In an article on mounts for astrophotography we briefly mentioned the fact that a fork mount, when operated in its native alt-az mode, had a problem called Field Rotation when used for long-exposure astrophotography.
What’s that about? Fork mounts are very popular, and good ones have computer-controlled tracking that seems very smooth and accurate. No problems are apparent when using them visually. What’s this “field rotation” thing?
Here is an explanation.
Again, let’s emphasize that this problem applies to fork mounts only when they are used in their normal alt-az mode. A fork mount on an equatorial wedge does not have this problem. And, this problem is apparent only when doing long-exposure astrophotography; you’ll never notice it when looking “live” through an eyepiece.
Simulation of Field Rotation on a Fork Mount
Here is a video simulation of the sky around the constellation Cassiopeia, from about 5 PM to 10 PM on a December evening. Each frame represents 15 minutes of elapsed time. You can see the constellation slowly moving across the sky, rotating around Polaris, at the North Celestial Pole.
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Now, let’s imagine we have a camera attached to a fork mount telescope. This is a very large camera with a very wide field, so we can fit all of Cassiopeia in the frame. The field of view of the camera is shown as a rectangle in this video. (In reality, Cassiopeia is too large to photograph through a telescope, but it helps this example to use a familiar target.)
This is a good quality fork mount, well-aligned, and it is tracking the central star in Cassiopeia perfectly. Notice how the central star stays perfectly centred in the frame all night.
The frame remains oriented perfectly horizontally all night, because fork mounted telescopes always remain oriented horizontally. (i.e. the top of the telescope tube remains the top, wherever the telescope is pointed.)
Observe Cassiopeia in the frame closely. Although the central star remains perfectly centred, you can see that the rest of the constellation is rotating around it as this very long exposure progresses.
Let’s make that easier to see. Here we zoom in on the camera frame. This is exactly the same video – nothing is changed, except that we are seeing only what the camera sees.
The fork mount is doing its job perfectly – the centre star in our target is remaining perfectly centred. However, because the sky is rotating and the camera is not, the other stars in the frame are rotating as this long exposure proceeds.
If we were taking a long-exposure photograph, the centre star would be perfect, but all the other stars would show up as circular arcs. The longer the exposure, the longer the arcs would be.
Why an Equatorial Mount Doesn’t Have Field Rotation
An equatorial mount doesn’t suffer from field rotation, assuming it is properly polar-aligned. This applies to any equatorial mount, including traditional equatorial mounts like the one shown here, and fork mounts on equatorial wedges.
This is because when an equatorial mount tracks an object across the sky, the telescope does not remain horizontal, the tube rotates. Observe this equatorially-mounted telescope closely, and note how the “top” of the tube changes as the scope moves across the sky.
Here is our camera field simulation again. This time, however, the camera is on a telescope on an equatorial mount.
Note how the frame of the camera rotates as the target is tracked across the sky. Now, note that all the stars of our target are remaining in the same relative position inside the rotating camera frame.