ESPaDOnS: an Echelle SpectroPolarimetric Device
for the Observation of Stars at CFHT

Guider

ESPaDOnS has its own acquisition and guiding camera [photo].

There are 3 ways to guide when observing with ESPaDOnS:

1. By default, the ESPaDOnS guider is used, and controlled by the observer.
2. If the science target is too faint (fainter than about magnitude 12.0) for the ESPaDOnS guider, or if the ESPaDOnS guider has difficulties doing a good guiding (for whatever reason), it is possible to do Cass guiding with a star offset from the science target; this is under OA control.
3. If neither ESPaDOnS guiding or Cass guiding is satisfactory (eg, there are dec oscillations and the star is not kept in the hole), guiding can be performed manually by the observer, by stopping all guiding, and using the Guider Control button "Center Star" to re-center the target every 30-60 seconds.

The rest of this page documents the ESPaDOnS guider.

At the entrance of the polarimeter module, there is a tilted mirror [photo] that reflects the light of the field into the guiding camera, located on the side of the polarimeter. At the center of this tilted mirror are the star and sky holes.

The field of view is about 100 arcsec. The shortest exposure time is 0.5 seconds. A neutral density filter can be used to attenuate the light of bright stars; the setting for the neutral density filter is on the same window as for the guider control.

As shown on the Guider window of the GUI, the acquisition and guiding camera can be used 4 ways:

• as an acquisition camera that shows the field of view, with integration times that can be changed to suit the brightness of the target; when going on a target, this is usually the first mode used for the guider, to find and center the target of interest
• as a guiding camera, calculating the guiding corrections needed, but without sending corrections to the telescope; this mode can used to see if the guiding is stable
• as a guiding camera, sending guiding corrections to the telescope

It is possible to guide on the target (which is in the star hole) [image], or on a nearby target (offset guiding with the Cassegrain guiding camera) [image]. This is done by defining the position of the guiding zone (represented by a dashed circle in the 2 previous images), using the GUI.

If for some reason the ESPaDOnS guider does not work well, it is possible to use the Cassegrain guiding camera. The ESPaDOnS camera can then be used in acquisition mode only, while the guiding is taken care of by the Cass camera.

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