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Celestron C9.25 + Baader ASTF 240 Solar Filter, Ed Astle

Celestron C9.25 + Baader ASTF 240 Solar Filter

Description

I use the C9.25 for "white light" solar observing.

The Sun would frazzle the secondary mirror without the Baader filter, which is only available in OD5 in this size. Boo!
Baader do sell OD3.8 A4 sheets but they're not big enough. Boo!

Why boo?
For lucky imaging and acquiring a good quality set to stack I want the shortest exposure time possible to get them wobble-free images.
OD = optical density. It's a measure of light blockage. The bigger the number the higher the blockage.
OD5 = 5 zeroes = 100,000 so a transmission of 1 / 100000, or 0.001%
OD3.8 = 3.8 zeroes = 6309 so a transmission of 1 / 6309, or 0.016%
(use the 'ten to the power of x' button on a calculator; 10 to the power of 3.8 = 6309. Conversely Log(6309) = 3.8)

OD3.8 has far higher transmission than OD5 so I could use shorter exposures, which would give me more "luck".
OD3.8 is dangerous. Using it visually will permanently damage an eye so is for photographic use only.
The bigger the aperture the more the damage, so Baader stopped selling extremely dangerous sizes. Only normally dangerous OD3.8 sizes are available.
Baader OD5 is totally safe for visual and photography.

After properly collimating the C9.25 I was able to see amber/straw colours visually around sun spots.
My goal is to capture "solar granulation".

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Celestron C9.25 + Baader ASTF 240 Solar Filter, Ed Astle

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