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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?

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    As you've already posted on that question, please try and edit your question to explain what you don't understand about the answers there. – Philip Kendall May 5 '16 at 17:20
  • Well phillip kendall...what i want to know is the question thatci have asked...i watched a video by forest tanaka..it was on short multiple exposures. But if i connecty dslr yo a telephoto lens and shoot. What would b preferably better shorter or longer exposures..??...a 30 second exposure gove me star trails without using any computerised mount or should i frop the exposure time to below 10 seconds..???..this is what ibwant to know. – dwayne dias May 5 '16 at 18:18
  • Possible duplicate of Deep Sky Photography with telescope without Tracking – scottbb May 5 '16 at 18:25
  • @dwaynedias The accepted answer on that question says "the longer you can get away with exposing, the better in the long run." What's not clear about that? – Philip Kendall May 5 '16 at 18:34
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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.

  • This is impressive @michaelclark. So if i use a telephoto eyepiece (32 to 44mm) which doest not give you that vignetting effect. Can i take about a 20 to 30 second shot...but my telescope focal length is 120mm which is big. Wont this effect my exposure time...?? – dwayne dias May 6 '16 at 4:23
  • To take a 30 second exposure without the stars demonstrating trails you will need an angle of view of roughly half the width of the sky. Your setup sounds like it is much, much narrower than that. It's still not exactly clear to me how you are projecting the light from the telescope onto the camera's sensor (What is between the eyepiece and the sensor? Anything besides air? Your original question says you have a lens on the camera? You also haven't told us your camera's sensor size. But with a 1200mm reflector at the widest with a FF camera you're looking at about a 2º AoV. That's 1/2 sec max. – Michael C May 6 '16 at 4:32
  • And that is for a single uncropped image. When you do stacking the frames will be cropped so that only the part of the sky that is in all of the exposures will remain. You have to take that into account as well. Just how much that will be depends on the length of time between the first and last frame as well as what spot in the sky you are imaging. The stars near the celestial equator appear to move much faster than those near the celestial poles. – Michael C May 6 '16 at 4:35
  • I have gathered information of the equipment i am using. I have a nikon d5500 with a APS-C sensor size of (23.5mm x 15.6mm) much bigger than the t6s and t6i sensors. Telescope focal length of 1200mm, faocal ratio of f/6, the eyepiece is a super view telephoto projection eyepiece (1.25" and 32mm focal length). The eyepiece will be atached directly to tue t ring and so it will b at a very close distance to the eye lens come and sensor. – dwayne dias May 6 '16 at 5:38
  • The dslr and eyepiece setup will then b connected to the scope. If the focal lenght is still an issue..can i use a focal reducer...?? To give me a little longer exposures of 20 to 30 secs without any trails...?? – dwayne dias May 6 '16 at 5:42

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