Earth-bound astronomic viewing

sky transparency

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

magnification

the ability to funnel light into a small point for viewing is proportional to the area of the aperture (ie. square of its radius)

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

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 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

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.

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.

see in vision

for details see astronomic "seeing"

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.

Haze/Smog

The Jet Stream/Upper Level Winds

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.