Atmospheric Extinction: How Deep Can 20/15 Vision See?

[chasrob]

Let's say that you are aboard the ISS looking out a port hole. Far above the atmosphere, how deep should someone of say, 20/15 vision be able to see with the naked eye? 12th or 13th mag.?

I read where, in 1944 during the war blackout, amateur astronomers were able to pick out 8th mag. stars by naked eye.

 

[Drakkith]

12th or 13th mag is far too dim for a person to see with the naked eye. I don't know the exact number, but I doubt you'd be able to see more than a half mag or so fainter outside the atmosphere. Remember that a change in 1 magnitude represents about a 2.5 change in relative brightness.

 

[chasrob]

Only a half magnitude? I recall reading somewhere(it was many years ago, and not on the web--Sky and Telescope?) that outside the atmosphere you could see 3 or 4 mags deeper. That's how I came up with my wild guess--adding that to the wartime dark skies.

 

[Drakkith]

I have a VERY hard time believing that you could see 3-4 mags deeper. A magnitude 10 object is only 6% as bright as a mag 7 object, and a mag 11 is only 2.5% as bright. There's no way the atmosphere is attenuating over 90% of starlight.

 

[glappkaeft]

From http://www.asterism.org/tutorials/tut28-1.htm

"At elevations of 0.5 km, 1.0 km, and 2.0 km, the extinction effects are about 0.24, 0.21, and 0.16 magnitudes per air mass, respectively."

 

[glappkaeft]

I realized that this quote from the article actually fits much better (don't know how I missed the first time).

There are three factors that can be quantitively considered to assess the effect of extinction. Molecular absorption, mainly due to atmospheric ozone and water, is a minor one, about 0.02 magnitudes per air mass. More importantly, Rayleigh scattering by air molecules accounts for up to 0.14 magnitude increases per air mass. Finally, aerosol scattering (dust, water and manmade pollutants) adds about 0.12 magnitudes per air mass. The average total effect at sea level is the sum of these factors, in the order of 0.28 magnitudes per air mass at Standard Temperature and Pressure, (STP = 760 mm Hg, 00 C). Note that stellar objects are, therefore, 0.28 magnitudes brighter at the top of our atmosphere.

 

[chasrob]

Wow, those links are interesting; the refs too. One quarter, or a half magnitude difference only?

The HST can see 30th mag stars in the visual-
https://en.wikipedia.org/wiki/Apparent_magnitude#History

So, if I set up a replica telescope with a 2.5m mirror at sea level, I should be able to see to mag 28,29?

Don't think the Keck can do that, and it's on the top of a mountain.

I must have oversimplified something somewhere.

 

[glappkaeft]

You are probably not taking into account of the effects of sky glow. Even the best sites possible have an sky glow of 22 magnitudes/arcsec² due to air glow, zodiacal lights and scattered starlight. This has little effect on most objects but it is a real problem when you try to reach the faint stuff.

 

[Drakkith]

So, if I set up a replica telescope with a 2.5m mirror at sea level, I should be able to see to mag 28,29?

No. The HST uses a CCD camera and long exposure times to see as deep as it does. Your eye is not capable of this and would not be able to reach the same mag.

 

[russ_watters]

Note also the atmosphere doesn't just absorb light, it refracts it, which is what causes stars to twinkle. For our eyes, that effect can change how dim of a star we can see a little -- but not very much on a calm night. For a telescope, even on a calm night that impact is huge (because of their much higher resolution) and will greatly affect the dimmest star they can see.