Guiding

1️⃣ When does guiding become necessary?

As focal length increases, mount tracking errors project into fewer arcseconds per pixel.
At that point, periodic error, drift, and flexure become visible within a single exposure.

Rule of thumb:

• ≤ 100 mm — usually fine unguided

• 100–150 mm — borderline, depends on pixel size and mount, short subs

• ≥ 150 mm — guiding strongly recommended

• APS-C DSLRs or small-pixel OSC cameras: guiding may be needed even below 150 mm

This isn’t about perfection — it’s about preventing star elongation during the sub.

2️⃣ What guiding actually does

Guiding measures the motion of a reference star and sends real-time corrections to the mount to cancel:

• Periodic error

• Polar alignment drift

• Mechanical imperfections

• Flexure over time

It does not improve seeing and does not fix poor balance or overload.

3️⃣ What hardware and software are required?

A basic guiding setup consists of:

• Guide camera

• Guide scope or off-axis guider

• Mount capable of accepting guide corrections

• Control system, one of the following:

  • Laptop running guiding software

  • ASI Air (mount control via ST-4 or direct USB, depending on mount)

  • All-in-one guide cameras (camera + controller)

Connection note:
Some mounts require an ST-4 cable, while others accept guide pulses directly over USB.
Both methods work — the difference is convenience, not accuracy.

4️⃣ Guide scope and camera matching (important, often misunderstood)

You do not need extreme resolution in the guide system.

A good target is:

Guide scale ≈ main imaging scale

In practice:

• Small guide scopes (30–50 mm) work extremely well

• Modern guide cameras have ample sensitivity

• Oversampling the guide camera offers no real benefit

Guiding measures centroid motion, not image detail.

5️⃣ What guiding accuracy is “good enough”?

Guiding accuracy is measured as RMS error in arcseconds.

A practical target:

Guiding RMS ≈ 0.5 × main image pixel scale

Example:

• Main imaging scale: 1.5″/pixel

• Good guiding RMS: ~0.75″ (although <1" is fine)

Below this, guiding errors are usually invisible in the final image.

6️⃣ Declination matters (reality check)

Guiding accuracy:

• Is hardest nearest the celestial equator

• Improves towards poles

• Is affected by backlash, mount geometry, and balance

This is normal and unavoidable.
Expect worse RMS at low Dec, even with a good mount.

7️⃣ Key truth (no hype)

Guiding does not turn a bad mount into a perfect one.
It simply allows a mount to operate within its mechanical limits.

If the mount’s raw periodic error is large, guiding becomes mandatory — not optional.

Guiding — quick summary

• Guiding becomes necessary around 150 mm, sooner with small pixels

• You need a guide camera, guide optics, and control software

• Match guide scale roughly to imaging scale

• Aim for ~0.5 pixel RMS, not zero

• Expect performance to vary with declination