These may include slow tracking errors as well as vibrations of the telescope structure at higher frequencies. The WHT autoguider system operates at a closed loop bandwidth of approximately 0.1 Hz. Tracking errors occurring at higher frequencies are uncorrected, so that the spectrum of image motion at frequencies higher than 0.1 Hz is relevant to the long exposure PSF width. The JOSE wavefront sensor data provides measurements of the power spectrum of image motion. The first order Zernike terms measure the component of image motion which is common to all subapertures of the wavefront sensor, ie.
Tilts result from both tracking errors and from large scale atmospheric seeing distortions.
The figure below shows the averaged power spectrum of the first order Zernike terms for the JOSE data set. In general the telescope and atmospheric contributions to the spectrum cannot be distinguished. However, a narrow spike in the power spectrum due to a known oscillation of the WHT structure is visible at 2.7 Hz. Although this resonance contributes little power on average, the oscillation can have a significant effect on image width if it is strongly excited, for example by wind buffeting of the telescope structure. Such high frequency telescope oscillations are difficult to detect during normal astronomical observations.