Pupil Tracking Mode


Pupil Tracking (PT) mode was implemented to support SAM, but given the high demand from the community, it has been commissioned and offered in combination with imaging, SDI+, SDI+4 and other Coronagraphic modes.

In this mode, the telescope, independently from NaCo, tracks the pupil instead of the field. For example, a double star will be seen to rotate in subsequent frames while the pupil spiders will remain fixed. The advantage is that the spiders can be positioned in a way that prevents contamination of the scientific field of interest.

In addition, pupil tracking may be used to replace angular differential imaging, a high contrast imaging technique currently used with NaCo imaging, coronagraphy SDI+4 and SDI mode to reduce quasi-static speckle noise and facilitates the detection of nearby companions.

Pupil tracking is a more efficient and easier to use (because transparent to the user) feature for the most popular NaCo observing modes.


Pupil Tracking mode (also called Alt-Az) is set during acquisition. The rotator is then rotated to anoffset angle specified by the user in the template as the position angle on sky parameter. Note that this is the same parameter used to set the PA on sky for a target observed in field tracking mode. The meaning is different when the pupl tracking flag is set to true.

The figures below shows the position of the spiders at different position angles on the sky. A positive angle rotates the spiders clockwise. The spiders have a 180 degrees rotational simmetry, e.g. the spiders positioningis identical at PA=0 and PA=180.

The figure below shows images taken in PT mode with the spiders set at different positions angles.

The templates supporting modes with which PT is offered are:

  • NACO_img_acq_SAMMoveToPixel: acquisition for SAM - PT is compulsory.
  • NACO_img_acq_SDIMoveToPixel: acquisition for SDI+
  • NACO_img_acq_MoveToPixel: acquisition for imaging (no LGS)
  • NACO_img_acq_SDIMoveToMask: acquisition for SDI+4
  • NACO_img_acq_MoveToMask: acquisition for coronagraphy.
Note however, that while PT is optional for imaging, SDI+, SDI+4 and coronagraphy, it is compulsory for SAM.

More work is needed to characterize the mode, in particular how effectively PT can be used with coronagraphy, SDI+and SDI+4 modes and how it performs if compared with other techniques such as angular differential imaging.

Interested users should mention their need for this particular mode in the proposal form under Sections 8 and 14.

As of P86, pupil tracking is offered in SM/VM for direct imaging only (and APP). Associated to short DIT, the observed drift close to zenith can be taken into account during postprocessing.

Pupil traking associated with SAM/SAM-Pol or any coronographic masks is only offered in VM.