How do camera settings for astrophotography differ from daytime settings? The answer is not a simple comparison. For daytime photography or even astrophotography using camera lenses instead of telescopes, the aperture, shutter speed, and ISO settings each affect the others. The f-stop setting controls the aperture or amount of iris opening. A lower f-stop translates into a wider aperture, meaning more light can get through to the sensor. Because more light goes through a larger aperture, shorter shutter speeds (exposure times) are possible, which is useful for moving targets to prevent blurring. And because a wider aperture will cause a brighter image, the ISO setting can be lower.
F-Stop vs Focal Ratio
In astrophotography using a telescope, the relationship of these settings still holds, but because the aperture (diameter of the telescope) is not variable, the f-stop setting is fixed and is referred to as the focal ratio, which is the focal length of the scope divided by the aperture. For example, a telescope with a focal length of 1000 mm and an aperture of 100 mm has a focal ratio of 10, indicated by f/10. Similar to an f-stop, the focal ratio has an effect on the required shutter speed (in astrophotography, this is referred to as exposure time). A higher focal ratio (similar to a high f-stop) means the aperture is small relative to the focal length and exposure times must be longer to achieve the same amount of light capture. But in contrast to lens photography, the focal ratio cannot be adjusted and can only be compensated for by longer exposures. A lower focal ratio, f/2.5 for example, is similar to a low f-stop and can use shorter exposure times to achieve the same amount of light capture.
Astrophotography ISO Settings
How does the DSLR or mirrorless digital camera ISO setting come into play with cameras using telescopes as lenses? Dedicated astrophotography cameras don’t have an ISO setting, but the ‘gain” setting behaves similarly. This is a very complicated topic. Contrary to what you will see in many articles about this subject, increasing ISO setting DOES NOT mean the sensitivity of the camera sensor is increased. Sensitivity is inherent to the particular sensor and cannot be adjusted. Increasing the ISO setting only affects the way the data is read out. If you raise the ISO setting from a sensor’s native 100 to ISO 200, you are doubling the read-out from each pixel. If the pixel had stored 5 electrons, it would be read out as 10 electrons, which would increase the apparent brightness of that pixel in the resulting image.
The topic of ISO settings is further complicated by the dynamic range of the sensor. The bit depth of the sensor determines the maximum reading of each pixel. For example, a camera sensor with a bit depth of 14 means the pixel value can be read anywhere from 0 to 214
= 16384. Let’s say the dimmest parts of an image have a read value of about 1000. Doubling the ISO for the same exposure will result in a read value of 2000. This means that the ability of the sensor to display the contrast between the darkest and lightest areas is reduced by 1000. The net result is that you get a brighter image, but the difference between the darkest and lightest parts of the image is reduced by about 1000/16384, or about 6%. The end result is that in post-processing you will have less ability (or range) to display subtle variations. The math is not quite that simple and the change in dynamic range varies for each sensor, but the concept of reduced dynamic range with higher ISO settings remains.
What About Digital Noise?
Choosing the best ISO setting for your camera when used with a telescope is also complicated by the concept of noise. This topic is beyond the scope of this article but also varies with the camera’s sensor. For my Canon EOS Ra camera, the sweet spot for my purposes lies somewhere between ISO 1200-1600. The bottom line is that you really want to achieve the best combination of light gathering in the most efficient time interval to be able to bring out detail in post processing. Your best bet is to try various settings and see what works best for your setup. A good place to get some ideas for where to start is at https://www.photonstophotos.net. There are many charts available for a wide variety of cameras based on actual testing.