The STRAP system
Introduction
The STRAP system
(see also [AD 03]
and [AD 04])
is a self-consistent system. STRAP encompasses a photon-counting
visible sensor, the housekeeping and the control
hardware/software. The input is a signal from a Guide Star focussed on
the sensor unit, the output is a command signal, optionally analog or
digital, which can drive most commercial steering-mirror drivers. The STRAP
board needs to be inserted in a standard VME 19" - 6HE rack with a CPU,
a LAN connection, a MAC4-INC motor controller and a VME4SA DC motor servo-amplifier.
STRAP as delivered consists of two separate units:
- A photon counting sensor head composed by an APD quad-cell with
its lenslet array. The image of the tip-tilt natural guide star is
focussed on the lenslet array. The sensor head electronics
transforms each photon detected into a TTL pulse. The four TTL
signals of the quad-cell are passed to the STRAP VME board. A
variable aperture diaphragm is present to limit the sky background
if necessary and its micro-motor + encoders are controlled by one
channel of the MAC4-INC and VME4SA boards. Also a neutral density
wheel is present to limit the flux below a predefined threshold.
- A VME DSP-based control board, located on the VME rack at a
distance not greater than 15m. It controls the loop in real
time, it performs all the necessary housekeeping and data
gathering for the diagnostics. The Local Control Unit on
the same rack dialogues with the board via the VME bus, and uses
the data for diagnostic and setup. An RS232 serial output port
on the STRAP is also available to give a similar access to the
DSP board for servicing and diagnostics. The control board can
drive a steering mirror unit for the feedback, digitally via the
VME bus or directly with an analog or digital output signal. The
former is used to drive directly the M6 steering mirror on the AT
(M2 tip-tilt mirror via a dedicated LAN on the UT).
The STRAP units may be integrated in VLT instruments for field
stabilization, to remove the atmospheric and environmentally induced
tilts. STRAP is to be used also for the VLT and VLTI adaptive optics,
in the UTs and ATs.
Specific Requirements for the Auxiliary Telescopes
On the Auxiliary Telescopes, the STRAP system is involved in Field
Stabilization and Chopping activities and interacts with M6.
The host LCU runs 4 control loops:
- Field Stabilization: monitors APD sensor data and M6 corrections
- M6 Recentering: counter-offsets the telescope axes for tracking
drift compensation of M6
- Chopping: controls phase switching for APD and M6
- Translation Stage tracking: controls the position of the APD
sensor for field rotation and differential refraction compensation.
All required functions for the APD and M6 control are directly
available at LCU level in the STRAP Software.
In order to perform some operations and in particular during system
configuration and maintenance, it is necessary to have the
APD working both in:
- Closed loop, i.e. sending actual corrections to M6
- Open loop, i.e. performing all necessary calculations,
but without sending corrections to M6
Since M6 and the APD sensor are located on 2 different planes, rotating
with time, the STRAP system must support the update on-the-fly of the
Interaction Matrix.
Last modified: Mon Nov 22 10:18:49 MET 1999