If your stars have moved 40 pixels from the first image to the last, either you're not guiding or your guiding is doing a horrible job. And in fact, the length of the streaks in "walking noise" is far more than that. For field drift to be the issue, you'd have to have a significant, noticeable change in position of the stars from the first image to the last. And when you're done stacking, you'd have to see a darker edge to the image where the stacked frames did not align, which would be equal to the length of the noise streaks. Never happens.
That doesn't have to be the case. You can have field drift even in the presence of perfect guiding. That is why this is so often confusing and hard to wrap your mind around.
Differential flexure has nothing to do with guiding, per se. It is the movement of one optical axis with respect to another during imaging. That is the very definition of differential flexure. Any time you are guiding with one optical system and imaging with another, you can have this field drift due to the differential flexure.
Imagine that could save each of your guide exposures over a four hour session in addition to saving your images gathered by the tethered main scope. At the end of the four hours, you stack all of the guide exposures. Because you are guiding, they will always line up and do so perfectly if guiding is perfect. Now consider the other main optical system which is tethered to your guide scope and is also imaging.
Over the four hours of your imaging system use, the optical axis of the main scope can slowly diverge from the optical axis of the guide scope. It could be that one of the scopes has a movable mirror such as in an SCT allowing it to tilt during movement across the sky. It could be that the mounting rings on the guide scope can allow a slight flexure under gravity as the mount tracks across the sky. It may even be due to using a long refractor whose tube can bend ever so slightly under its own weight. There are also cases where the focuser can droop a few ten-thousandths of an inch due to gravity during that four hours. In all cases, the optical axis of one scope does not point to exactly the same place when compared to the optical axis of the other scope over time.
That divergence of optical axes over time is the definition of differential flexure. Even though the guiding can be perfect from the viewpoint of the guiding system, the field in the other scope can move a number of arc-seconds over a period of hours as their axes diverge. That shows up as field drift in the main scope's images.
The usual cure for field drift due to differential flexure is to use an off-axis guider. That way, both the guide camera and the imaging camera see the exact same field drift and guiding prevents star drift over time. (Except in the case of field rotation due to polar misalignment which can also cause field drift and walking noise even with an OAG. In such a case, though, it is much, much less severe and not usually very noticeable.)
Edited by jdupton, 14 August 2019 - 03:06 PM.