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Whether one takes pictures with film or with a digital system of
some type (CCD camera, digital single lens reflex camera, or web
cam), it requires considerable time and experience to produce
high quality images. CCD cameras have incredible light
sensitivity, and they can photograph astronomical objects much
fainter than film. A 24-inch telescope today can take CCD images
that rival photographs taken with the Palomar 200-inch telescope
in the 1950's and 60's. However, CCD's are very unforgiving.
While exposure times are less with CCD imaging than with film,
the guiding accuracy required is greater! CCD chips are small
compared to film, and the image is greatly magnified on a
computer monitor or a print. As a result, any star trailing from
an inaccurate drive, any image degradation from less than an
exact focus, or any image degradation from poor seeing are
greatly magnified.
To produce accurate, "true color" images, it is necessary to
balance the amount of red, green, and blue that is combined into
the final color image. This is best done with exposures that
reflect the relative red, green, and blue sensitivity of the
imaging receptor, film or CCD camera. In our experience, it is
much more difficult to correct color balance after the fact.
Sophisticated programs, such as Mira, MaxIm DL/CCD, CCDSharp,
and PhotoShop, allow you to process and combine images in a
variety of ways, emphasizing one color and reducing another;
even so, if you have too much green and not enough red, for
example, it is nearly impossible to completely remove a subtle
green tint from white stars and the sky background. At the
Grasslands Observatory, relative exposures of red=1, green=1.5,
and blue= 2 seem to work well. One unpleasant surprise that
everybody encounters when taking CCD images through colored
filters is how much longer the exposure has to be to produce the
same image data counts as those obtained without a filter being
present. Typically, an exposure must increased by a factor of
six or more!
As every astrophotographer knows, it is very frustrating to
discover that a slight gust of wind 23 minutes into a thirty
minute exposure ruined it. Fortunately, CCD imaging gives you
immediate feedback on how well you are doing, and you can
immediately discard a bad image and take another one or several
ones of shorter length, keeping those which are good and
discarding those which are substandard. |
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