Use Case:  Make Pointing Error Measurement

Description: Measure the pointing errors for a number of stars, according to predefined parameters. The output is a measurements file. No model calculation or update is done.
Three types of model creation are be supported:

• total pointing model build-up
  It calibrates all reproducible pointing errors using a large number of stars. It may use, in addition, temperature sensors if required. The model shall contain a telescope-dependent part and a station-dependant part.
• medium pointing model update
  It uses a relatively small numbers of stars and calibrates out medium term variations.
• small pointing model update
  It calibrates out the 3 angular offsets associated with the non-reproducible positioning errors of the AT on the a specific station.

Role(s)/Actor(s):
Primary: Operator, Maintenance
Secondary: Tracking axes, FAS

Priority:Critical

Performance:

Frequency:

• total pointing model build-up
  During provisional acceptance and redone whenever necessary
• medium pointing model update
  At start of night when time-frame allows (avoid loosing dark time)
• small pointing model update
  When AT has moved to a new station.

Package Assignment:  Pointing Modelling.

Preconditions:

• Telescope ready to preset and take images from FAS or FSS

• Select a number of stars with reachable positions
  Postcondition: Enough stars are available to build a model
• ATCS presets to each star

  with Astronomical Preset (PRS/PRSCOOR)

  Postcondition: Telescope is tracking on selected star
• ATCS Prepares FAS Setup (FAS/SETUP) to setup FAS CCD for one image acquisition with display and centroid calculations
  Alternate Course: ATCS Prepares FSS Setup (FSS/SETUP) to setup FSS Strap for one sequence of images with display and diagnostics
  Postcondition: Detector setup is available
• ATCS make error measurement for each star
  ATCS Starts Field Acquisition (FAS/START) to take one image with display and centroid calculations
  Alternate Course: ATCS Starts FSS Acquisition (FSS/STRTOLP) to take one sequence of images with display and centroid calculations
  Postcondition: Diagnostics are available
  Exception Course: Error measurement fails (e.g. more than one or no star visible)
• ATCS stores error measurement of each star

• Select a number of stars with reachable positions.
  Postcondition: Enough stars are available to build a model
• ATCS presets to each star

  with Astronomical Preset (PRS/PRSCOOR)

  Postcondition: Telescope is tracking on selected star
• ATCS moves star to pointing axis reference point by offsetting telescope

  with Offset Alpha/Delta.

  Alternate Course: Several stars are visible
  Exception Course: No star is visible
  Postcondition: Star is on reference point
• ATCS make error measurement for star
• ATCS store error measurement of star

Alternate Course: Several stars are visible

• Select star that is closest to reference point and has the right brightness
• Move star to pointing axis reference point by offsetting telescope.

Exception Course: No star is visible

• Search for star by moving the telescope with offset steps
  Exception Course: No star found
  Postcondition: A star is visible in the field
  
• Move star to pointing axis reference point by offsetting telescope.
  Alternate Course: Several stars are visible
  

Exception Course: Error measurement fails

• Continue with next star

Exception Course: No star found

• Continue with next star

Postconditions:

• Pointing error measurements made for sufficient number of stars.

Issues to be Determined or Resolved:

Notes:

Last modified: Thu Jun 28 13:50:31 UTC 2001