Download 3. Daylight Mapping

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MEMORANDUM
Doc: MAD-TRE-0038
Date: 10.08.07
From: C. Arcidiacono, J. Farinato, R. Ragazzoni, M. Lombini, G. Gentile and E. Diolaiti
Subject: Layer Oriented Wavefront Sensor Coordinate Mapping
Version: 1.0
1. INTRODUCTION
The scope of this technical report is to define the procedure for coordinate mapping of
MAD and LOWFS systems at the VLT Nasmyth platform, during daylight, and on sky.
The following MAD functions move in Equatorial Coordinates (Right Ascension and
Declination):
 CUP1 (CUX1 & CUY1)
 CUP2 (CUX2 & CUY2)
 PUP[1,2,3,4,5,6,7,8] (PUX1-8 + PUY1-8)
 IRCP (IRCX & IRCY)
The motion of the functions can be either absolute (RAabs: hh mm ss.s, DECabs: dd mm
ss) or relative in arcseconds along RArel and DECrel.
Additionally the offset of the Optical Derotator (ODER) should be determined in order to
have the correct (user defined, but fixed somewhere hard coded in the control software to
be 0, see mdiINS_PUP.c, SIGN_*=0) coordinates orientation.
Finally the Acquisition Camera (AC) displays the full FoV and its images have to be
mapped in Equatorial Coordinates too.
We consider here only relative coordinates mapping where the motion is in arc seconds.
In principle we will not use CUP2.
2. CONCEPT
The basic idea is to refer the AC (Acquisition Camera), IR (CAMCAO) and PU (Pyramid
system) to X and Y motions of the Calibration Unit 1 (at the entering F/15 focal plane) in
daylight, and to the motion of the telescope during on sky operations.
The motion in daylight will be first measured in mm and then translated in RA and DEC
arcseconds, supposing Y corresponding to DEC and X to RA and axis perpendicular: the
optical derotator will be moved in order to realize this condition.
The X & Y motions of the 8 pyramids will refer first to a global coordinate system and
then this system is mapped to Calibration Unit 1 one.
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In order to properly map the AC we build a coefficients matrix to taking into account
translation & axis rotation and distortion (stretching up to the second order), with respect
to Calibration Unit1 first and then to telescope coordinate system.
For the IR will be computed the axis rotation and centring with respect to the optical axis,
with respect to Calibration Unit1 first and then to telescope coordinate system.
For each Pyramid will be computed axis centring, rotation and axis directions (the latter
is already known), with respect to a global pyramid coordinate system where the 4
quadrants are defined. The global coordinate system then is with respect to Calibration
Unit1 first and then to telescope coordinate system.
3. DAYLIGHT MAPPING
3.1. Preliminary operations
For daylight mapping the input FoV (before the Optical Derotator) is not rotating and is
not possible to make it rotating. Without loosing of generality, we simulate an
instantaneous situation where the input FoV is oriented with North pointing vertically
(i.e. along gravity vector) upward w.r.t. the MAD bench. When pointing on the sky, this
situation is achieved with the telescope looking at any point on the sky laying on the 2
central meridian of the observatory (i.e. the vertical meridian passing through Zenith and
identifying the North and South points on the horizon). The following conditions must be
set before and maintained during the laboratory mapping process:
 RTC in Open Loop status
 DMs in “flat position”;
 TT-Mount in aligned position (from optical alignment);
 CUP1 with single mode central fibre illuminated;
3.2. Origin of Coordinates
The origin of the coordinates (OoC) is set to be CUP1. All the coordinates’ mapping is
relative to CUP1 position.
The CUP1 is placed with its central fibre at optical axis (position given by optical
alignment). All the other functions (PUP[1,2,3,4,5,6,7,8], IRCP, CUP2) are placed in
order to be centred with the central fibre of CUP1. For PUP[1,2,3,4,5,6,7,8] (PUP
hereafter) the centring template on fibre will be executed, for IRCP a user defined
position will be chosen (could be the one from optical alignment).
Once the functions have been centred on the fibre their encoder positions will be
recorded. For each function these encoder positions represent the origin of relative
coordinates (RArel=0”, DECrel=0”). For the AC the OoC will be the position on the CCD
of the image of the fibre of CUP1 when located on the optical axis.
3.3. Coordinates Orientation
The relative coordinates orientation is defined with the North (DEC positively increasing)
pointing upward on the IR camera detector as displayed by the RTD.
Important note: the direction of the East (RA positively increasing) is not known (ASK
TO ENRICO MARCHETTI: DONE! EAST is at left with NORTH pointing up)
MAD will point on the sky. For this reason the mapping will be implemented for both the
condition of East pointing rightward and leftward w.r.t. the North direction.
3.4. Coordinates Scale
The reference for the relative coordinates scale is CUP1: 0.582 mm/arcsec or
1.719arcsec/mm. All the other functions will base their scale w.r.t. CUP1.
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3.5. Mapping Procedure
Remind that the input FoV is simulated to have the North up. The function CU2M must
be out.
3.5.1. ODER Offset
1. Record the position of the fiber’s image (IRorigin) on the IR camera detector (IR
camera static at its origin coordinate position);
2. Move CUY1 (negative direction to move the fiber upward) by the equivalent of
25” or 14.550 mm and record the position of the fiber’s image on the IR camera
detector (not moving the IR camera);
3. Rotate ODER in order to align the 25” shift along the vertical of the detector
pointing upward with an accuracy of ±1px (at the 25” off axis point); this step
must be repeated iteratively (back-forward from 0” to 25”) until the accuracy is
reached; record the ODER offset position.
3.5.2. IRCP Mapping
For the IRCP the ODER offset is always kept at its new position (i.e. giving North up on
the IR detector) and it is not tracking for FoV rotation. The function CU2M must be out.
1. Measure the plate scale on the IR camera detector (arcsec/px) using the data
obtained in Sect. 2.5.1, step 3;
2. Set East direction: put back CUY1 to 0” and move CUX1 by +25” or +14.550
mm (or -25” or -14.550 mm) and record the position of the fiber’s image on the
IR camera detector (not moving the camera);
3. Put CUY1 up by 25” CUX1 to 0” and move the camera both in IRCX and IRCY
in order to put the fiber’s image on IRorigin with an accuracy of ±1px and record
the IRCX and IRCY position; repeat this measurement for a 33 grid of CUP1
positions spaced by 25” centered at CUP1 OoC;
4. Measure IRCP scale (arcsec/mm) and orthogonality of the CUP1 motion (possible
optical distortion be present);
5. Compute the IRCP to CUP1 conversion matrix.
3.5.3. PUP Mapping
For the pyramid mapping we have to fix first the position and angle of the absolute
coordinate system to which all pyramid xy coordinate system is referred. The most
common choice is to have the absolute system to be the projection of the CUP1 coodinate
system (North Up, East left, Centered in the Optical axis = OoC). This choice can be
accepted if the 4 quadrants defined by the pyramid stage motions are similar to the 4
quadrants defined by the CUP1 axis projection at the F/20 entering focal plane through
the optical derotator, moved to align the CUP1 to the IR camera axis. Otherwise an
arbitrary absolute coordinate system will be defined in order to be similar to the one
defined by the mechanical quadrants.
The absolute coordinate system is needed by the collisions prevention control software.
For PUP the procedure below must be repeated for eight times, one per Pyramid. The
ODER offset is kept at its new position (i.e. giving North up on the IR detector) and it is
not tracking for FoV rotation. The function SELO must be in and SESH and CUM2 out.
3.5.3.1. Case A
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We suppose that all pyramids can be positioned on the optical axis. For each pyramid we
set a grid of 33 point 20” arcsec spaced (11.64mm) covering the corresponding quadrant
and in which one corner is the OoC (0,0)
1. The East direction is set: east is at left!
2. Repeat on the 33 grid of CUP1 positions spaced by 20”: move the PUP both in
PUX and PUY up to catch the fiber’s light and center it; record the PUP positions;
3. Measure PUP general plate scale (arcsec/mm), it is supposed to be the same for all
the pyramids.
4. Compute for each PU# axis origin (Tx and Ty) and rotation (theta) with respect to
the global pyramid coordinate system. Being FoV centre achievable for all
pyramids it is rationale to define the global coordinate system (needed for
collision control) identical to the projection of the CUP1 movement and centre
(TxA=0,TyA=0);
5. Finally compute the global PU coordinate system to CUP1 conversion matrix.
3.5.3.2. Case B
In this case we suppose that the FoV centre is not achievable by all pyramids. In this case
a similar grid 33 grid 20” spaced should be defined, but in this case one corner cannot
be the centre of the FoV and TxA and TyA could not be 0.
Mapping then is identical…. To Case A.
In principle will be possible to take into account of field distortion on the F/20 Focal
plane by using a coordinate transformation:
x' 
n  3 n 3
 a
i 0,n j0,n
y' 
i, j
xi y j
j, j
xi y j
n  3 n 3
 a
i 0,n j0,n
But, in principle, aberrations are small and we will fix a10 = 1 and 0 all the other
coefficients.
3.5.4. CUP2 Mapping – NOT USED
For the CUP2 the ODER offset is always kept at its new position (i.e. giving North up on
the IR detector) and it is not tracking for FoV rotation. In this procedure the position of
the fiber’s images on the IR detector obtained in Sect. 2.5.2, step 3 are used. The function
CU2M must be in.
1. For each position of the fiber’s image: move CUX2 and CUY2 until the fiber’s
images is superimposed to the one obtained in Sect. 2.5.2, step 3 with an accuracy
of ±1px;
2. pay attention to North and East directions; record CUP2 position;
3. Measure CUP2 plate scale (arcsec/mm);
4. Compute the CUP2 to CUP1 conversion matrix.
3.5.5. AC Mapping
A simmetric matrix define the aberration of the Acquisition Camera:
x' 
n  3 n 3
 a
i 0,n j0,n
y' 
i, j
xi y j
j, j
xi y j
n  3 n 3
 a
i 0,n j0,n
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Using this matrix we cannot take into account of decentering of the optical axis w.r.t.
CCD central pixel and camera rotation, then this two parameter has to be previously
computed and then used as x and y to compute x’ and y’.
For the AC the ODER offset is always kept at its new position (i.e. giving North up on
the IR detector) and it is not tracking for FoV rotation. The functions SESH, SELO and
CU2M must be out
1. Put a single mode fibre on the optical axis; take note of the corresponding pixel on
the camera;
2. Set North direction: move CUY1 up by 50” or 29.100 mm (CUX1 on 0”) and
record the position of the fiber’s image on the CCD;
3. Set East directions: they correspond to the ones of put CUY1 at 0” and CUX1 at
±50” or ±29.100 mm; record the position of the fiber’s image on the CCD;
4. Measure AC plate scale (arcsec/px);
5. Compute the AC to CUP1 conversion matrix (a00 will be zero).
4. ON-SKY MAPPING
4.1. Preliminary Operations
Differently from daylight training, in on-sky calibration the input FoV (before optical
derotator) is rotating and its orientation change with telescope pointing direction. For this
reason it is crucial that the FoV is stabilized for rotation tracking, that is, the ODER must
be fully operative at the moment of the implementation of this procedure.
The following conditions must be set before and maintained during the on-sky mapping
process:
 DMs in “flat position”;
 TT-Mount in aligned position (from optical alignment);
 CUP1 with single mode central fiber illuminated;
 CUP2 with single mode central fiber illuminated;
 Optical Derotator tracking, no specified offset is required;
 Telescope pointing and tracking a bright star far from Zenith to avoid eventual
derotation errors.
 The on-sky calibration will result easier if the laboratory one has been already
implemented and it will be possible to skip some procedure’s steps.
4.2. Origin of Coordinates
The origin of the coordinates is set at the position of central fiber of CUP1 located at the
AD’s optical axis.
To set the origin of coordinates for SHP, IRCP, CUP2 and AC will be the same ones
obtained with optical alignment (see Sect. 2.2). In any case it will be useful to quickly
check the correctness of those positions with the following steps:
1. With IRCP at its OoC acquire a long exposure (averaged) image of the star
(CU2M out);
2. Offset the telescope pointing to put the star at IRorigin with an accuracy of ±1px;
3. Move PUP to their OoC position and check if the star is centered measuring a
long exposure tilt with the Pyramid Sensor; ± 0.05 pixel offset per axis is
acceptable (SELO in, SESH out);
4. If the PUP is shifted by more the above accuracy re-center them with the
centering template and record the new PUP encoder positions; these are the new
OoC for PUP;
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5. Acquire an image of the star with the AC (SELO, SESH and CUM out) and
compare its position to the AC OoC; if the position is shifted by more than 0.1
pixel record the new position; this is the new OoC for AC;
6. Put CUP2 on its OoC (fiber light on, CU2M in) and acquire an image with the IR
camera; if the position is shifted by more than 1px then center CU2P and record
the new position; this is the new OoC for CUP2.
4.3. Coordinates Orientation
The relative coordinates orientation is defined with the North (RA positively increasing)
pointing upward on the IR camera detector as displayed by the RTD. The direction of the
East is retrieved accordingly.
4.4. Coordinates Scale
The reference for the relative coordinates’ scale is the telescope offset pointing. All the
other functions will base their scale w.r.t. telescope pointing offsets.
4.5. Mapping Procedure
4.5.1. ODER Offset
1. Put the IR camera at its origin position;
2. Move the telescope to put the star located at IRorigin on the IR detector;
3. Set the North direction: offset the telescope DEC -25” (South) and record the position
of the star’s image on the IR camera detector (not moving the IR camera);
4. Rotate the Optical Derotator in order to align the 25” shift along the vertical of the
detector pointing upward with an accuracy of ±1px (at the 25” off axis point); this
step must be repeated iteratively (back-forward from 0” to 25”) until the accuracy is
reached;
5. record the Optical Derotator Offset
4.5.2. IRCP Mapping
From this point forward the Optical Derotator Offset is always kept at its new position
(i.e. giving North up on the IR detector) while it is continuously tracking for FoV
rotation. The function CU2M must be out.
1. Measure the plate scale on the IR camera detector (arcsec/px) using the data
obtained in Sect. 3.5.1, step 3;
2. Set East direction: go back with telescope to 0” DEC offset, then offset it RA -25”
(West) and record the position of the star’s image on the IR camera detector (not
moving the camera);
3. Move the camera both in IRCX and IRCY in order to put the fiber’s image on
IRorigin with an accuracy of ±1px and record the IRCX and IRCY position; repeat
this measurement for a 33 grid of star’s positions spaced by 25” centered at
origin;
4. Measure IRCP scale (arcsec/mm);
5. Compute the IRCP to RA/DEC conversion matrix.
4.5.3. PUP Mapping
For PUP the following procedure must be repeated for eight times, i.e. one per Pyramid.
The ODER offset is kept at its new position (i.e. giving North up on the IR detector)
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while it is continuously tracking for FoV rotation. The function SELO must be in and
SESH and CUM2 out.
1. Offset the telescope by DEC -50” (South); move the PUP# both in PUX# and
PUY# up to catch the fiber’s light and center it; record the PUP# XY positions;
repeat this measurement for a 33 grid of star’s positions spaced by 50” centered
at the origin;
2. Measure PUP plate scale (arcsec/mm);
3. Compute the PUP to RA/DEC conversion matrix.
4.5.4. CUP1 Mapping
For CUP1 the ODER offset is set to the value obtained for laboratory mapping and it is
not tracking for FoV rotation. This operation can be done also in daylight provided that
the position of the star on the IR detector obtained with 25” North (Sect 2.5.1, step 3) and
25” East (Sect 2.5.2, step 2) offsets have been saved. CU2M must be out.
1. Switch on the CUP1 single mode fiber located at its center;
2. Put IRCP at its OoC;
3. Move CUP1 until the fiber’s image on the IR detector will be coincident to the
one of the star with 25” North (DEC+) offset; repeat iteratively up to an accuracy
of ±1px; record the CUP1 position;
4. Move CUP1 until the fiber’s image on the IR detector will be coincident to the
one of the star with 25” East (RA+) offset; repeat iteratively up to an accuracy of
±1px; record the CUP1 position;
5. Measure CUP1 plate scale (arcsec/mm);
6. Compute the CUP1 to RA/DEC conversion matrix.
4.5.5. CUP2 Mapping – NOT USED
For CUP2 the ODER offset is set to the value obtained for laboratory mapping and it is
not tracking for FoV rotation. This operation can be done also in daylight provided that
the position of the star on the IR detector obtained with 25” North (Sect 2.5.1, step 3) and
25” East (Sect 2.5.2, step 2) offsets have been saved. CU2M must be in.
1. Switch on the CUP2 single mode fiber located at its center;
2. Put IRCP at its OoC;
3. Move CUP2 until the fiber’s image on the IR detector will be coincident to the
one of the star with 25” North (DEC+) offset; repeat iteratively up to an accuracy
of ±1px; record the CUP2 position;
4. Move CUP2 until the fiber’s image on the IR detector will be coincident to the
one of the star with 25” East (RA+) offset; repeat iteratively up to an accuracy of
±1px; record the CUP2 position;
5. Measure CUP2 plate scale (arcsec/mm);
6. Compute the CUP2 to RA/DEC conversion matrix.
4.5.6. AC Mapping
For AC the ODER offset is kept at its new position (i.e. giving North up on the IR
detector) while it is continuously tracking for FoV rotation. The function SESH, SELO
and CUM2 must be out.
1. Offset the telescope by DEC -50” (South) and record the position of the star’s
image on the CCD; repeat this measurement for a 33 grid of star’s positions
spaced by 50” centered at the origin;
2. Measure AC plate scale (arcsec/px);
3. Compute the AC to RA/DEC conversion matrix.
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5. CONCLUSIONS
The procedure for MAD coordinates mapping has been fully described. Both day-light
and on-sky mapping have been considered.
This document has to be intended in continuous review since the procedures are
undergoing to continuous review.
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