photo:ast_viewing
astronomic viewing conditions
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sky turbulence:
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need to see what the cloud cover is like, even at night - check
infra-red images
see also:
Earth-bound astronomic viewing:
object brightness
atmospheric effects
extinction of incoming light making the celestial object fainter - see
sky transparency
competing brightness of the atmosphere (
sky brightness) which reduces the contrast between it and the light from the celestial object and thus, when there is no contrast the object will no longer be visible. Light from the atmosphere is due to a combination of
natural sky glow, moonlight, and
light pollution. Typical values magnitude per sq. arc sec of sky are 17 for urban, 19 for rural and 21 for alpine.
atmospheric turbulence results in almost random changes to the refractory path of the incoming light as a result of changes in the refractive index of different air masses due to differing temperatures and air pressure resulting in
"seeing" effects
optical system
resolution
light-gathering power
the ability to funnel light into a small point for viewing is proportional to the area of the aperture (ie. square of its radius)
when compared to the naked eye with an effective aperture of approx. 1/4 ” (ie. light-gathering power = 1), telescopes with the following effective apertures will have light-gathering powers of:
thus the resulting
limiting magnitude of stars visible in good conditions ranges from 10.3 for a 50mm aperture, to 13.3 for a 200mm aperture, etc.
threshold contrast
unlike light-gathering power, telescopes of increasing aperture quickly approach a limit to its threshold contrast - the ability to display an object of given brightness on a given background sky brightness
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an 8” aperture telescope has nearly 200x better threshold contrast than the naked eye,
a 16“ is approx. 300x, ie. only 50% better than a 8”
a 64“ is 1000x better than naked eye but only 5x better than an 8”
a 2048“ is ~3000x better than naked eye but only 15x better than an 8” despite being 256x larger!
other factors
light sensor system:
sky brightness
light pollution
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if the air around us was completely clean and pure, free of all dust, pollutants and light, this issue wouldn't be so crucial. But that's sadly not the case. All the dust and pollution that is suspended in the air scatters light in all directions.
the primary sources of this photonic pollution are street/city lights (like outdoor building lighting) and the moon (full moon being the worst). To minimise the effects of the moon, move to higher altitudes to decrease the amount of air particles which scatter the moonlight and impair viewing.
light pollution reduces the detail and brightness of objects.
light pollution can be easily seen by the lighting up of clouds at night
A typical suburban sky today is about 5 to 10 times brighter at the zenith than the natural sky. In city centers the zenith may be 25 or 50 times brighter than the natural background.
The night sky from light-polluted areas can be quite bright, and naturally acquires the color of the predominant source of light pollution. It is a reddish-orange for sodium vapor lighting, and greenish for mercury vapor lighting.
unfortunately, in some countries such as Japan and England, soon no-one will be able to see the Milky Way without going to another country because of light pollution as there will be no rural areas more than 100km from an urban area.
natural sky glow
The moonless night sky at a remote location far from any man-made light pollution is, however, still not completely black. To most people who are fully dark adapted, it appears a dark gray, but it may also have some faint color.
The dark night sky is illuminated by a natural skyglow that is composed of four parts:
Airglow is the brightest component and is caused by oxygen atoms glowing in the upper atmosphere which are excited by solar ultraviolet radiation. Airglow gets worse at solar maximum. Airglow can add a faint green or red color to the sky background. The color may be vivid if there is a strong aurora occurring.
Interplanetary dust particles reflect and scatter sunlight and make up the zodiacal light and gegenschein.
At night starlight is scattered by the atmosphere, just as sunlight is during the daytime. Air molecules scatter short blue wavelengths more, which is why the daytime sky is blue. The night sky also has a very faint blue component from scattered starlight.
Countless stars and nebulae in our own galaxy also contribute to the brightness of the night sky, most easily seen in the form of the Milky Way.
dark adaption and exposure to bright lights
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in short:
you cannot see faint objects such as nebulae in colour unless the telescope aperture is at least 16“
it takes ~ 30 to 60min for your eyes to adapt to the dark and this process must restart if there is significant exposure to lights, especially bright lights (minimise this by using faint red lights but if you can see that it's red on the paper your looking at, it's too bright)
if you go out on for long on a sunny day, expect to lose about three-quarters of a magnitude in your magnitude threshold the succeeding night—after extended exposure to high-intensity scenes (beach, snow-skiing on sunny days), it takes more than 24 hours to become fully dark-adapted! The usual half-hour or hour won't do. Wear “glacier glasses” when outside during daytime.
Good seeing
What does "good seeing" mean?
The atmosphere is a complex and ever changing mass of air which can drastically affect how well you can see with your telescope. To the naked eye, on what would appear to be a clear night, stars and planets might look just fine. But through a telescope, focusing may actually be next to impossible.
Observing planets, planetary nebulae or any celestial object with details at high power requires excellent seeing conditions. The seeing is the term used in astronomy to quantify the steadiness or the turbulence of the atmosphere.
When we look at planets, we need high power to see all the fine details but most of the time we are limited by turbulence occurring in the telescope (local seeing) and/or in the atmosphere.
During a night of bad seeing we are usually limited to see only two bands on the Jupiter disc and we can hardly use power over 100-150x. On excellent seeing conditions we can use high power and see many bands, white spots, festoons and details in the great red spot. Excellent seeing with high quality telescopes can also show details on the largest moon of Jupiter, Ganymede. What we are seeking is the best nights where we can boost our telescopes to their limits… which reach as high as 50X per inch diameter for quality telescopes… which means 500x for a quality 10-inch ( 25cm ) instrument.
A night of exceptionally good seeing, a night where the detail seen on Jupiter causes observers to swoon and swear, is thought to be rare. It would be boon to a know in advance when good and bad seeing might occur.
factors affecting seeing:
When and Where is "Good Seeing" Possible?
It is imperative your telescope temperature has stabilised and there are no heat sources to create hot air currents in your light path otherwise all is lost!
The best time to observe is just before dawn, when the air is stillest after the Earth has given off it's heat over night.
Looking through the least amount of atmosphere by observing when the object is overhead or at it's highest and preferably from high altitude (>1500m above sea level) and at least 10m from ground level or at least on grass rather than concrete.
As you use a telescope on different nights, you will find every night is different depending on the weather, pollution, heat, humidity and dust.etc. One night you won't be able to use more than 200x magnification, then on the next night you can. There are different ways to tell roughly. How much the stars twinkle is one way or finding out the UV (ultra-violet) rating for the day on the weather is another. After a while you can tell just by looking at an object you know.
"Airy Disk":
the disc-like image of a planet or star (or any point source) which is seen through an optical system with a circular aperture.
the majority of the light from the object is within this disc, and this is what limits the resolving power of a telescope.
it is a series of concentric rings around a bright star and the ability to see it indicates excellent optics and seeing conditions.
the central disk is known as the Airy disk and it's size in inversely proportional to the size of the telescope objective.
That is why a large telescope can see more detail under perfect conditions than a small one.
Because of physical limits the Airy disk is the smallest detail that can be seen at maximum magnification and the smaller it is, the less it intrudes on the detail. Makes little difference when looking at a star which can never be resolved because of distance but when looking at the surface of Mars or the Moon, every feature is just a lot of Airy disks all jumbled together and the larger they are, the fuzzier the image.
measuring "seeing":
Professional astronomers and more advanced astro-amateurs evaluate the seeing with a scale 1-10. Through a telescope, they measure the star diameter which usually ranges from bad seeing at 5-8 arcsec to excellent seeing at 0.5-0.2 arcsec. Astro-amateurs, can also use a qualitative way to measure the seeing. They look through their telescope at the zenith for a 2-3 magnitude star at about 30-40X per inch diameter ( 300-400x for a 10 inch telescope ) and from the look of the diffraction pattern they estimate the seeing on a scale I-V.
the seeing can be rated through astro-amateur telescopes with the following guidance incl. arc-seconds diameters:
V ….. Perfect motionless diffraction pattern….<0.4“
IV….. Light undulations across diffraction rings…..0.4-0.9”
III….. Central disc deformations. Broken diffraction rings…..1.0-2.0“
II…… Important eddy streams in the central disc. Missing or partly missing diffraction rings…..3.0-4.0”
I……. Boiling image without any sign of diffraction pattern……>4“
sky transparency
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 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
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top inversion layer of stratus cloud usually lies at elevation 500-2000m
1st 1km of atmosphere is called the planetary boundary layer
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 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 |
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:
how much extinction:
the best possible sky transparency is where aerosols are neglible in which case the baseline values for extinction are:
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?
in Japan for CCD astro-photography:
Usually I take my CCD images from Tokyo using my 12 inch Newtonian + HPC-1 CCD. However, from Tokyo, I can not take images of low altitude objects, for example, NGC253. Also, during Spring and Summer, the sky of Tokyo is not very clear. This is the reason why I developed this compact mobile system of a Celestron 5” + SBIG ST-7 and Takahashi EM200 mount. The image quality using this system is very close to those using 12 inch system, if I bring it to Mount Fuji.
I developed a compact mobile system based on Celestron C8. With three handmade reducers and Celestron's x0.63 reducer, it works at:
F=10.0, 8.7, 7.5, 6.1 with AO-7 + ST-7E
F=5.0 without AO-7, but with self-guiding by ST-7E
this system gives as good a result as the 12“ system.
planning for an astronomy night
photo/ast_viewing.txt · Last modified: 2013/02/01 23:49 by gary1