If you have an older telescope, especially one from before the 1990’s, check whether it came with small threaded filters that screw directly into the eyepiece. They often came with two of them. One was marked MOON and the other was marked SUN. The Moon one works great to reduce the glare of a full Moon.
It’s the SUN filters that are so dangerous. These are the Lawn Darts of the astronomy hobby, if the Lawn Darts had sharpened hypodermic needles at the front.
The original concept was this: Your telescope captures the sunlight and focuses it on the dark SUN filter that is screwed into the eyepiece at the back of the telescope. You place your delicate eye up to the eyepiece, only a few inches from the searingly hot SUN filter.
If curious, the recorded demonstration is at the start of this video: https://youtu.be/rtyLH5fBaJE
As long as the glass didn’t crack, you were fine. Unfortunately… they cracked a lot. Like, a whole lot. Sometimes they even shattered. This would instantly expose your retina to the full blinding light of a 3 or 4 inch magnifying glass, instantly causing solar retinopathy and likely blindness.
If you have one of these SUN filters, it is recommended that you smash & trash them so that nobody can ever accidently try them out.
I tested an old threaded solar filter while recording it with a camera. It reached 326 degrees Fahrenheit and in less than 12 seconds after installation, the filter cracked from the heat buildup and suddenly allowed bright sunlight through.
*** I think this is the only known video of one of these cracking in real time: https://youtu.be/rtyLH5fBaJE
Safe Alternatives:
- Solar Eclipse Glasses (by themselves, NOT in combination with telescopes, binoculars, or cameras).
- Front-mounted solar filters specifically designed for your telescope. These stop 99.999% of the light energy before it even has a chance to enter your telescope.
Summary:
Solar filters for telescopes should go on the front of the telescope, before the sunlight is concentrated. Never put a solar filter at the eyepiece end. If you have one of these old threaded SUN filters, smash it & trash it.
Have a safe and exciting solar eclipse experience!