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When you are trying to track down guiding errors with your astrophotography, look for flexure first. The basic rule of thumb is that if it can move, it will move. Any movement in your imaging train can wreck your pictures and make star trails and wild jerking movements.
Currently I’m using my Nikon 80 to 400mm zoom lens for most of my astrophotography. It’s an ok but not perfect lens. I’m waiting to buy a decent astrophotography telescope for wild field views. In the meantime, I’m struggling to get long exposures in excess of 5 minutes in the 200 to 400mm range. I have done up to 20 minutes with this lens, but only if I mount the guide camera directly to the top of the lens. In addition, I’m getting flexure in the lens itself due to the single mounting point design that was intended to be used in a camera tripod.
Differential Flexure
This refers to a difference in flexing from your optical train and the guide camera. If I try to mount the guide camera off to the side, then I get differential flexure and can not take shots longer than 5 minutes. Even at 5 minutes, the shots are inconsistent and many are ruined.
The Fix
I have finally got a system where I can consistency take long sub exposures. The solution was to add a second support bracket that bolts onto the universal vixen plate. The difference this one change made is dramatic. I can no longer feel any give at all when I press my finger down on the imaging camera. Prior to the bracket, you could see visible movement as the camera train flexed on the tripod mounting bracket.
In addition to this bracket, I went back to a piggy back system where I mounted the guide camera right onto the lens. The reason I took it off to begin with is that I did not want to damage the vibration reduction switch on the lens. Unfortunately, this is the only spot I could find to mount my custom 3D printed guide camera mount.
By make these two small changes, you can see the difference both in the final images and by looking at the RMS error reading on the ASIAir guide camera graph. The following images shows the difference is guiding corrections between having the guide camera piggy backed, versus having it off the the side.
Conclusion
Working out the bugs in your auto guiding setup has a lot of moving parts. For me, the biggest issue is flexure and I think this is the first place you should start. In the video above, I made a standardized test and show just one change at a time. This is important so that you can narrow down the root cause of your issue.