Why can’t I use a ND filter to photograph the transit of Venus?

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Summary: Although you can probably use (a combination of) photographic neutral density (ND) filters to reduce the same amount of light as a typical solar filter (e.g. Baader solar filter), it is not SAFE for your eyes as it will not filter out the harmful Ultraviolet (UV) and infra-red (IR) light.

The table below shows the reduction in light intensity obtained by using different filters — both photographic ND filters of various Optical Density (OD) and the Baader solar filter. Just from the standpoint of intensity reduction, it is evident that one would require a 12-stop ND filter (OD 3.8) to match the Baader Photographic AstroSolar film. However, the following reasons¬†undoubtedly makes the photographic Neutral Density filters unsuitable for solar photography:¬†

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Gearing up: Received Baader AstroSolar film

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The last time I pointed my camera towards the Sun to photograph the 2009 Total Solar Eclipse, which is also a “once in a lifetime experience,” I had used a piece of a $9.00 Mylar Thermal Blanket instead of a proper solar filter in front of my lens. Well, that was not by choice but by chance as I was carrying a Mylar thermal blanket with me. Mylar blankets, which are made by vacuum depositing a very precise amount of pure aluminum vapor onto a very thin film substrate (Mylar), reflects upto 97% of radiated heat. This makes them very effective for preventing body heat from escaping. They are commonly used by campers as “thermal blankets” and also used in a lot of solar projects.

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