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NAOMI: the adaptive optics for the auxiliary telescopes of VLTI

Profile image of Pablo GutierrezPablo Gutierrez

2018

Abstract

The New Adaptive Optics Module for Interferometry (NAOMI) is ready to be installed at the 1.8-metre Auxiliary Telescopes (ATs) at ESO Paranal. NAOMI will make the existing interferometer performance less dependent on the seeing conditions. Fed with higher and more stable Strehl, the fringe tracker will achieve the fringe stability necessary to reach the full performance of the second-generation instruments GRAVITY and MATISSE. All four ATs will be equipped between September and November 2018 with a Deformable mirror (ALPAO DM-241), a 4*4 Shack– Hartmann adaptive optics system operating in the visible and an RTC based on SPARTA Light. During the last 6 months thorough system test has been made in laboratory to demonstrate the Adaptive Optics and chopping capability of NAOMI.

Figures (20)
Figure 1: (left) AT2 on the CO station of the Very Large Telescope Interferometer (center) cut view of the AT and (right) cut view of the ROS with including in it the STar Separator. The NAOMI WES lays under the STS. NAOMI is integrated to an existing system and as such makes it very complex. Few example are shown here. Figure 1 (center and right) shows the different areas in which NAOMI will be physically implemented. The Corrective Optics replaces the M6 mirror position at the level of the azimuth of the telescope. The wavefront sensor and its relay optics are implemented in the lower part of the Relay Optical Structure (ROS) and replaces the STRAP sensor. The electronics is also implemented in several areas, one cabinet goes at the azimuth level, another other goes in the ROS. The last one and the RTC electronics goes in the Signal Cabinet implemented in the Transporter. NAOMI is also connected to remote workstation and data server implemented respectively in 2 different Computer Room. The first one being in the VLT Control Building and the second one in the VLTI building as seen on Figure 2. The NAOMI software is interfaced the same way as the STRAP system with the Auxiliary Telescope and the VLTI Instrument Supervisor System.
Figure 1: (left) AT2 on the CO station of the Very Large Telescope Interferometer (center) cut view of the AT and (right) cut view of the ROS with including in it the STar Separator. The NAOMI WES lays under the STS. NAOMI is integrated to an existing system and as such makes it very complex. Few example are shown here. Figure 1 (center and right) shows the different areas in which NAOMI will be physically implemented. The Corrective Optics replaces the M6 mirror position at the level of the azimuth of the telescope. The wavefront sensor and its relay optics are implemented in the lower part of the Relay Optical Structure (ROS) and replaces the STRAP sensor. The electronics is also implemented in several areas, one cabinet goes at the azimuth level, another other goes in the ROS. The last one and the RTC electronics goes in the Signal Cabinet implemented in the Transporter. NAOMI is also connected to remote workstation and data server implemented respectively in 2 different Computer Room. The first one being in the VLT Control Building and the second one in the VLTI building as seen on Figure 2. The NAOMI software is interfaced the same way as the STRAP system with the Auxiliary Telescope and the VLTI Instrument Supervisor System.
Figure 2: NAOMI network interfaces showing the connection to the VLT and VLTI Computer room.
Figure 2: NAOMI network interfaces showing the connection to the VLT and VLTI Computer room.
Figure 3: Control Hierarchy Layers: distribution of ATCS and NAOMI SW components NAOMI software is a component of the Auxiliary Telescope control software. It is composed of 4 parts, the Telescope Control Software, the Instrument Control software, the RTC and finally the Detector Control Software. It is implemented in three hierarchical layers as seen on Figure 3.
Figure 3: Control Hierarchy Layers: distribution of ATCS and NAOMI SW components NAOMI software is a component of the Auxiliary Telescope control software. It is composed of 4 parts, the Telescope Control Software, the Instrument Control software, the RTC and finally the Detector Control Software. It is implemented in three hierarchical layers as seen on Figure 3.
The NAOMI software is deployed on three workstations and one Local Control Unit as shown on the Figure 4 Figure 4: NAOMI computational nodes (<<WS>>, <<LCU>> stereotypes) and controlled devices (<<<Device>> stereotype) are shown in blue. NAOMI SW components deployed on each computational node are shown as attributes: NAOMI TCS, NAOMI ICS, NAOMI RTC, and NAOMI DCS. Physical location of the computational nodes and devices are labelled with <<VLTComputerRoom>>, <<VLTIComputerRoom>>, <<SignalCabinet>>, <<ROS>>, <<Altitude>> stereotypes.
The NAOMI software is deployed on three workstations and one Local Control Unit as shown on the Figure 4 Figure 4: NAOMI computational nodes (<<WS>>, <<LCU>> stereotypes) and controlled devices (<<<Device>> stereotype) are shown in blue. NAOMI SW components deployed on each computational node are shown as attributes: NAOMI TCS, NAOMI ICS, NAOMI RTC, and NAOMI DCS. Physical location of the computational nodes and devices are labelled with <<VLTComputerRoom>>, <<VLTIComputerRoom>>, <<SignalCabinet>>, <<ROS>>, <<Altitude>> stereotypes.
Tr he RTC Box design is based on 3 pipelines processing stages; the Wavefront Processing, the Wavefront reconstruction and the Control. The Wavefront processing unit manages a window at 64*64 meta-pixels on a 4*4 sub-aperture geometry and depending of the mode will use 2 different sub-aperture window size of 6*6 pixels (in closed loop) of 16*16 (in acquisition mode). The block diagram for the WPU is given in Figure 6. he wavefront reconstruction is performed by Matrix Vector Multiplication delivering a result in the modal space. It is based on 12 sub-aperture for the WFS. The system can reconstruct up to 21 modes but NAOMI system delivers its best performances with 14 modes. The control is based on 2 driver; the Infinite Impulse Response driver and the Anti Vibration Control driver. The HR driver takes care of the AO loop and of the chopping when the AVC driver focus on rejecting vibrations at particular fearniianpisae Tha TIR hlack diacram 16 chaum an Biaqnire 7 Tr
Tr he RTC Box design is based on 3 pipelines processing stages; the Wavefront Processing, the Wavefront reconstruction and the Control. The Wavefront processing unit manages a window at 64*64 meta-pixels on a 4*4 sub-aperture geometry and depending of the mode will use 2 different sub-aperture window size of 6*6 pixels (in closed loop) of 16*16 (in acquisition mode). The block diagram for the WPU is given in Figure 6. he wavefront reconstruction is performed by Matrix Vector Multiplication delivering a result in the modal space. It is based on 12 sub-aperture for the WFS. The system can reconstruct up to 21 modes but NAOMI system delivers its best performances with 14 modes. The control is based on 2 driver; the Infinite Impulse Response driver and the Anti Vibration Control driver. The HR driver takes care of the AO loop and of the chopping when the AVC driver focus on rejecting vibrations at particular fearniianpisae Tha TIR hlack diacram 16 chaum an Biaqnire 7 Tr
Figure 8: NAOMI RTC Box IIR open loop Chopping Block Diagram A pre-tilt is made on the mirror before the start of the chopping. It can run a chopping at up to 5Hz. The block diagran for the open loop chopping is shown on Figure 8.
Figure 8: NAOMI RTC Box IIR open loop Chopping Block Diagram A pre-tilt is made on the mirror before the start of the chopping. It can run a chopping at up to 5Hz. The block diagran for the open loop chopping is shown on Figure 8.
Figure 9: (left) 3D model of the Corrective Optics, (right) photo of the C.O. on a test bench. ica a eS ee — - - / _ N to 20 degC). The ALPAO DM7?241 provides the chopping and atmospheric turbulence correction capability. The device has a tip-til wavefront stroke of up to 50 xm PtV at ambient temperature. Its capability decreases linearly at lower temperature. This is calibrated in NAOMI. Piston stability and control is the second feature to study when a DM is used for interferometric purposes. The piston introduced by the DM is estimated by the influence functions calibrated in the laboratory. the DM shape is controlled piston-free to few percent. Even though the influence functions are expected to evolve with time, the piston ratio, (influence function piston over piston-free influence function amplitudes) is assumed to remain constant. hence the need of external measurement devices in operation is not foreseen.
Figure 9: (left) 3D model of the Corrective Optics, (right) photo of the C.O. on a test bench. ica a eS ee — - - / _ N to 20 degC). The ALPAO DM7?241 provides the chopping and atmospheric turbulence correction capability. The device has a tip-til wavefront stroke of up to 50 xm PtV at ambient temperature. Its capability decreases linearly at lower temperature. This is calibrated in NAOMI. Piston stability and control is the second feature to study when a DM is used for interferometric purposes. The piston introduced by the DM is estimated by the influence functions calibrated in the laboratory. the DM shape is controlled piston-free to few percent. Even though the influence functions are expected to evolve with time, the piston ratio, (influence function piston over piston-free influence function amplitudes) is assumed to remain constant. hence the need of external measurement devices in operation is not foreseen.
Figure 10 shows the 21s modes measured on one of the 5 deformable mirrors delivered for NAOMI as well as the be: obtained flat with a Wavefront of 14 nm RMS.
Figure 10 shows the 21s modes measured on one of the 5 deformable mirrors delivered for NAOMI as well as the be: obtained flat with a Wavefront of 14 nm RMS.
Figure 11: (Left): SHS configuration maps with in red the 16x16 pixels used in the full-geometry configuration, ir green the 6x6 pixels used in the AO-geometry. (right): WFS acquisition with Flat reference
Figure 11: (Left): SHS configuration maps with in red the 16x16 pixels used in the full-geometry configuration, ir green the 6x6 pixels used in the AO-geometry. (right): WFS acquisition with Flat reference
Figure 12: (left) WFS assembly. starting from the top: the PCU in brown, the lenslet mount in deep blue, the camera in grey, the Detector Assembly interfacing in green with the Field Stabilization stages in blue, (right) photo of the WES in the test bench and with the Neutral Density Filter Wheel on top of it.
Figure 12: (left) WFS assembly. starting from the top: the PCU in brown, the lenslet mount in deep blue, the camera in grey, the Detector Assembly interfacing in green with the Field Stabilization stages in blue, (right) photo of the WES in the test bench and with the Neutral Density Filter Wheel on top of it.
Figure 13: WFS mode#2 with acquisition frequency at 500Haz and EM gain=3 The detector selected for NAOMI is a commercial electro-multiplicative CCD camera: ANDOR ixon ultra 897. Thi camera has a chip with 512x512 pixels with a pixel size of 16 um. To increase the frame speed and the pixel size th camera will be operated in a 2x2 binned mode (implemented at low level). This means a camera of 256x256 mega-pixel: and pixel size of 32x32 um. This implies a pupil size diameter of 2.048 mm for the pupil area of 64x64 mega-pixels. Th area of interest will be read in a customized crop window mode in order to read the detector at the required speed (5O0( Hz). The camera offers a RON < | e- when the EM gain (L3 gain) is applied. Gain factors up to 1000 can be applied The dark of the camera has been measured with different gain and frequencies as seen on Figure 13 for one of the mode: used by NAOMI. The figure shows the mean of all frames, the standard deviation of the frames from which the mea has been subtracted, and the histogram of the darks frames values are given.
Figure 13: WFS mode#2 with acquisition frequency at 500Haz and EM gain=3 The detector selected for NAOMI is a commercial electro-multiplicative CCD camera: ANDOR ixon ultra 897. Thi camera has a chip with 512x512 pixels with a pixel size of 16 um. To increase the frame speed and the pixel size th camera will be operated in a 2x2 binned mode (implemented at low level). This means a camera of 256x256 mega-pixel: and pixel size of 32x32 um. This implies a pupil size diameter of 2.048 mm for the pupil area of 64x64 mega-pixels. Th area of interest will be read in a customized crop window mode in order to read the detector at the required speed (5O0( Hz). The camera offers a RON < | e- when the EM gain (L3 gain) is applied. Gain factors up to 1000 can be applied The dark of the camera has been measured with different gain and frequencies as seen on Figure 13 for one of the mode: used by NAOMI. The figure shows the mean of all frames, the standard deviation of the frames from which the mea has been subtracted, and the histogram of the darks frames values are given.
Figure 14: Transfer function for DM mode 3 (focus) and mode 14. The system test has been made on a test bench simulating the Auxiliary Telescope including the ROS. The turbulences have been produced by the Deformable mirror itself using a bases of 120 modes. Basically 10 times more than what is used for the correction (14 modes) The transfer functions of the system has been measured and compared to the theoretical one as seen in Figure 14. The linearity of the system has also been measured as seen in Figure 15. 7. SYSTEM TEST RESULTS
Figure 14: Transfer function for DM mode 3 (focus) and mode 14. The system test has been made on a test bench simulating the Auxiliary Telescope including the ROS. The turbulences have been produced by the Deformable mirror itself using a bases of 120 modes. Basically 10 times more than what is used for the correction (14 modes) The transfer functions of the system has been measured and compared to the theoretical one as seen in Figure 14. The linearity of the system has also been measured as seen in Figure 15. 7. SYSTEM TEST RESULTS
The ratio of correction on turbulences has been measured. The rejection functions are very similar for the 14 modes, anc in these simulated atmosphere conditions that are close to the limit ones specified for NAOMI, the cut-off frequency is around 23 Hz. The Figure 16 shows the ration of the PSD between closed loop and open loop for the 14 modes used by NAOMI. Figure 17 shows the same ratio for 5 different rotation of pupil. It demonstrate the efficiency of the numerica de-rotation implemented in NAOMI.
The ratio of correction on turbulences has been measured. The rejection functions are very similar for the 14 modes, anc in these simulated atmosphere conditions that are close to the limit ones specified for NAOMI, the cut-off frequency is around 23 Hz. The Figure 16 shows the ration of the PSD between closed loop and open loop for the 14 modes used by NAOMI. Figure 17 shows the same ratio for 5 different rotation of pupil. It demonstrate the efficiency of the numerica de-rotation implemented in NAOMI.
Figure 16: Ratio of the PSD in CL and OL for turbulence injected on the DM (equivalent to 1.4” seeing).
Figure 16: Ratio of the PSD in CL and OL for turbulence injected on the DM (equivalent to 1.4” seeing).
Figure 17: OL/CL rejection functions of the 14 corrected modes and for 5 position of the derotator implemented in the test bench to produce the rotation of pupil which will be seen by the WFS of NAOMI once implemented in the AT.
Figure 17: OL/CL rejection functions of the 14 corrected modes and for 5 position of the derotator implemented in the test bench to produce the rotation of pupil which will be seen by the WFS of NAOMI once implemented in the AT.
Figure 18 shows the Strehl ratio performance in the H band for different modes and different flux for a seeing of 1.1”. It shows that the performance is better that 55% of Strehl ratio in high flux. NAOMI also obtain a tip-tilt residual better than 150 milliarcsec in faint mode (1 photon/sub-aperture/ms). Figure 18: SR vs flux for the different NAOMI modes and AO loop configurations. Seeing = 1.1”, Tau0 = 2.5 ms.
Figure 18 shows the Strehl ratio performance in the H band for different modes and different flux for a seeing of 1.1”. It shows that the performance is better that 55% of Strehl ratio in high flux. NAOMI also obtain a tip-tilt residual better than 150 milliarcsec in faint mode (1 photon/sub-aperture/ms). Figure 18: SR vs flux for the different NAOMI modes and AO loop configurations. Seeing = 1.1”, Tau0 = 2.5 ms.
The most important parameter for NAOMI is probably its capacity to inject constant flux in the spatial filters or the fibers of the interferometric instruments. The performances are shown on Figure 19 and compared to an open loop system and to a pure tip-tilt corrector system (equivalent to STRAP). It is clearly seen that we have a much higher coupling efficiency and much more stable.
The most important parameter for NAOMI is probably its capacity to inject constant flux in the spatial filters or the fibers of the interferometric instruments. The performances are shown on Figure 19 and compared to an open loop system and to a pure tip-tilt corrector system (equivalent to STRAP). It is clearly seen that we have a much higher coupling efficiency and much more stable.
Figure 20: (left)Closed-loop chopping performance with turbulence. Only the TipTilt content of DM and the WF‘ are shown. Chopping was configured with a frequency of 5 Hz and a stroke of 6 arcsec. The DM is in a fixed position when ON_SKY and correct the turbulence when ON_STAR.. (right) performance of NAOMI reclosing the loop
Figure 20: (left)Closed-loop chopping performance with turbulence. Only the TipTilt content of DM and the WF‘ are shown. Chopping was configured with a frequency of 5 Hz and a stroke of 6 arcsec. The DM is in a fixed position when ON_SKY and correct the turbulence when ON_STAR.. (right) performance of NAOMI reclosing the loop

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