DIY solar filter help

bjn70 wrote:

I'm thinking about stacking an nd10, nd2, and 2 circular polarizers(1.5) each. For a total of 15 stops. Will this do?

Maybe nobody cares anymore, maybe you already found this out, but 2 circular polarizers stacked and oriented so they are blocking each other will get you in the neighborhood of 9 stops. Someone else may be able to do the calculation for me- in my test I used ISO 100, f32 and 1/8000 sec. and the sun was still overexposed slightly, but in a full shot of the sun I could see sunspots so I thought maybe I wasn't overexposed by too much.

Strangely though, for the 2 circular polarizers to block each other they not only had to be rotated to exactly the right positions but they had to be facing the right direction. I had the rear filter facing forward and the front filter facing rearword and that worked. With both filters facing the same direction it didn't work at all.

Once upon a time, I thought circular referred to the disc shape being able to rotate -- that is NOT what it means. Circular polarizers ARE DESIGNED NOT TO LEAVE LIGHT POLARIZED -- the focus and exposure sensors in SLRs are usually behind semi-silvered mirrors that act as polarizers, so the circular polarizers are designed to avoid having light exit polarized in order to avoid blacking-out the focus and exposure sensors. Some older, cheaper, polarizers were not designed for SLRs with semi-silvered mirrors, and they can dramatically cut the light passed when stacked.

I believe the AstroSolar film is definitely the way to go. First off, it's just 2 air-interface surfaces, not multiple like stacked filters. Second, my understanding is that the TWE (transmitted wavefront error, which is related to flatness) is the critical thing for image quality; a typical filter is about 1/4 wave -- AstroSolar film claims just 1/20 wave, which is more like a $1000 filter. Third, the film is just 0.01mm thick, so there should be virtually no CA introduced -- even a perfect flat glass filter introduces CA due to its thickness (light travels through a region with a different refractive index from air -- this is what all the thick sensor coverglass discussions are about, although it's far less of an issue with a filter in front of the lens). Fourth, no polarization issues. Fifth, it's MUCH cheaper than even crappy filters.

Anyway, aside from photographing the Sun, the AstroSolar film looks like a good general-purpose very-extreme ND filter. I haven't tried it for non-Solar photography yet, but I expect it to be great for things like getting slow exposures to blur flowing water lit by bright daylight....