I was experimenting with solar projection with my 8" scope. I first stopped up the aperture to about 3". All was well. Then I went for full aperture. The images were sharper.
But then the images went foggy. And I couldn't get them in focus. And smoke started coming out.
I disconnected the Kellner and noticed the lens was fogged up. Turns out that while the eyepiece housing was metal, as it should be for solar work, the eyepiece lenses had a plastic spacer. Which melted. The spacer is the black thing to the left of the lenses.
The lenses cleaned off fine with acetone. But I wonder how I can find a metal replacement spacer.
So, that's a warning for solar work: don't assume that just because the eyepiece tube is metal, the inside is metal, too.
I re-glued the lenses with high-temperature RTV. Works fine as an eyepiece (except for a bit of the RTV showing in a shadowy way near the edge of field). I hope the high-temperature RTV would let me use it as a solar projection eyepiece. As a test, I glued two black pieces of metal together with the RTV as a test, and then held it at the prime focus of the telescope, and sure enough pretty quickly the test RTV started smoking and softened.
ReplyDeleteBut a very kind person on Cloudynights is sending me an all-metal 25mm Kellner.