How to make a solar filter

Caution: Never look at the sun without protecting your eyes!

My solar filter is made of exposed black&white film which has a layer of metallic silver.
This shall in theory block all wavelengths and should therefore be safe for viewing the sun.
However, I do not guarantee this!!! If you want to be 100% safe you should buy a filter from your astronomy dealer.
NB! Colour films consists of organic dyes and lets through radiation outside the visible spectrum! Never look at the sun through colour film.
You can also buy ready made filter film like for example Baader AstroSolar^TM

What you need:

Access to a darkroom
Basic knowledge of developing black&white film
Kodak Kodalith Ortho Film Type 3 (or similar from other manufacturers)
Developer and fix as recommended by film manufacturer

The film were exposed and developed to give a suitable density. One has to experiment with several exposure times in order to find the correct density. Test the density first by viewing a strong light bulb (e.g. 60W) through the filter in a room where this is the only light source. If you can just see the bulb through the filter you are quite close to correct density. Now you can take a quick look at the sun - be careful!. If the sun looks bright you have to make a new film with longer exposure time.
Now place the film in front of your camera lens and point the camera towards the sun. Make a series of exposures at different shutter speeds (e.g. from 1/15s up to 1/1000s at f:8) in order to find the right exposure. Develop the film and see which exposure setting that give the best result. Slides film is best for this purpose.

Again: Always use extreme caution when observing the sun!

Now you are ready to make a mount for your lens or for your telescope. See pictures.
First I made a tube to fit over the telescope tube. I cut strips of thin cardboard (or thick paper) and wound this around the telescope tube. Apply glue between the layers of paper. Add at least 5-6 layers of paper. Be sure not to spill glue on the telescope! When the glue is dry you have short tube that fits the telescope tube. Then cut a quadratic piece of cardboard and cut a circular hole in the centre approximately the same diameter as the front lens of your telescope. Glue this to your cardboard tube. Fix the solar filter to this plate with tape in all four corners (so that a sudden wind won't flip the filter away). Add another piece of cardboard in order to protect the filter from scratches.

Picture of ETX-90 with Solar Filter mounted

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