The Sun rises over Pine Mountain Vista in the Allegheny mountains of Pennsylvania. This image shows the effects of atmospheric refraction, extinction, scattering and reddening.

Light that reaches our telescopes from the Sun, Moon, stars and planets must first pass through the Earth's atmosphere.

The Earth's atmosphere is made up of mostly nitrogen (78%) and oxygen (21%) gas, and water vapor. It is the most concentrated closest to the ground and becomes increasingly thinner as we go up in altitude until we reach the vacuum of space.

Wind, turbulence, and temperature gradients in the Earth's atmosphere affect light as it passes through. This has a tremendous impact on our view of celestial objects and greatly influences the quality of high-resolution planetary imaging.

The atmosphere causes seeing, scintillation, refraction, dispersion, extinction, scattering and reddening effects in images.

• Seeing - Seeing describes the steadiness of the atmosphere that allows fine details to be seen in celestial objects.

  The double star Almach shows the effects of bad seeing from the jet stream.

  Bad seeing causes random motion and spreading of the image. If the seeing is not good, details will be blurred.

• Scintillation - Scintillation causes rapid changes in brightness in the celestial object being viewed. This is why stars twinkle at night.

• Refraction - Whenever light travels through a medium, such as air, or water, or glass, it slows down compared to its maximum speed in a vacuum. The speed of light is different in different mediums. When light travels from one medium to another, it bends and changes direction as its speed changes. This is called refraction. The amount that light bends in a certain medium is called the medium's refractive index.

  Refraction causes a change in the apparent position in the sky of a celestial body. For example, when we see the Sun on the horizon at Sunrise, it is actually about one solar diameter below the horizon, but because the light is bent, or refracted, by the atmosphere, it appears above the horizon.

• Dispersion - Different wavelengths (colors) of light bend different amounts as they move through the atmosphere. This is called dispersion. Longer red wavelengths bend the least. Shorter blue wavelengths bend the most.

  Venus displays atmospheric prismatic dispersion.

  Light from celestial objects in the night sky has to pass through the Earth's atmosphere where it is refracted, and the different wavelengths are dispersed. This is called atmospheric prismatic dispersion. This is why we can see a blue fringe on top and a red fringe on the bottom of a star or planet that is low in the sky.

  The lower the altitude of an object, the greater the angle of incidence to the atmosphere, the greater the bending of the light, and the greater the dispersion of its constituent colors.

• Extinction - Extinction causes the brightness of celestial objects to diminish. Extinction is caused by absorption and scattering of light in the atmosphere.

  Light is absorbed by water vapor, gases, aerosols, and compounds such as ozone in the atmosphere. Some wavelengths, such as gamma, x-rays and most ultraviolet, do not even reach the ground.

  Extinction is influenced by three factors: atmospheric transparency, observer elevation above sea level, and the altitude of the celestial object above the horizon.

  Transparency can vary depending on a number of factors such as the amount of dust, smoke, humidity, smog and aerosols in the air. Transparency is usually better in the winter than in the summer.

  If we are on top of a high mountain, we have less of the Earth's atmosphere to look through, so extinction is less. As we view closer to the horizon, the brightness of celestial objects diminish more because we are looking through more atmosphere.

  Environmental factors such as forest fires, volcanic dust, and man-made pollution can also significantly affect extinction, as well as naturally occurring haze from plants. "Heat haze" in summer is caused mostly by plants and trees.

  When we look directly overhead, we are said to be looking through one airmass. At 45 degrees above the horizon, we are looking through 1.41 air masses. At 30 degrees, 2.00 air masses; at 20 degrees, 2.92 air masses; at 10 degrees, 5.6 air masses; and for an object on the horizon, nearly 40 air masses.

  The dimming of light from a celestial object due to extinction can affect the exposures that we need to use to shoot planetary images. At a sea level observing location, objects at the zenith are dimmed by about 0.28 magnitudes. An object with an elevation of 12.5 degrees above the horizon is dimmed by 1.28 magnitudes, or 2.5x more than an object at the zenith.

• Scattering - Scattering causes light to diffuse in the atmosphere. Rayleigh scattering is the main cause of scattering of light in the atmosphere. It is caused by particles such as atoms and molecules in the atmosphere smaller than the wavelength of light. Rayleigh scattering scatters blue light more than red, which is why the sky is blue in the daytime.

  Other things in the atmosphere can also cause scattering, such as smoke.

  Scattering hurts astronomical observations because it lowers contrast, both in the object and between the object and the background sky.

• Reddening - Rayleigh scattering is also why celestial objects appear more red when they are closer to the horizon.

  The Sun is really white. Since blue wavelengths are scattered more, and there is more atmosphere for the light to pass through when the Sun is on the horizon, only red wavelengths remain, giving the Sun it's red color when it is rising or setting.

  Reddening affects all celestial objects, not only the Sun.

Seeing and Transparency

Seeing is not the same thing as transparency.

Seeing refers to the steadiness of the atmosphere that reveals high-resolution details on the Sun, Moon and planets.

Transparency refers to the clarity of the atmosphere and how transparent it is and affects the visibility of faint objects. On a night of good transparency, you can see fainter stars and deep-sky objects. With good transparency during the day, you can see farther at ground level. Transparency is usually good in the winter after a front comes through bringing in clear, crisp, cold air.

Seeing and transparency are frequently inversely related. Seeing is usually better on hot muggy summer nights with the air is very humid, hazy and dense, but stable. Seeing is usually worse in the winter when the air is very clear and transparent and the stars twinkle like crazy. The seeing will also usually be bad when the jet stream is overhead.

Transparency is not that critical to planetary photography, except that it may dim the brightness of an object requiring a longer exposure.

Seeing and Planetary Photography

Of all the atmospheric effects, seeing affects high-resolution planetary photography the most. Good seeing is critical for high-resolution, high-magnification planetary photography.

Poor seeing is caused by thermal turbulence that is caused by temperature differences in air that is in motion. It may occur in the atmosphere or in the telescope itself.

It is not air movement itself that causes poor seeing. Indeed, a smooth laminar flow off the ocean can produce excellent seeing. It is temperature differences in air that is in motion that produce optical turbulence.

The atmosphere is a chaotic mix of wind and temperatures. Regions of the atmosphere with different temperatures have different densities. Mixes of air with different densities have different refractive indexes. When regions of air with different temperatures meet, a boundary layer forms between them. This boundary layer breaks up into increasingly smaller eddies, each of which, in the overall chaotic mess, bend light when it passes through these areas of different refractive indexes. The end result of this turbulence is that the light moves around, an effect called oscillation, and flickers, an effect called scintillation. Oscillation and scintillation combine to give us "seeing".

Since we are looking through less atmosphere when an object is higher in the sky, the seeing is usually better. As we observe closer to the horizon, the seeing is worse.

Turbulence near the ground and inside a telescope dome will cause large slowly undulating seeing with large changes in the quality of the image. Turbulence in the upper atmosphere will cause small scale fast changes in image quality that destroy fine planetary detail by creating a swollen, fuzzy image.

Seeing effects are caused by thermal turbulence in four general locations and these effects must be reduced as much as possible to achieve good results in planetary imaging. We will discuss how to do this in Chapter 3.

• Inside the Telescope and In the Immediate Vicinity of the Telescope

  When a telescope is first set up, it is usually warmer than the ambient air temperature. The objective is not at the same temperature as the surrounding environment so it radiates heat through convection to reach equilibrium.

  It is the air currents and convection at the boundary layer between the mirror and air that cause poor seeing inside the telescope itself. In reflectors and SCTs, this effect is worse because the light has to go through this boundary layer twice before it reaches the focal plane.

  This heat can move inside the telescope itself as it tries to escape, causing tube currents that degrade the image.

  Even the heat from an observer's body or breath can make its way into the light path and corrupt the seeing. To demonstrate this, simply defocus the telescope and put a hand in front of the objective, or in the scope tube, and examine the out-of-focus image of a star.

  Telescope domes can also be problematic, especially at the dome slit where the light that enters the telescope must first pass. Heat escaping from the dome can cause poor seeing as it mixes with the outside air.

• In the Local Geography

  Air flowing over the landscape can become turbulent when it encounters hills, mountains, trees, man-made structures and sources of heat. Convection is caused by heat absorbed during the day radiated from the ground and buildings.

  Pavement, concrete, buildings, houses, and even cars will absorb heat during the daytime and radiate it during the night. When this heat radiates, it creates convection and poor seeing.

• In the Mid-Level Atmosphere

  The air in the mid-levels of the atmosphere, from a couple of hundred meters above the ground to several kilometers high can be affected by geographic features such as mountains, cities, and large bodies of water.

  Laminar air flow that is usually smooth, such as air that has passed over a large thermally stable body of water will become disrupted when it hits a mountain range. Seeing can be good on the windward side of the mountains and poor on the leeward side.

  Observing sites downwind from large cities can have poor seeing due to winds disrupted by the heat released in the urban environment.

  Observing sites surrounded by large bodies of water can have very good seeing, such as islands in the Caribbean and the Keys in south Florida, and even the peninsula of Florida itself.

• In the Upper Atmosphere

  The jet stream is the main cause of atmospheric turbulence, and poor seeing, in the upper atmosphere.

  The jet stream is a fast flowing current of air located in the tropopause, between the stratosphere and troposphere at an altitude of about 4 to 10 miles (7 to 16 kilometers) above sea level. It is normally located above 30 degrees latitude and usually flows from west to east at high speeds. There are usually two jet streams in each hemisphere, the polar jet and sub-tropical jet.

  The jet stream is caused by the Earth's rotation and solar heating of the atmosphere. They usually form near boundaries of air masses with large differences in temperature, such as between the polar regions and warmer air near the equator.

  The jet stream usually is few hundred miles wide with a thickness of a 3 miles or less. Winds in the jet stream can be between 50 and 250 miles per hour (90 - 400 kilometers per hour).

Seeing Scales

Visual astronomers have traditionally used scales to subjectively rate the seeing, usually on a scale of 1 to 10 with 1 being the worst, and 10 being the best.

The most famous of these is the Pickering Scale, named after William H. Pickering (1858-1938) of Harvard College Observatory. Pickering observed with a 5-inch refractor. Here is his scale:

1. Star image is usually about twice the diameter of the third diffraction ring if the ring could be seen; star image 13 arcseconds (13") in diameter.

2. Image occasionally twice the diameter of the third ring (13").

3. Image about the same diameter as the third ring (6.7"), and brighter at the center.

4. The central Airy diffraction disk often visible; arcs of diffraction rings sometimes seen on brighter stars.

5. Airy disk always visible; arcs frequently seen on brighter stars.

6. Airy disk always visible; short arcs constantly seen.

7. Disk sometimes sharply defined; diffraction rings seen as long arcs or complete circles.

8. Disk always sharply defined; rings seen as long arcs or complete circles, but always in motion.

9. The inner diffraction ring is stationary. Outer rings momentarily stationary.

10. The complete diffraction pattern is stationary.

A rating of 1 to 3 is considered bad seeing, 4 to 5 is poor, 6 to 7 good, and 8 to 10 is excellent.

There are two main problems with this scale. The first is that it is subjective. The second is that psychologists have found that humans can not easily differentiate on that fine of a scale, they can usually only tell 5 different grades. For visual observing, a better scale would be something like the following...

A Simple Seeing Scale

1. Awful

2. Bad

3. Average

4. Good

5. Excellent

This scale is also subjective. An "average" rating for my normal observing location, which typically has bad seeing, might be a "bad" rating for Florida, which normally has good seeing.

Measuring Seeing

We don't have to be subjective in our assessment of the quality of the seeing. We can actually measure it.

Although a star is essentially a point source in the sky, it forms a disk with a measurable size at the focal plane of a telescope due to the diffraction of light by the telescope aperture. We will discuss this in more detail in the next section, but for right now, we can use this fact to measure the seeing.

The size of a star that can be formed by an optical system is governed by the telescope's aperture. The disk that forms due to diffraction is called the Airy disk or spurious disk. It can be calculated with a simple formula.

Distortions due to seeing will make this disk larger than the one calculated by theory. This is called the seeing disk, or point spread function.

The diameter of the seeing disk can be used to measure the seeing. In poorer seeing, the distortions are more, causing the star to move around more, resulting in a larger seeing disk.

The full width at half maximum (FWHM) of the seeing disk is the metric used to measure the seeing. The FWHM is the width of the disk where the star's intensity is 50 percent of its maximum brightness value, measured in arcseconds.

For example, the theoretical FWHM diameter of a star formed by a scope with 11 inches of aperture is about 0.4 arcseconds. Under average seeing conditions, long exposures will typically record a star that is 2-3 arcseconds in diameter. In really bad seeing, it may grow to 5 to 10 arcseconds.

For high-resolution planetary photography, we will be using short exposures with lucky imaging, and hopefully working as close to the theoretical size as possible. It takes truly excellent seeing to do this however.

Median Seeing at Professional Observatories

Here are some numbers on the median seeing sizes at some professional observatories. All are FWHM.

Seeing at Dome C in Antarctica is about the best measured on the planet. However, it is located at 10,607 feet above sea level in Antarctica at one of the coldest places on Earth. When it is dark in the winter, temperatures can reach -84°C! During the Austral summer, the Sun is up 24 hours a day. So, while the seeing may be incredible there, it's not the kind of place you would want to build a vacation home.

• European Southern Observatory (ESO) - La Silla, Chile

  • 0.84 arcseconds

• Roch de los Muchachos - La Palma, Canary Islands

  • 0.69 arcseconds
  • Less than 0.54 arcseconds 50% of the time in the summer
  • 0.4 - 0.46 arcseconds from June to July

• European Southern Observatory (ESO) - Cerro Paranal, Chile

  • 0.66 arcseconds

• Gran Telescopio Canarias (GTC) - La Palma, Canary Islands

  • 0.65 arcseconds
  • Less than 0.5 arcseconds for 20 to 30% of the time
  • Less than 1.0 arcseconds for 80% of the time
  • More than 2.0 arcseconds for 1 to 2% of the time

• Keck - Mauna Kea, Hawaii

  • 0.55 arcseconds

• Dome C - Antarctica

  • 0.27 arcseconds
  • 0.15 arcseconds for 25% of the time
  • 0.07 arcseconds best recorded

Seeing at most average observing locations that amateur astrophotographers use runs around 2 to 3 arcseconds at night and about 5 arcsecond during the daytime.

At observing sites with very good seeing, like the Florida Keys, seeing at night can run around 0.5 to 1 arcseconds.

Seeing, Luck, Patience

Even if your scope optics, focus and collimation are perfect, and your technique is impeccable, if you don't have great seeing, you will not get great pictures. Lucky imaging can work wonders, but it can't work miracles.

Even after optimizing your micro and local seeing conditions, periods of excellent seeing are actually rare at most observing locations. You have to have patience and wait for periods of good seeing. These may come randomly during the course of a single night. This is why you have to wait and monitor the seeing and shoot when it gets good. Even at locations with good seeing, at some points during the night it may get even better.

If you live where the seeing is usually bad, like I do, you may have to wait for those rare and precious nights, maybe only three or four times a year, when the seeing is good, to do really high-quality high-resolution planetary work.

It takes persistence to do the highest quality work. The world's best planetary photographers go out every chance they get. But don't expect to get killer results every time you go out. As my friend Joe Stieber says, if you don't fail once in a while, you're not trying hard enough.

Once you have all of the technical aspects mastered, quality high-resolution planetary photography is all about good seeing and the patience to wait for it.