Glenn Chaple's Observing Basics: Screw-in solar filter hazards

January 2010: Readers provide firsthand evidence that these filters crack under the Sun's heat.

Contributed by Glenn Chaple

Published: November 23, 2009

This screw-in solar filter cracked while Dennis O’Connell was turned away from the eyepiece of his secondhand 60-millimeter refractor some 40 years ago.

Photo by Dennis O’Connell

Last July, I asked readers to help me document the risk of using solar filters that screw into the eyepiece. For decades I crusaded against screw-ins, warning people to avoid them at all cost. Yet I never heard of one actually failing, so I began to wonder if I might be passing along an urban (or, in this case, cosmic) legend. I wanted firsthand evidence.

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