UK Astronomy Technology Centre	Chris Tierney
Royal Observatory, Edinburgh	31st March 2001

pickoff.dl

The pickoff assembly is the collection of the more important, and inter-dependent, components of the wavefront sensor. It comprises five stages, each driven by a steppermotor:

The pickoff x-stage determines the x position of the wavefront sensor pickoff mirror.

The pickoff y-stage determines the y position of the wavefront sensor pickoff mirror.

The fore-optics stage determines the position of the fore-optics carriage, which holds a collimating lens, the filter and ADC assemblies and the lenslet selection wheel.

The lenslet wheel stage determines the orientation of the lenslet selection wheel. The lenslet selection wheel houses four lenslet arrays, one full-aperture doublet and one reticule.

The CCD stage determines the position of the CCD carriage, which holds the cameras that NAOMI uses to record lenslet images.

All of these devices, except the lenslet selection wheel, utilise a linear (optical) encoder to proivide accurate, reliable position feedback to the software. These four linear stages index to limit switches, whilst the (rotary) lenslet wheel stage indexes to a home switch.

Details of the inter-dependencies of these stages are given below in the section Parameter input/output.

Safety

All the NAOMI mechanism control screens display two buttons - a red "Emergency" button and a green "Safe" button - in the top right corner of the screen.

Left clicking an "Emergency" button will immediately stop any motion by any part of the wavefront sensor or calibration unit. Thereafter, any demands for motion are rejected, until the user left clicks on any of the "Safe" buttons. Many mechanisms will then demand to be re-indexed before allowing the execution of any other motions.

Note: This safety feature has no effect on the motion of the Durham deformable mirror (DM) or fast beam-steering mirror (FSM), nor any component of the electronics associated with NAOMI.

For more information, see the NAOMI mechanism software safety page.

Commands to the pickoff assembly

Before any operations are performed on this assembly, it must be initialised by left-cicking the Mode button "Init", followed by Directive "Start". The user should see the status display "Command accepted - record state is busy". The record state will return to "idle" when initialisation is complete.

For all commands, in normal operation the user should see the status display "Command accepted - record state is busy" when the command starts. When the command is complete, the record state should change to "idle".

A status of "Command rejected" indicates that the command was not started. A status of "record state is error" indicates that the command did not complete succesfully. In both cases an error message will indicate the cause of the failure.

After initialisation, the mechanism should be indexed. This is started by clicking the "Index" Mode followed by "Start" Directive. The assembly then embarks on a somewhat complicated procedure designed to ensure that, no matter what the current configuration of the instrument, the five stages are able to index to their correct locations. These locations are:

The pickoff x-stage will index to its (absolute) upper limit. This can only be achieved if the fore-optics stage is clear of this location on the linear slide. In order to achieve this, fore-optics stage is moved to its (absolute) upper limit which, in turn, requires that the CCD stage has been moved clear of this location. Thus, the first operation required is to move the CCD stage to it's lower limit, and then move the CCD stage clear of the fore-optics upper limit.

The pickoff y-stage will index to its upper limit switch.

The fore-optics stage will index to its lower limit, with the pickoff x-stage in place at its upper limit. This position is not the same as the (absolute) lower limit of the fore-optics stage.

The lenslet wheel stage will index to its home switch.

The CCD stage will index to its lower limit, with the fore-optics stage in place at its index position. This position is not the same as the (absolute) lower limit of the CCD stage.

The procedure, therefore, is:

CCD stage to lower limit
CCD stage to safe location
Fore-optics stage to upper limit
Index pickoff x-stage to upper limit
Index fore-optics stage to lower limit
Index CCD stage to lower limit
Index pickoff y-stage to upper limit
Index lenslet wheel to home switch

Following the index procedure each device is moved, in turn, to a default (safe) location from which normal operation can resume. The default stage (names) positions and velocities are loaded into the pickoff assembly screen.

The interested reader is advised to examine the NAOMI hardware to discover why such a convoluted procedure is required.

After index operation has been performed, the assembly is now available for normal use. The user should left-click the "Move" Mode button. Thereafter, clicking the "Start" Directive button will cause the assembly to move the pickoff stages to their specified locations. The assembly attempts, through the use of named positions, to maintain focus at the CCD cameras as the pickoff mirror is moved, or the lenslet selection altered (see below).

The "Stop" Directive button can be clicked at any time to bring the assembly to a controlled stop.

Parameter input/output

The five "Demand Position" input fields are provided for the user to configure the assembly. Input to the four linear stages is in mm, with the lenslet wheel positions in degrees. "Demand Velocity" fields can also be modified by the user, although the assembly will attempt to calculate appropriate velocities if allowed to do so. The following values should be used as the demand positions for each device:

Pickoff x-stage: the required x offset (mm) of the pickoff mirror from center-of-field.

Pickoff y-stage: the required y offset (mm) of the pickoff mirror from center-of-field.

Fore-optics stage: any required offset (mm) from the position at which the collimating lens is focussed on the pickoff mirror.

Lenslet wheel: the wheel angle (degrees) for the required lenslet selection.

Camera stage: the focal length (mm) of the selected lenslet, plus any required additional offset.

Given these demand positions, the assembly uses the following algorithm to determine the required locations of each device:

(pickoff x-position) = (pickoff x-demand) + (pickoff x-base)

(pickoff y-position) = (pickoff y-demand) + (pickoff y-base)

(fore-optics position) = (fore-optics demand) + (collimator focal length) + (fore-optics offset) + (pickoff y-demand) + (pickoff x-position)

(ccd stage position) = (ccd stage demand) + (ccd focal length) + (ccd stage offset) + (fore-optics position)

(lenslet wheel position) = (lenslet wheel demand)

where: (pickoff x-base) and (pickoff y-base) are the distances from the pickoff mirror index positions to the centre-of-field position, (collimator focal length) and (ccd focal length) are the focal lengths of the collimating lens and camera focaussing optics, respectively, and (fore-optics offset) and (ccd stage offset) are pre-determined offsets related to the mechanical configuration of the wavefront sensor.

The net result of this calculation is that, provided the camera stage demand position correctly reflects the focal length of the selected lenslet and the fore-optics stage demand position is input as zero, the CCD cameras should maintain good focus on the guide star image whilst the pickoff mirror is moved across its field-of-view.

"Motion Velocity" input fields are provided for the user to specify the required motion velocity for each device. The user can leave these at their default (post-index) values if desired, or modify them within the velocity limits of each device.

For the user to specify each velocity in this way, the selector menu at the bottom of the screen should instruct the assembly "Do not calculate motion velocities, using...". Right-click and hold to modify this selector if required.

If the user changes the selector menu to "Calculate motion velocities, using..." the assembly will fill the velocity fields, before every move, with the values required for all five devices to complete their motions in the time specified. This can be helpful if the user requires the wavefront sensor to maintain focus during a long move of the pickoff, but requires careful selection of the motion time if the derived motion velocities are to remain within allowed limits.

The "Current Position" indicator fields display the eventual position, from the index location, for each device in the assembly.

Named positions for the pickoff assembly

Operation of the pickoff assembly is greatly facilitated by the use of named positions in the "Demand Position" fields.

The specifications of these named positions, along with values for the offsets used by the assembly (see above), are determined by the software during initialisation using the pickoff assembly lookup table. Details of the lookup tables used by the NAOMI mechanism control software are available on the NAOMI mechanism control software lookup tables page included in this documentation. We describe the named positions for the pickoff assembly in more detail here.

At the time of this writing, the defined "named position" string are as follows:

The Pickoff x-stage has one named position. "center" has the numberical equivalent 0.0 and places the pickoff at the center of field in the x direction.

The Pickoff y-stage has one named position. "center" has the numberical equivalent 0.0 and places the pickoff at the center of field in the y direction.

The Fore-optics stage has one named position. "focussed" has the numerical equivalent 0.0 and locates the fore-optics stage such that the collimating lens is focussed on the pickoff mirror.

The Lenslet wheel has six named positions, for selecting each of the installed lenslet arrays (and other components mounted in the wheel). Accepted strings are "reticule" (3.042), "f195_1.1" (63.35), "doublet" (123.255), "f95_1.1" (183.5), "f195_1.1_offset" (243.255), "f95_2.2" (303.369).

The Camera stage has named positions for each of the lenslet wheel named positions, giving the focal lengths of the installed components. The names are "reticule" (209.0), "f195_1.1" (209.0), "doublet" (211.0), "f95_1.1" (105.0), "f195_1.1_offset" (209.0), "f95_2.2 (209.0).
In normal use, however, the string "focussed" should be used. This causes the assembly to select the correct focal length, based on the (valid) named position contained in the lenslet wheel demand position field.
Two further names are provided for engineering purposes: "pupil" has the numerical equivalent 0.0 and "near_pupil" has the equivalent 10.0. They are used to place the camera stage at (or near) the pupil focus of the wavefront sensor.

With the strings "focussed" in place for the fore-optics and camera stages, the user is free to alter the named lenslet wheel selection and modify the pickoff x and y positions, confident that the software will keep the wavefront sensor optics in focus.

Menu items

Five items are provided in the Device displays menu - to access the deviceControl screens for each of the pickoff assembly devices.

Status and Higher displays menus give access to further assembly information and the top-level screens, respectively.

Pickoff x-stage device

Pickoff y-stage device

Fore-optics stage device

Lenslet wheel device

CCD stage device

Up to Wavefront sensor to return to the WFS top-level men screen.

Up to Mechanism control to return to the NAOMI mechanism control top-level menu screen.