# Atmospheric extinction

1. Feb 20, 2014

### 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.

2. Feb 20, 2014

### Drakkith

Staff Emeritus
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.

3. Feb 22, 2014

### 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.

4. Feb 22, 2014

### Drakkith

Staff Emeritus
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.

5. Feb 22, 2014

### 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."

6. Feb 23, 2014

### glappkaeft

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

7. Feb 23, 2014

### 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.

8. Feb 23, 2014

### 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.

9. Feb 23, 2014

### Drakkith

Staff Emeritus
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.

10. Feb 23, 2014

### Staff: Mentor

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.