Chapter 1: Introduction to Wide

Transcription

Chapter 1:
Introduction to Wide-Field
Astrophotography
1.1 Beginnings
The word photography means “writing with light.” When we photograph
something, we use various physical properties of optics, electronics, and
chemical reactions to capture the object’s image and record it on a medium,
whether that medium is a transparent plastic carrier (the negative), paper (a
print), or a magnetic surface (digital imagery). When we apply these processes to celestial objects, we call it astrophotography.
Eventually almost everyone who owns a telescope is struck with the
desire to take celestial photographs. The beauty of the celestial sphere is too
much to resist, and we succumb to the temptation of capturing the grandeur
and spirit of what we see in the sky. If a person is already a shutterbug, his
camera bag will likely have the needed gadgetry to begin photographic
explorations of the cosmos.
Photographing celestial objects through a telescope is one of the most
demanding types of photography. Fortunately there is a way for novices to
achieve success quickly and to capture on film the celestial delights that lure
their gaze upward: in wide-field astrophotography with ordinary cameras,
often known as “piggyback” astrophotography, one can start modestly and
build the skills needed for more ambitious projects, yet achieve satisfying
results. Basic piggyback photography does not require a lot of fancy equipment: beginners can start with any camera capable of time exposures and any
equatorially mounted clock-driven telescope or tracking platform. If you do
not already have a telescope, later in this chapter we will see how this can be
accomplished with a simple homemade, hand-operated star tracking device.
Astrophotography is full of surprises. With your camera, you may discover a supernova millions of light years away, like Jack Newton in Victoria,
British Columbia. Or, like Ed Szczepanski from Houston, Texas, you might
find a new comet. Moreover, there is something special about starlight that
has traveled longer than recorded civilization, or has been streaming toward
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2 Chapter 1: Introduction to Wide-Field Astrophotography
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Figure 1-1: Is this a plate from Barnard’s classic Atlas of Selected Regions of the Milky
Way? No, this beautiful image of the Sagittarius Star Cloud and the “Great Galactic Dark
Horse” was taken with an inexpensive 50-mm f/1.7 lens on a Minolta SRT102 camera and
hypered Kodak Technical Pan. This image demonstrates the versatility of the common cameras available to the amateur astronomer. Photo by Alan Sifford.
us since before humanity was on this planet: it carries a sense of immortality.
If it went through such lengths to enter your camera, it deserves to be preserved and admired.
Some authorities have suggested that special skills are required to photograph the night sky successfully; they warn that celestial photography is
difficult and filled with such unusual jargon as “reciprocity law failure” and
“hypersensitizing.” Yes, new skills are required. But celestial photography
is not especially difficult, it is just different from terrestrial photography.
Indeed, the most important attribute for those entering the field of sky photography is the patience to learn a new skill.
Beginners in the art of celestial photography should start simple and
work their way up. Even if money is no object, a beginner should not be mesmerized by the hype in magazine ads for the latest telescopic wizardry. As
with any endeavor, diving in without first mastering the basics will quickly
lead to frustration.
Astrophotography has two objectives: first, we want to record and share
Section 1.2: The Bare Basics 3
the beauty of the celestial vistas that we love; and second, we want to enjoy
the technical challenge of mastering a new skill.
This book assumes that the budding astrophotographer already has
some experience with normal pictorial photography. Learning astrophotography will be much easier if the novice is already familiar with 35-mm cameras, interchangeable lenses, film speeds, and the techniques of film
processing.
1.2 The Bare Basics
Wide-field skyshooting is astrophotography in its purest and original form,
for it derives from the pioneering Milky Way photography in the late 1800s
and early 1900s by Max Wolf and Edward Emerson Barnard. Their
equipment was the state of the art for their time, but today’s amateur astrophotographer can capture on film the same fascinating celestial targets that
Wolf and Barnard did with today’s inexpensive and readily available
equipment.
Indeed, beginners can start their astrophoto adventures with little additional equipment beyond that used for regular photography, and interesting
sky pictures can be accumulated without having to shovel vast amounts of
cash into a new hobby. Because of the fast new color slide films, a darkroom
is no longer necessary for quality astrophotography. Even electronic
imaging does not have to be expensive for the beginner: if you have your pictures of celestial objects digitized onto a Kodak Photo CD, you can view and
study them on your home computer.
The temptation for the beginner to do deep-sky photography—of galaxies, for example—through a telescope should also be avoided. Nothing
will kill a novice’s enthusiasm faster than running into a wall of technical
unknowns. A ladder of experience must be climbed first through wide-field
astrophotography. Fortunately, this ladder is short and good results can be
achieved quickly. The concepts and techniques learned in wide-field work
are critical to success in later, more complex astrophoto work. Basic widefield astrophotography is often regarded as the first step in a learning process
that eventually leads to deep-sky photography. However, wide-field astrophotography can also be considered a field of specialization in itself.
Many experienced amateurs actually take better sky photos than professional astronomers. Indeed, professionals no longer have the time or incentive to do film-based astrophotography. The field is in the hands of amateurs,
many of whom are obtaining better images than observatories were
obtaining just 15 or 20 years ago. The fact is that state of the art professional
work is routinely surpassed by amateurs with modest equipment. Not only
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Figure 1-2: A wide-angle lens on a 35-mm camera is a powerful tool for recording large
areas of the sky. Here, a 35-mm lens and hypered Kodak Technical Pan records all of Orion,
including the semicircular nebula known as Barnard’s Loop and the pumpkin-like Lambda
Orionis nebula at the “head” of Orion. The Rosette Nebula in Monoceros is to the left.
Photo by Edward Szczepanski.
Section 1.2: The Bare Basics 5
Figure 1-3: Simple star trails taken with an unguided, tripod-mounted camera will teach you
about the characteristics of sky, the camera and lens, and the film used. Here, the stars circle
Polaris over the Texas Star Party. This photo was taken with a 20-mm f/3.5 lens on Kodak
Ektachrome Elite II 100. Photo by Robert Reeves.
that, amateurs are portable. They can travel to where the sky is clear, while
professional observatories are anchored to one location.
But if wide-field astrophotography is so simple, what is the purpose of
the rest of this book? Like any endeavor, wide-field astrophotography has
many levels. Amazing results can be achieved with simple equipment, but
there is much more that can be accomplished with advanced techniques. The
theory and practical application of these will comprise the majority of this
text. The actual level of an individual’s involvement in this field is governed
by one’s interest and available resources.
1.3 How I Got Started
I was inspired to begin celestial photography by a 1957 article in National
Geographic Magazine about the then-ongoing Palomar Observatory Sky
Survey (POSS) being conducted with the 48-inch Schmidt telescope there.
The giant Schmidt produced negatives 14 inches square encompassing
a field of view six degrees on a side. The detail recorded on these plates was
astonishing. The infinite depth of space and the precision and beauty of the
endless sweeping star vistas captured my imagination.
Inspired by the Palomar images, I took my family’s WWII-vintage Voightlander camera to the roof of the garage, pointed it at the brightest star in
the sky, braced it in place with blocks of wood, and snapped a one-second
exposure of what I later learned was the planet Jupiter. Emboldened by this
first step, I lengthened the exposures to a half-hour.
When I saw the prints from that first roll of 120-format Verichrome
Pan, I discovered, to my joy and astonishment, that my initial images had
turned out well. With a maximum aperture of f/4.5, I had succeeded in
recording Jupiter with that first one-second snap. In fact, seemingly hundreds of stars piled up on the longer-exposed negatives, and I could clearly
trace the arcs of stars circling the celestial pole and identify the pattern of
constellations as charted on star maps.
The attraction the Milky Way holds for me was summed up quite nicely
by Edward E. Barnard in the introduction to his classic work An Atlas of
Selected Regions of the Milky Way:
…the Milky Way reveals all its wonderful structure, which is so
magnificent in photographs made with the portrait lens. The
observer with the more powerful telescopes, and necessarily
more restricted field of view, has many things to compensate him
for his small field of view, but he loses essentially all the wonders of the Milky Way…. It was these views of the great structures in the Sagittarius region of the Milky Way that inspired me
with the desire to photograph these extraordinary features, and
Section 1.4: Why Do Astrophotography? 7
one of the greatest pleasures of my life was when this was successfully done at Lick Observatory in the summer of 1889.
Today, the Milky Way remains the magnet that draws me toward deep
space and is the heart and soul of my involvement with astronomical photography. It offers so many challenges and rewards that one can spend a lifetime
devoted to the photographic study of this stellar highway and never run out
of material.After many years of portable telescope piggyback photography,
I finally completed a domed observatory located far from city lights. It
became operational in 1987 and is devoted to long-exposure Schmidt
camera photography of the Milky Way. The dome helps protect the instruments from heavy summer dew as well as from buffeting by the cold winter
wind.
I must admit that I don’t miss being buffeted by the wind. And the
images produced by the Schmidt are amazing. But I lost something when I
went under the dome—the grandeur of the dark night sky. Astrophotography
“indoors” with only a slice of the sky visible through the three-foot-wide
dome slot had made me homesick for the stars. Over the course of the hour
between each succeeding exposure, the sky moves quite a bit: old friends set
in the west and new ones rise to greet me in the east. I found it necessary
between exposures simply to go outside and get reacquainted with the
changing stars.
Consequently I now look forward to skyshooting away from the dome.
Astrophotography with ordinary 35-mm cameras and a portable telescope
has taken on a whole new joy for me after all these years because I can see
the stars again. Because manually guiding a simple wide-field shot is not as
rigorous as guiding the Schmidt camera, I again have the opportunity to look
around and enjoy the view. An ST-4 autoguider is still in the future for me
because of the basically primitive equatorial mounts I use, so I remain
“trapped” at the guiding eyepiece. But since I am outdoors again, I don’t
mind. I can see the stars once more.
1.4 Why Do Astrophotography?
One reason we chase the beauties of the sky with a camera is because photographs communicate the astounding celestial majesty to others. We love the
sky and wish to share it with others. Through photography we can capture the
constellations, the star clouds of the Milky Way, and delicate smudges and
puffs of nebulae and galaxies. This is a beauty we love, and with a camera we
can introduce it to a wider audience.
Photography extends past the brightness range the human eye can see.
The average naked eye can detect stars down to the 6th magnitude, encom-
Figure 1-4: The author’s humble beginning in long-exposure wide-field astrophotography
was in 1964 with a Yashica J-5 on an equatorial mount, normally used with the Criterion K2B 4-inch reflector. The unguided combination produced surprisingly good images with fiveminute exposures on Kodak High Speed Ektachrome film. Photo by Robert Reeves.
passing 6000 stars with a brightness range of about 250 to 1. A one-inch
aperture camera lens can extend this to include 5 million stars with a brightness range of about 60,000 to 1. Indeed, exposures of 20 seconds will show
all the stars plotted on Sky Atlas 2000.0, and an exposure of 10 to 20 minutes
will chart all objects seen in average amateur telescopes.
The celestial objects that can be recorded with a standard 35-mm
camera are exciting. What seems so familiar to the eye becomes a thrilling
new area of discovery once you realize the camera can detect far more than
the eye. When you see the results of your first astrophoto session, you will
not be able to wait to get out in the country again to further explore the sky
with your camera.
The challenge of preserving and sharing the visible as well as recording
the invisible is another of the great driving forces impelling the astrophotographer. Successfully recording objects that are invisible to the eye leads to a
sense of discovery that is a great reward for the long hours at the telescope.
Section 1.5: An Astrophoto Philosophy 9
Figure 1-5: Orion shines brightly from a clear dark sky and poses for an 8-inch Schmidt
camera mounted atop a Celestron-14 guide scope. Photo by Robert Reeves.
Sharing the “discovery” with others is the icing on the cake.
The joy of tinkering with scientific apparatus and the desire to master a
technical skill also promote a high degree of satisfaction. But in spite of all
its technicalities, astrophotography can go beyond the nuts and bolts of hardware and be aesthetically satisfying as well. Shooting a unique record of
one’s favorite portion of the sky can be artistically very rewarding. There is
a great deal of pride in displaying a fine photograph of the sky and knowing,
“I did this, that is my picture.” A universe beyond description and imagination awaits those who dare to reach for it.
1.5 An Astrophoto Philosophy
To get the most out of photographing the sky, it is best to adopt a philosophy that will guide you toward your photographic goals. It should be
your own personal justification for pursuing an avocation that is not commonly practiced by the average person. Moreover, your philosophy should
not only explain to you why you do astrophotography, but how you do it
and how far you are willing to pursue it. You should believe in it or your
ultimate photographic results will suffer from lack of commitment.
It does not take too much thought to realize that most of your coworkers and neighbors are not out under the stars at odd hours of the night,
often facing uncomfortable weather or hungry insects, to chase a barely vis-
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Figure 1-6: The Summer Triangle, composed of the bright stars Deneb, Vega, and Altair,
makes an inviting wide-angle target. Dick Mischke imaged this asterism along with the Great
Rift in the Milky Way south of Cygnus using hypered Kodak Technical Pan and a Nikkor 28mm lens stopped to f/4.
ible celestial object. Most of my acquaintances think I am thoroughly off my
rocker for doing so. But I ignore their jests and do not bring up their television-addicted couch-potato life style. This is because my astronomical activities are guided by a calling from within my soul. I do astronomy because I
want to, not because it is a fad, a professional or educational obligation.
I do my astronomy primarily to satisfy one person—me! I want to
record the natural beauty of the constellations, to chart the river of stars that
makes up our Milky Way galaxy, to see the unseen, and record these vistas
on film to enjoy again and again. If I am satisfied with what I do, there is a
trickle-down effect. Others around me are exposed to new ideas and new
worlds through my enthusiasm for astrophotography.
To succeed at astrophotography, my philosophy dictates that I pursue
projects that I can actually accomplish within the range of my equipment’s
capability. This does not preclude experimenting to see what is possible with
my equipment; indeed, my philosophy encourages me to push the limits to
see what it is capable of. But realistic goals must be maintained as well. A
beginner with a short telephoto lens is not going to be able to match the
achievements of an experienced person using a Schmidt astrograph.
Biting off more than you can chew is a sure way to become discouraged
Figure 1-7: The amazing recording power of modern 35-mm films is demonstrated by this
40-second unguided exposure of Comet Hale-Bopp, seen through a pine forest in Spain. To
image the comet, Marc Vallbe exposed Fuji Super G 800 film though a 50-mm f/1.7 lens.
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Figure 1-8: Subtle details in the dagger-like IC 434 nebula surrounding B 33, the Horsehead
Nebula, and in NGC 2024, the Flame Nebula, were captured by Chuck Vaughn using a 500mm f/4 Olympus lens and hypered Kodak Technical Pan.
in a new endeavor. It is my hope that this book will guide the reader through
the initial experience-gaining steps and build his self-confidence to pursue
the full extent of his or her astrophotographic capability with whatever
equipment he or she has. It must be realized that any new skill takes time to
master. Simply spending money to buy fancy equipment does not automatically guarantee success. One must learn how to use that equipment effec-
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Figure 1-9: One of the largest nebulae in the sky, IC 1396 lies in the northern Milky Way in
Cepheus. Chuck Vaughn took this picture on hypered Kodak Technical Pan through an Olympus 350-mm f/2.8 lens.
tively for the project one is doing.
Critical analysis of the photographic results is also an important part of
my astrophoto philosophy. If something did not work, I have to figure out
why in order to achieve success on another try. Bad results can be very
instructive if carefully analyzed. Was it an equipment problem, a procedural
error, or was the attempted photo simply beyond the capacity of the equip-
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Figure 1-10: The graceful Seagull Nebula, IC 2177 in Monoceros, was photographed with a
90-minute exposure through an Olympus 350-mm f/2.8 lens on hypered Kodak Technical
Pan. Photo by Chuck Vaughn.
ment? Recognizing that I can make mistakes is important. Simply blaming
the film, the photo lab, or the camera for poor results will perpetuate the
recurring error if I am the one who really made the mistake.
It is essential to persevere when the inevitable astrophoto failure rears
its unwelcome head. Good astrophotography is a learned technical skill.
Time, patience, and practice are necessary to master any new skill. Publica-
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Figure 1-11: The huge Rosette Nebula, NGC 2237 in Monoceros, is a popular target for
astrophotographers. Chuck Vaughn imaged the nebula with hypered Kodak Technical Pan
and a 350-mm f/2.8 Olympus lens coupled to an Olympus 1.4 teleconverter specifically
designed for this lens. The resulting is equivalent to a 500-mm f/4 lens.
tions and tutorials can show you what to do, but the experience gained by
practice is the real teacher.
One person’s great picture may be the next person’s disaster. I, for one,
tend to seek out the faintest possible nebulosity in the deep reaches of the
Milky Way. This renders familiar objects like the Lagoon or Eagle Nebula as
overexposed masses on the background of fainter nebulae. Some people find
these exposures unacceptable because they do not show the shape and beauty
of familiar objects. On the other hand, their photos, exposed only long enough
to reveal the classic forms of popular sky targets, strike me as abysmally
underexposed because they do not show the hidden detail that lurks between
the Messier classics. Their beauties are my disasters, and vice versa.
Even the most seasoned skyshooters make their share of mistakes. My
personal cross to bear was the seven separate 40-minute Schmidt camera
exposures of the North America Nebula required before obtaining the shot I
was satisfied with. Over the course of two years, my battle included guiding
errors, fogged film, scratched film, and misframing or completely missing
Figure 1-12: Jane Boyce’s first attempt at astrophotography resulted in this stunning keeper.
Although she was a celestial novice, she applied basic photographic principles to successfully capture this lifelike image of Comet Hyakutake with a tripod-mounted 35-mm camera.
Her two-minute exposure was on Kodak Gold 400 film through a 50-mm f/1.7 lens.
the invisible target. But in the end, the effort was worth it. The resulting
exposure rivals a Palomar Sky Survey photograph for recording faint nebulosity (Figure 5-3 on page 154).
Even the best astrophoto is useless if it is not seen. Achieving photo perfection on the negative is meaningless unless that image can be viewed, analyzed for science, or simply enjoyed. When I succeed in capturing an
exceptional view of the sky, my philosophy dictates that I share it with
others. If it is a good print, it is shown to audiences, it is displayed, posted on
my Internet web page, or illustrates an article. I encourage others to do the
same and not to be afraid of having their images “ripped off.” That is not
likely to happen, because the only realistic venue is publication in the relatively small amateur astronomy magazine and book market where economics dictate relatively little in the way of monetary reward.
Good astrophotography is a natural ambassador for astronomy and science in general. I have yet to see someone who was unmoved by a beautiful
color portrait of our Milky Way. My efforts are thus geared toward capturing
the views of our universe and spreading the faith by sharing them with others.
Figure 1-13: All stars visible to the naked eye can be easily recorded on modern fast 35-mm
films using short exposures with a fixed camera. Here Sagittarius rises over the bright skyline
of San Antonio, Texas, yet Kodak Ektachrome 200 film recorded the stars in spite of moonglow and the bright urban sky. The Moon is the disk to the upper right of the “spout” of the
“Tea Kettle.” Photo by Richard Mischke.
Figure 1-14: Fast 35-mm emulsions like Kodak PPF color negative film and a 28-mm lens
are a natural combination for recording such wide-angle phenomena as the zodiacal light.
Thomas Krajci recorded this scene with a 20-minute exposure with the lens stopped to f/4.
1.6 Getting Started
A snapshot is generally regarded as a hand-held exposure lasting a fraction
of a second, while a time exposure is longer than a second. Most of us are
familiar with snapshooting terrestrial objects. However, crossing the line
to time exposures used in astronomy opens a whole new world of photographic opportunities and their accompanying challenges.
The first step for the novice in wide-field astrophotography is taking
star trails with a camera on a fixed tripod. Shooting star trails is as much an
initiation rite as an educational experience. Everyone has seen pictures of the
classical swirl of stars circling Polaris. It is an image that is ingrained in the
mind of all amateur astronomers. The beginner can learn much about the
camera and film, the sky, and the motion of the sky by taking similar star trail
photographs.
An image of star trails gently arcing across the sky will demonstrate
celestial motion better than anything else. But if a familiar object or landmark is placed in the foreground, a good star trail photo can also be quite
scenic. Using the same rules of composition that govern terrestrial photography, we can place our house, trees, or familiar landmarks in the photo and
convert a stark scientific photo into something beautiful and aesthetically
pleasing. An example is a short exposure of Orion. In itself, the image is
Section 1.7: If You Can See It, You Can Shoot It 19
interesting because it shows a star pattern we are familiar with. But if the
same star field is viewed out the dome of an observatory with the telescope
illuminated in the foreground, the image of Orion evokes the thrill of
astronomy and a feeling of imminent discovery (see Fig. 1-5).
1.7 If You Can See It, You Can Shoot It
When I became a serious astrophotographer in the early 1960s, the
selection of color film was extremely limited and the most sensitive films
were black-and-white emulsions in the ISO 400 (then called ASA 400)
speed range. Kodak’s fastest color film was High Speed Ektachrome, with
a rating of 160, and the fastest color emulsion available anywhere was
Ansco 500, an ISO 500 speed transparency film that does not even exist
today. Now we have both color transparency and print films with 800,
1600, even 3200 speeds. Moreover, the color sensitivity of modern films
shames their 1960s counterparts.
The fact is that with today’s 1600 and 3200 speed color films, a
beginner can take better celestial images with unguided, tripod-mounted 30second exposures than the advanced amateur could in the 1960s with halfhour, fully guided exposures. The sensitivity of today’s film is the key to
this; modern films suck up starlight like a sponge compared to those used 30
years ago. Indeed, several seconds of exposure on 3200 speed film with an
ordinary 35-mm camera’s lens wide open will record more stars than the
naked eye can see, and 30-second exposures can capture more stars than are
plotted on the Sky Atlas 2000.0 charts.
Introductory celestial photography has virtually been reduced to pointand-shoot by modern film technology. Those entering the arena of widefield astrophotography do not need to worry about the advanced technique
of film hypersensitization widely discussed in astronomy periodicals (and in
Chapter 11). Excellent results can be obtained with today’s off-the-shelf
films used right out of the box. If you can see the astronomical object, you
can photograph it with nothing more than a camera and tripod. More
advanced techniques marrying the camera to a tracking platform (in the form
of a telescope) and using telephoto lenses will easily follow.
1.8 The Recipe for Beginner’s Success
The technique for astrophotography of such wide-field objects as comets or
constellation star-patterns is very straightforward. First, load your camera
with fast film, 400 speed or higher. Next, attach the camera to a sturdy tripod.
Then compose the image the way you would arrange a terrestrial shot. Most
bright comets are close to the horizon, and a foreground object such as a
scenic tree line or known building can frame the image and give a point of
reference. The camera’s lens should be wide open, the focus set on infinity,
and the shutter speed at “T” for time exposure (or “B” if the shutter has to be
held open with a locking cable release). Finally, expose the film for 15 to 30
seconds, long enough to record a star-pattern or comet but not so long that
star trails will be evident or the comet’s image blurred.
For those acquainted with casual snapshooting, the first three steps are
not a challenge. However, the final step, the time exposure, is the unknown
for novice astrophotographers. Be warned that longer is not necessarily
better for a time exposure, even if you wish to record the long arcs of stars
circling Polaris during the course of a night. At some point the exposure will
become fogged from natural background sky brightness or light pollution.
1.9 Finding Your Sky Fog Limit
The maximum exposure time for an astrophoto is limited by sky fog.
Beyond a certain exposure length, the brightness of the sky washes out the
celestial image. The exposure limits dictated by sky fog are something that
must be determined by trail and error for each camera/film combination.
Four things naturally brighten even the dark country sky and can create
sky fog on film if the exposure is too long: (1) air glow; (2) aurora activity
that varies in intensity with the sunspot cycle (and reaches its peak with the
sunspot maximum); (3) zodiacal light, caused by interplanetary dust
reflecting sunlight; and (4) the faint light from countless unresolved stars,
nebulae, and galaxies.
Obviously someone shooting from an urban location with nearby street
lights will have to contend with artificial as well as natural light pollution and
therefore will reach the sky fog limit much more quickly than the photographer in a country setting. In general, it can be assumed that with a fast film,
400 speed or higher, we will be limited to about a 30 second exposure from
an urban location, whereas in the country, reaching the sky fog limit may
take 5 to 20 minutes.
Most suburban locations have some areas of the sky that are brighter
than others. Similarly, a country location may have directions with different
sky fog limits if there is a nearby town or city.
To determine the maximum exposure possible without objectionable
sky fog, load the camera with a roll of color slide film and place it on a tripod,
compose the view to include a recognizable constellation, and snap off a
series of bracketed exposures of increasing length. By using slide film, the
results can be examined directly without the bias of corrections performed
by automated printmaking machines.
If an urban location is being tested, begin with the lens aperture set wide
open and expose for 10, 20, 40, and 80 seconds. Then, as an experiment,
Section 1.10: Examining the Experiment 21
reduce the lens’s aperture first by one, then two f-stops, and repeat the series
of exposures. We will see the reason for this in the next section. If a country
location is being tested, start with 1-minute, then 2-, 4-, 8-, and 16-minute
exposures, each with the lens wide open, then again with it closed down one
and two f-stops.
After the film has been processed, closely examine first the series of
exposures made with the lens wide open. Determine which exposure shows
an image that is degraded by sky fog beyond the point of acceptability. To
get acceptable celestial images from this location using the same camera
under similar sky conditions, the exposure must be no longer than the longest
test shot that was not ruined by sky fog. If you compare the full aperture
series with the series taken with the opening reduced one f-stop, you probably will find the best urban results come with 1000 speed or faster film with
15- to 20-second exposures and the lens stopped down one f-stop.
1.10 Examining the Experiment
Let’s go back and look at the series of sky fog test exposures made with
the aperture reduced by two f-stops. A slide projector will allow critical
analysis of faint detail and grain structure, though it must be used to
evaluate only small portions of the negative, because projector lenses
rarely give a crisp, sharp view simultaneously over the entire field. If you
do not have a slide projector, view the film through a reversed 50-mm/35mm camera lens—it will give you an excellent close-up of an entire 35mm frame.
Most lenses show stars as fine points only at the center of the field of
view; at full aperture, bright stars closer to the edge will be elongated into
fuzzy football shapes or even smeared into a comet-like form. We will see
why this happens in Chapter 4. However, as your series of test exposures
should demonstrate, closing a lens down two f-stops will dramatically
improve star image quality at the edge of the field, even with expensive
“brand name” lenses. Most lenses are perfectly adequate at wide aperture for
terrestrial photography of extended objects, but pinpoint stars are tough
objects for any lens to image accurately across the entire field at full aperture. In fact, no lens can do this at f/1.4. Even a very expensive Leica lens
will display some degree of spherical aberration at full aperture.
In most cases, the improvement in image quality from stopping the lens
down will actually allow dimmer stars to be photographed in spite of the
smaller lens aperture. This is because images of the faintest stars are out of
focus at full aperture, and thus can’t make the emulsion record them as
points. There are of course times when it is desirable to expose with the lens
wide open, such as while recording meteors, aurorae, or dim earth satellites.
a
b
Figure 1-15: This photo series of the Sagittarius/Scorpius area taken with a 35-mm f/1.4
Nikkor lens illustrates the dramatic improvement in image quality achieved by simply stopping down the lens. The sequence is (a) 5 minutes at f/1.4, (b) 10 minutes at f/2, (c) 20 minutes at f/2.8, and (d) 40 minutes at f/4 on hypered Kodak Technical Pan. At f/1.4 the stars are
Section 1.10: Examining the Experiment 23
c
d
so aberrated they are unacceptable. Astigmatism, coma, and severe vignetting degraded
images (a) and (b); in fact star images in (a) are so aberrated by spherochromatism that they
are unacceptable. However, the same lens stopped to f/2.8 or f/4 produces superior results.
Photos by Dick Mischke.
But most wide-field astrophotos are intended to record vistas of constellations, star fields, and the Milky Way; blobby, comet-like star images at the
edge of the field will rob an otherwise excellent photo of its snap and impact.
1.11 The Moving Sky
Because the Earth rotates west to east on its axis, celestial objects seem to
move east to west in the sky. Thus exposures with stationary cameras show
trailed star images.
The nearer a star lies to the celestial equator, the longer its apparent path
around the sky during the course of a day. Thus, stars nearer the equator
seem to move faster than stars nearer the celestial poles that (as a star trail
photo centered upon Polaris will show) describe comparatively small circles
in 24 hours. Consequently, the further a star-field is from a celestial pole, the
shorter the exposure time before star trails become obvious on it.
Lens focal length also affects the trailing of stars. With wide-angle
lenses, star motion will be negligible during short exposures, but with telephoto lenses, stellar movement—as well as everything else—is magnified.
Fortunately, these trailing effects can be mathematically determined in
advance. The formula for the length of a star trail along the celestial equator,
where apparent star motion is greatest, is:
L = F × E × 0.00007
(1.11.1)
where
L = trail length (in the same units as F),
F = focal length, and
E = exposure time in seconds.
For a 50-mm lens and a 10-second exposure, the length of the star trail on
the film will be L = 50 × 10 × 0.00007 = 0.035 mm, or 0.00138 inches,
close to the resolving limit of medium-speed films, and well within the
resolving power of fine-grained films.
Computing a mathematical formula before each exposure tends to take
some of the fun out of skyshooting, so use Table 1-1 to determine how long
an exposure can be with a certain lens toward a certain part of the sky.
Because the resolution of various lenses differs and the degree of enlargement of the final image is not constant among all astrophotographers, Table
1-1 must be used only as a rough guide. Some experimental bracketing of the
exposure will quickly show when the star trailing becomes objectionable
with your system.
Table 1-1 also demonstrates the limitations of unguided cameras. The
short exposures needed to prevent trailed star images are too brief to record
Section 1.12: Record-Keeping is Vital to Later Success 25
Table 1-1
Unguided Exposure Times for Trail-Free Images
Declination
in
Degrees
Lens Focal Length
28 mm
35 mm
50 mm
85 mm
135 mm
60 to 90
67 sec
53 sec
42 sec
24 sec
14 sec
30 to 60
50 sec
40 sec
25 sec
15 sec
9 sec
0 to 30
22 sec
18 sec
13 sec
8 sec
5 sec
faint detail. For longer trail-free exposures, the camera must be mounted on an
equatorially driven telescope or tracking platform as described in Chapter 2.
1.12 Record-Keeping is Vital to Later Success
When doing your sky fog tests, begin a logbook of your astronomical photography. Although it may seem to be a drudgery to record a bunch of technical data about each photograph, take it from someone who wishes he had
been more diligent about this early in his astrophoto career: record keeping
is vital to later success. There has been more than one instance when I have
taken some excellent astrophotos but was lazy and didn’t record how I did
it. Later, I was unable to achieve similar results because I could not
remember the film/lens/exposure combination I had originally used.
Just as you keep a star chart with your telescope, you should keep a
notebook of astrophoto exposure records with your camera gear. Record
keeping is the only way to determine accurately how your photographic
system works in particular circumstances, and its importance cannot be
overemphasized.
Be sure to record the following data about the test exposures: target,
film type, lens, f-stop, any filters used, exposure duration, exposure location,
sky and weather conditions, and, later, the general result of the exposure.
Sometimes the method of film processing and any other subsequent printing
or image processing should be noted. Such records are vital to scientific discoveries. Should you find a comet or supernova, you would need to have the
exact time and date of the exposure.
1.13 Barn Door Star Trackers
An easy way for a beginner who does not yet have an equatoriallymounted telescope to break away from the limitations of short-exposure
unguided photography is to build what is known as a “barn door” star
tracker. This type of camera mount was originally developed in the early
1970s by the Scotsman G.Y. Haig. It is thus also known as the “Haig
Figure 1-16: Ron Dawes made this barn door tracker from two sheets of Plexiglas. Note that
he uses the jackscrew to lower the camera platform to the west instead of raising it from the
east (as used in the northern hemisphere). Photo by Robert Reeves.
mount.” Though deceptively simple and crude in appearance, a barn door
tracker is a very effective mount for portable wide-field astrophotography.
The prototype built by Mr. Haig was a lash-up of scrap lumber and
hardware pieces. In its most basic form, it consists of two horizontal wooden
boards hinged at one end. The hinge is the polar axis. The bottom board is
the fixed base of the tracker, the top board being driven upward around the
hinge by a jackscrew, or a pushbolt, to follow the motion of the stars (see
Figs. 1-17 and 1-18).
A barn door platform will allow your camera to track the stars either manually or automatically with an electric motor, depending upon how sophisticated you wish to make the mount. Because such a platform only needs to
support the camera, it does not need the massive equatorial mount, counterweights, large tripod, and other paraphernalia accompanying a complete telescope. Indeed, such a mount is small enough to fit on a large photo tripod.
Section 1.13: Barn Door Star Trackers 27
Figure 1-17: This barn door star tracker assembled by Tom Krajci rests directly on the
ground and is built for his particular latitude. The 4x riflescope is secured with a strong rubber band and illuminated with a battery-powered LED for precise polar alignment. The
tracker platform unscrews from the base, and can be disassembled into three pieces for easy
transportation in a duffle bag. Photo by Thomas Krajci.
Figure 1-18: Krajci’s star tracker uses a jackscrew turned manually at one RPM to drive it
in right ascension. To fine-tune the rate that the platform tracks the stars, the screws that
attach to the jackscrew mechanism have slotted holes. To track faster, the tracking mechanism
is moved closer to the polar axis; to track slower, it is moved further away. Photo by Thomas
Krajci.
Later in this section we will describe how a basic barn door star tracker
can be assembled in one evening using hardware store parts and simple hand
tools. For those who like to tinker we go on to discuss how to enhance the
performance of the tracker, but these refinements are not necessary for its
basic function.
The basic single-hinge mount does suffer from tracking error after
about 5 minutes of operation because of the changing geometry between the
hinged boards and the pushbolt, but the device will allow one to take good
wide-field images up to 10 minutes in length.
There are advanced variations of the barn door camera mount that use
multiple hinges to maintain extreme tracking accuracy for exposures lasting
up to an hour. Details of these more advanced mounts can be found in the
Handbook for Star Trackers by Jim Ballard.
Once you have built your star tracker, use a normal lens and load your
camera with fast color film that can be processed by a one-hour photo lab.
Attach your camera to the tracker with a ball-head swivel camera mount (see
Fig. 1-16). Polar-align the tracker by aiming the hinge at Polaris (or the south
celestial pole if you are in the southern hemisphere). Next, aim the camera
toward the bright areas of the Milky Way or your favorite constellation, and
close the lens down two f-stops to sharpen the star images. Open the shutter
and begin operating the tracker’s jackscrew drive mechanism. Follow the
stars for five minutes, then close the shutter. Congratulations! You just took
a guided wide-field astrophoto.
If you cannot wait to see the photo results and enjoyed being out under
the stars operating the camera and tracking the stars, then celestial photography is for you. If the resulting photos only whet your appetite for more
celestial exploration, you are ready for the more advanced wide-field astrophotography techniques discussed later in this book.
1.13.1 Construction Details
The best hinge for barn door trackers is the classic piano hinge. Use a hinge
at least 4, and preferably 6, inches long: the greater the length, the less play
in the hinge and the easier it is to get the two boards parallel. Piano hinges
can be found at good hardware stores in three-foot lengths; they must be
trimmed to fit the application.
The key to the barn door mount is the drive mechanism that propels the
top board in synchronous rotation with the sky. An ordinary !/4 x 20 thread
bolt passing through the bottom board 11.43 inches from the hinge and
turned at one RPM will raise the top board at the sidereal rate. These parameters are related by the formula:
Section 1.13: Barn Door Star Trackers 29
D = RPM × 228.56 × Pitch
(1.13.1)
where
D = distance from pushbolt to hinge in units of length
RPM = jackscrew revolutions per minute
Pitch = length of one jackscrew thread in same units of length as D.
We can confirm that D should be 11.43 inches for one revolution per
minute and a !/4 x 20 thread by making RPM = 1 and tpi = 20 in formula
1.13.1. The formula will be helpful for computing the proper distance D of
the pushbolt from the hinge when the barn door tracker is being propelled
by a motor that delivers more or less than 1 RPM. For a motor that turns
the pushbolt at 2 RPMs, for example, D would be 22.86 inches.
If need be, use a counterweight to prevent the weight on the drive screw
from being too much for the motor to handle.
The precise 11.42-inch distance between hinge and screw necessary
with a !/4 x 20 bolt and a one RPM drive rate for a barn door tracker to follow
the motion of the stars is almost impossible to achieve with hand woodworking tools. The next best thing is to make the jackscrew’s thrust location
adjustable. A slotted attachment point will allow the screw’s thrust point to
be moved to the ideal location (see Fig. 1-19). Thomas Krajci suggests that
a laser pointer can be used as a test rig to determine the precise location of
the thrust point. Attach the laser pointer to the tracker and shine it on a wall
about 50 feet away. Make sure the pointer is perpendicular to the wall. With
some elementary trigonometry, you will be able to calculate the distance up
the wall the pointer spot should travel in one minute.
For example, the rate that the stars move is 15.04 degrees per hour, or
0.250667 degrees per minute. To determine the distance the laser spot
should move up the wall, we multiply the tangent of the drive angle by the
distance from the hinge to the wall. In one minute of time the drive angle will
be 0.250667 degrees, the tangent of which is approximately 0.0044. Thus at
a fifty-foot distance from the wall, the laser spot should travel 50 × tan
(0.250667) = 50 × 0.0044 = 0.22 feet, or approximately 2.64 inches, in one
minute. A more easily measured distance would be 26.4 inches in ten minutes. If the spot moves too slowly, adjust the thrust point closer to the hinge.
If it moves too quickly, move it further away from the hinge.
With the laser pointer test, the tracker’s lag rate can be measured and
corrections applied at the proper time. For instance, for the first 12 minutes,
the tracker may follow the stars just fine at one RPM, its lag being negligible.
However, from 13 to 17 minutes, the bolt must be turned 366 degrees per
minute; from 18 to 21 minutes, the turn rate is 372 degrees per minute;
between minutes 22 and 23, it is 378 degrees per minute; and between min-
utes 24 and 25, the rate advances to 384 degrees per minute. After 25 minutes, the needed corrections begin to build so quickly that it becomes
impractical to attempt any corrective action.
Timing the turns of a manually driven right ascension pushbolt can be
as simple as giving a quarter turn every 15 seconds as gauged by a watch
second hand. On the other hand, one enhanced system uses a 12-point
clicking ratchet that is advanced one click every five seconds as timed by
tape-recorded tick sounds. Star images made with a 50-mm or wider lens are
quite forgiving of tracking errors. However, with even a short telephoto lens
the right ascension drive has to be corrected at 5-second intervals.
A H-inch T-nut on the bottom board will allow it to be attached directly
to a tripod tilt head. Latitude adjustment then can be made by tilting the
tripod head upward until the hinge angle equals the local latitude. Another
variation is an angle bracket matching your latitude, thus allowing the polar
elevation to be set by a bubble level.
To refine the aim at the pole, a gunsight for aiming the polar hinge can
be made by spacing a 0.335-inch hole 12 inches from a peep sight. This will
give a 1.6-degree field of view. Align the peep sight axis so it is parallel to
the polar hinge. To align the platform on the true pole as accurately as possible, place Polaris on the side of the sight hole opposite Alkaid, the end star
in the handle of the Big Dipper.
My advice is just to build one of these simple devices and have fun
experimenting with it. Sophistication is not necessary: barn door trackers can
take wonderful pictures with exposures of just five minutes. Precise tracking
is not needed except with longer focal lengths.

Chapter 1: Introduction to Wide

Transcription

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