  Real-time seeing estimation at the focus of a telescope is nowadays strongly
emphasized as this knowledge virtually drives the dimensioning of adaptive
optics systems and instrument operational aspects. In this context we study the
interest of using active optics Shack-Hartmann (AOSH) sensor images to provide
accurate estimate of the seeing. The AOSH practically delivers long exposure
spot PSFs -- at the critical location of the telescope focus -- being directly
related to the atmospheric seeing in the line of sight. Although AOSH sensors
are not specified to measure spot sizes but slopes, we show that accurate
seeing estimation from AOSH images can be obtained with a dedicated algorithm.
The sensitivity and comparison of two algorithms to various parameters is
analyzed in a systematic way, demonstrating that efficient estimation of the
seeing can be obtained by adequate means.
