Imaging Galaxies from the City
Imaging Galaxies from a city site was once an impossible task, but with the development of CCD cameras this can now be done routinely. Because galaxies emit their light across the visible spectrum in a uniform distribution, no filters have been available that would pass the light of galaxies while blocking the unwanted pollution of city lights. But what could not be done mechanically with filters in the past, can now be accomplished electronically with a CCD camera and a computer.
My observing site is perhaps the least favorable one could imagine for imaging galaxies. Located 25 miles south of downtown Houston, Texas and 25 miles north of the Gulf of Mexico, skies are usually visually limited to magnitude 3.5. However, even under these conditions it is possible to obtain images of galaxies with a CCD camera that are as good as one would expect to get from only a dark site. The only requirement is to take the necessary steps that can overcome the negative effects of the light pollution.
At any site the requirement for producing a good CCD image is the same, i.e. maximizing the signal to noise ratio of the image. The signal is the galaxy which is being imaged along with the unwanted signal of light pollution which is present. When imaging from the city, the portion of the signal which is associated with the light pollution is very large and must be subtracted out accurately to yield the object of interest. Subtracting out the light pollution would be relatively simple if it weren't for the fact that the part of the signal that is the galaxy and the part of the signal that is light pollution can not be known with certainty. This is because the measurement of the signal sensed by the CCD chip is only accurate to a degree which is defined by Poisson statistics, i.e. there is an inaccuracy in the value of the signal being measured. This inaccuracy in the measured value of the signal is random noise and prevents the exact value of the light pollution and the exact value of the galaxy from being known with certainty. If the signal from light pollution is large, then the random noise associated with it will be large and this random component from light pollution will overwhelm the signal from the galaxy. The result when imaging from a light polluted environment, is an additional large component of random noise introduced which degrades the image. This is the effect which must be overcome when imaging from the city.
In order to separate the light pollution from the galaxy, the contribution to the total signal from the light pollution must be known accurately. This means that the randomness of sensing the light pollution (and galaxy) must be minimized and this can be accomplished with longer exposures. Therefore, with a long exposure and a fast signal accumulation, the signal from the light pollution and the signal from the galaxy will be accurately known and the light pollution can be truncated accurately.
Maximizing the rate of signal accumulation is done by optimizing the telescope and the CCD camera for the accomplishment of this task. A telescope with a focal ratio of f/5 will accumulate a signal at a rate 4 times faster than one with a focal ratio of f/10. This clearly illustrates that focal reducers are in order with typical schmidt-cassegrains. A less understood aspect of increasing the speed of an imaging system is choosing a CCD camera which has relatively large photosites (pixels). Typical pixel sizes for CCDs range in size from 9 microns to about 25 microns. Additionally, in many CCDs these pixels can be "binned" (combined) to produce an effective pixel size of 35 microns or more. Since the speed of the pixels is proportional to their area, the 35 micron CCD would have a speed 15 times that of the 9 micron CCD. An f/5 telescope combined with a CCD camera which has 35 micron pixels will therefore accumulate a signal 60 times faster than an f/10 telescope used with a CCD camera that has 9 micron pixels. This is a huge advantage for the signal in overcoming the random noise component present due to light pollution. While the noise component is very large in a light polluted site, it is minimized under certain weather conditions.
The amount of light scattered at any observing site can fluctuate greatly from night to night depending on weather conditions. Light pollution is only a problem because the light is scattered by the atmosphere. If you were an amateur astronomer living on the Moon, any nearby lights would not be a problem since there is no atmosphere to scatter the ambient light. From your Earth based city site, imaging galaxies should be done on nights when the ambient light is scattered the least by atmospheric conditions. The main components which scatter light besides the air molecules themselves are industrial pollution, water vapor and dust. On nights when these components are the least present, galaxies (or any faint object) will have a higher signal to noise ratio and can be imaged in a shorter time. These light scattering components are minimized under certain weather conditions such as after the passage of frontal systems. Cold fronts are frequently accompanied by wind and rain which blows and washes dust and pollution out of the air. The amount of water vapor in the air is also lowered because cold air can hold less moisture than warm air. At my site in Houston, the summer skies can get very transparent when southwest winds bring dry clean air in from the Mexican deserts. To take advantage of nights of minimum light scattering, know which local weather conditions bring you the cleanest and driest skies. Take advantage of nights when there is less light present to be scattered such as moonless nights or nights when a nearby sports stadium is not being used. At my site, stars about one-half of a magnitude fainter can be observed after midnight when local light pollution is less prevalent .
Overcoming light pollution when imaging from a city site will always require exposure times that are considerably longer than exposures made from a dark site in order to achieve comparable results. The proper selection of telescope focal ratios, a CCD camera with large photosites, and imaging during favorable weather conditions, will result in fine images. Thanks to CCD technology, there is no longer the requirement to pack up all your equipment and head to a dark site in order to participate in this exciting aspect of amateur astronomy.
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