Determining the proper exposure time is as much an art as a science.  For deep sky imaging, suffice to say that longer is always better, at least until light pollution starts to overwhelm the image.  From a very dark location, or with a narrowband light-pollution filter (such as a Hydrogen-Alpha filter), the only limit may be the camera software, which usually limits exposures to 3600 seconds (1 hour).  For planetary imaging, the right exposure time can be more critical as there is a balance between short exposures to avoid atmospheric turbulence and longer exposures to get the most image data possible.  Some suggestions for starting points are given below.

Note:  For an in depth discussion of exposure times, complete with fun mathematical equations, visit the Imaging Theory section.  For beginners and math-phobics, the section below will give sufficient information.

Deep-Sky Exposures

If your CCD camera is unguided, the exposure times will be determined by the accuracy of the mount’s drive and the focal length of the telescope.  A telescope with a 400mm focal length on a fork-equatorial mount might track for 60 seconds, unguided.  A 2000mm focal length on the same fork mount might only be capable of only 10-15 seconds.  A telescope with a moderate focal length on a high-precision German equatorial mount may track unguided for 2 minutes or more.

While the individual exposures may be limited to just a couple minutes, longer exposures are always better, so stacking multiple short exposures is a good idea.

For guided CCD cameras, you can take an exposure that is just about as long as you want.  In this case the limitations will be light pollution and software.  Usually software limits exposure times to less than 1 hour, although this would be considered an exceptionally long CCD exposure.  There are rarely times when you need to exposure this long, and again you can always stack multiple long exposures just as you would short exposures.  Light pollution will eventually overwhelm the image at all but the darkest locations, so there is an unknown variable that must be determined by experience.  Try simply taking longer and longer exposures until the background light becomes a problem.  Then stack as many of the longest good exposures as you can!

Using Other’s Times

A great way to figure out an appropriate exposure time is to look at all the available images online or in books.  If you know the exposure time used to take an image you like, you can fairly easily figure how long an exposure is required with your equipment to achieve a similar result.  If you consider that factors like pixel size and quantum efficiency are, for the most part, negligible due to the relatively small variations from camera to camera, the only thing that matters is exposure time and focal ratio and the math is pretty easy.

For example, you find online an image of your favorite galaxy which was taken through a telescope operating at f/7.  The exposure time was 20 minutes.  If your telescope has a focal ratio of f/10, you need an exposure of 40 minutes to get the same results, because f/10 is half as fast as f/7.  However, the other factors do come into play.  To determine a more accurate exposure time, use the calculator below.

Equivalent Exposure Time Calculator

Planetary Imaging

Note:  This section refers to planetary images taken with a CCD camera.  The most popular method for planetary imaging is using a webcam, which is covered in the Webcam section.

The trick with planetary imaging is to take as short an exposure as possible, just about the opposite of deep-sky imaging.  However, a very short exposure will have a lot of noise in it, and a longer exposure contains more information than a short one.  More information is good for post-processing.  Finding the balance is the tricky part.  Again, it depends greatly on the telescope being used, the subject being imaged, and the focal ratio of the telescope.

Below are some good starting points based on the planetary target and the focal ratio of the telescope.  Factor such as pixel size and quantum efficiency (which are less important) have been ignored, so some experimentation is suggested.

Note:  Exposures will be about 3x longer if RGB filters are used to take color images.

Mars

Focal Ratio

Recommended Exposure Time (sec.)

10

0.01 – 0.02

20

0.05 – 0.10

25

0.08 – 0.16

30

0.11 – 0.22

40

0.20 – 0.40

50

0.30 – 0.60

60

0.45 – 0.90

Note:  Mars experiences a wide variation is brightness depending on how close it is to Earth, so a range of recommendations are given based on whether Mars is at opposition (closest to Earth) or slightly more distant.  When Mars is most distant it is too tiny to image.

Jupiter

Focal Ratio

Recommended Exposure Time (sec.)

10

0.01

20

0.05

25

0.08

30

0.11

40

0.20

50

0.30

60

0.45

Saturn

Focal Ratio

Recommended Exposure Time (sec.)

10

0.10

20

0.30

25

0.50

30

0.70

40

1.20

50

1.90

60

2.70

Some CCD cameras will not exposure shorter than about 0.1 second.  If this is the case with your camera, and a shorter exposure is required, try stacking one or more neutral density or polarizing filters between the CCD and telescope to cut down on the brightness of the target.  This works for the Moon as well.