Why You Need Guiding
If you have ever looked at a 30 second exposure and noticed your stars were not round little dots but short streaks, you have already met the problem guiding solves. Even the best equatorial mounts do not track the sky perfectly. Minor mechanical imperfections, atmospheric refraction, and polar alignment errors all add up. For exposures longer than about 30 seconds on most setups, your stars will trail without some form of active correction.
Autoguiding uses a second, smaller camera to watch a guide star and send correction commands to your mount in real time. Think of it as a tiny co pilot that nudges the steering wheel every fraction of a second to keep you pointed exactly where you want to go.
Guide Scopes vs Off Axis Guiders
There are two main ways to pick up that guide star:
- Guide scope: A small refractor mounted alongside your main telescope. Simple to set up, works with almost any optical tube, and the most popular choice for beginners and many advanced imagers.
- Off axis guider (OAG): A prism that picks off a small patch of light from the main imaging train. No flexure between the guide and imaging cameras, but the setup is fiddlier and the guide star selection window is smaller.
For most backyard astrophotographers, a guide scope is the right starting point. This guide focuses on that path.
Choosing a Guide Scope
The key spec is focal length, and shorter is generally better. Here is why: a 50mm guide scope typically has a focal length around 200mm. That gives you a wide field of view, making it easy to find a guide star anywhere in the sky. A longer guide scope (say 400mm or more) narrows the field, and you will sometimes struggle to find a suitably bright star near your target.
Popular options include:
- William Optics 50mm UniGuide (f/4, 200mm FL) – lightweight, slide base adjustment, extremely popular
- SVBONY SV106 50mm (f/4.3, 220mm FL) – budget friendly, helical focuser
- Orion 50mm Mini Guide Scope (f/3.2, 155mm FL) – very short FL for wide FOV
- ZWO 30mm Mini Guide Scope (f/4, 120mm FL) – ultra compact for shorter FL imaging rigs
The rule of thumb: your guide scope focal length should be shorter than your main scope, ideally less than half. My own setup uses a Meade 800mm guide scope (an older approach), which is actually on the long side. Modern 50mm or 30mm guide scopes are much easier to work with.
Choosing a Guide Camera
Your guide camera needs to do one thing well: detect stars quickly. Key considerations:
- Sensor sensitivity: Modern CMOS guide cameras from ZWO, QHY, and SVBONY are sensitive enough to find guide stars in seconds. Look for cameras with low read noise and decent quantum efficiency.
- Pixel size: Smaller pixels sample the star image more finely, which helps guiding accuracy. Most guide cameras have pixel sizes between 2.9 and 4.5 microns, which pairs well with short FL guide scopes.
- Cooling: Not needed for guide cameras. They take short exposures (1 to 5 seconds) where thermal noise is negligible.
- Popular models: ZWO ASI290MM Mini, ZWO ASI220MM Mini, SVBONY SV305, T7C (all monochrome preferred over color for guiding)
Monochrome cameras are strongly preferred for guiding because they produce sharper star images and have better sensitivity than color cameras at the same price point.
Setting Up PHD2 Guiding
PHD2 is the free, open source guiding software used by nearly every astrophotographer. Here is the basic workflow:
- Connect your equipment: In PHD2, connect your guide camera and mount. Select your mount type (EQMOD for most Sky Watcher and Orion mounts, ASCOM for others).
- Calibrate: Point near the meridian and celestial equator, then click the guide button. PHD2 will move the mount in each direction to learn how much correction each pulse command produces. This takes about 30 to 60 seconds.
- Guide: Select a guide star, click the green guide button, and PHD2 keeps that star locked on the same pixel. You will see a graph of your corrections in real time.
- Tune: If your RMS error is consistently above 1 arcsecond, adjust your aggressiveness settings or increase your guide exposure time.
Integration with NINA is straightforward. In NINA, add the PHD2 equipment item, connect it, and your guiding starts and stops automatically with your imaging sequences.
My Setup
On my rig, I use an older Meade 800mm guide scope with a focal reducer, paired with a Meade DSI Pro monochrome guide camera. It works, but it is definitely a vintage approach. If I were starting over, I would grab a William Optics 50mm UniGuide and a ZWO ASI220MM Mini. That combination is lighter, easier to balance, and finds guide stars faster.
The key detail from my setup that matters: guide scope rings and mounting hardware. Even slight flexure between your guide scope and main scope will cause guiding errors that no software can fix. Use solid mounting rings, check that nothing shifts when you slew, and tighten every connection.
Troubleshooting Common Issues
- Cannot find a guide star: Increase your guide exposure to 2 to 4 seconds. Try a different star. If using a long FL guide scope, consider switching to a shorter one.
- RMS error above 1 arcsecond: Check for cable snags, balance issues, or loose connections. Verify polar alignment. Try reducing aggressiveness in PHD2 from the default 0.7 to 0.6.
- Stars still trailing despite guiding: Your guide scope may be shifting relative to the main scope (differential flexure). Tighten rings, reduce hanging cables, or consider an OAG.
- Guiding starts fine then degrades: This is usually cable drag or a shift in balance as the scope tracks across the sky. Route cables carefully and re balance your rig with all equipment attached.
Quick Reference: Guiding Numbers
| Metric | Good | Great |
|---|---|---|
| RMS error | Under 1.0 arcsec | Under 0.5 arcsec |
| Guide exposure | 1 to 4 seconds | 2 seconds (typical sweet spot) |
| Calibration steps | 25 to 40 | 30 (enough but not excessive) |
| Aggressiveness | 0.6 to 0.8 | 0.65 to 0.7 |
| Max dec backlash | Under 100ms | Under 50ms |
Getting Started Checklist
- A guide scope (50mm is the most versatile starting point)
- A monochrome guide camera (ZWO ASI220MM Mini or similar)
- PHD2 installed on your imaging laptop
- ASCOM or INDI drivers for your mount
- Solid mounting rings and a dovetail or finderscope shoe
- A USB cable long enough to reach your guide camera without tension
- Patience. The first calibration always takes a few tries.
Guiding is one of those skills that transforms your astrophotography overnight. The first time you see perfectly round stars in a 5 minute exposure, you will understand why we bother. Start simple, learn PHD2, and upgrade your guide scope later if you need to. Clear skies!
