Autoguiding for Astrophotography: Setup and First Steps

If you've tried astrophotography and noticed your stars turning into short streaks instead of sharp points in longer exposures, you've hit the tracking accuracy wall. Even the best equatorial mounts have small imperfections in their gears (called periodic error), and these imperfections cause your camera's aim to drift slightly over time. For exposures under about 30 seconds, it usually doesn't matter. For the 2-5 minute exposures that deep-sky imaging demands, it's the difference between sharp stars and blurry ones.

The solution is autoguiding, a system that watches a star in real time and sends tiny corrections to your mount to keep it tracking perfectly. It's the single most impactful upgrade you can make to your astrophotography setup.

How Autoguiding Works

The concept is simple:

1. A small camera (the guide camera) looks at a star through a small telescope (the guide scope) or an off-axis guider.
2. Software on your laptop measures the star's exact position several times per second.
3. If the star drifts from its expected position, the software sends a correction signal to the mount's motors.
4. The mount adjusts, bringing the star back to the correct position.

This feedback loop runs continuously throughout your entire imaging session, correcting for periodic error, wind gusts, cable snags, atmospheric refraction, and any other source of tracking drift. The result: pinpoint stars even in 5-10 minute exposures.

What Gear You Need

An autoguiding setup requires just three components:

1. Guide Camera

A small, sensitive camera with a USB connection. Popular choices include:

• ZWO ASI120MM Mini: The most popular guide camera in astrophotography. Compact, sensitive, and about $100. It works.
• ZWO ASI220MM Mini: Newer, more sensitive sensor. Around $150. If you're buying new, it's worth the upgrade.
• QHY5L-II-M: A solid alternative around $130. Same class of performance.

Monochrome cameras are preferred for guiding because they're more sensitive than color cameras, you only need to track a point of light, not capture color information.

2. Guide Scope or Off-Axis Guider

The guide camera needs optics to see stars. You have two options:

• Guide scope: A small refractor (typically 30-60mm aperture, 120-240mm focal length) mounted on top of your main telescope. Simple, affordable ($50-150), and easy to set up. This is what most beginners use.
• Off-axis guider (OAG): A small prism that picks off a bit of light from your main telescope's optical path and redirects it to the guide camera. More precise (no differential flexure) but harder to find guide stars because the field of view is tiny. Better for advanced setups.

3. Guiding Software

The software reads the guide camera, calculates drift, and sends corrections. The standard is:

• PHD2 (Push Here Dummy 2): Free, open-source, and excellent. It's called "Push Here Dummy" because the original version was designed to be that simple, you just push a button and it guides. The software has evolved enormously and now includes sophisticated algorithms, but the basic operation remains straightforward.
• MetaGuide: A Windows alternative with some unique features.
• ASIAIR: ZWO's all-in-one device that runs guiding (and image capture) without a laptop. Convenient but locks you into the ZWO ecosystem.

Step-by-Step: Your First Guided Session

Here's how to set up autoguiding from scratch using PHD2 and a guide scope:

Before You Go Outside

1. Install PHD2 on your laptop (free from openphdguiding.org).
2. Install drivers for your guide camera (usually from the manufacturer's website).
3. Connect your mount to the laptop via USB, ASCOM, or INDI. PHD2 needs to be able to send pulse-guide commands to the mount.
4. Run PHD2 and configure the Equipment Profile: select your guide camera, your mount connection, and enter your guide scope's focal length.

At the Telescope

1. Polar align your mount as accurately as you can. Autoguiding corrects for tracking errors, but poor polar alignment causes field rotation over time, which guiding can't fix. A polar alignment error under 2-3 arcminutes is ideal.
2. Attach the guide scope and camera to your main telescope. Make sure everything is secure, any mechanical shift during the session will ruin guiding.
3. Focus the guide camera. Point at a bright star, start a live preview in PHD2, and adjust the guide scope's focuser until stars are sharp, tight points.
4. Select a guide star in PHD2. Click on a moderately bright, unsaturated star in the frame. PHD2 will lock onto it.
5. Run the calibration. PHD2 will pulse the mount in each direction, watch how the guide star moves, and calculate the relationship between pulse commands and star movement. This takes 1-2 minutes.
6. Click "Guide" and watch. PHD2 will show a real-time graph of corrections. After a few minutes of settling, your guiding should stabilize to under 1 arcsecond RMS error, more than good enough for most imaging setups.

Troubleshooting Common Issues

What "Good" Guiding Looks Like

PHD2 displays your guiding accuracy as an RMS (root mean square) error in arcseconds. Here's a rough guide to what numbers mean in practice:

• Under 0.5" RMS: Excellent. You can shoot at very long focal lengths (2000mm+) with sharp stars.
• 0.5-1.0" RMS: Good. Perfect for most imaging setups up to about 1500mm focal length.
• 1.0-2.0" RMS: Acceptable for shorter focal lengths (under 800mm) but may show slight elongation at longer focal lengths.
• Over 2.0" RMS: Something needs attention, check balance, connections, and polar alignment.