I am also interested on using direct control through the AUX RJ12 port of my AVX.
Seems that the hbg3 project you refer (rtr.ca/hbg3/) is alive but at this moment kits and presoldered boards are "out of stock". As a DIY project, if not starting from a kit, I would say it is not easy (double sided pcbs, SMD soldering...).
Another possibility is to use the celestron_aux driver instead of the celestron_hc. There is a lot of information on this driver in the indi site but when you look at github there is a big warning about being beta code and not suitable for unattended work.
I would say these two approaches are very different and use different protocols. HBG3 mimics the Hand Control, so uses the same indi driver used when the HC is in the middle. the celestronaux driver talks directly the AUX protocol.
It would be great to know the current status of these direct PC-telescope connection for someone actually using them.

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