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Technical Notes | Low cost solar film cell
 
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Solar filter
The visual and photographic Astro-Solar films by Baader 
come rather tightly rolled with a protective paper sheet as a
separator.

Thinking of ways to mount this to my AP155 dewcap, it was my wife who came up with this idea: "Why not use one of those embroidering frames? They come in different sizes - surely one will match the dewcap diameter?"

These frames are made of two concentric wooden hoops: the inner one is a solid closed one, the outer one is split and has a thumbscrew arrangement to tighten it onto the inner one thereby compressing and holding the embroidering cloth (or in this case the Astro-Solar film).
Armed with the dustcap for sizing, I went shopping and found that the frames go in inch-sizes, just like telescopes. The 8" size seemed about right and I bought 2 frames for a grand total of 1.200 Chilean Pesos (2 U$!).

The rest was easy. It took no more than 5 minutes to assemble the film in the "cell":


Lay out the film with the protective sheet "convex" side down on a sheet of paper. Using a carpet knife and  the frame as a guide cut through the three layers leaving at least a 1/4" of film protruding.
Remove top paper sheet, put some paper glue (glue stick) to the inner hoop and carefully center it on the film cutout: this guarantees a stress-free mounting of the film which is important for optical quality. Leave to dry with some weight on it for a minute or so. Then turn over, mount the outer hoop and gently tighten the thumbscrew.
Putting a self adhesive strip of "Tesa Moll" (the foamy stuff that is used against drafty doors and windows) on the inside of the cell proved to give just the right tightness to fit the "cell" to the AP155's dewcap:  in fact it is tight enough that when grabbing only the filter cell you can safely lift the whole dewcap. Whatever fixing method you choose, make sure that the filter cannot be accidently removed by a bump or a wind gust.

Folks, it took longer to write this posting than to actually assemble the two filter cells.


By the way, although Santiago winter sky was hazy with smog and the sun near its lowest, my first 6" full aperture view of the sun was breathtaking!

For the source of the AstroSolar Safety film please consult:

http://www.baader-planetarium.com/sofifolie/sofi_start_e.htm

   
Solar filte in AP155
 
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