Package: Autoguiding

Description:  The Autoguiding package takes care of correcting low frequency telescope position errors. Such errors can be caused by inaccurate coordinate or refraction calculations, flexure or vibration in the structure, temperature variations, wind buffeting. Corrections are applied to the telescope axes, altitude and azimuth. The Autoguiding is not supposed to correct errors with frequency higher than 1 Hz, like variations in atmospheric or local seeing..

Package name: ag

Inheritance: Standard Package

Deployment: AT Control Workstation: atagwsControl process. Sensor LCU: agServer and agLoop processes

Use Case diagram: 

Class diagram:

What follows is a Level 0 class diagram. The Rational Rose model for the AG package contains more detailed diagrams.

Architecture:

The Autoguiding package provides all the same functionality of the VLT TCS Autoguiding package and maintains the same architecture.

For a detailed description of the VLT Autoguiding software, refer to the TCS Auto Guiding and Field Stabilisation Design Description[RDV06].

The Rational Rose model for the ATCS project contains also a complete description of the VLT Autoguiding system in the VLT sub-package inside the AG package.

The Autoguiding technique is based on the selection in the field of view of a guide star, which is bright enough to allow to measure reliably its position by means of a CCD camera in the Field Acquisition Sensor. The computed position errors are then used to move the telescope in a closed loop control.
Autoguiding functions are started automatically, if the guiding type parameter has been set to <auto guiding>, after the telescope preset has been completed.
A complete sequence of actions is then automatically executed, which means that generally no commands are necessary during the observation time. Nevertheless it is possible to control/initiate various autoguiding functions or to configure its behavior by means of a set of explicit commands.

Note:The term guide probe is used in this context in a generic way and does not imply the presence of an independent guide probe installed on a robotised arm.
For the auto-guiding of the ATs, the guide probe is just the on-axis Field Acquisition Sensor with the possibility of defining a guide box in any area of the FAS field. This box moves tracking the given sky coordinates according to field rotation and to the relative motions between the target and the guide star, due for example to differential refraction effects (the target will be typically observed in the infrared, while the guide star is in the visible.

The only significative difference, with respect to Autoguiding, between VLT and ATs is the fact that the FAS is fixed in the center of the field and not on a moving guide probe. It is also receiving light from a beam splitter and does not vignet the field.

The ATs do not have a derotator.

On the other hand, the ATs Tracking Axis software (see Tracking Axis package) handles transparently the FAS (and the FSS) as a guide probe. In particular it automatically changes the reference point of the FAS to put it always on the theoretical position of the guide probe.

With respect to the VLT adapter, it provides also the current position of the guide probe in XY pixel coordinates in the reference frame of the FAS and stores the reference position in a public accessible database point.

This has the following architectural implications:

• The default FAS reference position is not any more read from the Mode Switching instrument configuration data, according to the selected focus, but is retrieved from the probe software whenever needed.
• No reference position must ever be sent to the FAS software, since it is always automatically set by the Tracking Axis software.

The Autoguiding package is composed of a WS part and an LCU part.

The WS part receives all autoguiding commands, which can be user commands or internal ATCS commands. Most of these commands are passed to the LCU part for action.

The LCU part consists of a command interpreter, some data handling functions, functions for CCD handling, some image processing, centroiding, evaluation of position errors, and sending of position correction commands. These commands are sent directly to the LCU part of tracking in the Alt and Az LCUs. This direct sending of commands only takes place if autoguiding is active, and if the corresponding LCU part is authorized by the WS part to do so. At any time, the LCU function is prepared to receive a `stop sending corrections' command, so that the WS part of autoguiding is in full control of what is going on.

The direct sending of commands from one LCU to other LCUs is an accepted exception of VLT rules. This is a (soft) real-time activity, and must bypass the WS for performance reasons.

On the workstation side, the following design changes with respect to the VLT Autoguiding software are required:

• A new class is created:
  atagwsCON_GUIDE_PROBE: public agwsCON_ADAPTER_VLT.
  This class is responsible for all the interfaces with the adapter software. In the case of ATs, we expect the same interfaces provided by the VLT adapters
  This class must then just be a subclass of agwsCON_ADAPTER_VLT and add the methods necessary to retrieve the reference position via direct database read (in the same way the agwsCON_SERVER class accesses the TCCD error vectors when the ErrVector() method is called). atagws gets the current reference position (probe/atp) to calculate offset for DETGS
• A new class is created:
  atagwsCON_SETUP: public agwsCON_SETUP.
  The following methods are overwritten/implemented:
  • GetDefaultRefXY() is overridden to load the reference position from the probe using the interfaces provided by atagwsCON_GUIDE_PROBE
  • SetWin() is overridden to NOT store the reference position in the setup file to be sent to the TCCD control software
• The keywords for the reference position are removed from the TCCD defauls setup file.

On the LCU side, no code change or extension is necessary.

Command Definition Table: 
The CDT for Autoguiding is built by including for maintenance convenience a numbere of separate CDTs:

• CDTs for agwsControl WS coordination process:
  • agwsControl.cdt
  • agwsControl_MAINT.cdt
  • agwsControl_PUB.cdt
  • agwsControl_START.cdt
  • agwsControl_TEST.cdt
• CDT for agServer LCU process:
  • agServer.cdt

Reuse from VLT TCS: 90%

The Autoguiding package provides all the same functionality of the VLT TCS Autoguiding package and maintains the same architecture.

Issues to be Determined or Resolved: None

Last modified: Tue Feb 26 10:50:12 UTC 2002