Field Rotation

Why does it happen?

Someone wrote:

Does anyone know a good reference for explaining how the telescopic field of an ALT-AZ mounted telescope actually appears to rotate as the telescope is moved?

If you don't need the math, it's actually quite simple to explain. In short, the amount of rotation is the difference between "north" and "up."

Suppose you're looking at two stars with the same right ascension—say, the westernmost pair of stars in the square of Pegasus. (That's in the northern hemisphere. Find a similar pair of stars in the southern hemisphere.) When they rise in the east, depending on your latitude, those stars are side-by-side, more or less—something like the following:

```          beta
*

alpha
*

+------------------+
horizon
```

When they cross the meridian, though, beta will be nearly directly above (that is, toward the zenith from) alpha, as follows:

```               beta
*

alpha
*
```

And when they set in the west, beta will now be to the upper right of alpha, like so:

```                 beta
*

alpha
*

+------------------+
horizon
```

If you were to track alpha Pegasi using a (very wide-FOV) alt-az mounted scope (or a pair of binoculars), you would see beta Pegasi make a broad, circular arc above alpha. That is the field rotation at work. There is nothing magical about being at the same right ascension, by the way; the entire field around alpha will appear, from the point of view of an alt-az scope, to rotate clockwise throughout the night.

A GOTO scope essentially figures out the instantaneous alt-az coordinates of a given object. By using the same algorithm to figure out the instantaneous alt-az coordinates of a position just north of that object, it can differentiate and determine the amount of field rotation. (Yes, I know that a GOTO scope can work in equatorial mode—didn't the Compustar only work that way?—but most of them nowadays are sold to users who will put them in alt-az mode.)