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EUROPEAN Organisation Europäische SOUTHERN Européenne pour Organisation für OBSERVATORY des Recherches Astronomiques astronomische Forschung in dans l'Hémisphère Austral der südlichen Hemisphäre ESO, Karl-Schwarzschild-Straße 2, D-85748 Garching bei München 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. 1 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. 2 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 33 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 3 We suppose that all pyramids can be positioned on the optical axis. For each pyramid we set a grid of 33 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 33 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 33 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 j0,n y' i, j xi y j j, j xi y j n 3 n 3 a i 0,n j0,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 j0,n y' i, j xi y j j, j xi y j n 3 n 3 a i 0,n j0,n 4 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; 5 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 33 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) 6 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 33 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 33 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. 7 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. 8