Page last updated 7/14/01
The cardinal rule in solar observing is safety. The focused light of the sun from even a small telescope contains enough heat and light to do permanent damage to you eyes from even the briefest exposure. Only observe the sun with proper filtration. If you don't know if what you are thinking of trying is safe then DON'T DO IT!
I don't claim to be an expert in high resolution solar photography but I have learned over the years that there are some keys to success. None of these "secrets" are original with me. I've read all of this somewhere or another. But I've also read a lot of stuff that doesn't work. The best books that I know of for high resolution amateur solar photography are High Resolution Astrophotography by J. Dragesco, 1995 and Sunspots by R.J. Bray and R.E. Loughhead, 1964. Bray and Loughhead's book is written at a professional level but the techniques that they used are not at all out of reach of today's amateurs.
The following notes are the things that I know by personal experience to work. I have listed them in my idea of their order of importance. I first wrote this on 6/4/99. This was before I started using video for white light imaging of the sun in January, 2000. Video has allowed me to do much better imaging than film allowed and has changed a lot of my thinking. I think that contrast between the photographic video "truths" is interesting and may be useful to others. So, rather than delete what I wrote before I've interleaved the video view point below.
Aperture
Photographic: The optimal aperture is around 5 inches. Anything much smaller than that and you cannot resolve the important features. Anything much larger and daytime seeing actually reduces your resolution relative to a smaller telescope. I use a 4.5 inch lens.
Video: I have yet to find the limit. Not long after I started using video I realized that I was routinely acheiving the diffraction limit of my 4.5 inch refractor. This was very satisfying on the one hand. But it was also somewhat frustrating because I knew that a bigger telescope would have captured more detail. Consequently, I built the specialized Newtonian reflector described here. Even with 8 inches of aperture, so about a 0.5 arcsecond diffraction limit, it is not uncommon for the video images to be diffraction limited. I can already feel a case of aperture fever coming on.
Light Reduction
Photographic: The best way to reduce the light from the sun is with an optical quality front aperture filter. Unfortunately, these are hard to come by. I ended up buying a surplus optically flat fused silica window and having it custom coated. This is the most expensive element in my system. I should note that this is not the only way to skin this particular cat. Others have gotten excellent results with Mylar filters and Herschel wedges.
Focusing
Photographic: Focus is critical and in the daytime it tends to change due to thermal changes in the telescope. You must have a way of precisely focusing. I use an Olympus OM-1n camera with a focusing screen that has a clear center with a cross hair and a magnifier (actually with inexpensive video camera to provide even more magnification).
Monitoring Seeing
Photographic: You must be able to see the moments of good seeing and take the photographs only then. I use a beam splitter that feeds an inexpensive video camera. The beam splitter allows me to always see exactly what the film will see.
Magnification
Photographic: You must use enough magnification to resolve the interesting features. I use a scale of about 14 arcseconds per millimeter, which is almost f/130 with my lens! I use a projection lens that I salvaged out of a high-end microfilm printer to magnify the prime focus image (i.e. eyepiece projection).
Exposure
Photographic: Exposures must be short enough to capture the very short instants of good daytime seeing. I use 1/250 second.
Refractors vs. Reflectors
Photographic: There are volumes written on the subject but I am in the camp that favors refractors over reflectors for solar observing. Refractors generally have closed tubes and the light passes through the air in the tube only once.
Film
Photographic: The film that is used must be as fine grained as possible. This is the way that you can get excellent resolution and still have enough field of view to image large spots. Kodak Technical Pan is the best film on market today for this.
Number of Exposures
Photographic: If the seeing is good you should take several exposures (even an entire roll for a particularly interesting spot) at the moments when the seeing is best. Invariably you will find that one or two are better than the rest.
Camera
Photographic: A camera with a locking reflex mirror reduces vibrations at the time of the exposure.
Mount
Photographic: A solid equatorial mount will help to reduce vibrations and aid in watching for the moments of good seeing.
Observatory
Photographic: A telescope that is mounted in the open, ideally surrounded by water or grass, is less susceptible to local seeing effects than one that is mounted in an observatory.
Telescope Tube
Photographic: To prevent air turbulence in the telescope tube, the telescope must come to thermal equilibrium with the outside air. This is facilitated if the telescope is stored in an unheated/uncooled room when not in use (e.g. a garage if cleanliness is possible).
I am always interested in learning about other people's experiences. Please send me E-mail with your ideas and/or images.
Art Whipple (squalodon@comcast.net)