Astrophotography with a telescope — fewer long exposures or more shorter exposures for astrophotography?

Question

I have an 8" GSO Dobsonian (1200mm focal length) telescope and I use a Nikon D5500 with a UHC (ultra-high contrast) broadband filter. I connect my camera with a 32 n 42 mm telephoto lens. Can I take multiple shots (about 50 to 100) of 20-30 second exposure, and stack them using a combination of light, dark, and bias frames and avoid star trails? Or should I settle in with 8-10 second exposures with more number of shots and stack them? Can someone please help me out with this?

Answer 1

In order to determine the longest shutter time you can use without getting star trails you must calculate the angle of view (AoV) your images have with the setup you are using. With a camera attached to a telescope there are several variables that will affect the angle of view shown in the resulting images.

This is a little difficult to calculate because the focal length of a telescope is different than the focal length of a camera lens when calculating angle of view. About the closest analogy that might be used to explain the difference is that with each different eyepiece one uses on the same telescope you are doing pretty much the same thing as attaching a camera with a different sized sensor to the same photographic lens. A more powerful eyepiece has the same effect as a smaller sensor by showing only the center of the entire field of view projected by the telescope's primary lens or mirror. With a camera mounted there is also the consideration of the amount of vignetting (if any) caused by the adapter tube between camera and telescope being narrower than the camera's field of view.

The narrower the field of view the shorter your maximum exposure time per frame can be before the Earth's motion causes the stars, as seen through the non-tracking telescope, to trail. The larger you intend to display your final image for viewing also reduces the maximum shutter time, as blur that isn't noticeable at smaller display sizes can become quite noticeable at larger display sizes.

Also, the longer you shoot sequential frames the more must be cropped from the edges of the first and last frames when stacking them to show only the sky common to both of those frames and all others taken in between them. This results in a higher sensor size to viewing size magnification ratio because less and less of the total sensor area is used as the time difference between the earliest and latest frames gets larger.

With cameras that use the 35mm film format or a FF sensor, the general rule of thumb is to use a shutter time of 600/focal length (Tv=600/FL) when the intended display size is about 8x10 inches. So a 24mm lens (84º diagonal AoV) would allow a shutter time of 25 seconds before star trails would begin to be noticeable in an 8x10 print. Note that a 16x20 print would halve the shutter time to 12 seconds. A 4x5 print would double the shutter time to 48 seconds. A 100mm lens (24º diagonal AoV) would allow a shutter time of only 6 seconds for an 8x10 display size. A 600mm lens (4º AoV) would allow a shutter time of only 1 second.

As you can see from the above examples, translating the 600/focal length rule to angle of view means that for a single frame you can use about 1 second of exposure time for each 4º of diagonal angle of view (Tv=AoV/4) when you are planning to view the resulting image at about 8x10 inches.