Solar Filter for Telescopes Answered
Having seen some predictions of increased solar activity over the next two years, I decided to make a filter so I can check it out with my trusty telescope.
To start out, I constructed the rig shown in photo A, and practiced with it by cutting disks of plain glass. Make a table like that in photo C so you can make a continuous score on the glass, without stopping. Put oil on the wheel of the grass cutter. Be sure that the surface you work on is flat. You can skip this if you want a square filter.
After scoring, your glass should look like that in photo B, 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. 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.
Then, make 8 radial scores from the circle to the edge of the glass and propagate those cracks in the same manner as the disk. When you do it right, you get a result like photo D, 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. Or, you can just cut the mirror in a square instead of a circle. Photo E shows an uncut and a finished cut mirror.
The mirror came out of the back of a discarded projection TV. Some of the mirrors are plastic films, some are second surface, 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.
WARNING: Use of a mirror with a thin reflective coating can result in eye damage if used to view the sun through a telescope. I have access to a smaller commercial filter to use as a guide.
This filter produced a bright blue image of the sun, so I added a 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.
The Filter is mounted to a screw-in lens cap.