Guiding with auxlilary telescope achieved

Guiding with auxlilary telescope achieved

30 minute exposure of the Helix nebula (Sloan r')

18-07-2010

Active guiding for WiFSIP on STELLA-1 has been achieved during this 30 minute exposure of the Helix nebula.

Correcting for inevitable tracking errors on Stella-I is achieved by permanently monitoring the field-of-view using the auxiliary telescope. After correcting for field rotation, an offset can be calculated and after some filtering in a PID, it can be applied at the telescope. The image still shows slightly elongated stars, which is caused by three different reasons: First, the differential pointing model between the auxiliary and the main telescope has not been measured. This causes slight drifts, which leads to elongated stellar images on very long exposures. Second, the telescope drive still suffers from high-frequency oscillations mainly in the azimuth axis. This leads to elongated images also present on short-time exposed images. Third, the bad focus of the auxiliary telescope makes determination of the centroid noisy, which leads to a general increase of the PSF of the star. This effect, if unbiased, does not cause elongation of the images, but merely an overall increase of the seeing.