Editor's Note:
This month we have a fine article by Jamey Jenkins about
white light solar photography. This is a topic that I have read
little about, so I found this a particularly interesting article.
I hope you enjoy it as much as I! ~johnb
A Midwest Amateur's Techniques for Solar Photography
~Jamey Jenkins
Introduction
With the current solar cycle approaching its predicted peak, interest in
solar observing among amateur astronomers is also increasing. Many
observers will for the first time gaze upon the face of this nearest star
and be fascinated by the ever changing and dynamic aspects of its huge
sphere of rotating gases. Many will also be inspired to obtain a
photographic record of what they see and experience. I wish to share
with you various tools and techniques that I have found to be useful in
the pursuit of white light solar photography. I am assuming that you are
experienced enough to have done at least some casual solar observing and
are familiar with objective filters, projection techniques and the
like.
A Warning
Every book, article, or discussion of solar observing must
carry this warning because of the inherent danger in viewing the sun.
Permanent injury to your eye in the form of at least partial blindness
can occur from sunlight concentrated to burning strength in a lens
system. Be on guard to protect uninitiated friends and don't forget to
cap the finder on your telescope. Above all watch out for children who
will try to look through anything. Now that that is out of the way it is
refreshing to know that solar observing and photography can be made safe
if certain precautions are taken and proper filtration is used.
The white light photographer can, with care, select tools successfully to
record many features on the face of the sun. These features may include
umbral and penumbral structure, photospheric faculae, granulation, limb
darkening and pores, to name but a few possibilities. Of course
prominences and some other features are in the realm of narrower band
observations and are not possible with white light equipment. Nevertheless
the aspiring solar photographer will find more than enough
challenges with white light photography!
Instrumentation
Astronomy is comparable to many other vocations in that
the results you obtain are often in direct proportion to the quality of tools
you use in your work. Deep sky observing is better accomplished with one
type of telescope while planetary work is best performed with yet
another. A long focus refracting telescope, fully baffled with a well
made primary lens, is the ideal solar telescope. The refractor, when
correctly baffled, is capable of producing contrasty sharp images.
Reflectors on the other hand are susceptible to scattered light and
increased image deterioration due to the central obstructions and tube
current situations. The long focus refractor, typically f/15 to f/20, will
use fewer components to produce a given image size when compared to the
typical Newtonian of f/5 to f/8 design. There are always the exceptions,
but the long focus refractor will generally be the preferred instrument
for solar study.
My telescope is a home assembled 5-inch f/18 refractor using a doublet
lens manufactured by the D & G Optical Company. It is fully baffled with
a straight through design which keeps the optical components to a
minimum. I utilize a 2-inch rack and pinion focuser at the tail end to
provide steady support for the camera system or other accessories. The
tube is painted white to reflect heat, and I do not use a separate finder
choosing to watch the sun's shadow on the ground when locating it in the
sky. The 5-inch is a compromise between cost and practical application
of the telescope. I have found that atmospheric turbulence usually
limits the observation of objects to 1 or 2 arc seconds (such as solar
granulation) during the daytime. Consequently, an objective which can
exceed a resolution of 1 arc second is wasting resolution on average
when used for solar observations. Larger objectives with a greater dollar
value would hardly ever be permitted to perform to their full potential.
You will rarely find aperture fever among solar observers; Our concern
tends to be the quality of the image at the focal plane.
The image of the sun formed by my telescope at prime focus varies from
approximately .828 inch in the summer to .855 inch in the winter
(northern hemisphere). For full disc photography this fits nicely into a
35mm film frame but for detailed photography it is necessary to enlarge
the prime focus image several diameters. I use a Dakin Barlow
manufactured by the Vernonscope Company as a projection lens. Often
photographers report that barlows tend to make their images go soft, but
fortunately that has not been my experience. Mating the barlow with a set
of extension tubes, I have a range of focal ratios available from f/38 to
f/64, sufficient for high resolution photography with a fine grained film.
The camera body I use is a Mirex Laborec, a cousin to the Miranda Laborec
popular years ago. The Laborec features clear screen focusing and a low
vibration reflex mirror and shutter system. Minimizing vibration from
mirror bounce and shutter slap in essential to high resolution
photography. Although difficult to find on the market these days, the
Laborec is ideal for an astrophotographer utilizing film as the detecting
medium. Several other cameras come to mind which also do well in this
regard, one being manufactured under the Olympus brand.
The telescope is mounted on an 8-inch diameter pier in my backyard with
an equatorial head from a vintage Cave Optical mounting, 1-1/2 inch axles
and clock driven. One lesson I learned at the beginning of my telescope
making experience is that overkill in mounting your telescope is good.
The general idea being that mass will lessen vibrations and will be more
effective in supporting your optics with less effort.
Filtration
Reduction in the brightness of the sun is necessary for safe
direct observation of any features on its disc. With the introduction
of objective filters in the last several decades, this task has been
simplified greatly. The quality of the filters may vary, and the
observer should take care to obtain a filter suitable for the
observations to be made. Large aperture glass filters with smooth surfaces
and parallel faces that provide photographic quality images are
very expensive by their nature. Many inexpensive glass filters
advertised today do not fall into this category and are useful for
casual sunspot observing at best. The trick is for the filter to
disturb the wavefront of the final image formed by the telescope as
little as possible. Besides glass there are also mylar filter products
on the market that can be used to advantage for solar observation. The
mylar, if thin enough, will disturb the wavefront only minimally and will
permit quality observations to be obtained.
After reviewing the excellent results obtained with mylar filters from
other amateurs I opted to go that route. For visual observing with my
5-inch refractor I use an off the shelf Tuthill Solar Skreen filter
which consists of two layers of aluminized mylar sandwiched together to
provide an equivalent neutral density of 5.0. For photography I use a
single layer of the Tuthill Solar Skreen and additional photographic
filters near the first focus of the telescope to further increase
contrast and reduce brightness. The mylar filters are fragile so it is
important to take care of them. It is nearly impossible to clean them
as you would glass filters. When not in use I keep my filters
in plastic bags stored inside cardboard boxes in a dry storage area,
getting them out only when observing or photographing the sun.
Film/Filter/Development Combinations
A proper choice of film, filters, exposure and development is paramount to
successful photography. Initially I spent several weeks testing various
combinations to see what results I could obtain from my location. Having
predetermined that the film I would be using was the standard 2415
Technical Pan and that the exposure I would prefer to use was 1/125 second
my task was to find a development and filter combination that would yield
optimum results with a given focal ratio. Having had experience
processing 2415 with Kodak's HC110 developer that was my natural choice.
The dilution B with about 12 minutes @ 68 degrees F would produce
negatives with sufficient contrast without blocking up highlights. Using
the barlow at its shortest projection distance yields a focal length of
190 inches at a focal ratio of f/38. Having determined my film selection,
preferred exposure, development scheme, and a starting focal ratio, I
needed to see how various colored filters would perform with these
parameters. Using the single layer Tuthill filter over the objective
required further reducing the sun's brightness at the film plane. Hence a
filter's exposure factor would play a roll in its selection unless I was
willing to vary the exposure to compensate for various filters. Avoiding
the narrower band filters I chose to stay with conventional off the shelf
photographic filters. Testing a number of filters independently and in
combination, I discovered that my best results were with a #58 dark green
filter. Further experiments showed that as the sun would gain altitude
during the summer I could fine tune my exposures with the addition of a .3
or .6 neutral density filter as needed. Having found a film, filter,
development combination that worked, I locked that into my observing
routine.
A Typical Shooting Run
A photo sequence generally begins with me having
completed a sunspot count and noting an interesting group or two that
is worthy of catching on film. If seeing conditions are poor (too low of
a sun, boiling atmosphere, strong winds) I generally don't bother
attempting photography. These conditions tend to make success quite
rare. If conditions are favorable, I will attach the filter scheme as
above, load the camera with film, and begin a shooting run. With my
pre-determined film-filter-development combination I have only two
variables left with which to contend. They are focus and seeing
conditions. By exposing film only when the seeing is steady and the
sharpest view presented much wasted film is avoided. This is
accomplished by monitoring the seeing through the viewing system of the
camera. This is why a clear focusing screen is useful. Those moments
when the atmosphere calms and fine detail is evident is the moment to
trip the shutter and expose the film. I use an air bulb release to
further reduce the chance for unwanted vibrations. Accurate focus must
be accomplished or all previous effort would be for naught. For
stellar photography replacing the film with a sharp straight edge and
performing a "knife-edge" test on a star is best for finding accurate
focus. Since this test cannot be done in the daytime on the sun, I have
relied on simple aerial focussing with the viewing system of the camera.
The clear focusing screen has a cross hair scribed on it which appears
to float above whatever is being photographed. When the crosshair and
the image are both sharp, focus is obtained. The nature of the eye
however is such that it can adjust its focus independently. It is
possible for the optical system to be out of focus slightly and our eye to
correct this, bringing it into focus. Because of this, I bracket focus
my exposures. Basically, each attempt at a photo consists of 3 separate
photos: one at apparent best focus and one each slightly on either side
of best focus. This assures me that one of my attempts will be in
correct focus.
Select the image below for the full size image:
Image Processing
Once a good negative has been obtained, additional
techniques can be employed to enhance detail already captured. Darkroom
work of unsharp masking, dodging and burning, etc. may be used to produce
photographic prints, or scanned images may be obtained to use with digital
applications on a computer. Adobe Photoshop is a powerful medium for
image processing. Digital images may be output as reflective prints or
on a film base as negatives or transparencies. Files may be shared with
others on conventional storage disks or displayed on internet sites for
mass distribution.
Conclusion
Solar photography is challenging in requiring a number of
factors to be in your favor for success. Persistence is the key to
obtaining good results and with such a dynamic and ever charging face, each
photograph of the sun is a unique and singular record of its appearance
at that moment. Therein lies the value of your photos.
As always correspondence with other observers is welcome, please feel
free to contact me at jenkins@prairienet.org.
Last Modified: 3/19/03 8:55p
This page:© Copyright 2005 by John A. Blackwell