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Comment by jakobdabo | Hacker News Reader

On a camera in manual mode if you increase the exposure time, you must compensate the increased amount of light coming to the sensor by decreasing the aperture or the ISO. Can anybody please explain, how is it possible to have such a long exposure without overexposing the image?

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Note that these images are made from many exposures that are then processed and combined, and the total exposure time is given. The individual exposure times were probably long, but I'm guessing on the order of minutes, not hours. (Actually, if you photograph stars with an exposure time that is too long, then you get star trails in the image.)

Trails appear very quickly, at focal lengths of around 20mm (typical dslr kit lens) they are obvious in 20-30 seconds, with a 100mm lens (start of telephoto range) they are obvious in 3-5 seconds. This of course depends on the res of your camera (I shoot with an 8MP and a 24MP).

To get rid of the trails you need a tracking mount. The better the mount the smoother and longer it tracks. Astrophotography is expensive...

> if you photograph stars with an exposure time that is too long, then you get star trails in the image

He was using a tracking mount. At 200mm you can't image for more than a few seconds without trailing.

If you want longer exposures than your lowest ISO can handle you can also use neutral density filters, which go over the lens and reduce the amount of light hitting the sensor.

https://en.wikipedia.org/wiki/Neutral-density_filter

By using very low ISOs. You want to use the lowest ISO possible because the lower the ISO the less noise in the picture, and noise in the night sky can be very noticeable since you are photographing a ton of tiny dots against a black background.

Astrophotography is less like photography and more like scientific data acquisition.

To specifically answer your question though, this is "1250 hours of exposures" and not "a 1250 hour exposure".

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