I'm posting this for those who have experienced some tracking issues where the guide star seemed to jump in RA or Dec by several pixels during a normal run. I experienced this last night after a 4 hour imaging session where tracking was on the order of 1/4 pixel. Traced it down to the camera cable rubbing on the pier strut. Just touching the cable actually sent the guide star back and forth by several pixels in both RA and Dec.
It turns out that while the mount is slowly advancing in RA the cable was slowly dragging across the pier strut in a stick-slip mode. This was enough to move the guide star by as much as 5 pixels. Since the mount is always moving in RA, this is the direction that the cable is being pulled and this is why the guide star moves primarily in RA.
In the attached screen shot you can see the first RA guide star anomaly marked by a red 1 arrow. The second one occurred about 5 minutes later (red 2 arrow). When I saw that I went to the mount and gently touched the heavy power cable coming from the STL11K camera. The guide star jumped around 5 pixels as marked by the green 3 arrows. Even though the camera, focuser and scope were properly tightened down, still a loose cable dragging will cause the guide star to jump around by several arc seconds.
So, the advice I have for imagers is to not hang the cable down. Rather tie it securely to the mount Dec axis before looping it down to the ground. Either that or run it inside the mount, but again secure it to the scope body so that it cannot tug at all on the camera body.
It turns out that while the mount is slowly advancing in RA the cable was slowly dragging across the pier strut in a stick-slip mode. This was enough to move the guide star by as much as 5 pixels. Since the mount is always moving in RA, this is the direction that the cable is being pulled and this is why the guide star moves primarily in RA.
In the attached screen shot you can see the first RA guide star anomaly marked by a red 1 arrow. The second one occurred about 5 minutes later (red 2 arrow). When I saw that I went to the mount and gently touched the heavy power cable coming from the STL11K camera. The guide star jumped around 5 pixels as marked by the green 3 arrows. Even though the camera, focuser and scope were properly tightened down, still a loose cable dragging will cause the guide star to jump around by several arc seconds.
So, the advice I have for imagers is to not hang the cable down. Rather tie it securely to the mount Dec axis before looping it down to the ground. Either that or run it inside the mount, but again secure it to the scope body so that it cannot tug at all on the camera body.
Attached Image:
