What solar filter to buy for the August 12 eclipse
The August 12 total solar eclipse is 55 days out. If you’re planning to photograph it, point a telescope at it, or just watch the partial phases safely, you need a solar filter. Prices are still normal and stock is still available. By late July neither will be true. Here’s what the numbers on the label mean, which filters are worth the money, and what I’m packing for Spain. The one number on every solar filter: optical density Every solar filter blocks a fraction of the Sun’s light. The spec you care about is optical density (OD), a logarithmic scale. OD 5 means only 10^-5 of the incoming light gets through — one hundred-thousandth. OD 4 passes ten times more. That factor of ten is the line between safe for your eyes and not. ...