Tracking & Auto-guiding can be perhaps one of the most confusing aspects of this hobby. It doesn’t help that the terms are used interchangeably (they mean very different things!). But trying to get auto-guiding to work properly can be a big challenge. Auto-guiding is key in taking long sub-exposures so it’s important to understand what it is and how it works!

The Short Version

Tracking – The mounts motors move your camera/telescope against the earths rotation to stay locked on to one part of the sky.

Auto-guiding – The same as tracking, but using a secondary camera to actively correct for mechanical errors.

The Long Version

Tracking is exactly that – just tracking. Your mount has motors and gears which it uses to move the telescope against the earths rotation. It rotates at the same speed as the earth but in the opposite direction – this is called the sidereal rate. It allows your telescope to focus on a fixed point in the sky (like a galaxy or nebula) and stay locked on to it. 

The problem is that nothing is ever mechanically perfect. Machining tolerances, wear/tear and other imperfections mean that tracking is not completely accurate. Eventually the mount will begin to drift and you’ll begin to see star trails and blurry images. The general rule is anything more than a minute and you’ll start seeing trails. This is a problem because for very faint targets, 1 minute just isn’t long enough. We need to expose for 3, 4, 5, sometimes even 6+ minutes per image to really capture faint details. How do we do that if our mount can only stay accurate for 1 minute? Autoguiding!

Auto-guiding is the next step up from tracking. It uses a secondary camera that usually sits on top of your main scope. This secondary camera and guiding software lock on to a star. If the mount starts to drift in error, the star will move in the field of view. This will be detected by the guiding software which will then send a command to the mount’s motors to correct the error. This means that any drifting errors are corrected on-the-fly. And it’s amazing! Using auto-guiding, I have been able to take exposures up to 8 minutes long without a single star trail! This is great if you’re using narrowband filters to capture faint targets!

Additional Equipment for Auto-guiding

In order to start auto-guiding you will need some additional hardware and software. You’ll need a guide camera, a guide scope, and some software called PHD2. The guide camera fits into the guide scope, and together they ride on top of your main imaging scope. The guide camera will detect the stars above and PHD2 will analyse the field of view. PHD2 will automatically choose a star and lock on to it, and if it detects movement in the field of view, it will issue corrective commands to your mounts motors. Have a look at the system layout below:

Something to consider if you decide to start auto-guiding (And you really should!) – your guide scope must be a similar focal length to your imaging scope. Think about it – when you zoom all the way in on your smartphone camera, the smallest movements become hugely amplified. Telescopes with very long focal lengths (1000mm+) will need very accurate guiding. Using a guide scope with a 200mm focal length will give poor results. This is because 200mm is not very zoomed in – so small guiding errors won’t be visible. But they definitely will be on your main scope! To fix this, use a guide scope that has a “similar” focal length of your main imaging scope. This ensures your guide scope has a similar sensitivity to vibrations and tracking errors as your main scope and can better correct.