Hi, a few months ago I bought a fairly cheap (around 40 euro's) 2nd
hand (self built) solar filter for my 8" scope. When I look through
the filter towards the sun I can see it very nicely. Up till now I
didn't dare to look dat the sun with this filter because I recently
found out there are 2 grades. 3.5 and 5 ND. Is there any way to tell
which grade it is?
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