Field Rotation

Field rotation smears stars in long-exposure photos, thanks to the Earth's rotation. Alt-az mounts like the Seestar’s don’t compensate for this rotation so stars at the edges of your image may appear as streaks instead of points.

Note that field rotation is most pronounced when an object is near your meridian1), as the object's altitude changes rapidly, leading to longer streaks. And it’s gets worse the closer you live to the equator. 2)

To minimize field rotation you can use a couple of strategies. Short exposure times helps, but the best solution is to limit your data collection 3) to times when your target isn’t close to the meridian.

Visibility plots can help you predict how an object's path changes throughout the night, offering a slick way to estimate field rotation. By drawing imaginary tangent lines on the visibility curve, you see how much field rotation will occur. The tangent line for a target at 2000 (8PM) and 0000 (midnight) basically overlap, so there will be very little problem.



This tangent line technique also makes it clear why capturing images during a meridian crossing will create the greatest field rotation.

If you start at 1600 (4PM) and end at 2200 (10PM) you’ll enjoy field rotation of almost 90º. So plan your sessions for when the target isn't near the meridian and is closer to the horizon where field rotation is less severe. But keep in mind that the atmosphere is very distorted near the horizon, so don’t go too low.

As a rule of thumb try to capture targets that are close to east (90º) and west (270º), not north (0º) or south (180º), above 30º from the horizon and below 80º. Obviously, that’s not a firm rule. In other words, YMMV.

An excellent web tool that lets you calculate the safe exposure times for your latitude is at https://apximhd.github.io/field-rotation/field_rotation.html.

By the way, many post-processing software packages can help correct field rotation, but it's not always a perfect solution. Just sayin’.

If you get tired of the field rotation issue you can buy an equatorial mount for less than $100 and use the Seestar's EQ Mode.

1)
An imaginary line you are on that extends from the North Pole to the South Pole
2)
There's an excellent post by Kelly Flanagan the spells out the details with some excellent illustrations.
3)
Astrophotography uses a telescope as a funnel to direct light into a camera sensor that detects photons, and converts their intensity into a number, which is stored as data for later processing