There are two ways to interpret this graph.
One is to presume a constant exposure time with a variable test star.
The other is to presume a constant test star and to vary the exposure time.
In either case the more steep the slope the more easily the effect of a given
quantity of photons may be measured. See light Pollution.
Film, Obtaining Maximum Sensitivity
Black and white film requires a certain minimum number of photons to complete the chemical reaction that produces the dark pigment. Less than this number and the reaction proceeds part way but then drops back after time. To help to overcome this problem the film is stored in a special blend of hydrogen gas and an inert gas. This process will lengthen the time that the film pigment will remain in the partially reacted state until another photon arrives to complete the reaction. This process is called hypering the film.
The CCD is sensitive to a broad band of light colors. The CCD is usually connected to a computer in order to read out the charge stored at each pixel. The field of pixels is read line by line, pixel by pixel. The reading of the CCD produces an amount of noise. Due to the reading noise problem the exposure time for low light levels has a minimum exposure time. It is possible to add together many individual exposures in the computer improving the image and allowing the person guiding the telescope to rest between exposures.
CCD's have variations in their surface that are very hard to reduce during the manufacturing process. Fortunately these variations can be compensated in the computer processing of the image. First a dark image is taken with no light in order to establish the zero light level for each pixel of the CCD. Next an image is taken of a white background to expose each pixel the same. Using these two readings the computer is able to find a factor to multiply the signal from each pixel to 'normalize' the reading from the pixel.
The noise level in a CCD can be greatly reduced by cooling the CCD.
In the past the fate of several major observatories has been sealed
by the growth of light pollution. Many of us cannot understand why
cities and the public cannot seem to understand that their light directed
at the sky serves them no purpose. Light fixtures which direct the
light only toward the ground are readily available.
There are other things that also help. Incandescent lights, light bulbs,
generate a broad range of colors that can be more easily subtracted from CCD data
in the computer. The sodium vapor lamp, those yellow street lamps, are helpful in that
their light is mostly of a particular yellow spectral line which can be subtracted from
most data fairly easily.
Upward facing mercury vapor street lamps are the worst case. They have many spectral
lines across the spectrum effectively hiding astronomical data.
Much recent astronomical work is in the near infra-red spectrum looking for red shifted
nova and quasars. The effect of light pollution is less in the infra-red. Partly because
infra-red is less scattered by the atmosphere and therefore less of it is scattered back
toward the telescope. That plus the ability to effectively subtract light pollution out of
CCD data is a big factor in saving our older observatories located near to metropolitan
Detectors Influence the Evolution of Telescope Design
The earliest telescope lenses were of poor optical figure causing astronomers to stop them down. This produced sharp images but reduced the light making them useful only for bright objects. Cameras were unknown until the mid 1800s and even then they were not considered for astronomy because of their insensitivity. These factors probably influenced the designed focal length of the telescopes of the era.
Some of the early work with astronomical photography was done with the Crossley telescope. With the advantage of the large image field of the photographic plate realized telescopes were designed to have large image field with low off axis aberrations. The largest CCD chips are less than 2 inches on a side and small, brighter images were desired. This leads to very low F# telescopes with enough light to use high resolution spectrographs. The APF Telescope Now it has been found possible to have curved arrays of CCD chips allowing wide field with high sensitivity.
Return to calling page