I always keep a neutral density filter on my camera, if it gets scratched it is more easily replaced than a camera lens.
"ND" refers to neutral density filter. This is a grey filter. The ND
rating of solar glass filter or Baader film is typically 5 or higher. The number means that the material passes 10^-5 in all wavelengths (infrared, visual, and uv). So an ND-5 filter passes 1/100,000 of the Sun's light; ND-6 passes 1/1,000,000. For visual observing ND-5 is the minimum. The ND-3.8 is for photographic work, although my experience with a DSLR and an ND-5 filter is that I could only get a useable exposure at the highest ISO speed. This leads me to believe the ND-3.8 is something for wet film photography, not digital solar photography.
Since permanent vision loss is a possible outcome, only buy from reputable dealers. I bought the Baader ND film directly from Baader Observatory because they claim to have high quality control on their product.
http://www.baader-planetarium.com/sofifolie/sofi_start_e.htm
If you read solar film advertising literature closely, Baader is the only company that states they quality control their product, i.e. - that Baader film is ``CE Tested.'' This means that an independent lab tested the product to assure that it meets European Union import health and safety requirements.
For solar observing, I own a 4.25 inch (Celestron) glass filter. When white light observing, my 10" Newt has a solid cover with a smaller hole built-into it. The 4.25 glass filter goes over that.
The other approach for white light observing with an 8 or 10 inch scope is the lower cost, but equal safe Baader film, to fabricate a full aperture cover. I do not have any comments on whether the full aperture or the 4.25 glass filter approach gives you better resolution. I go with the 4.25 glass filter approach because the glass filter is easier to store and to inspect. It is not clear to me whether when using a Baader film over 8 or 10 inches of aperture you also should use a supplemental ND filter on the eyepiece.
Maybe others will have comments.
With either approach, a necessary safety precaution is to keep a permanent marker handy and to inspect your solar filter before each use by holding it up to Sun and looking for pinhole leaks. Use the permanent marker to cover up any pinhole leaks. Otherwise those little pinhole leaks become mini-high powered lasers that scar your retina.
Clear Skies - Kurt
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