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I'm contemplating trying to make one of these star trackers, but after everything I've read, including the two star tracking related stackexchange posts, I don't know if I understand how they work. (If anybody has made this tracker, I'd love to hear how it worked out for you).

My understanding is that you set the exposure of your dslr to 5 minutes and then the star tracking mount will automatically rotate with the stars (assuming motor and properly set up), but what I don't understand is:

  1. How are you able to take a picture for 5 minutes while the camera is moving and not have the picture be blurry?

  2. The pictures taken with the 5 minute exposures/mounts only compare to 5 minutes with no mount, and not 15-25 seconds with no mount–how much better are these pictures and how so?

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  • \$\begingroup\$ The kind of tracker you're talking about is also known as a barn-door tracker. Searching for DIY barn-door trackers will give you lots of results, including more advanced versions that don't have the "tangent error" that limits the tracker you linked to to about 5 minutes. \$\endgroup\$
    – scottbb ♦
    Commented Sep 15, 2016 at 19:12

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How are you able to take a picture for 5 minutes while the camera is moving and not have the picture be blurry?

The stars are moving across the sky. More accurately, the Earth is rotating beneath the stars. The mount moves the camera very smoothly at the same speed as the sky appears to move from the Earth's surface so that the camera stays pointed at the same spot in the sky as the Earth rotates underneath it.

The pictures taken with the 5 minute exposures/mounts only compare to 5 minutes with no mount, and not 15-25 seconds no mount–how much better are these pictures and how so?

Longer exposures help in several ways:

  • You can use a narrower aperture. This allows you to use the "sweet spot"of the lens you are using. Most lenses are softest and show the effects of aberrations the most when used at their widest aperture. Narrowing the aperture by two or three stops usually gives you the lens' best performance. It also allows you to avoid the need for very expensive, very wide aperture lenses.
  • You can use a lower ISO setting. By collecting more light with a long exposure time you don't need to amplify the signal from the sensor as much. The problem with amplifying the signal too much as that you also amplify noise along with the signal (light). Less amplification is particularly helpful with reducing read noise which is in the same spot in every frame.
  • You can take multiple exposures and combine (stack) them while covering the same part of the sky without repositioning the camera between each shot. This is another technique used to increase the signal to noise ratio by minimizing the random noise that appears in different spots in each frame.
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  • \$\begingroup\$ I prefer taking multiple shots since I can remove shots where there's a plane or other random sources of light that can appear in a few shots. I'm not sure if this is removed in a very long exposure t hough? \$\endgroup\$
    – unsignedzero
    Commented Sep 15, 2016 at 5:42
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    \$\begingroup\$ With stacking you can still use a mount to take longer exposures than without a mount. This helps with read noise by increasing the signal-to-noise ratio. Since you don't need to reposition the camera between each frame (or lose resolution and angle of view by cropping each frame differently) you can take more images in a shorter time period. It takes a good while for most planes to cross the area covered by wider angle lenses. So with longer exposure you don't have to discard as many frames. Also, the more frames you stack the dimmer the plane will be in relation to the stars. \$\endgroup\$
    – Michael C
    Commented Sep 15, 2016 at 5:48
  • \$\begingroup\$ Because you are using a narrower aperture and a lower ISO, the plane won't be as bright to start with. This is further compounded by the fact the plane spends very little time over any one pixel as it moves faster than the sky. The stars, on the other hand, stay over the same pixel(s) the entire exposure so the longer exposure makes them much brighter compared to the plane that spend much less time over any particular pixel. \$\endgroup\$
    – Michael C
    Commented Sep 15, 2016 at 5:50
  • \$\begingroup\$ I forgot about that part since the plane will at some point disappear and we'd see only the stars. \$\endgroup\$
    – unsignedzero
    Commented Sep 15, 2016 at 5:51
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    \$\begingroup\$ Planes are similar to meteors in that respect. photo.stackexchange.com/questions/30493/are-meteors-dim/… \$\endgroup\$
    – Michael C
    Commented Sep 15, 2016 at 5:51

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