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NAOMI
NAOMI is the Adaptive Optics system for the WHT, installed at the GRACE Nasmyth focus.
NAOMI stands for `Natural-guide-star AO for Multi-purpose Instrumentation'. The Adaptive correction is performed by a 76-element (228 degrees of freedom) segmented deformable mirror. The distortion of the wavefront, and its tip-tilt movement, is measured using the Wavefront Sensor (WFS), which consistes of a Shack-Hartmann wavefront sensor (built at RAL, based on an 80x80 EEV CCD39). The most important lenslet array in the Shack-Hartmann is a 10x10 array, and similarly the DM segments are laid out on a 10*10 grid with the corners missing.

The tip-tilt motion, also caused by the atmosphere, are removed separately by a Fast Steering Mirror (FSM), at the rate of several tens of times per second. Meanwhile, every second, any trend in the tip-tilt motion in one particular direction is corrected by sending guiding packets to the telescope.

More information: NAOMI Homepage · Techical Information

In twilight run SloDAR!

Startup
TCS:
Check that the Nasmyth Turret is in the GRACE position. If not, park at zenith, close the petals and press the GRACE button.
On the TCS::
  • USER> STATION GRACE_IRDEROT
  • USER> INSTRUMENT NAOMI
  • USER> AGSELECT NAOMI
  • Zeroset as normal
  • USER> CAL LAST
  • USER> FOCUS 98.60 (Ingrid, 2011)

Preparation

Telescope focus:
A change of focus expands/contracts the Shack-Hartmann spot pattern, which should fit exactly into the WFS grid. The SA will perform the focus run.

Rotator center and calibrate:
First night after an instrument change only, unless the pointing is bad. Notes say that the rotator centre is not done by the TO (???) since a pickoff mirror inside NAOMI can be accurately positioned at the optical rotation axis of the Nasmyth focus. However in July 2011 the rotator centre was measured by the TO using the acquisition TV (AG3) (see orientation below), and the calibrate done on this position.

Observing

Warning: Never slew or even offset when the AO loop is closed!!
Otherwise you will mess up the Deformable Mirror (DM), which is a bit delicate! The loop can be opened and closed manually at the TopGui control or using "loop open/close" command on taurus.

Acquisition:

uDAS TV:
For acquisition move the NCU (Naomi Calibration Unit) to AcqCam-position.
The FOV is approximately 3' in diameter. With PA=0 North is to the left and East is at the bottom.

The arrows indicate the direction a star moves on uDAS TV when in XY handset mode:

       +------+
       |      |              PA=0:  +--------+
       | <--- |                     |        |
       |      |                     |        |
       |      |                   N |        |
+------+------+------+              |        |
|      |      |      |              |        |
|  |   | ---> |   ^  |              +--------+
|  v   |      |   |  |                   E
|      |      |      |
+------+------+------+

uDAS TV commands and startup:
The camera (AG3) can be used as a conventional science camera as far as uDAS is concerned:
DAS> run AG3 2 "acq of NGC1162"
DAS> finish AG3
The first command will save the image on /obsdata..., the second forces immediate read-out.

  • On LEO or CETUS, log in as 'whtobs'.
  • Type 'obssys' or 'obssys dev' in the pink window and select option 'manual camera selection'.
  • Type 'startobssys' in the pink window.
  • To start the camera running as a TV then type 'startudasag AG3' in pink window, followed by option '9'.
  • Windows and binning can be set up using the usual uDAS syntax. This is useful as it speeds up read-out and therefore acquisition,
    e.g. window AG3 1 "[400:780,200:600]".
  • The reference position for acquiring on the WFS is x= 575 and y= 476 (01/07/2005) (no window, no binning).
Move the star to the reference position on the TV. Once the object is acquired the NCU is moved to Science-position. Now the star should appear in the Wavefront Sensor. You might need to tweak the position according to the instructions of the person in charge of NAOMI.

There is a new tool called Adquisition Tool running on Taurus, this new tool allows the astronomer to center the objects in the center of OASIS or INGRID. This Tool uses the acquisition camera AG3.

IMPORTANT: When the Acquisition tool is going to be used is it necesary to stop the whatever acquisition from the computer that is running AG3. PC close to the Taurus monitors

The arrows indicate the direction a star moves on the WFS when in XY handset mode:

       +------+
       |      |
       |   ^  |
       |   |  |
       |      |
+------+------+------+
|      |      |      |
| -->  |   |  | <--  |
|      |   v  |      |
|      |      |      |
+------+------+------+

Autoguider:

Note that switch-box for autoguider has to be set to NAOMI!
The NAOMI Autoguider is sourced from an RS232 line from the Navis computer in the control room. Images are piped directly to the display tool. Guiding packets come from the AO loop, specifically from drift found in the Fast Steering Mirror (FSM), which takes out pupil motion (also called Tip-Tilt). Since there is no guiding when the AO loop is open, you don't need to turn AUTOGUIDE OFF, even when re-acquiring (unless AO loop has crashed).

The guide star can be the target itself ("self-referencing"), if the latter is sufficiently bright and sufficiently pointlike (< 1.5 arcsec, say, to be smaller than the sub-aperture guide boxes on the wavefront sensor). The guide star cannot be brighter than V = 4, or K ~ 5, because of saturation of the wavefront sensor or science detector respectively. There should be no stars of similar magnitude within ~ 5 arcsec of the guide star, to avoid confusing the wavefront sensor.

  • Closing guiding loop:
    USER> AUTOGUIDE ON 50 50
    The numbers are "pseudopixels", 50 is the center position of the Fast Steering Mirror.
  • Closing AO loop:
    The AO loop is closed using either either button on TopGui or
    SYS>loop close

Observing with the AO guide star off-axis:
The WFS assembly includes a "pick-off" mirror (a probe) that allows the AO guide star to be followed by the WFS off-axis, while the science target is on axis.
  • You start with the pick-off on axis and star light on the WFS.
  • Move the telescope to put the science target on axis.
  • Move X and Y positions of the pick-off to compensate for the telescope offset and to pick up the star light again. Moving the telescope North requires a +y movement of the pick-off, moving East a +x movement.
  • It is a good idea to do these movements step-by-step.
Miscellaneous
NAOMI Observing "modes":
NAOMI has 3 different Shack-Hartmann lenslet arrays, the choice depending on the seeing. The "good seeing array" and "poor seeing array" both have 10x10 lenslets, but the latter have much shorter focal length, so that they produce less spot movement. This should be used iif natural seeing is > 1.2 arsec approx. There is also a 4x4 lenslet array for "even worse" seeing.

Additionally, the WFS can be read out with 3 different binning arrangements: "full frame" (6x6), 4x4 and "quad-cell" (2x2), these numbers referring to the pixels assigned to each Hartmann spot. Increased binning allows you to increase signal-to-noise on fainter stars AND to read out faster (which should help with poorer seeing, but in NAOMI, has not yet been proved to do so...).

Each combination of lenslet array and binning is referred to (by NAOMI designers) as a "mode".

The most important ones are modes 1 and 2, while mode 8 was also tested (didn't work). Mode 1 corrresponds to ["good seeing" lenslets + full frame readout] and mode 2 is ["poor seeing" lenslets + full frame readout"]. Mode 8 is [4x4 lenslets + quad cell readout].

NAOMI Optical Optimisation:
The shape of the Deformable Mirror (DM) and the response of the Wavefront Sensor (WFS) are optimised using this procedure (see Frank Gribbin's notes for more detail):

  • Remove relative tilt  between the segments of the DM, using "Fisba" laser interferometer. The result is that the segments should be parallel, but may have step ("piston") differences.
  • Run "Simplex" (also called "Nathanisation") using the White-light interferometer, to remove "step" error between DM segments. This program allows quick read ("grab" mode) of INGRID, so that you can see the image becoming a sharp point.
  • Run procedures to remove remaining "staircase" errors, so that the image on INGRID consists of a single point ("staircase" causes multiple images).
  • Now that the DM is "tuned" to be flat and ALSO compensates for static optical aberrations in the INGRID optics, we can proceed to measure WFS "offsets" (distance of spots from "nominal" centers, which is where they would be if there were no optical aberrations...).  This is done using a pinhole light source. The "offsets" are different for each Shack-Hartmann lenslet array and each binning "mode" of the WFS readout. Once measured, they give the WFS its "starting points" and allow it to send corrections appropriate to the optical setup in use.
  • Test the whole setup using the "wavefront reconstructor" algorithm. This makes sure the response of the DM to tilt is to remain parallel. The result should be a stable image on INGRID when the FSM is moved a small distance.
Now ready to be adaptive!



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Last modified: 14 July 2011