Overall structure of the NAOMI EPICS software

Positioning of the NAOMI mechanisms is achieved with stepper motors, which are controlled using OMS VME-8 and VME-44 stepper motor controller cards. The one exception is the NCU mask mechanism (see below), which uses a DC motor, controlled from a XYCOM XVME-240 digital I/O card, to achieve in/out positioning. Other VME cards present in the system are as follows.

• The XYCOM XVME-240, mentioned above, is for control of TTL levels (switching of lamps, DC motors, shutter etc) and the sensing of input levels (NCU mask in/out, reading of A/D converter for the NCU photodiode).
• The VMIC VMIVME-4145 Waveform Generator card is for generation of the time-dependent voltages required to drive the NCU tip-tilt calibration mirror.
• The VMIC VMIVME-3230 Thermocouple Board is used to measure the temperature at thermocouples distributed at various locations on the instrument.

A device driver for the XYCOM card is included in the Gemini release of EPICS used for NAOMI, and one for the OMS cards was developed at HIA for the GMOS mechanism control records (although this was modified slightly for the NAOMI project). Software for control of the VME-4145 and VME-3230 cards has not yet been integrated into the NAOMI mechanism control software.

We deal now with each component in more detail.

Wavefront Sensor components

• Pick-off X and Y positioning - two stepper motors position the NAOMI Wavefront Sensor Pick-off mirror. The mirror is used to select a guide star from the NAOMI field-of-view, who's light will be passed into the wavefront sensor toward the NAOMI cameras.
• Fore-optics and Camera Stage positioning - The NAOMI CCD cameras are mounted on a linear slide which moves along the same axis as the pick-off X motion. An additional stage, between the two, holds the NAOMI "fore-optics" comprising a filter wheel, two AtmDC prisms and a lenslet wheel. Both these stages must be moved in response to any motion of the pick-off mirror, in order to maintain focus at the CCD cameras.
• Lenslet selection - The selection of which lenslet to use for NAOMI wavefront sensing is made using a single rotary stage, mounted on the fore-optics stage mentioned above. The lenslets must maintain accurate registration with the CCD camera pixel axes (and binning windows). The lenslets are not all of equal focal length, requiring that the camera stage position be dependent upon the current lenslet selection.
• Filter selection - Selection of the WFS filter is achieved using a single rotary stage.
• Atmospheric Dispersion Correction (AtmDC) - The NAOMI AtmDC comprises two prisms, which are rotated using rotary stages.
• Opto-mechanical chassis - The OMC holds several components of NAOMI, and is really a separate major component in its own right. However, the small amount of EPICS software required for the OMC has been subsumed into the software for the WFS. The only requirements for automated motion here are X and Y positioning of the Deformable Mirror (DM). This is performed in order to maintain alignment of the DM segments with the lenslet array spots that are seen at the NAOMI cameras.

Nasmyth Calibration Unit

• Beamsplitter - The beamsplitter is driven by a stepper motor between in and out positions. With the beamplitter in-beam, light from the NCU calibration source will be seen by the wavefront sensor (ie. the NCU is deployed).
• Motorised mask - This mask is driven into or out of the NCU beam by a DC motor. Limit switches at either end of the travel are used to determine whether the mask has been deployed (or removed) successfully.
• Calibration source - The NCU calibration lamp is switched on or off by a TTL voltage generated by the XYCOM-240 card. Light from the source passes through a shutter, which acts as a variable attenuator when rotated by its stepper motor, into an integrating sphere. A measure of the light intensity within the integrating sphere is obtained from a photodiode mounted in the sphere. The photodiode voltage is amplified, measured by an A/D converter and recorded in software via the XYCOM-240 card. Light from the integrating sphere then passes through the plane of the NCU mask and on to the beamsplitter.

General structure of the EPICS database for each component of NAOMI

• For mechanisms which contain one or more steppermotors, use is made of the GMOS assemblyControl and deviceControl records. Each stepper motor (device) is controlled and monitored using a single deviceControl record (see the description of GMOS records). These records use the OMS device driver to generate motion in the motors, accepting as input a demand position in engineering units meaningful to the user. The records allow the user to datum (index) the mechanisms using home or limit switches, and to move or jog (step at a constant rate) the mechanisms. They also allow the developer to configure the use of encoders, to set the velocities at which each device must move or index and to specify the whether the system should monitor the power or brake status of each device.

  Over this, related devices are grouped together into assemblies, each one of which uses one assemblyControl record to sequence the actions of its component devices. The assemblyControl records add any required offsets to the user-demanded positions for each device, and perform any additional calculations required to maintain devices in their correct relative positions. For instance, the correct position of the Fore-optics stage is dependent upon the x/y position of the pick-off, whilst the correct position of the CCD stage is dependent upon both the position of the Fore-optics stage and the focal length of the selected lenslet array. This calculation is performed within the "pick-off" assemblyControl record.

• Devices which don't utilise any stepper motors do not need to use the deviceControl or assemblyControl records. Control over these devices, such as the DC motor for the NCU mask, is implemented using the Gemini general-purpose subroutine (genSub) record and standard EPICS records like the binary input record.