- our ability to view celestial objects from earth is determined by several
related physical circumstances:
- object brightness:
- 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
- full moon overhead = -12.5 which results in 0.267 lumens/sq.m
luminance onto earth's surface = 100,000 x brighter than zero
- 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
- 5th magnitude which results in 0.0000000265 lumens/sq.m
luminance onto earth's surface = 0.01 x as bright as a zero
- 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)
- 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"
- optical system:
- 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
- optical quality
- "seeing" conditions
which impose the greatest limitation on a telescope and usually is
the limiting factor in apertures > 4"
- 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:
- 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.
- 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
- see see http://www.mapug-astronomy.net/AstroDesigns/MAPUG/VisualDet.htm
- 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
- 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:
- see Light Pollution for
- 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.
- 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
- The dark night sky is illuminated by a natural skyglow that is composed of
- 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
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
- 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
- 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
- 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"
- 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
- 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
- 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
- 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):
||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 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
- 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
- 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 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
- visual assessment:
- "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
- Simply note the relative amount and brightness of the
scattered light close to the Sun when there are no clouds
- Compare the scattered light level with weather patterns (wind
direction, humidity, etc.), time of year, and with nighttime sky
- 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
- 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
- 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
- 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
- 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.
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
- 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
- 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
- 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
- consider locating at high altitude although the cold may
outweigh the benefits
- avoid nights of full moon which dramatically impede ability to see
- avoid windy nights which will make telescope shake too much