June 2001 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - June 2001 Issue

The Telescope Nut
by Jeff Baldwin

Solar Viewing

Some of us enjoy looking at the Sun, even though we have been told since we were babies not to look at the Sun. And again, here we go: Don't look at the Sun! Looking at the Sun is a terrible and stupid thing to do if you don't obey the proper safety techniques.

As gross as it sounds, one of my favorite astronomy labs at my high school is to fry a cow's eye at the focus of a telescope to illustrate the results of looking at the Sun through a telescope. It is very gross: the eye blisters, ruptures and ignites, all in about 1 to 2 seconds. This will convince you very quickly.

The trick is to find ways of observing the Sun safely, and there are many. Indirect viewing is obviously the safest. Reflecting the Sun onto a wall a couple of hundred feet away with a small (1" to 2") mirror works, but the results are low in resolution. Punching a small hole in a large card, holding it up in the air, and looking down at the sidewalk where the shadow of this card is will show the Sun projected onto the sidewalk. Again, low resolution.

The next step upward to resolution and thrill, as well as being the next step upward toward danger, is to point a slow telescope, say an f/15 refractor, at the Sun and projecting the Sun's image from the eyepiece onto white shaded flat material--otherwise known as solar projection. This is a fine way for a group to safely view the Sun, but it can be hard on expensive eyepieces due to heat build-up within the eyepiece, and the threat of a youngster looking into the telescope while your back is turned is very frightening.

The next level upward in resolution, thrill, and (again) danger, is direct viewing. This is done ONLY with a full aperture solar filter. This filter is mounted on the front of the telescope, is labeled specifically for this use, is clean and free of pinholes or scratches, and is attached so that it can not be removed or fall off.

Please note: the screw-into-the-eyepiece solar filters that come with many inexpensive telescopes are burned eyeballs waiting to happen. The heat from the Sun will crack these pieces of junk and you will lose your vision.

There are a few types of full aperture filters on the market, some specifically for photography, some for visual, some for both. The filters come in mylar or plastic, as well as metal deposition glass filters. One might think that the glass filters are better, but glass filters are usually made from window glass and may or may not be flat enough for the sharp viewing you are after. You might also think that mylar filters are bad because they are made from such a flimsy material. However, the mylar solar filters tend to be excellent. Never use mylar balloons or mylar space blankets for solar viewing. Even though you might think that the Sun is filtered out comfortably, you may be getting excessive doses of infrared or ultraviolet light that can permanently damage your eyes. You might not know it when it happens, but you will later. So use only mylar material specifically made for solar viewing.

There are two other dangerous methods of viewing the Sun. One is depositing candle carbon onto glass and looking through the glass at the dim Sun. Another is using black and white photographic film that has been fully exposed and developed. I'm not going to say it won't work or that it is not safe, but can you honestly say that it is? If not, don't try it. There are welding glasses available that will block out the Sun safely, yielding a green image. These filters are also made from glass that may be only as flat as plate glass. This would be for "zero power" viewing, held by the hand in front of the eye during eclipses, rather than for telescopic use.

Here is a telescope design that is great for solar viewing. The light passes through an optically smooth glass mounted at a 45° angle at the front of the telescope. This glass is coated with aluminum on the inside surface. The light then passes through to an uncoated primary mirror, then reflects back to the aluminized 45° glass plate, which reflects the light up to the eyepiece. The aluminum removes most of the light, but not enough. The uncoated mirror reflects 4% of the light rather than the usual 88%, which is a filtration of 20X. The reflection to the eyepiece by the aluminized 45° glass removes about 12%, and a glass filter can be used in the eyepiece to adjust the brightness if the 45° glass doesn't remove enough. If the 45° glass breaks or falls out, light isn't even reflected up to the eyepiece; a fairly safe system.

I haven't built one of these, so if you do and it doesn't filter out enough sunlight and you fry your eye, don't sue me. However, it would be fairly easy to test it along the way to see if there will be enough filtration. There would also be no secondary mirror obstruction, which would be beneficial to those high resolution solar observers.

A great filter has hit the market. It is from Baader Observatory and is referred to as the Baader Filter. Astro-Physics Inc. sell this material. At first it appears to be mylar, but it isn't. It is a different plastic, very smooth, smooth enough for Astrophysics to recommend using it. Eric Reichenbach has made a filter out of it, so you might ask him for his opinion.

To observe flares and prominences, you may want to invest in a tunable hydrogen a filter. They run about $2500. You stop your telescope down to about f/30, place a red filter in the remaining aperture, and then place the tunable filter at the focus. It is made of plates that are coated with material gapped at the interference thicknesses that will pass a specific wavelength. Heating it to the exact temperature needed allows it to be tuned to the wavelength of the Sun's excited hydrogen. You will then be looking at the chromosphere rather than the photosphere. The telescope needs to be f/30 or slower, so that the angle of attack of the light waves through the filter will not make them appear different in wavelength from the interference thicknesses of the filter. You can't use this on an f/5.6 system unless you stop it down.

There is a new school telescope on the market, the Sunspotter, selling for about three hundred dollars. It is a solar projection telescope that appears to be very safe and easy to use, as well as durable.

It is a birch plywood triangle with a hole cut in it to hold a 60mm f/11 refractor lens. The light passes through to an opposing vertex, reflects off a mirror to another vertex, then up to the top vertex, then back down through an eyepiece mounted in another wood board, which projects the Sun onto a flat card on the bottom side (see diagram).

This whole triangle can slide around in a circular birch plywood carrier on legs, which can be moved around in azimuth to locate the Sun. It's ingenious. It's safe. It's gonna take off in classrooms all across the country (I hope).

Be careful.

Clear Skies!...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.