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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.


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


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.


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...


The filter is mounted to a screw-in front lens cap. I too had that image and took care that it wood not happen.

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


OK. But it seems more appropriate at the bottom.

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.

Yes, I suppose so. What do you reckon to optical-quartz for an eye-piece in that situation?


You don't need quartz, just eyepieces without cemented lenses. The simpler the better.

I'm a little curious. Why did you do this as a forum topic, rather than creating a regular Instructable? It's worth Featuring if it's written up. The pictures are great (though in a real I'ble it'd be better if each was a separate image, rather than mosaicked.

The Instructable format is often presented in 2 or more pages. I want everything on one page.

Also, the PDF files are often corrupted. The PDF "fixer" programs that I have tried won't make these readable.

Also, just this forum format took 15 minutes to create. The instructable format would probably take much longer.

Well, you've certainly got two or even three pages worth of content here. Visually, since the content won't fit in one screen, users have to keep scrolling up and down to go from your pictures to the text and back again. By breaking the information into smaller chunks, and having the images associated closes with each piece of information, the reader is better guided in the process.

How much of that time was spent doing the images, or composing the original content? A simple cut-and-paste (if you had already written this somewhere else) shouldn't have taken more than a couple of minutes. I've created I'bles from pre-existing content in about 10 minutes, because I could just cut text and drop it into each step.

If the PDFs are corrupted, would you consider creating a bug report, and attaching one of the corrupted files (you can upload arbitrary files to your "image library" and attach them the same way as PNGs or whatever)? That would let the staff know that there is a problem, and give them an opportunity to fix it.

It is the internet time that I have to keep low. I get 10 hours of super slow dial-up from Netzero. And the web is getting slower every year. Many of the PDF's take a long time to download. Uploading them would take longer.

I noticed that I can upload instructables a piece at a time rather than all at once. Perhaps I'll start working on that.

I wanted all the content on one web page, not one screen. I tried putting the pictures by the associated text and failed. It works on some other forums.

Ah! You don't need to upload PDFs. You use the text-based forms interface to create your Instructable, step by step. You can stop and come back to it indefinitely, both before and after "publishing" (i.e., making it public). The PDF is generated by the I'bles servers directly from your content.

The system here is not set up to support either raw HTML or "bbcode" with embedded images. It's based on something called CKeditor. A lot of us don't like it, but what can you do ;-)

:-) Let me know if you need a hand with any of the technical stuff. This is a really great "practical science" project; thank you so much for making it available!