Astrophoto Quick Reference
Some of you
might not be familiar astrophotography techniques, or even with some
conventional photography basics. This document will give you in little
detail some of the basics in photography/astrophotography. It is not
intended to be a detailed guide, but only a short reference.
A. Conventional Photography
- SLR Camera :
35mm Single Lens Reflex cameras are the amateur photographers favorite.
SLR practically means that a mirror and a prism are used so that
the viewfinder can look directly through the camera's interchangeable
lens. Also, SLRs, in contrast to "compact" fixed-lens
cameras, offer full access to exposure settings. A serious amateur
can settle with nothing than an SLR, mechanical or electronic.
- 35mm : The "35mm"
in camera specs refers to the film size. 35mm is the standard film
type that most compact and SLR cameras use. More expensive, mostly
professional cameras use larger format film that can deliver more
detail for the same ISO rating.
- Cable Release : A camera set
up on a tripod can still give blurry images if the human hand presses
the shutter button. To resolve this, a "cable release"
(actually a metal cable) that is attached to the camera is used.
Some cameras go even further, eliminating the vibrations caused
by the mirror (which moves up during the exposure) by offering a
mirror lock function.
- ISO/ASA rating (speed)
: The "ISO" (used to be ASA) rating, or "speed"
of a film indicates how sensitive to light a film is. There are "slow"
films (ISO less than 100) that have little grain and can deliver large
prints without loosing much quality. However, they require good lighting
conditions (and possibly a tripod) as the exposure times are greater.
Then, there are "normal" speed films (ISO 100 - 200) that
most people use, as they are suitable for everyday situations and
have good quality. The ISO 200 is more flexible, as it works better
in darker scenes without noticeably degrading the quality. Finally,
there are "fast" (ISO 400 - 800) and "ultra fast"
(ISO 1000 - 3200) that are aimed to "special" photographic
sessions. Although ISO 400 films are also very good for "everyday"
photo shots, faster films are used in situations where extra light-gathering
power is required, like astrophotography, underwater photography,
etc. The fastest film available is the Konica SR-V or SR-G 3200.
- Color Sensitivity : This aspect
of film performance is usually neglected in conventional photography.
Of course, photographers know which films exhibit "colder"
colors (like Kodak Ektar), in contrast to the "warmer" ones
(like Kodak Gold) and use each to their advantage. In some areas of
astrophotography though, color sensitivity is crucial. For example,
one of astrophotographers favorite films was the Fuji Super-G, that
had superb Green (and Blue) sensitivity. An ISO 400 Super-G could
capture nebulae and comet tails that some ISO 1000+ films couldn't.
- Focal Length
: The focal length determines the field of view and magnification
of a lens. To simplify things, let's say that a 50mm lens corresponds
to 1x, a 100 mm to 2x etc. Apart from the usual 35-80 mm lenses that
are mostly used there are "telephoto" lenses with large
focal lengths that are used when the subject is far away. Also, there
are "wide-angle" lenses (FL < 35mm) that give a very
large field of view. Be careful, as the more wide-angle a lens is,
the more it distorts the image. Finally, there are special kinds of
lenses, like the "Macros" that are like telephotos, but
can focus on a short distance providing a view of the Microcosmos,
and the "fisheye" ultra wide-field lenses (less than 15mm)
that can capture the entire sky (180°) in one frame.
- f/ratio : The ratio of the focal
length by the aperture is called "f/ratio". It is symbolized
as "f/x" where x is the derivative of FL/Apert (e.g. f/2,
f/4.5, f/16). A large aperture lens concentrates more light on the
film and has a smaller f/ratio. Lenses have a variable aperture. The
f/ratio number on the lens is the minimum, with the full aperture.
But, when there is a lot of light, or when more depth of field is
required the f/ratio must be increased. Short f/ratios are considered
f/1.2 to about f/2.8, which have a short depth of field, long f/ratios
are considered f/8 or more.
- Exposure time
: This determines for how long the camera shutter is going to stay
open for light to gather on film. The little light, the slow film,
and the long f/ratio can lead to long exposures. The subject cannot
move during the exposure, or the photo will be blurred. Generally
, exposures of 1/30 sec or less can be handled by most. Very steady
hands can even shoot at 1/15 to 1/8 sec. From then on, a tripod or
a mount is required. Of course, in other situations (like sports)
even shorter exposures are required, as a tennis ball would appear
just as a yellow streak in everything more than 1/1000 sec. For astrophotography,
on the other hand, exposure times of a few seconds to a few hours
are common. Most cameras are equipped with a photometer that helps
determining the "correct" exposure time. Photographers use
the photometer as a reference and sometimes deliberately "underexpose"
or "overexpose" their subject. Obviously, photometers are
useless in astrophotography.
- Bulb setting
: If you cycle through your camera's exposure time settings you will
come up with a "b" or "bulb" setting. This setting
is for long exposure photography and the shutter remains open for
as long as the button is being pressed. The bulb setting is usually
used in conjunction with a cable release.
- Multiple Exposures
: Normally, when you take a picture the film is rolled (automatically
or manually) to the next frame. However, some cameras let you take
two or more pictures on the same frame, a process called multiple
exposing. For example, the full moon
shines on the sea when it is quite high in the sky (more than 40°),
so the moonshine and the moon cannot fit a telephoto's field of view.
The solution is to photograph the moon and the moonshine separately,
but on the same film position. If you come across a picture with a
large moon just above the shining sea, don't be fooled, it is a double
exposure and such a scene cannot be witnessed.
Camera & Tripod
Be warned that the most important element of
astrophotography is a dark sky. In urban areas even the most expensive
equipment give mediocre or disappointing results.
- Star Trails :
Arm yourself with a tripod, a camera with cable release and a roll
of normal film. Aim the stars and shoot for a few minutes (this
depends on your taste and the darkness of your sky) to a few hours
(under dark skies with long f/ratios). Develop the film and witness
the motion of the earth around its axis.
- Stable Tripod wide-field :
After you are fed up with star trails, try shooting the constellations
with a medium or wide-angle lens. Keep the exposures short, try
up to 15-20 secs for a 50mm lens, 25-30 secs for a 35 mm lens. Use
the full aperture of the lens (lowest f/ratio), although lower quality
lenses give better results if they are stopped down by one step.
Experiment with films and exposures. Choose fast films (e.g. ISO
800) and fast lenses (short f/ratios) to capture the most stars.
Meteors : Using one of the above two techniques, on nights
with meteor activity (meteor showers) you can possibly capture a
meteor on your film. This depends only on your luck. Just select
a part of the sky that pleases you and shoot. Star-trail method
(i.e. exposures longer than a minute) are preferred, but if you
are lucky enough to capture a bright meteor in a few seconds exposure,
the result will be more pleasing. Be careful not to point the camera
to the meteor shower's originating constellation. For example, if
you happen to catch a perseid meteor in Perseus, it's bound to have
a very short trail. Try selecting a part of the sky that is more
than 50 degrees away.
- Telephoto/Astroscapes : A
telephoto on a sturdy tripod can capture magnificent views of the
sun, moon and planetary conjunctions. Also, these astronomical targets
can be combined with beautiful landscapes, which can even be temporarily
lit up with an external flash or a flashlight during long exposures.
Eclipses : Telephotos are good for solar/lunar eclipses.
A short lens can also deliver pleasing results if the event is accompanied
with an interesting landscape. Also, multiple exposures can be used
(for example a lunar eclipse
shot every few minutes with the camera fixed).
Fisheye : There are special lenses that let you capture
the entire night sky on a single frame (with great distortion though).
These lenses are very expensive. A cheap alternative can be created
with a spherical (convex) mirror mounted horizontally
and a camera on tripod above it and aimed towards the ground. This
way, the camera and tripod will also be recorded on the frame, which
can be acceptable considered the cost of fisheye lenses.
Camera & Telescope
: This is the favorite technique for people with telescopes that have
simple clock drives (or even no drives at all). Piggybacking consists
of mounting a camera with a lens on an equatorial telescope and using
the telescope only for tracking the stars. Piggyback photography can
increase dramatically the exposure time for crisp images of stellar
objects. Compared to the few seconds with a fixed tripod, telescope
guiding can be used to accomplish exposures several minutes long,
or even hours limited only by sky conditions, polar alignment (of
the telescope) and inaccurate tracking. Generally, the low magnification
used (camera lenses) is quite tolerant to minor tracking errors.
- Prime Focus
: This is a more advanced technique. In prime focus astrophotography
the telescope's optical system is used as a giant telephoto lens.
The camera is placed instead of an eyepiece using a T-adapter. Prime
focus photography is perfect for large deep-sky objects, such as nebulae,
star clusters etc. Of course, it requires perfect polar alignment
and excellent motor drives, or the results will be poor.
- Eyepiece Projection
: Planetary or narrow-field deep-sky (planetary nebulae, distant galaxies,
globular clusters...) photography require more magnification. An eyepiece
used in front of the camera which attaches to it with a tele-extender.
Relatively short exposures (a few secs) of planetary targets can be
taken with a simple clock drive. Lunar photos can even be taken with
a camera on a tripod next to the eyepiece and no motor drive (exposures
about 1/30 sec). Nevertheless, most eyepiece projection targets call
for long exposures. In those cases the perfect polar alignment and
the constant corrective guidance through a second special "reticle"
eyepiece are crucial, as the large magnification does not forgive
any guiding errors.