Telescope Knowledgebase » Uncategorized » Goto Telescopes

Goto Telescopes

Question
Photography has been a hobby of mine for 35 years and I’ve noticed very big differences in lighting between different types of terrain and soil, and between low and high altitude (up to 17,000 feet). I’ve never made it to the Moon, but in the desert there is much more reflection from foreground objects lighting the subject. And the astronauts’ suits are a bright white while the albedo (reflectivity) of the “light background” of lunar soil you mention is 7%, which is actually extremely dark. And I’ve noticed that at higher altitudes where the atmosphere scatters less sunlight, the sky is much darker near the Sun so it doesn’t affect the exposure unless the Sun is actually in the scene, i.e., backlighting is much less of a problem. That’s what’s noticed at high altitude observatories like Mauna Kea, too. And telescopes in space aren’t affected by sunlight at all unless it’s actually shining into the telescope tube (which is just a big camera lens) — the sky is dark unless the telescope is pointed right at the Sun. It looks like photography in a vacuum is different than what my bit of knowledge on the subject here on Earth would suggest. How about you? What you say sounds plausible. Do you really think, however, that sunlight falling on ground is within the dynamic range of a suit lit by such ground?
Anwser
Sorry, I don’t think I understand the question. However, the main point (along with the fact that the suit is way lighter than the dark lunar surface) is that the Sun doesn’t cause the backlighting effects you’re used to here on Earth. As I said, observatories at high altitudes have a very different environment even though about 2/3 of the atmosphere is still above them. They get above most of the particulates and water vapor, though (which is why the best infrared sites are high — drier air) and that’s what causes much of the scattering in the atmosphere. There’s still the Rayleigh scattering that gives us the blue color of the daytime sky so even there you have a light source that goes from horizon to horizon. Get rid of that light source and thinks change dramatically. Look, for example, at the solar views from the LASCO C3 choronograph on the SOHO satellite at http://sohowww.nascom.nasa.gov (click on The Sun Now, then LASCO C3). An occulting disk covers the Sun’s brighter region (photosphere and chromosphere) in order to show the corona to best advantage without sunlight going directly into the telescope. There in the solar corona, within a few degrees of the Sun, are stars. Their “Best of” page has some pictures of planets near the Sun, including the spectacular conjunction that occurred recently — all invisible to those of us on Earth because of the effect of our atmosphere. The bottom line is that backlit subjects in the open on Earth are surrounded by a bright light source. On the Moon, they can have direct sunlight on their backs but still have a dark sky background. Exposing a very bright subject surrounded by well-lit but dark material and a black sky isn’t that hard. But, like you, I haven’t been to the Moon in quite a while.