How to Set Up Autoguiding with PHD2

What is Autoguiding and Why Do You Need It?

Autoguiding uses a second camera to monitor a star in real time and send corrections to your telescope mount, keeping it tracking accurately over long exposures. Without autoguiding, even good mounts will show star trailing in exposures longer than about 30 to 60 seconds due to periodic error in the mount's gears.

PHD2 (Push Here Dummy 2) is the most widely used autoguiding software in astrophotography. It is free, open source, and works with virtually every guide camera and mount on the market. The current version is 2.6.14, updated in December 2025.

If you are shooting deep-sky objects and want to take exposures of two minutes or longer, autoguiding is not optional. It is essential.

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What You Need

Guide Camera

A small, sensitive camera dedicated to autoguiding. Popular choices include:

• ZWO ASI120MM Mini (~$149): The most popular budget guide camera. Compact, lightweight at just 60 grams, and sensitive enough for most guide scopes. The monochrome sensor with 75% peak quantum efficiency detects faint guide stars reliably.

• ZWO ASI174MM Mini (~$249): A step up with a larger sensor that captures more sky, making it easier to find bright guide stars in sparse fields.

• ZWO ASI290MM Mini (~$199): Excellent sensitivity with very low read noise. A strong all-around choice.

Monochrome cameras are preferred over color for autoguiding because they are more sensitive per pixel, which means shorter guide exposures and more responsive corrections.

Guide Scope or Off-Axis Guider

You need optics for your guide camera. There are two approaches:

Guide scope: A small refractor (typically 30mm to 60mm aperture, 120mm to 240mm focal length) mounted on top of your main telescope using guide scope rings. This is the simpler and more affordable option. A 50mm guide scope in the $50 to $100 range works well for most setups.

Off-axis guider (OAG): A small prism that picks off light from the edge of your main telescope's image circle and redirects it to the guide camera. This eliminates differential flexure (the guide scope and main scope shifting relative to each other), but finding guide stars can be harder due to the small pickoff area.

For beginners, a guide scope is the way to go. It is easier to set up and more forgiving.

Cables and Connections

Your guide camera connects to your computer via USB. The mount receives guide corrections through one of two methods:

ST-4 cable: A physical cable from the guide camera's ST-4 port to the mount's autoguider port. This is the traditional method and works independently of any other software.

Pulse guiding (ASCOM): Software-based guiding where PHD2 sends corrections to the mount through its ASCOM driver over USB. This requires your mount to be connected to the computer via ASCOM but eliminates the need for an ST-4 cable.

Pulse guiding via ASCOM is generally preferred for modern setups because it is simpler to connect and integrates well with imaging software like NINA.

Installing PHD2

Download PHD2 from openphdguiding.org. It runs on Windows, macOS, and Linux. The installer is straightforward with no complex dependencies.

Creating an Equipment Profile

On first launch, PHD2 opens the New Profile Wizard. This is the best way to configure your setup.

Step 1: Guide Camera

Select your guide camera from the dropdown list. If you are using a ZWO camera, select "ZWO Camera" (you will need the ZWO ASI drivers installed). PHD2 supports cameras from ZWO, QHY, Atik, Starlight Xpress, Altair, ToupTek, and many others.

Step 2: Mount Connection

Choose how PHD2 will communicate with your mount:

• On-camera (ST-4): Select this if you are using an ST-4 cable. PHD2 sends guide pulses through the camera's ST-4 port.
• ASCOM mount: Select your mount's ASCOM driver. This is the preferred method for modern setups.
• INDI mount: For Linux users running an INDI server.

Step 3: Guide Scope Parameters

Enter your guide scope's focal length and your guide camera's pixel size. PHD2 uses these values to calculate the guide rate and calibration step size. Getting these numbers right matters for accurate calibration.

For example, if you are using a 50mm guide scope (200mm focal length) with a ZWO ASI120MM Mini (3.75 micron pixels), enter 200 for focal length and 3.75 for pixel size.

Step 4: Build a Dark Library

The wizard offers to build a dark library. Accept this option. A dark library subtracts hot pixels and thermal noise from guide frames, giving PHD2 a cleaner image to work with. Cap your guide camera, set the same exposure length you plan to guide at (typically 2 seconds), and let PHD2 capture a set of dark frames.

Connecting and Starting

With your profile created, the main PHD2 window shows a simple toolbar. The workflow is:

1. Connect equipment. Click the USB icon to connect your camera and mount. Green indicators confirm each connection.

2. Start looping. Click the green play button to start continuous guide exposures. You should see stars appear in the display. If the image is too bright or too dark, adjust the exposure time (1 to 3 seconds is typical) and gain settings.

3. Select a guide star. You can click on a star manually, or use the Auto-select feature (under the Star menu) which picks the best star based on brightness, saturation, and proximity to the edge. Avoid stars that are too bright (saturated) or too close to the frame edge.

4. Calibrate. Click the PHD2 guide button (the crosshair icon). PHD2 first runs a calibration sequence where it pulses the mount in each direction and measures how the guide star moves. This teaches PHD2 the relationship between guide pulses and star movement on your sensor.

5. Guide. After calibration completes, PHD2 begins guiding automatically. The graph display shows real-time tracking corrections in both RA (right ascension) and Dec (declination) axes.

Understanding the Guide Graph

The guide graph is your primary feedback tool. It shows the tracking error in arcseconds over time for both RA (blue, typically) and Dec (red).

Good guiding looks like a tight band oscillating close to the zero line, typically within plus or minus 1 to 2 arcseconds total. For most imaging setups, RMS error below 1 arcsecond is excellent, and below 1.5 arcseconds is perfectly usable.

RA oscillations that look like a sine wave indicate periodic error from the mount's worm gear. PHD2 should track these out smoothly. If the corrections are lagging behind the error, your guide exposure may be too long.

Dec drift that trends steadily in one direction usually indicates a polar alignment error. PHD2 can correct for this, but significant drift eats into your correction budget. Improving polar alignment is always better than relying on Dec guiding to compensate.

Calibration Tips

Calibration is the most critical step. A bad calibration leads to bad guiding no matter what settings you use.

Point near the celestial equator and meridian. Calibration works best when the mount is pointing at declination close to zero and near the meridian. Avoid calibrating near the celestial pole, where the mount's RA and Dec axes do not produce clean orthogonal movements.

Get enough calibration steps. PHD2 should take at least 8 steps in both the west and north calibrations. If it finishes in fewer steps, reduce the calibration step size in the Brain settings (the gear icon). The New Profile Wizard usually sets a reasonable starting value.

Do not calibrate through clouds. If the guide star disappears during calibration, PHD2 will produce garbage results. Wait for clear skies.

Re-calibrate when you change equipment or flip the mount. If you swap guide cameras, change the guide scope, or significantly change declination, recalibrate. However, PHD2 can automatically adjust calibration for meridian flips if you check "Reverse Dec output after meridian flip" in the Brain settings.

Optimizing Your Guiding

Once you have basic guiding working, there are several ways to improve performance.

Use the Guiding Assistant

PHD2's Guiding Assistant (under the Tools menu) analyzes your mount's behavior and recommends optimal settings. Let it run for a few minutes while guiding is active. It measures your mount's periodic error, backlash, seeing conditions, and recommends RA and Dec aggressiveness values, minimum move thresholds, and whether to enable Dec guiding in one or both directions.

This is far better than manually tweaking settings based on guesswork.

Adjust Aggressiveness Carefully

RA aggressiveness controls how much of the detected error PHD2 corrects on each guide pulse. The default of 100% (full correction) works for many mounts, but mounts with significant backlash or poor gear quality may benefit from lower values (60 to 80%).

Dec aggressiveness is often best at lower values (50 to 70%) because Dec backlash can cause overcorrection. The Guiding Assistant will tell you if your mount has significant Dec backlash and may recommend guiding Dec in only one direction.

Set a Minimum Move Threshold

The minimum move setting tells PHD2 to ignore corrections smaller than a specified amount. This prevents the software from chasing seeing (atmospheric turbulence) rather than actual tracking error. A value of 0.2 to 0.3 arcseconds works for most setups, but the Guiding Assistant will recommend a specific value.

Dithering

Dithering moves the guide star by a small random amount between imaging exposures. This shifts hot pixels and walking noise patterns to different positions in each frame, which your stacking software can then reject. Most imaging software (NINA, SGP, APT) can trigger dithering through PHD2 automatically between exposures.

Troubleshooting Common Problems

Stars look elongated despite guiding. Check that your guide scope is securely mounted and not flexing. Tighten the guide scope rings and make sure all connections are solid. Also check your main imaging scope's focuser for tilt or slip.

PHD2 loses the guide star frequently. Your guide exposures may be too short, or the guide star may be too faint. Try increasing the exposure time to 3 or 4 seconds. If clouds are intermittent, there is nothing PHD2 can do.

Large Dec corrections that oscillate. This is usually Dec backlash. Run the Guiding Assistant and follow its recommendations. You may need to guide Dec in only one direction.

Calibration fails or produces odd results. Make sure the mount is tracking at sidereal rate, is properly unparked, and is not pointed near the pole. Also confirm your guide scope focal length and pixel size are entered correctly.

The guide graph shows sudden large spikes. Wind gusts, cable snags, or someone bumping the tripod. Make sure cables have enough slack and are routed so they do not pull on the guide scope as the mount tracks.

Putting It All Together

A typical autoguided imaging session looks like this:

1. Set up and polar align your mount
2. Connect all equipment and launch PHD2
3. Slew to your target using your mount controller or imaging software
4. Start PHD2 looping and select a guide star
5. Calibrate (if needed) and start guiding
6. Begin your imaging sequence in your capture software
7. Monitor the guide graph periodically

With practice, this entire process takes 15 to 20 minutes from setup to first guided exposure. Once everything is dialed in, the same equipment profile and calibration can be reused night after night with minimal adjustment.

After your imaging session, check your exposure data with ExifGrabber to review the settings you used across your frames. Consistent metadata across a night of shooting is a good sign that your automation is working correctly.

Autoguiding transforms what is possible in astrophotography. Exposures of 5, 10, or even 15 minutes become routine, unlocking faint nebulae and galaxies that are invisible in shorter unguided exposures. PHD2 makes it accessible to everyone, and it will not cost you a thing.