Exposure time
The shutter speed, or exposure time, is the duration during which the camera sensor is exposed to light. In astrophotography, this duration can range from a few fractions of a second for the Moon and bright planets to several cumulative hours for fainter objects such as distant galaxies or faint nebulae.
Accumulating Photons
A photographic sensor records the photons that reach it. The longer the exposure time, the more photons accumulate on the sensor, and the more faint objects become detectable. This is the fundamental principle of long-exposure photography: light that is too faint to be perceived in a fraction of a second becomes visible after several minutes of accumulation. This is what makes it possible to photograph galaxies millions of light-years away with an amateur telescope.
Exposure Time Limitations
Increasing the exposure time without taking precautions leads to several problems. The Earth’s rotation causes the stars to move across the field of view, creating streaks of light instead of points—this is known as star trail. To compensate for this movement, astrophotographers use motorized mounts that track the movement of the sky. Sensor noise also increases over time, limiting the effective gain. This is why we generally stack many short exposures rather than a single very long one.
Importance in Astronomy
Long exposure times have revolutionized astronomy. Photographs taken over several hours, or even tens of hours combined, reveal structures invisible to the naked eye— galactic halos, filaments of nebulae, and objects of magnitude 25 or fainter. The Hubble Deep Field, one of the most famous images inastronomy, is the result of an exposure accumulated over several days, pointed at a seemingly empty region of the sky.
Concrete example
The Rosette Nebula, with an apparent diameter of 5 degrees, is almost invisible in white light even with a good telescope. With an H-alpha filter and cumulative exposure times of 5 to 10 hours, it appears in all its splendor— a red-pink gas cloud 130 light-years in diameter.
Frequently Asked Questions
How long should an exposure last in astrophotography?
It depends on the object, the telescope, the sensor, and the sky. Generally, it’s better to take many exposures lasting a few minutes each rather than a single long exposure, to better manage noise and defects. For very faint objects, exposures can be accumulated over several nights.
What is exposure noise?
Exposure noise is the random noise introduced by the sensor during exposure. It increases with sensor temperature and exposure time. To reduce it, astrophotographers cool their sensors and subtract calibration images called “darks.”
Does the Moon interfere with long exposures?
Yes. The Moon illuminates the sky enough to increase the sky background and reduce the contrast of faint objects. Astrophotographers generally schedule their sessions around the new moon phase for white-light objects, or use narrowband filters that remain effective eveneven in the presence of the Moon.