Periodic Error (PE): The Truth Behind the Mount Hype

Periodic Error (PE): The Truth Behind the Mount Hype 

1) What Periodic Error Actually Is

Periodic Error (PE) is the mechanical wobble in an equatorial mount caused by imperfections in the worm gear driving Right Ascension (RA).

• It repeats every worm cycle

• It is predictable

• It exists in every mount, cheap or expensive

PE is measured in arcseconds peak-to-peak (e.g. ±20″ = 40″ total).

YouTube gurus say: “This mount tracks perfectly unguided”
❌ No mount does.

2) Worm Error Is Sinusoidal — Calm Phases & Bad Phases

PE is not constant drift. It’s a sine wave:

• Flat sections → little movement (great subs)

• Steep sections → rapid drift (ruined subs)

This is why:

• One 60s sub looks perfect

• The next is trashed

• Then three good ones again

YouTube only shows the good phase.

YouTube gurus say: “I get 60s subs unguided every time”
❌ They don’t — they just don’t show the rejects.

3) Work it out: Time to drift 1 pixel (worst case)

To find your maximum unguided sub length, calculate how long Periodic Error takes to move a star by 1 pixel during the worst (fastest) part of the worm cycle.

Example (your real setup)

Camera: Canon 1300D
Focal length: 105 mm
Pixel size: 4.3 µm
Mount PE: ±25 arcseconds
Worm period: 10 minutes = 600 seconds

Step 1 — Pixel scale (arcseconds per pixel)

Pixel scale ("/pix)
= 206.265 × Pixel Size (µm) / Focal Length (mm)

Pixel scale
= 206.265 × 4.3 / 105
= 8.45 "/pix

So 1 pixel = 8.45 arcseconds.

Step 2 — Worst-case PE drift speed (steepest part of the sine wave)

PE is roughly sinusoidal. The maximum drift speed is:

Worst-case drift rate ("/s)
= (2 × π × PE amplitude) / Worm period (s)

Worst-case drift rate
= (2 × π × 25) / 600
= 0.262 "/s

Step 3 — Time to drift 1 pixel (worst case sub limit)

Time for 1-pixel drift (s)
= Pixel scale ("/pix) / Drift rate ("/s)

Time for 1-pixel drift
= 8.45 / 0.262
= 32 seconds

✅ Result: worst-case, your stars drift ~1 pixel in ~32 seconds.
That’s why unguided 30s subs at 105mm are the maximum potentially “safe”, and longer subs only work during the calm part of the worm cycle (the part YouTube cherry-picks).
This is the BEST sub length one can expect at 0 degrees declination. With real world imbalanced loads, individual mount worm cycle, and wind will reduce this. I actually found 20s only usable looking at stars 10x at 0 declination. Higher declination extends this.

A reality check: published PE is often optimistic

The Periodic Error numbers you see online are best-case samples, not guarantees.

Manufacturers usually quote:

• Well-behaved units

• Factory-tuned examples

• Ideal balance and temperature

• No payload flex

Real-world mounts are often worse.

Example: Star Adventurer “good” vs “bad” unit

It’s common to see Star Adventurers measured at:

• ±25″ → good unit

• ±35–40″ → average / unlucky unit

That’s up to ~1.6× more PE amplitude, which directly increases drift speed.

What that does to your subs (same setup as before)

Canon 1300D
105 mm
Pixel scale = 8.45 ″/pix
Worm period = 600 s

If PE = ±25″

Worst-case drift:

• (2 × π × 25) / 600 = 0.262 ″/s

• 1-pixel drift ≈ 32 s

If PE = ±40″

Worst-case drift:

• (2 × π × 40) / 600 = 0.419 ″/s

• 1-pixel drift ≈ 20 s

Why this matters

This explains why:

• One person gets clean 30s subs

• Another can barely manage 15–20s

• Both use the same mount

They don’t have the same PE amplitude.

4) Why Big Pixels + Short Focal Length Win

Big pixels and short focal lengths mean:

• Larger arcseconds per pixel

• More drift tolerated before stars elongate

• Longer unguided subs possible

Example:

• 24–50 mm lens + DSLR → 10–30″/pix

• 200 mm scope + small-pixel OSC → 0.8–1.2″/pix

Same mount.
20× difference in tolerance.

This is why DSLRs are widefield monsters
And why YouTube always shoots wide.

5) Why Guiding Becomes Mandatory (Even with “Good” Mounts)

High-read-noise DSLRs

• Need longer subs to beat read noise

• Long subs expose PE brutally

• Unguided → walking noise + star elongation

Low-read-noise OSCs

• Can shoot shorter subs

• BUT some have have tiny pixels

• Tiny pixels magnify PE

Result:

• Both require guiding beyond ~150 mm

YouTube gurus say: “Low read noise means you don’t need guiding”
❌ Pixel scale still exists.

6) Declination Makes Everything Worse

Tracking speed in RA scales as:

cos(declination)

Effects:

• Near the celestial equator → PE is worst

• Near the pole → PE is reduced

This is why:

• Orion is MUCH harder than Polaris

• Youtube gurus never mention declination affecting PE
  ❌ Physics disagrees.

7) Why Even a 50 mm f/5 Refractor Needs Guiding

Typical setup:

• 250 mm focal length

• DSLR pixel ≈ 4.3 µm

• Pixel scale ≈ 3.5″/pix

With a modest +-30″ PE mount:

• 1-pixel drift happens in ~15–20s

You must guide to:

• Smooth worm error

• Break fixed-pattern noise

• Allow RN-beating subs

If focal length is over ~150 mm
Guiding is not optional

8) Why Low-PE Mounts Are So Expensive

To reduce PE mechanically, manufacturers must:

• Hand-lap worm gears

• Use hardened metals

• Tighten tolerances to microns

• Reject imperfect assemblies

This is why:

• Mount price rises exponentially

• PE halves → cost doubles (or worse)
  
  

Guiding flips the economics

• £250 guide setup

• Corrects any PE

• Can perform as well as unguided £5,000 mounts

Guiding is software fixing hardware
And it works, but costs setup time and more gear.

Final Reality Check

Unguided imaging works when:

• Short focal length

• Large pixels

• Calculated short subs

• Modify for Declination

• Manufacturer's never publish PE, usually its worse than stated online.