Luxo .
I understand the point you're making, but it doesn't match my experience.
I have been using exactly the same setup (same telescope, same camera, same 3 nm narrowband filter, and the same guide settings) for almost two years without any significant guiding issues. The problem only started about 4–5 months ago, after recent ASIAIR software updates.
I've also been in regular contact with ZWO technical support for over a year regarding these issues, and they have never told me that DUO and AIR cameras are not intended or recommended for guiding through narrowband filters.
If that were actually the case, it would be very surprising, considering that the ASI2600 Duo and ASi 2600 AIR is one of ZWO's flagship cameras and is heavily marketed for narrowband astrophotography. I have also never seen any statement in the official documentation saying that narrowband guiding with the DUO/AIR should not be used.
Also, I am certainly not the only person using the ASI2600 Duo or the ASI2600MC Air with narrowband filters. If you look on AstroBin or Telescopius, you will find many images captured with these cameras using narrowband filters. I also personally know several people who have been using this exact configuration successfully for years, without any guiding issues, until about five months ago.
Of course, a 3 nm filter reduces the number of available guide stars, and everyone understands that. But that's very different from saying that the camera is not suitable for narrowband guiding.
The fact that the exact same hardware worked reliably for years, and that many users started experiencing the same issue only recently, strongly suggests that this is a software-related problem rather than a fundamental limitation of the DUO/AIR design.
Also, it's not that guiding doesn't work at all. It still works, but I now have to use BIN 2, a gain above 550, and 3 second guide exposures to achieve reliable guiding. Until about 5–6 months ago, with the exact same Sky-Watcher 200/800 Newtonian and the exact same setup, I was using BIN 1, a maximum gain of 400, and 2 second guide exposures without any issues. The only thing that has changed is the ASIAIR software. That is a significant change despite the hardware and optical setup remaining exactly the same.
