Introduction
Sky transparency factors
light extinction
- causes of atmospheric extinction
measuring sky transparency:

sky transparency and atmospheric extinction

see also:

Introduction

sky transparency means how clear the atmosphere is for viewing distant or celestial objects
observing deep sky objects such as faint galaxies and nebulae requires excellent sky transparency.
it appears that indeed the southern hemisphere is cleaner as far as aerosols, presumably because there is both less human activity and just less land to generate dust from.

Sky transparency factors

sky transparency varies with:
- altitude:
  - transparency is almost solely a function of altitude: the higher the better. However, for visual observing, if you go too high, you'll lose visual sensitivity simply because not enough oxygen is getting to your brain. The optimum altitude range seems to be from about 1500 up to perhaps 3000 meters (5000 to 9000 feet). Below 1500m, the amount of crud increases dramatically, and above 3000m most people have at least mild effects from lack of oxygen. Visual observing from Mauna Kea without bottled oxygen is pretty crummy. Remember that astro-observing is mostly at the threshold of acuity, so even small physiological effects from altitude (or ill health etc) will have pronounced effects on your vision in these circumstances
  - in general, temperature falls by 5-7 degC for every 1km elevation (the lapse rate) up to the tropopause at 10-15km, so at Mt Buller in summer it gets pretty cold still!
  - inversion layers are those with a negative lapse rate, where temperature rises with elevation
  - top inversion layer of stratus cloud usually lies at elevation 500-2000m
  - 1st 1km of atmosphere is called the planetary boundary layer
    - the wind's direction & speed are affected by the roughness of the ground
- moisture content of airmass:
  - with a humid airmass the transparency is reduced significantly. With a continental airmass from the arctic, relatively cold and dry conditions prevail, allowing the sky transparency to be at times be as good as in the semi-desertic regions. Good forecasts of such rare starry evenings will clearly be useful to the amateur astronomer.
  - moisture is the only element affecting sky transparency which can be both measured and forecast all across the globe. It is often the most important factor in reducing sky transparency locally.
  - a muggy summer day with a whitish sky is the best example of this moisture effect.
- industrial pollutants causing smog which appears as brown haze above large cities and is carried to the country by the wind.
- aerosols such as volcanic ash, pollen, sea salt and smoke from forest fires also contribute to reduced sky transparency
- auroras

light extinction

even in the best skies, the atmosphere is not completely transparent and results in extinction of light
in the best skies, looking at zenith where the atmospheric mass through which one is viewing is arbitrarily given the value of 1.0 airmass, the extinction, as long as it's not cloudy, is something less than about 0.5 magnitudes per airmass in the yellow part of the spectrum where the eye is most sensitive. So a cloud-free atmosphere makes the stars a few tenths of a magnitude fainter (at the zenith) than they would be from space.
for objects within 20 deg of horizon, extinction becomes a progressively important factor (in addition to poor seeing):

degrees altitude	extinction (stellar magnitudes) compared with zenith in areas with minimal smoke, industrial pollution.	ASA correction factor for film photography
0.75	8.78	0.00032
1	6.58	0.0024
1.5	4.39	0.018
2	3.29	0.049
5	1.32	0.298
7	0.94	0.423
10	0.66	0.546
15	0.44	0.671
20	0.33	0.739

causes of atmospheric extinction

extinction is due to:
- Raleigh scattering:
  - happens because the sizes of air molecules are not a lot different from the wavelengths of visible light
  - Rayleigh scattering also depends on altitude: higher places have less air to cause the scattering.
  - the amount of scattering changes as the inverse fourth-power of the wavelength: the scattering is way higher in the blue than in the red.
  - This is why:
    - landscape scenes taken with infrared film look like there's no atmosphere: very little scattered light at these wavelengths compared to regular pictures.
    - the sky is blue: the blue part of Sunlight getting scattered much more than the redder wavelengths
- absorption of light caused by the ozone layer at 20km altitude:
  - The main effect here is a small additional extinction right in the yellow-green.
  - The result is to flatten out the extinction curve in this part of the spectrum. Since the source of this is so high in the atmosphere, it is a nearly-fixed additional extinction for any site regardless of altitude.
- aerosols including moisture, dust, smoke, industrial pollution:
  - The Canary Islands have a serious local source of dust—the Sahara. Extinction may be 0.5 and higher in summer just from high-level sand suspended over the summit where the telescopes are.
  - summer in south-east Australia is often plagued with bushfire smoke for weeks.

measuring sky transparency:

photometric methods:
- units of measure:
  - sky transparency from a given location can be measured in terms of magnitudes of star brightness lost per airmass thickness
  - usually these numbers apply specifically for the standard V passband. The dark-adapted eye response is somewhat to the blue of this, so we see higher extinction, but nobody measures it there, so it's easiest just to calibrate things relative to the V-band extinction. (The “dark-adapted visual” value is about 0.03 larger than the V extinction.)
  - extinction for points other than the zenith can be approximately predicted as the airmass increases in proportion to the secant of the zenith distance, so at 30 degrees above the horizon [60 deg zenith angle], you're looking through the equivalent of two atmospheres worth of air
visual assessment:
- astronomers evaluate sky transparency with the faintest star visible to the unaided eye.
- in semi-desertic regions such as Arizona, one can see stars as faint as 6.5-7.0 magnitude. At mid-latitudes and in the more humid eastern regions, sky transparency is limited to the 4.5-6.0 range in the countryside.
- zenithal limiting magnitude (this is also greatly affected by sky brightness and effects of twilight):
  - 4.5-5.0:
    - Milky Way and Zodiacal light invisible. Typical conditions found in suburbs of major cities. Passing clouds are easily seen due to being lighted up from surrounding lights.
  - 5.1-5.5:
    - The indistinct Milky Way faintly visible only near the zenith. Zodiacal light invisible. M31, the Andromeda Galaxy, is barely discernible.
  - 5.6-6.0:
    - The Milky Way is now more easily seen, but lacks detail. M13, the Great Hercules globular star cluster can now be just glimpsed when near the zenith. The Zodiacal light is still invisible. The Milky Way from Auriga through Orion still invisible.
  - 6.1-6.5:
    - The Milky Way is now obvious and some detail can be glimpsed. The Zodiacal light is now barely visible, but not obvious. The Milky Way from Auriga through Orion is faintly visible. There is still noticeable skyglow along the horizon due to distant towns and cities.
  - 6.6-7.0:
    - Much structure is visible in the Milky Way. The Zodiacal light is an obvious cone of light. The major constellations are less obvious due to “noise” caused by the large number of faint stars now visible. Passing clouds appear as dark moving masses as they block the natural skyglow or the Milky Way. A few sources of skyglow are still visible.
  - 7.1-7.5:
    - Incredible! The Milky Way contains an enormous amount of structure all the way to the horizon and you can easily see your way around by it's light. The Zodiacal light now encircles the entire ecliptic. There are no sources of skyglow along any part of the horizon. Many meteors are visible.
- using the Southern Cross as a reference object:
  - Many of you may have noticed that dark patch sitting right next to the Southern Cross. The patch is known as the Coal Sack and many people think that this region is devoid of stars and totally obscured by dust. However embedded within this inky blackness are a couple of faint stars that help you to gauge the extent to which you have good transparency or dark sky on any particular night. The brightest star in the Coal Sack Nebula glows at magnitude 5.3 and if you are able to glimpse this star, the transparency of your site is quite good. The general area all around the Crux (Southern Cross) region is a good reference area for judging your limiting sky magnitude.
“Flagstaff thumbnail test”:
- On a casual basis, examine the sky close to the Sun by holding your thumb at arm's length to block the Sun.
- It's easy to do this often, since takes only a few seconds.
- Simply note the relative amount and brightness of the scattered light close to the Sun when there are no clouds interfering.
- Compare the scattered light level with weather patterns (wind direction, humidity, etc.), time of year, and with nighttime sky quality.
- If you do this in a consistent way (same time of day, or same solar elevation), pretty soon you'll see what the range of variation in crudiness is, and even be able to predict the sky quality on the night following, and know when extraordinarily good or bad conditions have arrived.
- You'll also see some interesting near-Sun atmospheric phenomena you probably didn't know about. Likewise, try it with a bright Moon at night.
deep sky reference objects:
- Choose reference objects of gradually decreasing magnitude. For example, look at three galaxies with a magnitude of 10.5, 11.0, and 11.5. This will give you a good idea of “how faint you can go” during a particular observing session. When your faintest reference object becomes difficult to see, then you know you have reached the limit for that night.
- Select galaxies as your reference objects. The reason is that there are plenty of galaxies throughout the sky, which means you will have a better chance of finding the magnitudes you require near a bright star. Do not use deep sky objects that are stellar in nature, such as open clusters or globular clusters, as these can withstand poor sky transparency and light pollution much more than diffuse objects like galaxies.

how much extinction:

the best possible sky transparency is where aerosols are neglible in which case the baseline values for extinction are:
- at sea level, the value is around 0.25 mag. per airmass; for 2000m it's around 0.11 or 0.12, for Mauna Kea it's 0.09.
thus, in Victoria:
- one would be advised to view from as far inland as possible, preferably north of the Dividing Ranges and away from light pollution, although there is the problem of dust storms and bush fires causing impaired transparency particularly in late summer/early autumn.
- To go to altitudes > 1500m you will be confined to the alpine region in north-eastern Victoria such as Lake Mountain (1433m), Mt Buller (1804m), Mt Buffalo (1723m), Mt Baw Baw (1563m) and the others in the Alpine National Park whereas in western & central Victoria the only three options > 900m are Mt Macedon (1011m but perhaps too close to Melbourne), Mt William (1167m) in the Grampians and Mt Lhangi Gheran near Ararat.
- what is the seasonal effect of dense, extensive eucalypt forests on alpine aerosol content and thus extinction?

Table of Contents

sky transparency and atmospheric extinction

Introduction

Sky transparency factors

light extinction

causes of atmospheric extinction

measuring sky transparency:

how much extinction: