Pempro PPEC

I’ve spent a lot of time guiding both worm-driven mounts (like the EQ6-R) and looking into the newer strain-wave designs (like the AM5). Since guiding questions come up constantly, here’s a proper breakdown of what’s going on mechanically, what PEC/PEMPro can and can’t do, and how to tune guiding for each mount type.

What is Periodic Error?

On a worm-gear mount like the EQ6-R, the worm takes about 7.5 minutes to complete one rotation. Any machining error, eccentricity, or uneven gear mesh introduces a repeatable tracking error over that cycle. It shows up as stars drifting back and forth in RA in a sinusoidal pattern.

Factory EQ6-R mounts usually ship with ±10–20 arcseconds of periodic error. Premium worm mounts can be under ±5 arcseconds.

Periodic Error Correction (PEC)

PEC records several worm cycles, then plays back the inverse of the error curve each time the worm rotates. On the EQ6-R it’s called Permanent PEC (PPEC) because the curve is saved in firmware until you retrain it. A good PEC run can cut periodic error by 50% or more, reducing the guider’s workload.

Where PEMPro Fits In

PEMPro takes this further by:

• Recording multiple worm cycles with your guide camera.

• Averaging them to filter out seeing and random noise.

• Modeling the actual mechanical error.

• Uploading a smoothed, high-fidelity curve into the mount.

Compared to in-mount PEC training, PEMPro produces much cleaner and more effective curves with fine control.

EQ6-R Guiding Strategy

• Worm period: ~478–480 s, with secondary harmonics ~120 s.

• PEC options: Either (A) train PPEC with PEMPro and use low-aggression guiding, or (B) disable mount PEC and let PHD2’s Predictive PEC handle it. Don’t run both strongly at the same time or they’ll fight.

• Exposures: 1.5–2.5 s to average out seeing without missing the worm cycle.

• RA settings: With PPEC on > Hysteresis ~35–45% aggression. With PEC off > Predictive PEC ~50–65% aggression and let it learn the cycle.

• DEC settings: If backlash is small, Auto ~35–50% aggression. If backlash is bad, guide one direction only (north or south) and bias polar alignment so drift stays consistent.

• Min-move: Start RA ~0.25″, DEC ~0.30″, refine with Guiding Assistant.

AM5 and Harmonic Drives

Strain-wave mounts like the AM5 use a harmonic gearbox instead of a worm. They’re compact and backlash-free, but the tracking error is more complex:

• A low-frequency, semi-repeatable component (~3–5″).

• High-frequency gearbox ripple at tens of Hz, which is semi-random.

That high-frequency noise is the killer: PEC can’t predict it, and guiding can only partly keep up. The net result is that PEC has limited benefit compared to a worm mount.

Guiding strategy for AM5:

• Exposures: short, ~0.5–1.0 s, to catch mid-frequency drift before stars smear.

• RA: Hysteresis or Predictive PEC with low aggression (30–45%). Raise min-move if you see fast RA oscillation.

• DEC: Auto, 25–40% aggression. Backlash is almost nil, but overshoot is common if you push too hard.

• Min-move: ~0.30″ RA, 0.35–0.45″ DEC, then tune from logs.

I am sure stainwave and Active optics (e.g. Starlight Xpress AO) is a killer combo, since they correct at 5–10 Hz, but they eat 30–40 mm of backfocus. Ive never owned a strainwave but I do own active optics..

PEC and Guiding Together

PEC and guiding complement each other, but you have to balance them:

• If you run a strong PEC curve and aggressive guiding, the two can clash showing up as RA ping-pong.

• Better approach: let PEC carry most of the worm error, then lower RA guiding aggression so it only catches drift and residuals.

• This works best with very accurate polar alignment (sub-arcminute) so DEC corrections stay minimal.

  
  Don’t Forget the Role of Stacking

When you look at guiding graphs, remember that stacking software aligns on star centroids. A lot of the small-scale, high-frequency error never makes it into the final image because pixel rejection tosses it out. Unless guiding is really poor, the stacked result won’t show every wiggle your guider reported.

Still, smoother guiding = fewer outliers, more usable subs, tighter stars, and less reliance on rejection.

Balancing and Backlash Offset

One of the biggest tricks with worm mounts is deliberate imbalance. If you balance perfectly, the gears can float inside the backlash zone, causing oscillation or runaway corrections.

• In RA: always bias east so the worm is pushing uphill.

• In DEC: bias so the load always falls one way, then guide north-only or south-only if needed.

This preloads the gears, removes oscillation, and gives the guider a stable system to work with.

Real-World Results

With tight polar alignment, slight off-balance, good PEC strategy, and tuned guiding, I see repeatable all-night RMS between 0.34″ and 0.19″ depending on seeing. That’s well below critical sampling for most setups. I don’t lose subs, and I can comfortably run exposures up to 3600 seconds. At that point, my camera not guiding becomes the limiting factor.

Complete Picture

• Correct min-move keeps guiding smooth.

• Deliberate off-balance preloads the gear train and prevents oscillation.

• Sub-arcminute polar alignment reduces DEC intervention.

• Stacking software does the final cleanup.

Where did the time 1.5 hours to write that up lol hope it helps!!

Clear skies !!