Description: This package is modeling the interaction layer with the CCD Software. It provides a high level control of the Field Acquisition Sensor, which supplies an image of the stellar field from the Coudé Focus. The detector is a Technical CCD as used on the UTs for Autoguiding and Field Stabilization. The image size is 400x290 pixel.
Package name: fas
Inheritance: None
Deployment: The CCD software is deployed on both the AT Work Station and the Sensor LCU.
Architecture: ATCS interfaces to the FAS completely through the standard CCD software [AD 27], via SETUP file and DCS commands.
The Use Cases identified in the previous phases resolve in sending the corresponding setup to the CCD software, complying the CCDDCS dictionary ESO-VLT-DIC.CCDDCS. In addition, the commands START, STOP and WAIT shall be sent according to the courses of the Use Cases for initiating, terminating or waiting for an exposure.
This package is a logical wrapper on the CCD Software. It implements a boundary class for each relevant command/function of the CCD Software. The VLT autoguider module agws provides all the necessary boundary classes for its interaction to the CCD; a similar layer must be provided from within the package opt.
The CCD Window is another Tracking Axis; unlike the UTs, the CCD
detector is not mounted on a robotized arm (guide probe), therefore
the location of the acquisition window used in autoguiding must
follow (thus acting as a guide probe) the position of the reference
point, computed by the tracking process running on this LCU. Since
the CCD software does not support the redefinition of the location of
the window while running in a loop, but only the Reference point,
special handling will be needed for moving the window during
autoguiding: i.e. the acquisition loop will be stopped, the setup will
be modified with the new window position and the acquisition will be
restarted. Since approx. 15 pixels represent 1 arcsec, the image of a
point like object will need a window size of about 45 pixels, as
calculated in the Notes of the Use Case
Set
FSS Translation Stage, the maximum field rotation is 7marcs/s in
the Coudé plane when 60" off-axis. This implies that the
tracking speed of the FAS window shall be of the order of 0.1
pixel/s. Consequently it shall be possible to guide for a couple of
minutes without moving the window. The update rate becomes lower as the
guide star is closer to the center of field.
The
reference point can be updated at any time and is expressed in
fraction of pixel as computed by the tracking loop.
The class acmFASWindow:atpAxis is implementing the methods for the update of the Reference Point on the CCD Detector (see the section Tracking Axis for details). It interfaces with the CCD Software at LCU API level via the function ccdconSetup().
Reuse from VLT TCS: 100%; The CCD software covers all of the necessary functionality. The CCD interface class of the autoguider module agws shall be taken as an implementation template for the package opt.
Issues to be Determined or Resolved: None
Notes: The pixel scale is approx. 0.07"/pixel with:
Last modified: Mon May 29 09:20:25 METDST 2000