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
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
Now you are ready to make a mount for your lens or for your telescope.
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