# What is the most measurable electromagnetic wave coming from a small, dark, and not necessarily hot object in sky? [closed]

Optical fibers are recently used to generate some spectroscopic surveys of the universe map based on some projects such as DESI, MOONS, SDSS-V, etc. The current fibers are often sensitive to visible light, so they can efficiently used to observe huge bright objects like galaxies, stars, and so on. However, there are some optical fibers (say, multi-channel ones) which are sensitive to the other parts of the EM spectrum rather than visible light. In particular, I'd like to know the feasibility of using fibers to detect small dark objects (compared to stars and galaxies) in the sky, e.g., a satellite in the LEO orbit. Since the satellite reflects a very trivial amount of light, I doubt that a fiber can detect its coming visible light. So, I'm wondering what other types of EM wave is radiated from a fairly small, dark, and not necessarily hot object in the sky (which can be potentially a better source to detect the object compared to the visible light).

## closed as too broad by Kyle Kanos, Buzz, John Rennie, ZeroTheHero, Qmechanic♦Jan 15 at 21:56

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• Do you have to measure the EM radiation from the satellite itself? Because if you don't, you could try similar methods used to detect planets: looking for periodic variations in intensity. – Kyle Kanos Jan 11 at 12:58
• @KyleKanos: No. Actually the EM radiation is about to be measured by the fibers mounted at a ground telescope. Can you explain which EM type is measured for the purpose of that "periodic variation detection"? – Roboticist Jan 11 at 13:21
• – Kyle Kanos Jan 11 at 13:22
• If all you want to do is detect something, you don't want optical fibers and a spectrograph, you just want an imager. To improve the detectability of "dark" objects, you want a bigger telescope so you can gather more light. – Peter Erwin Jan 12 at 10:32
• Also, the advantage of observing in the optical is that the Earth's atmosphere is relatively transparent and dark (at night, anyway). Cool objects might be better detected in the near- and mid-infrared, except that the atmosphere tends to be both more opaque and brighter (due to its own thermal emission), making it harder to see faint objects. – Peter Erwin Jan 12 at 10:34