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II. DSLR Astrophotography
with a Tripod
Astrophotography defies common sense, because for much less
money and effort you can enjoy better results in books, popular
magazines, and on the web. Astrophotography is also quite
challenging. It requires time, effort, and considerable luck to
produce a good picture. Black and white astrophotography with a
tripod has been performed for the past one hundred fifty years.
Color photography is a more recent phenomenon with successful
professional color astrophotography being introduced in 1959
(Miller, 1959; Carpenter, 1959).
Since then, tripod based color astrophotography has become very
popular. Wally Palcholka, for example, has published multiple
tripod color astrophotographs in leading periodicals, and his
images of Hale-Bopp and Mars taken with simple tripod SLR
techniques have won Picture of the Year Awards from Time
and Life (Palcholka, 2004). Fast color films permit
spectacular shots of the night sky with a simple SLR camera, a
tripod, and time exposures of seconds to hours. Short exposures
produce round, point-like stars, while longer exposures give
star trails.
A
one-minute exposure on ISO 400 color film and a wide-angle lens
gives an excellent color view of the night sky (figure 1):
Figure 1.
January 1984. All the Planets and the Moon were visible in the
same quadrant of the sky. Sixty-second exposure with a 17mm f/4
lens on Ektachrome 400 film. T Hunter.
Film astrophotography has
several distinct disadvantages compared with digital camera
techniques. Film has to be purchased, stored, and then
developed after the exposures have been made. There is
inevitably a long lead time between the exposure and the
result.
Film itself is expensive,
development and printing are expensive, and there is an expense
and effort required to convert film images into digital data for
computer processing. Many times, I have spent all night doing
astrophotography only to be greatly disappointed with the
results the next day after obtaining prints from the local photo
store. The images were out of focus, there was star trailing
when none was intended, the sky had an ugly green or reddish
glow, or the object of interest was only partially on the
frame. I once spent an entire evening trying to photograph
Comet Halley near the Pleiades only to find out the next day the
film had not wound in the camera (Hunter, 1986).
DSLR 35 mm cameras offer all
the advantages of a 35 mm SLR camera with none of the drawbacks
of film and many, but not all, of the advantages of digital
imaging. Their imaging chips usually have 6 or more
megapixels. High resolution images are possible with any
medium as long as the size of the resolution elements (pixels in
the case of imaging chips and grain in the case of film) are
supported by excellent optics, accurate focusing, perfect
tracking, freedom from vibration, and excellent seeing
(Covington, 1999):
Approximate Resolution
Medium Dots per inch (dpi)
Pixels per mm
TV
screen
25-75 1-3
Computer
Screen 70-100 3-4
Photographic
Print
150 6
Sharp
Print 300
12
Sharp 35mm
Negative/slide 2500 100
Nikon D100
chip 127
The above table shows the resolution elements of the Nikon D100
digital camera compare favorably with those of film, except the
chip is smaller than a 35 mm frame. If we assume
that a standard quality color print has a spatial resolution of
300 dots per inch (dpi) or better, then the 3000 x 2000 image
scale for the Nikon D100 should be able to produce acceptable
prints with an image scale of 10 x 7 inches. In reality,
some astronomical images may be enlarged considerably beyond
this up to 24 to 30 inches square depending on their
composition. If there is a homogenous dark background with
foreground stars of nebulosity, considerable image enlargement
may be performed with no resulting subjective loss of detail.
Ordinary daylight scenes generally will not stand such
enlargement, but it is fair to stay that the present high
quality DSLR cameras are widely used for portrait and landscape
scene enlargements on the order of 8 x 10 inches and larger (Schedler,
2004).
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