NAOMI mirror-flattening
NAOMI technical information ·
WHT ·
Astronomy ·
ING
Introduction
NAOMI corrects errors in the incoming wavefront by moving in piston, tip
and tilt the individual
segments of the deformable mirror (DM).
The mirror must be flattened before observing begins, by configuring
the individual elements so that the DM produces
on the science detector an image of a focal-plane pinhole
which is as near diffraction-limited as possible.
This configuration is stored and can be recalled as a starting position
whenever required.
The configuration is not quite physically flat, because it
also corrects for any
wavefront errors not common to the wfs and science-detector
light paths (non-common-path errors).
The flattening procedure is described below, and a recipe is given
on the NAOMI setup page.
When NAOMI is first set up, this full procedure (laser flat, white-light flat
and simplex) is required. A simplex through broad-band H filter usually
serves well for both J and K bands.
From one afternoon to another, the mirror is fairly stable, and it probably
suffices to re-simplex each afternoon from the previous day's simplex.
The degradation of mirror shape during the night seems (Dec 01) to be largely
in tilt, and can be recovered by floating the Nasmyth bench and
laser-flattening with z gain set to zero.
Several bizarre mirror-flattening problems have been traced to lack of
spectral diversity.
E.g. if the white-light flattening is carried out using light passed by
a coloured dichroic (rather than a transparent plate), it usually fails.
If a simplex is carried out through a narrow-band (rather than broad-band)
filter, it can yield a 'split' mirror
(some areas raised with respect to others, when compared to the
broad-band simplex) and multiple images on the science camera.
It's not yet clear whether mirror shape degrades faster when the mirror
is in constant use (good weather) than when it's idle (dome closed).
Laser flattening
The DM is viewed in monochromatic light by the double-pass FISBA interferomter
(light exits and returns through same lens, and is compared with the reference
beam via a beam-splitter).
The FISBA PC analyses the image, and adjusts the elements in tip-tilt and piston
to minimise the number of fringes across each element.
This leaves the mirror flat except for an n/2-wavelength ambiguity in piston
positions.
The manual tweaking takes ~ 10 minutes, the software
procedure ~ 5 minutes.
White-light flattening
The white-light flat resolves the ambiguity in piston positions left by the
laser flat.
The mirror is illuminated by white light emerging from the focal-plane pinhole.
The system is first configured (using a lenslet array with a half-cell shift)
such that each spot includes light from 2 DM elements adjacent in the Y direction,
and alternate rows of the DM are stepped in piston (the intervening rows remain
stationary) to allow the relative piston shifts between elements adjacent in Y
to be determined.
This step is then repeated in the orthogonal direction (by moving the DM 3 mm
parallel to its surface), with alternate columns of the DM
then being stepped in piston.
This procedure yields a mirror which is close to being physically flat.
The image of the pinhole on the science camera should now be nearly diffraction-limited,
with some distortions due to defects in the alignment.
The white light may not differ very much from the laser flat, particularly
if the laser flattening was started from a previous white-light flat.
In this case, most of the DAC values will be identical to those in the
laser flat.
This procedure takes about 15 minutes.
Simplex optimisation
Non-common-path errors are fixed differences in the
wavefront distortion between the WFS and science arms.
They are removed by configuring the mirror to optimise the image of
the pinhole on the science detector. In practice, this step also removes
residual misalignments in NAOMI.
For each of 228 random configurations of the DM, the procedure
measures the square of the intensity I^2 falling within a fixed aperture
on the detector. These 228 configurations can be represented by 228
points in a 228-dimensional space (228 = 76 * 3).
The point with the worst I^2 is selected, and its parameters reflected
through the origin of this space. The I^2 for this is measured, i.e.
one configuration is deleted, one added.
This step is repeated several thousand times.
This simplex algorithm (developed at CERN) is model-free, and assumes
only that there is a minimum to be found.
After optimisation, the spots will no longer be on a regular grid
on the WFS, but will still fall within the sub-apertures.
These are now the positions to which the spots should be driven to achieve
an optimum image with NAOMI, and the offsets from the centres of the cells
on the WFS are recorded.
The non-common-path errors were formerly (ELECTRA) removed by
adding zernike terms to the DM manually to optimse the PSF, but the simplex
optimisation gives better results (the Zernike
terms may need to be of very high order).
The white-light flat is needed before simplex optimisation to avoid
the latter getting stuck in a local minimum.
Simplex optimisation takes a minimum of 20 minutes.
De-staircasing
There remains a staircase effect on the DM: the segments are all
parallel, but there can be a smooth gradient in piston position across
the mirror. This is invisible to the laser.
It manifests itself on the science images as
spots along a cross centred on the pinhole image, at the same
positions as spots from the DM interference pattern, but
*asymmetrically* distributed (e.g. all left or all up).
The intensity of the spots is typically < 1% of peak intensity.
The staircase is removed by tweaking the gradient of the DM in software
while viewing the pinhole image on the science camera.
De-staircasing takes a few minutes.
NAOMI technical information ·
WHT ·
Astronomy ·
ING
