A fork mount,
when operated in its native alt-az mode, had
a problem called Field Rotation when
used for long-exposure astrophotography.
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.
So what is
Here is an
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
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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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 centered in the frame all night.
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.)
Cassiopeia in the frame closely. Although
the central star remains perfectly centered,
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
centered. However, because the sky is
rotating and the camera is not, the other
stars in the frame are rotating as this long
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
Why an Equatorial
Mount Doesn't Have Field Rotation:
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
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
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's point of view, the target is not
rotating at all, and all of the stars will
come out perfectly in a long exposure.
Credit: Richard McDonald