loading

Solar Filter for Telescopes

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.


Picture of Solar Filter for Telescopes
Filter2.jpg
sort by: active | newest | oldest
1-10 of 18Next »
lemonie5 years ago

What advantages over "projecting" the sun onto a screen?

L
Image intensity for one thing. The stuff that's very popular for solar imaging with a telescope is called Baader Film,
I was thinking of direct-observation rather than photography. It's not that clear.

L
ShutterBugger (author)  lemonie5 years ago
I just like to mess with things and try stuff.

I have this image of my filter falling off and spending the rest of my life looking more like a pirate, than I do already...

L
ShutterBugger (author)  lemonie5 years ago
The filter is mounted to a screw-in front lens cap. I too had that image and took care that it wood not happen.
~Bob~

I thought you would, but would you add that in bold at the top of this?

L
ShutterBugger (author)  lemonie5 years ago
OK. But it seems more appropriate at the bottom.
~Bob~
ShutterBugger (author)  lemonie5 years ago
When the concentrated sunlight hits the eyepiece, there will be lots of heat that can damage the eyepiece. It can burn the cemented lens elements.

The mirror keeps all that heat out.
~Bob~
Yes, I suppose so. What do you reckon to optical-quartz for an eye-piece in that situation?

L
1-10 of 18Next »