Making Solar Filters for Telescopes

Table of contents:

Introduction. Inspired by a storm warning.
step 1 Materials.
step 2 Score pieces of scrap glass.
step 3 Separate the pieces.
step 4 When you get good, switch to the mirror.
step 5 Conclusion.
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Introduction;

Early in 2010, some NASA scientists predicted a solar storm to occur in 2012 or 2013. I decided to make a solar filter for my telescope and monitor solar activity. And, I thought, why not make notes and pictures to post on the web so everyone else can do the same.

I wanted a circular filter, a filter that I could mount to a screw-in front lens cap that came with my telescope.. so I started by studying an old October 1965 Popular Mechanics article on glass cutting called “All You Need To Know About Cutting Glass” by Walter E. Burton. Then, I set about working out a method and practicing with what I had on hand.

I didn’t want to use eyepiece projection because all that light and resulting heat is concentrated in the eyepiece. I had read that eyepieces can be damaged by the heat, with cemented lenses burning. I also didn’t want to use any eyepiece solar filters because there is said to be a danger of them cracking from the heat.

Here is a picture of the completed filter mounted on the telescope.

Photo A shows some of the tools you will need.

1. Piece of wood, plastic, or metal to use as a beam.
2. A “C” clamp to hold the glass cutter in place.
3. A suction cup to anchor the beam to the glass.
4. A nail or wire to connect the beam to the suction cup.
5. A set of milk crates or other platform to work on.
6. A piece of suitable “leaky” mirror.

To start out, I constructed the rig shown in photo B, and practiced with it by cutting disks of plain glass. The thickness of the beam is chosen to match the lower notch on the glass cutter. You will need a sharp cutter and use plenty of downward pressure. You will notice that the score in the picture is uneven. This is because the cutter was old and dull. That piece of glass did not break properly. I had to get an even, uninterrupted score to get a clean break.

Make a table like that in photo C so you can make a continuous score on the glass, without stopping. I used some weight in the milk cartons to keep it from moving. I used a flat metal plate on top for the working surface.

Put oil on the wheel of the grass cutter. Be sure that the surface you work on is flat, otherwise the glass may shatter. You can put a thin piece of cloth between the glass and the work surface to keep the glass from sliding. You can skip this if you want a square filter.

After scoring, , your glass should look like that in photo D, but with a uniform and even cut.

Next, put the scored glass on a soft surface like a mouse pad, with the scored surface wetted with water and face down. The water helps to propagate the crack. Press on the back of the score with a dull nail to start a crack. By continual pressing you can watch the crack propagate all the way around. When you have the crack all the way around, you can make radial scores to release the disk.

Return the glass to a hard surface and make 8 radial scores from the circle to the edge of the glass and propagate those cracks on a soft surface in the same manner as the disk score. When you do it right, you get a result like photo E, where the pieces were separated for illustration.

When you get good at this with regular glass, you can cut the mirror into a disk. With the mirror, you score the glass on the side that has the metal coating. This is because you can then see the score as you press on it.

Or, you can just cut the mirror in a square instead of a circle. Photo F shows an uncut and a finished cut mirror.

The mirror came out of the back of a discarded projection TV. Some of the mirrors from TV’s are plastic films, some are second surface, and some are first surface glass but the metal coating is too thin.

There are many variations, but you need to find a first surface glass mirror that reduces the light level by 12 stops (as measured by a LunaPro SBC light meter). Photographically, this is like reducing the light level from F1.4 to F64. This level of light reduction will be hard to find. Out of nine mirrors that I tested, only one was suitable.

Or, as was pointed out to me by Steveastrouk , you can use commercially available films like that at http://www.baader-planetarium.com/sofifolie/sofi_start_e.htm

WARNING: Use of a mirror with a thin and inadequate reflective coating can result in eye damage if used to view the sun through a telescope. I have access to a smaller commercial filter that I used as a guide for light transmission.

This home made filter produced a bright blue image of the sun, so I added a dark red filter at the eyepiece. This combination produced a normal yellow image of the sun. There were no significant sunspots visible, so I didn’t bother taking any pictures.