Beginning Astrophotography
By Dean Ketelsen,
Member
Getting Started
It really is easy to get started! All you
need is...
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A camera capable of
taking a time exposure, preferably with a "fast" lens.
Film
A tripod, the
sturdier the better.
A few accessories - a shutter release, a flashlight (astronomers red preferred), and a
notepad.
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That is it... and most of you are likely
well equipped!
Camera
Best is a manual camera which does not
need batteries to operate the shutter. Many electronic cameras
drain battery power during long exposures. If you use one, bring
spare batteries! You can do astro pictures with nearly any camera
so long as it does "B" or "T" time exposures. Most any name-brand
35mm camera will work well for you.
Lens
For starting out, normal, wide angle
or slight telephoto lenses are best. Single focal length lenses
are preferred rather than "slower" zoom lenses. Longer telephotos
and zooms can be used for shorter exposures of moon/planet
conjunctions.
F/stops explained: An F/2 lens's focal
length is twice the aperture. An F/4 lens's focal length is 4
times the aperture. For an 80 mm lens, the effective aperture at
F/2 is 40mm, for F/4 it would be 20mm. An F/2 lens is "faster"
than an F/4 lens. Think of it as finishing the exposure faster! To
help calculate exposures, a "stop" is defined as a factor of 2
increase or decrease in the amount of light. Because the area of
an aperture goes as the square, an F/2 lens passes 4 times the
light of an F/4 lens, so is 2 stops faster! A normal lens may be
F/2 or F/1.4. Lenses used by professionals may be F/1.2 for low
light levels. Regardless of their maximum aperture, most lenses
perform better stopped down a bit. Experiment to see where your
lenses perform best.
Film
You have an incredible selection of
film compared to just a decade ago. Today there are several films
that really are perfect for astronomical applications. Film
sensitivity is high and grain and resolution is very fine.
Reciprocity is a loss of film speed for long exposures. In other
words, you double the exposure and the film density does not
double. Fortunately, with today's films you need not worry about
reciprocity very much. Just realize it does affect some film, thus
is a factor is picking your favorite. Hypersensitizing (hypering)
is a film treatment that increases films sensitivity and reduces
reciprocity effects. A kit to do it yourself is expensive, but you
can buy treated film for less than twice the normal cost and that
is the way to go as you start out. Unfortunately, the hypered film
you buy has a limited life - color film lasts weeks to months, but
B&W lasts up to a year in the freezer. Spectral sensitivity is
also a concern. Many nebulae in the night sky are red from
hydrogen emission. However some films are very insensitive at
those wavelengths. A red-sensitive film is another factor.
Film Recommendations: For black and
white photography, the ultimate film is Kodak Tech Pan. It has
just about the finest grain available, has excellent red
sensitivity and when hypered is quite sensitive. Unfortunately, it
is a real dud unhypered. For the highest contrast, this film needs
to be developed in D-19 developer. However, I have not found a lab
that does that, so you need to process your own film, which is no
problem for B&W. This film exposed thru a red filter does AMAZING
work on hydrogen clouds in our galaxy.
For color negative film (prints), the
choice is less clear than for B&W. A couple years ago, the choice
was Kodak's PPF400, but it was discontinued. Shortly afterwards,
Kodak came up with PJ400, but it too was dropped, though there are
rumors that the special order film LE400 (for law enforcement) is
the same film. Kodak is recommending Supra400 as a replacement.
APML (Astro Photo Mailing List) amateurs are using up their stocks
of the above, but some preliminary tests show Supra 400 as a good
choice.
For slide films, again, the choice is
very clear. Kodak's Elite 200 Ektachrome is a great choice with
good sensitivity, fine grain, good red response and a neutral
background. It does not need hypering and is readily available at
Walgreen's on your way out of town for an observing session! This
is my film of choice with the public slide presentations that I
do, and the film that you are about to see projected. The only
thing out of the ordinary that I do is have it pushed 1 stop when
I get it processed. The reason I do that is that it increases the
contrast very slightly, and does not seem to increase the grain
size.
Processing
While B&W film is easy to process,
color chemistry has tight temperature tolerances and chemicals
don't last long enough to make it worthwhile. Most any lab will
develop your color films, but there are things to watch out for.
Many will see your dark slide as unexposed and will likely not
mount them or worse, cut and mount them incorrectly. Your color
negatives may look like they have dust specs to the technician and
they may not get printed. Also, even if they are well-exposed, the
exposure computer may introduce color shifts and improper
exposures from what you might expect. A good lab is a real
treasure. My favorite [in Tucson] is Photographic Works on Grant
near Dodge. They develop and mount slides in 3 hours and follow
directions well and enough professionals and astrophotographers
use them that they know what they are doing.
Tripods
The sturdier the better. Make sure
that you can point them upwards without the handle hitting other
parts of the tripod. Taller ones make it easier to look through
when composing. Like telescope mountings, a stiff sturdy mount
makes use of the camera that much easier and more efficient.
Cable release
The ones that you can lock with one
hand is great as it frees up your other hand for starting the
timer.
To the Top
When not tracking...
To record stars as points with a tripod
mounted camera (no tracking) so they show no apparent trailing the
exposure is found by dividing 650 by the focal length of the lens
used. For example, for a 50mm lens, 650/50=13 seconds. For a 28mm
lens, 650/28=23 seconds. If you want to get a little more
complicated, divide the result by the cosine of the declination,
since the apparent rotation of stars nearer the pole is slower. i.e.,
for the big dipper at +60 declination (cosine=.5) and a 35mmm lens,
650/35=18/.5=37 seconds.
When tracking...
To record to the sky limit with no
filters and the films recommended (hypered Tech Pan, Elite 200,
Supra 400), a guess at the maximum exposure would be 2 times the
F/number squared. i.e., for an F/2 lens, a very deep exposure will be
2X(2squared)=8 minutes. For an F/8 lens, 2X(8squared)=128 minutes.
You can always expose less for bright objects like the Orion Nebula,
but follow this relation for the maximum exposure.
Brighter objects...
Approximate exposure for the moon or any
fully sunlit scene is 1/film speed at F/16. i.e., with 200 speed film,
use 1/250 second at F/16. Shooting thru an F/10 C-8, correct
exposure on 200 speed film is about 1/125 second. A crescent moon
needs about a stop or two more exposure than full moon.
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Books
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Wide Field
Astrophotography, by Robert Reeves. Published by Willmann-Bell, 2000
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Astrophotography
for the Amateur by Michael Covington. Published by Cambridge
University Press, 1999
E-mail Newsgroup:
The Astro-Photo Mailing List (APML) is a
great group of astrophotographers who are constantly pushing the
envelope of what telescopes and film are attaining. To join, send
e-mail to majordomo@seds.org
and in the body of the message, say "subscribe astro-photo". Before
you know it, you will be getting 30 or more e-mails a day with folks
helping out with photo problems or posting their latest images.
Iridium Predictions:
http://www.heavens-above.com You will
need to know your latitude and longitude when you first log in. The
more accurately you know them, the better the predictions. GPS
coordinates are fine, or look them up in a topo map.
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TAAA Astro-Imaging Special Interest Group
TAAA
members can learn more by joining our
Astro-Imaging Special Interest Group. Meetings are held monthly
and participants are invited to view and share their astronomy
photography.
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