Go to GEM28 – Latest Update Part V

Finally I get acceptable results with my GEM28. Even if I have to adjust the DEC axis a little bit more, the performance yesterday was outstanding. My new setup seems to work fine.


  • Dual 50mm Guide Scope 205mm f/4.1 with helical focusser
  • Canon EF 400mm mounted with original Canon ring
  • Canon EOS 70Da
  • Lacerta MGEN II
  • New set of MGEN cables
  • Powered by 12V 22Ah Power Tank
  • Payload weight: 5.25kg
  • Counterweights: 6kg


  • Gear Meshing RA: -1/2 turn
  • Gear Meshing DEC: -1/1 turn
  • Guide Speed: RA 0.50x, DEC 0.25x
  • MGEN RA: NUM 1, Tol. 0.20, Aggr. 70%, Mode 2
  • MGEN DEC: NUM 1-2, Tol. 0.20, Aggr. 70%, Mode 2


I started with 10 minute exposures to record again the guiding performance.

After the recordings I started a regular session (IC 1805 – Heart Nebula). Surprisingly all sub exposures were usable. 100% of them! No more DEC trails on any sub. The DEC values still are outside of the sweet spot, but it’s on a good way 🙂

And the best, no more crashes or freezes. Most probably it was time to replace the MGEN cables after 6 years of use.

MGEN Analysis Results

I’m happy especially with the RA values. The DEC values are better too than in my last tests, but still not there where it should be.

The analysis of the MGEN generated events.csv shows particularly a restless DEC axis. The drifts and correction signals are frequently outside of tolerance.

Blue = Drift
Red = Correction signal
Green overlay = tolerance range

Exposure #3: CSV events

RA looks much better compared to DEC. Maybe I’ll decrease aggressivity to 60% to keep the drifts and corrections completely inside tolerance.

I’ll next adjust the DEC axis stiction, as suggested in case of a slow-responding DEC axis during autoguiding, well visible on the ramps. The MGEN increases the guide signal and duration more and more until the DEC axis suddenly moves. Over and over again…

Corresponding sub exposures to the MGEN graphs above:

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