but I didn't realize how much better it could get. Looks like I observe from a class 7 site, too. Thanks!

[tdvance]

Thanks. Apparently, the topic reply notification e-mails aren't coming through for me again, which is why I didn't realize you'd replied.

I've only seen Airy disks once or twice from my location ... I knew I have horrible seeing because of nearby mountains, but I didn't realize how much better it could get. Looks like I observe from a class 7 site, too. Thanks!

I usually estimate transparency on a 1-5 scale as the magnitude of the faintest star I can see (preferably near the zenith in a clear part of the sky). A star chart helps for that, of course. You can get a quick estimate (not at my house since there's light pollution in that direction) from the little dipper--the bowl stars are magnitudes 2, 3, 4, and 5 roughly, so you get a fast estimate based on how many bowl stars you see.

Todd

[stu]

That technique would probably be good in most cases, but for me at the observatory on campus, the Li'l Dipper lies back in the direction of campus, so the transparency isn't as good there as it is in the South-West quadrant of the sky, which is where I generally image (North of zenith is the roof, and East of the meridian lies glow from 40-mile-away Denver). Definitely not the best of locations ... :(

[Dave Mitsky]

Another way of estimating visual limiting magnitude is to do star counts in select areas of the sky. This site (http://obs.nineplanets.org/lm/rjm.html) has the required information.

Dave Mitsky

[stu]

Cool. Thanks.