3. The Sky
The atmosphere is literally an ocean of air that one has to peer through. It has two innate characteristics that greatly affect imaging, transparency and steadiness (seeing). After you have honed your equipment to the best possible optical and mechanical tolerances, atmospheric seeing will then be the limiting factor for resolution. Although the steadiness of the atmosphere is important in deep sky imaging, it is planetary imaging that pushes the limit of resolution.
The seeing at your site will be affected by geography and atmospheric dynamics on the large scale and by local conditions at your imaging site. Assuming that you do not have the option of locating a scope on a mountaintop above much of the atmospheric turbulence, local conditions comprise the factors that you can most readily control. The local seeing is modified by turbulence caused by convection currents that arise from objects that are giving off heat, usually from daytime solar absorption. Any objects that are potential heat radiators should be avoided. These range from a cement driveway to your neighbor's roof and even your own body heat. One of the best locations for steadiness would be a highly vegetated region such as a grassy field surrounded by trees. Here the ground has been somewhat insulated from daytime heat absorption and therefore has little excess heat available for nighttime radiation. Other great seeing locations are those surrounded by water.
For deep sky imaging the most favorable nights will be those of high atmospheric transparency. The atmospheric transparency will be best when the air is dry and free from pollution and dust. During the winter in the northern hemisphere, the sky is usually most transparent when cold fronts come through and bring cold, clean, and dry Arctic air to the imaging site (if you live in the temperate zone). Many times during the summer the air masses become stagnant and the atmosphere becomes laden with pollutants, haze, and dust. These conditions can persist for weeks but sometimes can be improved by a good rainstorm, which has the effect of cleansing the atmosphere by washing out dust and haze. An avid imager will learn to read the sky for transparency. A deep blue sky is an indication of good transparency as well as a bright yellow (not orange) setting sun. These nights of good transparency will allow considerably fainter objects to be captured.
a. Normal FWHM resolution. The term resolution refers to the granularity of detail that can be seen through the eyepiece or in an image. The diffraction-limited and sky-limited resolution of the smallest distinguishable object, a point source such as a distant star, is measured as the full-width half-maximum (FWHM) of the point-spread function (PSF). See the Glossary of Terms for FWHM and PSF definitions. Amateur CCD "snapshot" images of planets may achieve sub-arcsecond FWHM, but very few amateur deep-sky images are less than 2 arcseconds FWHM.
b. Light pollution. Just as with your eyes, light pollution adversely affects the ability of CCDs to see deep-sky objects. Fortunately, CCDs can integrate long exposures to overcome this to some degree, but the darker the sky the better the CCD results! The absence of light pollution not only improves the contrast between deep-sky objects and the sky background, but also improves the quality of sky flats (see Section 4, Taking Images) and the unprocessed color balance of filtered images for RGB composites.
The brightness of the sky background, as affected by light pollution andother atmospheric factors, is the key determinant in the ability to see and image deep-sky objects, particularly faint extended objects such as galaxies and nebulae. Sky brightness is measured in magnitudes per square arcsecond, which can be thought of as stellar magnitudes blurred smoothly across the sky. Moonless professional observatories on mountaintops far from city lights have sky backgrounds as faint as 21st magnitide (or even somewhat fainter), moonless rural skies many miles from city lights are usually about from 19th to 20th magnitude. Light-polluted suburban skies are about 17th to 18th magnitude (or worse!). Since the brightness difference between adjacent magnitudes is a factor of 2.5, the difference between a 17th magnitude and a 19th magnitude sky is 2.5 x 2.5, or a factor of 6.25. Other things being equal, this means you would need to make CCD integrations 6.25 times longer under a 17th magnitude sky than under a 19th magnitude sky to achieve the same results! See http://www.stanmooreastro.com/CCD_topics.html for Stan Moore's excellent "CCD Topics" webpage, which includes sky background calculators which you can use to determine the sky brightness at your location based on data from your own CCD images.
c. Sky foreground color. Light pollution and other atmospheric variables, such as dust and haze, add their own distinctive colors to the sky that the light from astronomical objects must transit. This is known as sky foreground color. Fortunately, there are simple image processing techniques that imagers can use on their color filtered images to remove this phenomenon so the true colors of deep-sky objects can emerge (see Section 5, Processing Images).