Re: [APML]: Just 6th magniture from earth - what from space?

[Jerry Lodriguss <jml@astropix.com>]

Chuck Vaughn said:
>> I can't remember where I read this but the naked eye limiting magnitude in
>> space is only a little more than a dark and dry high altitude site on
Earth.


Wil Milan said:
>It may be a bit more complicated than that. On Earth at higher altitudes
there
>is less atmosphere and therefore less attenuation, but the reduced oxygen
levels
>at altitude also reduce night vision substantially. I used to own an
>unpressurized Cessna aircraft and flying it at night one could see this
effect
>quite vividly: Even at relatively low flight altitudes (under 10,000 feet)
where
>oxygen is not required, taking at hit of the oxygen at night would cause
>everything to seem brighter. The effect is very noticeable, but unless one
has
>oxygen handy to make the comparison it would probably go unnoticed.
>
>The upshot is that if one were at a high, dark location (being in orbit would
>certainly qualify) *and* had an unreduced supply of oxygen (as an astronaut
>presumably would), I would think one could see quite a bit more than is
visible
>from typical low-altitude dark site.


I think the original question presupposes that you will have to have oxygen
in space, so the question of effects of oxygen depravation at altitude seem
moot. <G>

So, what are the differences between being in space and being on the ground?

One difference is you don't have any atmosphere to look through. Lets
ignore differences in individual's ability to see faint, because as you get
older your eye won't dilate as much and effectively reduces your aperture,
and there is a tremendous difference in different individuals anyway. Lets
say it's the same person making the observations on the earth and in space,
and in both cases that person is completely dark adapted. Lets also take
best case scenario for earth observation, darkest site at a reasonable
elevation, say 6,000 feet on the top of Masai Point in Arizona. 

If we argue that you could actually go higher to make the observation on
earth, then you have to decide where you will draw the line, since it would
not be very interesting to compare say, the view from on top of Everst with
oxygen to the view from space, heck, that's almost in space anyway! 

There is no local light pollution anywhere near Massai Point and the
elevation is reasonable for effects of oxygen deprivation to be relatively
minimal, yet high enough to be well above the crud in the lower atmosphere.
A reasonable approximation of the best case for a "ground" observation.

As Brain Skiff has noted in his excellent discussion of dark skies on
s.a.a, the sky only gets so dark, and it just doesn't get any darker.  So
you don't have to go to Australia or Chile, the sky is just as dark in
Arizona.

So looking directly overhead to minimize effects of atmospheric absorption,
you can see to a certain limit.  You would have to observe during solar
minimum for best case observation to minimize auroral glow, and lets take a
nice cold winter night where the atmosphere is very clear and transparent.

What do we have left?  Natural sky glow. You don't change stuff like the
zodiacal light, gegenschein, and Milky way since those are outside of the
atmosphere.

Basically, just like a telescope, your personal limiting magnitude is
determined by the aperture of your eye and the focal length. That doesn't
change in space!!!


David Rowe said:

>I looked up the transmission coefficient of light at sea level.  It is: 0.54
>at 400nm, 0.69 at 500nm, 0.74 at 600nm and 0.81at 700nm.  I have no
>information as to the quality of the atmosphere that was measured, but I will
>assume that it was very good (lets say 6.2 magnitude).  The human eye is most
>sensitive to light in the 500 to 600 nm range, so I take 0.70 as the eye
>averaged transmission coefficient. R=2.51^M where R is the intensity ratio
and
>M is in stellar magnitudes.  Then, the increase in limiting magnitude
would be
>0.39, i.e., about 6.6 magnitude in space.  This is not nearly as good as some
>claims Iv'e heard from Texas.

I think David is right here, it's not nearly as much as most people think.
On one hand you subtract atmospheric absorption and skyglow, but on the
other you add absorption by the window of your spacecraft or spacesuit.
I'd bet those are coated or tinted too to protect from excessive UV.

So, I'd bet the difference ends up being almost trivial. Very little would
be gained by observing from space in terms of naked eye limiting magnitude.

Jerry