Go to GEM28 – Latest Update Part II
UPDATE Sept. 22, 2021
It’s really an adventure with my GEM28. Differential Flexure is likely not responsible for my guiding errors, but at least I learned something new 🙂
The Differential Flexure hint was given on this German forum thread:
https://forum.astronomie.de/threads/staradventurer-2i-vs-eq5pro-vs-eqm35-vs-ioptron-gem28.312432/post-1610995
2 months ago I started this thread about my GEM28 on astronomie.de (also in German):
https://forum.astronomie.de/threads/ioptron-gem28-erste-erfahrungen.309790/
I’m currently also subscribed to many GEM28 threads on CloudyNights and yesterday there was posted a very interesting overall summary about the GEM28 and their DEC guiding problems in this thread:
https://www.cloudynights.com/topic/787254-gem28-experience/#entry11375443
Like I already heard a few times before, iOptron mount parts basically are very well manufactured, but one has to find the sweet spot regarding gear meshing and belt tension by yourself. Or you’re lucky and get a well build one 😉
Since my GEM28 today almost crashed/freezed again, even if I used the original power supply, I’m still convinced that my GEM28 has some kind of fault:
Within 20 minutes I got first massive DEC trails on one exposure and then RA trails on a second. In both cases the GEM28 didn’t react anymore to signals from MGEN right after the exposure. Fortunately the GEM was still controllable by handcontroller all the time and I was able to correct the drifts and continue shooting. I’ve still no idea what leads to this…
BTW: the dark upper left corners are caused by the L-2 clip filter, since it does not stay in place while moving the lens (at least my L-3 filter fits perfect). At some point the filter is trapped between DSLR and lens and it becomes a real challenge to remove the lens from the DSLR. It already damaged the plastic around the filter glass… luckily the glass itself shows no traces of damage… and also the lens seems to be OK.
Problem solved! I recently found this video: https://www.youtube.com/watch?v=-55fjwC1Ttg
It’s actually aluminum, not plastic. The pressing force can be adjusted with a small screwdriver. Now they stay perfectly in place, regardless of the orientation 🙂
UPDATE Sept. 17, 2021
Differential Flexure Challenge
The dual mount for parallel attachment has arrived but unfortunately they delivered the wrong guidescope… so, for a first test I still will use my old guidescope. Today’s weather cast looks promising 😉
The guider camera cable and DSLR shutter cable are attached now to the RA unit to achieve a shorter cable loop to avoid pull from the cables.
It was a bit time consuming to balance the dual mount… Balance can be achieved by moving the lens/DSLR and/or the guidescope back and forth in the dove tail clamps. Additional the whole dual mount can be moved left/right within the dove tail sattle on the GEM28.
UPDATE Sept. 14, 2021
It seems my DEC problems are caused by Differential Flexure.
The guide scope or lens are drifting during the session relative to each other (in a range that is mostly not noticeable by eye). The reason can be various: pulling cables, insufficient attachment of components…
So I’ve ordered rings also for my guidescope and I’ll attach the lens and guidescope on a parallel dovetail adapter like this one: https://www.teleskop-express.de/shop/product_info.php/info/p223
I’m not sure at the moment, if Differential Flexure is really causing my problems that much. Why I didn’t had this problem already on the Star Adventurer? I still use the same lens, DSLR and guidescope…
More questions than answers at the moment… 🙁
Just a few words as introduction: I’m writing this post with a bit of frustration. I still have constantly bad results with my GEM28.
Previously I used the Star Adventurer L-bracket with my Canon EOS 70D, EF 400mm f/5.6L and the 50mm guide scope. Also the Star Adventurer 1kg counter weight was attached, to reach at least some balance. It was pretty unbalanced, but I reached 70-90% good results. The RA curves were much better than with the new setup.
Star Adventurer L-Bracket 
New setup total 4.2kg 
Full setup
A few days ago I recorded again with the new setup a few 12 minutes exposures to monitor once again the performance of my GEM28.
Surprisingly, the performance generally has deteriorated since I use this new ring/doveplate solution, altough its perfect balanced now. Especially RA performs very bad now. DEC always performed bad!
The Recordings
I have absolutely no idea what the DEC axis is doing all the time… it looks terrible!
MGEN Settings
Guider Setup:
162mm focal length, RA guiding speed 0.50x,
DEC 0.50x (same on GEM28)
Guiding:
Gain 9, Exp. 1000, Threshold 10
RA:
Num 1, Tol. 0.20, Aggr. 70%, Mode 2
DEC:
Num 1, Tol. 0.20, Aggr. 50%, Mode 2
Random Displacement:
Uniform Square, 5px
That night I reached 97% orthogonality during calibration.
Really, I don’t know what to do now, since I tried many different MGEN parameter combinations and also adjusted the worm gear meshing to various positions. No success at all.
RA seems to be relatively constant on the curves, but the RA std. values are too high. They are all above the tolerance, configured on MGEN (0.20). At least this value was much better with the unbalanced Star Adventurer L-bracket, attached to the GEM28.
DEC is struggling from the very beginning 🙁
To be honest, I get the best result without DEC guiding. Otherwise I got DEC trails on 10-30% of the taken exposures.
I recently found a YouTube video (https://www.youtube.com/watch?v=dqGZEtjeOL0), showing the adjustment of the gear meshing on a GEM28. I adjusted mine the same way now and looking forward for clear sky to test and record it.
I have not given up yet!
Besides that all, I like the haptic and the look 😉
