Table of Contents
astronomic viewing conditions
- sky turbulence:
- need to see what the cloud cover is like, even at night - check infra-red images
- see also:
Earth-bound astronomic viewing:
- our ability to view celestial objects from earth is determined by several related physical circumstances:
- object brightness
- atmospheric effects
- optical system
- light sensor system
- sky brightness
- dark adaptation
- sky transparency
- the brightness of astronomical objects such as stars is quantified by a logarithmic system using the unit “stellar magnitudes”:
- a difference of 5 stellar magnitudes equates to 100x brightness = ~6.5 f stops
- a difference of 7.5 magnitude equates to 1000x brightness = ~10 f stops
- a difference of 10 magnitudes equates to 10,000x brightness = ~13.5 f stops
- midday sun = -26.7 which results in 130,000 lumens/sq.m luminance onto earth's surface = 40,000,000,000 x brighter than zero magnitude
- full moon overhead = -12.5 which results in 0.267 lumens/sq.m luminance onto earth's surface = 100,000 x brighter than zero magnitude
- venus at brightest = -4.3 which results in 0.000139 lumens/sq.m luminance onto earth's surface
- sirius = -1.4 which results in 0.0000098 lumens/sq.m luminance onto earth's surface
- zero magnitude which results in 0.00000265 lumens/sq.m luminance onto earth's surface
- 1st magnitude which results in 0.000000105 lumens/sq.m luminance onto earth's surface = 0.398 x as bright as a zero magnitude
- 5th magnitude which results in 0.0000000265 lumens/sq.m luminance onto earth's surface = 0.01 x as bright as a zero magnitude
- 6th magnitude which results in 0.0000000105 lumens/sq.m luminance onto earth's surface
- relative brightness ratio = 10-(magnitude difference/2.5)
- magnitude difference = -2.5Log(relative brightness ratio)
- 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
- the ability to distinguish close objects such as resolving binary stars, rings on saturn
- this is dependent on:
- aperture diameter (ie. proportional to the diameter)
- optical alignment of the system (poorly aligned optics impairs viewing)
- optical quality
- “seeing” conditions which impose the greatest limitation on a telescope and usually is the limiting factor in apertures > 4“
- 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:
- 2“ = 64x; 3” = 144x; 4“ = 256x; 5” = 400x; 6“ = 576x; 8” = 1024x; 10“ = 1600x; 16” = 4096x; 32“ = 16384x;
- 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.
- 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
- 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!
- diffraction-related limitations
- optical artefacts
- see telescopes
light sensor system:
- visual - see dark adaptation and vision
- photographic film - see astrophotography
- CCD digital sensors - see astrophotography with digital cameras and CCD Astrophotography
- see Light Pollution for details.
- 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
- see in vision
- 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.
- for details see astronomic "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:
- The Jet Stream/Upper Level Winds
- local air disturbances:
- “surface layer seeing”:
- lower atmosphere temperature differentials and wind shear effects especially in the lowest 200m above ground level
- in general, the higher up in altitude, the better the seeing because there is less atmosphere to see through.
- telescopes with top of opening at least 10m above ground level can be expected to perform much better than those at ground level.
- rising heat:
- try to use a position which is not looking at the planets over the top of the house especially if the house heater is on. The heat fluctuations coming off the top of the house get between you and the planets.
- bad seeing caused by local effects, like a hot driveway, is properly called ground seeing.
- “dome seeing”
- poor seeing due to air refraction with telescope domes
- air disturbances “tube currents” within the telescope causing “mirror seeing”:
- Reflectors are notorious for their tube currents. Any open-ended tube should be ventilated as well as possible. Suspending a fan behind a reflector's mirror has become a popular way to speed cooling and blow out mixed-temperature air, otherwise a 10” Newtonian may take 2-3 hours to stabilise.
- It's easy to check whether tube currents trouble your images:
- Turn a bright star far out of focus until its a big, uniform disk of light.
- Tube currents will show as thin lines of light and shadow slowly looping and curling across the disk.
- if the out-of-focus star disk swarms with wrinkles that scoot across the view, entering one edge and leaving the other, then there is local seeing near the telescope.
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.
- 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.
- 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“
- 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:
- 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
- see clouds
- 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
- 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|
- 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:
- 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.
- The indistinct Milky Way faintly visible only near the zenith. Zodiacal light invisible. M31, the Andromeda Galaxy, is barely discernible.
- 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.
- 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.
- 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.
- 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?
- 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
- in the city:
- the severe light pollution and poor sky transparency alone will mean that you will never be able to see faint deep sky objects no matter what telescope you have. Brighter objects such as Orion nebula can be seen but not in their details which are fainter.
- you will be able to see:
- fairly good detail on the moon as well as most occultations
- bands on Jupiter,
- rings around Saturn
- polar ice cap and some other surface details on Mars at opposition as long as telescope can be cranked up to 250-300x magnification, but this will be limited by how good the seeing is as well as the telescope optic quality.
- select a good “dark sky” location:
- any site more than 50km from cities should have reasonable dark sky with little industrial air pollution
- consider mountains as seeing and sky transparency should be considerably better esp. if >1500m above sea level
- avoid sites where local lights or lights of passing cars will affect dark adaptation of your eyes (and also avoid dusty locations which will result in contamination of your optics)
- coastal areas although may have good seeing if ocean breezes, may have worse sky transparency due to sea salt, thus in general, an inland location is best.
- predicting a night with good sky transparency:
- the main criteria is low air humidity such as during a high pressure system, however, inversion layers that occur during these times can trap air particles and create smog such as occurs in Autumn around Melbourne.
- indicators for good transparency evenings when deep sky viewing is best:
- Watch the colour of the daytime sky, especially near the horizon. The bluer the sky, the darker the night will probably be. The white haze in a blue sky consists of microscopic water droplets that have condensed on tiny solid particles, primarily sulphate dust from distant factories and power plants. These particles are the precursors of acid rain. They do just as good a job of scattering artificial light at night. A deep blue sky in the afternoon should mean a transparent sky after dark.
- A windy cold front sweeping through a city can clear out local air pollution, leaving the night marvellously dark. The windiest city and suburban nights are often the darkest. A passing rainstorm or blizzard can also leave an unusually dark night in its wake.
- After a cold front passes - often with a heavy rain or snowstorm - the sky usually becomes very dark and crystal clear but, unfortunately, very turbulent. These clear nights, when stars twinkle vigorously and the temperature plummets, may be great for deep-sky observing but are usually worthless for the planets.
- predicting a night with good seeing:
- Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news.
- minimising the effects of poor seeing:
- minimise tube currents by ensuring telescope is allowed to cool to evening air temperatures
- minimise ground seeing effects by:
- locating telescope on grass rather than concrete and as high as possible off the ground
- avoid viewing across house roofs
- time your viewing so that object to view is near zenith when possible
- consider locating at high altitude although the cold may outweigh the benefits
- avoid nights of full moon which dramatically impede ability to see faint objects
- avoid windy nights which will make telescope shake too much
photo/ast_viewing.txt · Last modified: 2013/02/01 23:49 by gary1