The sensor in a DSLR can be set to mimic the sensitivity of film with ISO speeds ranging from about 100 to 1600. This setting varies the amplification that is applied to the analog signal coming out of the sensor before it is digitized.
Neither the lowest nor the highest setting is generally best; I normally work at ISO 400, in the middle of the range.
The ISO setting is a trade-off. The same exposure of the same celestial object will look better at a higher ISO than a lower ISO, as long as it isn't overexposed. (Amplifying the sensor's output gives it more ability to distinguish levels of brightness.) So a case can certainly be made for using ISO 800 or 1600 when imaging a faint object through a telescope.
But a longer exposure at a lower ISO will look better than a short exposure at a higher ISO. That is, capturing more photons is better than just amplifying the signal from the ones you already have.
This is demonstrated by a test that Canon published in Japan.2 With a Digital Rebel (300D), they photographed the galaxy M31 for 300 seconds at ISO 400, 150 seconds at ISO 800, and 75 seconds at ISO 1600. The first is quite smooth; the latter two are increasingly speckled with spurious color. This is not from hot pixels, but rather from irregularities in the sensor's response and noise in the amplifier.
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