I have so many questions, and we can't all be optical specialists. If ordinary window glass or enough atmosphere adequately filters UV, why doesn't the many layers of glass in a refractor do the same? As for IR, why can't we use a heat sensor/thermopile at points along the image train to assess safety levels? "Don't look at the sun" and "Trust reputable manufacturers" only gets you so far, and it seems to me that for our pursuits having basic instruments of assay should be as common as musicians having tuners.
It is unfortunately that many discussions simply cannot happen here due to the ToS and regarding commercial solar equipment and may safety discussions on this equipment involves knowing the parts inside and transmission of them, etc, which doesn't come from a white paper or commercial graph. It comes from people who opened these up and measured things. So all of that is forbidden around here. So yea, very unfortunate, and it all comes down to basically telling people something is safe or not and it gets left at that with some minor discussion. A super thread regrading safety would be great, but it would be ignored by the very people who need it, as they would just post first, and then get a link to it later from someone... The stickies here on this forum are ancient, nearly irrelevant, and the few good links in the stickies are completely ignored by the intended audience (evidenced by the threads that are started on the very subjects). Just being realistic about forum behavior.
Regarding your questions.... keep in mind, you cannot even look at the sun with your naked eye, and your pupil constricts to as small as it can go, and it's still too bright. So naturally, any optic that consolidates that energy into a small focused area is going to be much more intense. Refractor glass is about transmission, it refocuses light and consolidates it at the focal plane at very, very high transmission, it doesn't block light, it's purpose is to transmit all of it as best as it can. It's so good at it that the glass doesn't get hot, testimant to how good it transmits all of it (unlike a mirror, which does get hot, because it can't reflect it all and absorbs some and gets hot). Your eyes when looking around are seeing reflected light from objects. Not just staring at the star with the atmosphere and air as your filter. Looking at the reflected light around you after its already been filtered and lots of the UV & IR absorbed and visible light reflected, is nothing like looking directly at the star with your eyes--which is obviously not safe. So naturally, anything pointed at the sun that collects light would be even more so a problem in terms of what's unsafe about it. And even more intense. Thus, we need a way to decrease intensity and filter out the dangerous wavelengths that are invisible to our eyes, yet carry energy (UV & IR). Visible spectrum carries a lot of energy too, mind, more than UV does. But we can see visible spectrum and when its too intense we perceive it as too bright and our eye protects itself and shuts. So for visible spectrum, we simply need to lower intensity, which is easy, when its not too bright, its obvious to your eye, and you're fine. We cannot do that with UV & IR, its not visible spectrum to our eye, so we wouldn't know its too bright or intense. So we filter it out completely so that it's not even an option or question, or at least filter it to a very low amount. Remember, it has to be a low amount because an optic that consolidates that energy intensifies it over a smaller surface area, so its a lot more than one might think by the end of the imaging train of an optic. It's a lot easier to simply block IR all together, since you cannot see it anyway, than it is to try to measure thermal properties (which would be different in every single sample of a scope and different apertures, designs, etc, making it too variable, and too prone to risk). Again, so much simpler to block wavelengths that your eye cannot see, that's already taking out the energy and no need to measure it, it's just not there in a meaningful load, which is way simpler and safer for a population with zero information or education on the subject.
Even the moon is too bright to look at in a scope at low power with a big aperture! And the moon is absolutely reflecting IR at you!
Bottom line is, block UV & IR as much as possible and it will be a lot safer because you can't see those wavelengths. Then, grind transmission enough so that the image is not too bright to where you can look at it.
Here's a simple breakdown:
Our atmosphere already handles a ton for us. But a lot still comes through. Of what comes through, that graph breaks it down to what really matters for us visually or just thermally speaking. As long as we keep in mind that we cannot see UV & IR and that's why they're so dangerous, we wouldn't know they're too bright until it's too late and the damage was done, and understand that visible spectrum carries about the same amount of energy, but we can see it, so we know right away its too bright (blinking, pupil constricting, eye shutting, actual pain, etc) and we can also see when it's not too bright. It really becomes quite simple without any optics or physics knowledge. Literally as simple as, if you can't see it, it can harm you more. So block it! This is with respect to visual of course. Cameras don't care. So block UV & IR. And reduce transmission of all of it.
Very best,
Edited by MalVeauX, 02 September 2021 - 02:06 PM.
