Vibrating Image?

[Roy@Aldermaston]

This strange effect appeared for the first time last night when I was setting up for a session imaging M57. I had centred Vega in the SharpCap window using the reticule, when I noticed that the star was fluctuating - I mean moving rapidly backwards and forwards about the centre of the screen, but only in the horizontal plane. The scope had been balanced and polar aligned, and although there was some wind it was not enough to cause the scope to move. I tried adjusting the backlash settings, but there was no improvement. I know atmospheric conditions cause star images to shimmer on the screen, but this was just a horizontal movement, quite rapid at 10fps but slow enough at 5 fps to create a horizontally elongated star image - just like a sausage, but white!

I would be interested to know if anyone can explain what was causing this effect, and grateful for a solution.

Roy