WHT guiding

In general, autoguiding works very well at all WHT focal stations.

Below we report an ongoing investigation of an apparent 1-arcsec oscillation, period ~ 1 minute, at the Cassegrain focus.

In April 2009, Danny Steeghs (Warwick) obtained with WHT ISIS a series of short (2-sec) spectra of 2 objects placed symmetrically on the ISIS slit. In the figure below, the top left plot shows:

1. A slow drift in the position of one of these two objects, which might be ascribed to flexure between ISIS and the autoguider.
2. An oscillation of full amplitude ~ 1 arcsec (5 pixels) and period ~ 67 sec (see blow-up in middle plot in top row, and power spectrum at right):

   

   On 2009 Jun 29, Miguel Santander obtained a series of 10-sec J-band images of stars using LIRIS (0.25 arcsec/pixel)mounted at the Cassegrain focus, with the telescope autoguiding (in the optical). The star centroid positions (measured by Pablo Rodriguez) are plotted below (plot width ~ 0.008 days = 700 sec):

   	Star 1 (1418-06)	Star 2 (1440+40)	Star 3 (1415+29)
   X
   Y

   There is no evidence here for oscillations. However, there are surprisingly large drifts in position, up to 0.5 arcsec over a few minutes. Why?

   
   Chris Benn (crb@ing.iac.es)
   
   2009 Sep 17