Questions on Solar Observing

Jack Kramer

Those of us who have our e-mail addresses published on the LCAS web site often get questions from people who visit our site and ask our opinions about their astronomy-related concerns. The following are real questions all received from the same person; they are printed here because they reflect some potentially dangerous misconceptions about observing the sun. The answers that I gave are also included.

I have a few questions regarding solar observation that I thought you might be able to answer. Although I am a relative newcomer to amateur astronomy, I do have a Ph.D. in solid state physics, with which comes a rudimentary understanding of optics. All the questions refer to the 80 mm objective diameter and 400 mm focal length of my scope. I purchased a good, full aperture solar filter with it, which provides very nice views of the sun with the 10mm and 25mm eyepieces which were included. No problem. Here are the questions:

Question 1: Re: Finder Scope = 6 x 30. I had three overexposed 35mm color slides, which I stapled together, and rigged to fit over the end of the finder scope. I get a very nice blue image, which is very comfortable to view. The problem is that I read somewhere that there is danger due to IR when using color film in this way. My eye gives no complaints. My rationale is that, relative to the visual output, the sun puts out very little in the IR, and that coupled with the three layers of color slide, and the small aperture of the finder, will this give me the protection I need?

Answer: Regarding your finder, I too have heard that there is an element of danger in using overexposed slide film as a filter. The insidious thing is that certain wavelengths of light can damage your eye without your being aware of first. Let me suggest an alternative method. When I observe the sun, I sight in using the finder without a filter, but NOT looking through the finder -- I simply hold my hand behind the eyepiece of the finder. When the small image of the sun is centered in the silhouette of the finder tube, the sun is also about centered in the main telescope. Then I put the lens caps back on the finder.

Question 2: Re: Main Scope. Why can't I just stop it way way down, and look at the sun without my solar filter? According to my calculations, if I punch a teeny weeny pinhole of approximately 0.1mm diameter in my lens cap, and use that as a stop, the amount of light that enters would approximate that of the full moon without the lens cover, which is quite a comfortable viewing situation. What do you think?

Answer: NEVER LOOK AT THE SUN WITHOUT A FILTER!!! Forget your calculations -- you'd still be blinded looking directly at the sun through even a peephole. Try this experiment: take a piece of opaque material and punch a small pinhole in it. Now hold it so the sun can shine through it and you'll see an image of the sun projected on the ground or whatever surface is in line with the sun and the opaque material. Essentially, you have a pinhole camera that still transmits a lot of light. And from a purely practical standpoint, why would you want to stop down the telescope so drastically? Up to a certain point, the more aperture you have, the better the resolution of the telescope, and the more details you can see on the sun.

Question 3: Re: Solar Projection Method. When I opt for this approach, I would presumably be operating without the solar filter, with the full 80mm aperture. Is there a danger of damaging my Kellner eyepiece with this approach?

Answer: The solar projection method is the oldest way to observe the sun, and it works fairly well, especially if you want to trace on a piece of paper the sunspots you see on the projected image. However, you're right -- an eyepiece can become fairly hot and I've heard that this may eventually lead to deterioration of the cement that holds the lens elements in place. I don't think this would be a serious problem with your 80mm scope, but on my 10-inch reflector with its greater light-gathering power, the eyepiece got so hot that I couldn't touch it. I had an old "throwaway" eyepiece that I used until I got a solar filter. Also, one of the members in our club had his telescope catch on fire when the image of the sun wandered off the eyepiece and onto a plastic component in his focusing mount. With a good solar filter, you'll see as much, if not more, than with solar projection.