[dekhn]

I got lucky while testing my camera, which is mounted at prime focus in a refractor scope. I had it in Live View which meant the mirror was up, and I removed the solar filter for just a second.

The camera said "error", turned off, I replaced the solar filter, and it was fine. Much longer, and I'd have had a burn. The telescope really collects and concentrates a ton of light.

[lostlogin]

According to the thread above, the telescopic lens is safer to use than a shorter one. Only here would someone give a mathematical proof!

[stan_rogers]

No, a slower lens (higher f-number, or, actually, T-number) is safer than a faster lens. It's only a practical accident that a long "telephoto" lens is usually significantly slower than a lens closer to normal for the format (and that the long end of a consumer zoom is usually slower than the wide end). A 600mm f/4 lens is an eye-wateringly expensive hulking monster; a 50mm f/1.8 lens is the proverbial cheap-as-chips (with some exceptions), but will let about 5-6 times the amount of light through to be imaged on the sensor - it'll burn a smaller hole, but do it much more quickly. Should you be able too find a 600mm lens with the same T-number (similar to the f-number, but taking transmissivity losses into account), it'll do the job just as quickly, but much more thoroughly.

[dekhn]

Actually, this was a self-built telescope. It was a 3.5" apochromatic 900mm focal length lens adapted to a T-mount.

If you remove the camera, and point the scope at the sun, there's a good sensor-sized beam of intense sunlight at focus.