- Email: [email protected]
FALCON: Extending adaptive corrections to cosmological fields
New Astronomy Reviews 50 (2006) 382–384 www.elsevier.com/locate/newastrev
FALCON: Extending adaptive corrections to cosmological fields M. Puech a,*, F. Hammer a, P. Jagourel a, E. Gendron b, F. Asse´mat c, F. Chemla a, H. Flores a, P. Laporte a, J.-M. Conan d, T. Fusco d, A. Liotard e, F. Zamkotsian e a
GEPI – Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France LESIA – Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France Department of Physics, University of Durham, Rochester Building, Science Laboratories, South Road, Durham DH1 3LE, England, United Kingdom d ONERA, BP72 – 29 Avenue de la Division Leclerc, F-92322 Chatillon Cedex, France e Laboratoire d’Astrophysique de Marseille, 2 Place Le Verrier, 13248 Marseille cedex 4, France b
c
Available online 31 March 2006
Abstract We present FALCON which is an original concept for a next generation instrument which could be used on the ESO very large telescope (VLT) and on the future extremely large telescopes (ELT). It is a multi-object integral field spectrograph with multiple small integral field units (IFUs). Each of them integrates a tiny adaptive optics system coupled with atmospheric tomography to solve the sky coverage problem. This therefore allows to reach spatial (0.25 arcsec) and spectral (R P 5000) resolutions suitable for distant galaxy studies in the 0.8–1.8 lm wavelength range. In the FALCON concept, the adaptive optics correction is only applied to small and discrete areas selected within a large field. This approach implies to develop miniaturized devices, such as deformable mirrors (DM) and wavefront sensors (WFS) for wavefront correction. 2006 Elsevier B.V. All rights reserved. Keywords: Adaptive optics; Integral field spectroscopy; Micro-deformable mirror; Extremely large telescopes
Contents 1. 2. 3. 4.
FALCON: a new spectrograph in the NIR Adaptive optics on large fields of view. . . . The FALCON AO system . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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1. FALCON: a new spectrograph in the NIR FALCON is a proposal for a new multi-objects 3D spectrograph for the VLT or ELTs, for which the main scientific goals are the study of the formation and evolution of galaxies. Complete reviews of FALCON scientific drivers have been detailed both for VLTs (Hammer et al., 2001) *
Corresponding author. E-mail address: [email protected] (M. Puech).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.02.016
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382 383 383 384 384
and ELTs (Hammer et al., 2004). Briefly, the goal is to perform 3D spectroscopy of distant galaxies, with a redshift 0.5 < z < 3. In that case, the emission lines, such as [OII] or Ha are observed in the 0.8 6 k 6 1.8 lm wavelength range. Three-dimensional spectroscopy of such galaxies requires a high spectral resolution (3000 6 R 6 15000), as well as a high spatial resolution, the size of the structures to be resolved being in the 0.4–0.7 arcsec range (Ferguson et al., 2004). Adaptive optics (AO) corrections are required to improve the spatial resolution without losing in spectral
M. Puech et al. / New Astronomy Reviews 50 (2006) 382–384
SNR. A large field of view (FoV) is also required to fully benefit from the multiplex advantage and to avoid cosmic variance. The FoV should be at least equal to the clustering scales at all redshifts, i.e., 100 arcmin2. 2. Adaptive optics on large fields of view With FALCON we propose a new approach for adaptive optics. Instead of correcting the whole FoV, only the regions of interest are corrected, i.e., the IFUs performing the 3D spectroscopy of the observed galaxies. To do this, we plan to use several independent AO systems spread in the focal plane. Each IFU has its own AO system, which uses atmospheric tomography techniques (Ragazzoni et al., 1999; Tokovinin et al., 2001; Tokovinin and Viard, 2001): three wavefront sensors (WFS) per IFU measure the off-axis wavefront coming from stars located around the galaxy, and the on-axis wavefront from the galaxy is deduced from off-axis measurements and corrected thanks to an AO system within each IFU. The process of on-axis wavefront reconstruction from off-axis measurements is repeated as many times as there are spectroscopic IFUs. This new approach is often called ‘‘distributed adaptive optics’’ (DAO) or ‘‘multi-objects adaptive optics’’ (MOAO). This new AO approach is required because no existing solutions can be used with FALCON: with classical AO, a bright star (R 6 16) is required within the isoplanatic field to sense the wavefront. The main goal of FALCON being extragalactic astronomy, it will observe distant galaxies in directions far away from the galactic plane where the surface density of stars decreases dramatically (Bahcall and Soneira, 1981): this makes the sky coverage too low to meet the scientific goals; applying laser guide stars (Foy and Labeyrie, 1985; Le Louarn et al., 1998; Tallon and Foy, 1990; Rigaut and Gendron, 1992) to the FALCON case (few 10 IFUs spread over 25 arcmin) would require numerous LGS and as many tilt-stabilization systems using natural guide stars (NGS). This would create technological difficulties and increase by far the cost of such an instrument; with (multiconjugated adaptive optics) MCAO (Dicke, 1975; Beckers, 1989; Fusco et al., 1999; Le Louarn, 2002), it is not possible to correct a field as wide as 25 arcmin in diameter; with (ground layer adaptive optics) GLAO (Rigaut, 2002; Tokovinin, 2004; Nicolle et al., 2004), the correction, although valid over a wide field, is not sufficient for the FALCON purpose which requires a coupling of 40% in the H band to perform a good 3D spectroscopy. The FALCON DAO system has been investigated by numerical simulations. Considering only spatial aspects in J and H bands, (Asse´mat et al., 2003) and (Asse´mat, 2004) have shown that a light coupling in 0.25 · 0.25 arcsec2 of 30% in J band and 40% in H band can be reached
383
with a sky coverage of 50% at any galactic latitude, even from the galactic pole, by correcting at least 70 Zernike modes, and by using three NGS with a minimum magnitude R 6 17 within a radius of about 3 arcmin. Such a sky coverage (50%) is very high compared to the one generally provided by classical AO (a few percent). 3. The FALCON AO system The FALCON IFU assumes that a deformable mirror (DM) and its pupil relay optics can be miniaturized to be integrated into the spectroscopic IFU, i.e., the so-called adaptive button. The galaxy image is then sampled by a microlens array (0.125 arcsec/pixel) within every IFU, and the light is brought to the spectrograph using optical fibers. We assume that the wavefront sensors (WFS) can be miniaturized too, and fit into so-called WFS-buttons, which are located on neighboring guide stars (Fig. 1). We plan to use at least 10 IFUs, selecting 10 sub-areas in the whole 25 arcmin FoV. In the first design (Hammer et al., 2001; Puech and Saye`de, 2004), a hybrid corrector combined an adaptive lens (for tip-tilt and defocus modes correction) and a micro-DM (for higher order modes since micro-DM alone are usually unable to correct low order modes with the needed strokes). We are now studying designs with new generation micro-DM which can provide low order modes correction. In this kind of adaptive button, the IFU size is limited by the DM size, which approximately leads to a 50 mm large IFU. A completely different approach is to
Fig. 1. The FALCON concept: for every galaxy, the wavefront is measured by three WFS-buttons located on neighboring guide stars. The correction is computed by tomography and then sent to the adaptive button (the IFU) located on the galaxy which includes a micro-DM.
384
M. Puech et al. / New Astronomy Reviews 50 (2006) 382–384
assume that adaptive corrections are performed out of the focal plane and then use more conventional IFUs (without any adaptive component). This has the advantage of relaxing constraints on the DM size. Whatever the final design, the FALCON architecture is a departure from the usual closed-loop AO system, as there is no optical feedback from the micro-DM to WFS. Different solutions spanning from completely open loop to pseudoclosed loop (closed by ‘‘electro-mechanical analogy’’) can be imagined and are detailed in Puech et al. (in press). 4. Conclusion We plan to use the OKO device (Vdovin, 1998) as a demonstrator for the overall FALCON system. We currently think that, before the end of 2005, we will have proven the ability of the electrostatic DM technique to address this specific issue of an open loop controlled system. Many laboratories currently work on electrostatic DMs and we are strongly confident in the development of components able to address the pending stroke issue. The next step will be the definition of a prototype. References Asse´mat, F., Hammer, F., Gendron, E., et al., 2003. Optics in atmospheric propagation and adaptive systems VI. Proc. SPIE, 5237, 211. Asse´mat, F., 2004, PhD Thesis, Observatoire de Paris-Meudon. Bahcall, J.N., Soneira, R.M., 1981. ApJ 246, 122.
Beckers, J.M., 1989. Eso conference on very large telescopes and their instrumentation. In: Ulrich, M.H. (Ed.), ESO Conf. Workshop Proc., 2, 693. Dicke, R.H., 1975. ApJ 198, 605. Foy, R., Labeyrie, A., 1985. A& A 152, L29. Ferguson, H.C., Dickinson, M., Giavalisco, M., et al., 2004. ApJ 600, 107. Fusco, T., Conan, J., Michau, V., et al., 1999. Propagation and imaging through the atmosphere III. In: Roggemann, M.C., Bissonnette, L.R. (Eds.), Proc. SPIE, 3763, 125. Hammer, F., Saye`de, F., Gendron, E., et al., 2001. Scientific drivers for ESO future VLT/VLTI instrumentation. Proc. ESO workshop held in Garching, 139. Hammer, F., Puech, M., Asse´mat, F. et al., 2004. Second workshop on ELT. Proc. SPIE, 5382, 727. Le Louarn, M., Foy, R., Hubin, N., et al., 1998. MNRAS 295, 756. Le Louarn, M., 2002. MNRAS 334, 865. Nicolle, M., et al., 2004. Advancements in adaptive optics. In: Domenico, B., Calia, Brent, L., Ellerbroek, Roberto Ragazzoni (Eds.), Proc. SPIE, vol. 5490, pp. 858–869. Puech, M., Saye`de, F., 2004. Ground based instrumentation for astronomy. Proc. SPIE 5492, 303. Puech, M., Chemla, F., Laporte, P., et al., 2005. Astronomical adaptive optics systems and applications II. Proc. SPIE 5903, in press. Ragazzoni, R., Marchetti, E., Rigaut, R., 1999. A& A 342, L53. Rigaut, F., Gendron, E., 1992. A& A 261, 677. Rigaut, F., 2002. In: Vernet, E. et al. (Eds.), ESO Conf. Proc. 58, Beyond Conventional Adaptive Optics. ESO, Garching, p. 11. Tallon, M., Foy, R., 1990. Adaptive telescope with laser probe – isoplanatism and cone effect. A& A 235, 549. Tokovinin, A., Le Louarn, M., Viard, E., et al., 2001. A& A 378, 710. Tokovinin, A., Viard, E., 2001. JOSA-A 18, 873. Tokovinin, A., 2004. PASP 116 (824), 941–951. Vdovin, G.V., 1998. Adaptive optical system technologies. Proc. SPIE 3353, 902.
FALCON: Extending adaptive corrections to cosmological fields M. Puech a,*, F. Hammer a, P. Jagourel a, E. Gendron b, F. Asse´mat c, F. Chemla a, H. Flores a, P. Laporte a, J.-M. Conan d, T. Fusco d, A. Liotard e, F. Zamkotsian e a
GEPI – Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France LESIA – Observatoire de Paris, 5 Place Jules Janssen, 92195 Meudon, France Department of Physics, University of Durham, Rochester Building, Science Laboratories, South Road, Durham DH1 3LE, England, United Kingdom d ONERA, BP72 – 29 Avenue de la Division Leclerc, F-92322 Chatillon Cedex, France e Laboratoire d’Astrophysique de Marseille, 2 Place Le Verrier, 13248 Marseille cedex 4, France b
c
Available online 31 March 2006
Abstract We present FALCON which is an original concept for a next generation instrument which could be used on the ESO very large telescope (VLT) and on the future extremely large telescopes (ELT). It is a multi-object integral field spectrograph with multiple small integral field units (IFUs). Each of them integrates a tiny adaptive optics system coupled with atmospheric tomography to solve the sky coverage problem. This therefore allows to reach spatial (0.25 arcsec) and spectral (R P 5000) resolutions suitable for distant galaxy studies in the 0.8–1.8 lm wavelength range. In the FALCON concept, the adaptive optics correction is only applied to small and discrete areas selected within a large field. This approach implies to develop miniaturized devices, such as deformable mirrors (DM) and wavefront sensors (WFS) for wavefront correction. 2006 Elsevier B.V. All rights reserved. Keywords: Adaptive optics; Integral field spectroscopy; Micro-deformable mirror; Extremely large telescopes
Contents 1. 2. 3. 4.
FALCON: a new spectrograph in the NIR Adaptive optics on large fields of view. . . . The FALCON AO system . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
1. FALCON: a new spectrograph in the NIR FALCON is a proposal for a new multi-objects 3D spectrograph for the VLT or ELTs, for which the main scientific goals are the study of the formation and evolution of galaxies. Complete reviews of FALCON scientific drivers have been detailed both for VLTs (Hammer et al., 2001) *
Corresponding author. E-mail address: [email protected] (M. Puech).
1387-6473/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.newar.2006.02.016
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382 383 383 384 384
and ELTs (Hammer et al., 2004). Briefly, the goal is to perform 3D spectroscopy of distant galaxies, with a redshift 0.5 < z < 3. In that case, the emission lines, such as [OII] or Ha are observed in the 0.8 6 k 6 1.8 lm wavelength range. Three-dimensional spectroscopy of such galaxies requires a high spectral resolution (3000 6 R 6 15000), as well as a high spatial resolution, the size of the structures to be resolved being in the 0.4–0.7 arcsec range (Ferguson et al., 2004). Adaptive optics (AO) corrections are required to improve the spatial resolution without losing in spectral
M. Puech et al. / New Astronomy Reviews 50 (2006) 382–384
SNR. A large field of view (FoV) is also required to fully benefit from the multiplex advantage and to avoid cosmic variance. The FoV should be at least equal to the clustering scales at all redshifts, i.e., 100 arcmin2. 2. Adaptive optics on large fields of view With FALCON we propose a new approach for adaptive optics. Instead of correcting the whole FoV, only the regions of interest are corrected, i.e., the IFUs performing the 3D spectroscopy of the observed galaxies. To do this, we plan to use several independent AO systems spread in the focal plane. Each IFU has its own AO system, which uses atmospheric tomography techniques (Ragazzoni et al., 1999; Tokovinin et al., 2001; Tokovinin and Viard, 2001): three wavefront sensors (WFS) per IFU measure the off-axis wavefront coming from stars located around the galaxy, and the on-axis wavefront from the galaxy is deduced from off-axis measurements and corrected thanks to an AO system within each IFU. The process of on-axis wavefront reconstruction from off-axis measurements is repeated as many times as there are spectroscopic IFUs. This new approach is often called ‘‘distributed adaptive optics’’ (DAO) or ‘‘multi-objects adaptive optics’’ (MOAO). This new AO approach is required because no existing solutions can be used with FALCON: with classical AO, a bright star (R 6 16) is required within the isoplanatic field to sense the wavefront. The main goal of FALCON being extragalactic astronomy, it will observe distant galaxies in directions far away from the galactic plane where the surface density of stars decreases dramatically (Bahcall and Soneira, 1981): this makes the sky coverage too low to meet the scientific goals; applying laser guide stars (Foy and Labeyrie, 1985; Le Louarn et al., 1998; Tallon and Foy, 1990; Rigaut and Gendron, 1992) to the FALCON case (few 10 IFUs spread over 25 arcmin) would require numerous LGS and as many tilt-stabilization systems using natural guide stars (NGS). This would create technological difficulties and increase by far the cost of such an instrument; with (multiconjugated adaptive optics) MCAO (Dicke, 1975; Beckers, 1989; Fusco et al., 1999; Le Louarn, 2002), it is not possible to correct a field as wide as 25 arcmin in diameter; with (ground layer adaptive optics) GLAO (Rigaut, 2002; Tokovinin, 2004; Nicolle et al., 2004), the correction, although valid over a wide field, is not sufficient for the FALCON purpose which requires a coupling of 40% in the H band to perform a good 3D spectroscopy. The FALCON DAO system has been investigated by numerical simulations. Considering only spatial aspects in J and H bands, (Asse´mat et al., 2003) and (Asse´mat, 2004) have shown that a light coupling in 0.25 · 0.25 arcsec2 of 30% in J band and 40% in H band can be reached
383
with a sky coverage of 50% at any galactic latitude, even from the galactic pole, by correcting at least 70 Zernike modes, and by using three NGS with a minimum magnitude R 6 17 within a radius of about 3 arcmin. Such a sky coverage (50%) is very high compared to the one generally provided by classical AO (a few percent). 3. The FALCON AO system The FALCON IFU assumes that a deformable mirror (DM) and its pupil relay optics can be miniaturized to be integrated into the spectroscopic IFU, i.e., the so-called adaptive button. The galaxy image is then sampled by a microlens array (0.125 arcsec/pixel) within every IFU, and the light is brought to the spectrograph using optical fibers. We assume that the wavefront sensors (WFS) can be miniaturized too, and fit into so-called WFS-buttons, which are located on neighboring guide stars (Fig. 1). We plan to use at least 10 IFUs, selecting 10 sub-areas in the whole 25 arcmin FoV. In the first design (Hammer et al., 2001; Puech and Saye`de, 2004), a hybrid corrector combined an adaptive lens (for tip-tilt and defocus modes correction) and a micro-DM (for higher order modes since micro-DM alone are usually unable to correct low order modes with the needed strokes). We are now studying designs with new generation micro-DM which can provide low order modes correction. In this kind of adaptive button, the IFU size is limited by the DM size, which approximately leads to a 50 mm large IFU. A completely different approach is to
Fig. 1. The FALCON concept: for every galaxy, the wavefront is measured by three WFS-buttons located on neighboring guide stars. The correction is computed by tomography and then sent to the adaptive button (the IFU) located on the galaxy which includes a micro-DM.
384
M. Puech et al. / New Astronomy Reviews 50 (2006) 382–384
assume that adaptive corrections are performed out of the focal plane and then use more conventional IFUs (without any adaptive component). This has the advantage of relaxing constraints on the DM size. Whatever the final design, the FALCON architecture is a departure from the usual closed-loop AO system, as there is no optical feedback from the micro-DM to WFS. Different solutions spanning from completely open loop to pseudoclosed loop (closed by ‘‘electro-mechanical analogy’’) can be imagined and are detailed in Puech et al. (in press). 4. Conclusion We plan to use the OKO device (Vdovin, 1998) as a demonstrator for the overall FALCON system. We currently think that, before the end of 2005, we will have proven the ability of the electrostatic DM technique to address this specific issue of an open loop controlled system. Many laboratories currently work on electrostatic DMs and we are strongly confident in the development of components able to address the pending stroke issue. The next step will be the definition of a prototype. References Asse´mat, F., Hammer, F., Gendron, E., et al., 2003. Optics in atmospheric propagation and adaptive systems VI. Proc. SPIE, 5237, 211. Asse´mat, F., 2004, PhD Thesis, Observatoire de Paris-Meudon. Bahcall, J.N., Soneira, R.M., 1981. ApJ 246, 122.
Beckers, J.M., 1989. Eso conference on very large telescopes and their instrumentation. In: Ulrich, M.H. (Ed.), ESO Conf. Workshop Proc., 2, 693. Dicke, R.H., 1975. ApJ 198, 605. Foy, R., Labeyrie, A., 1985. A& A 152, L29. Ferguson, H.C., Dickinson, M., Giavalisco, M., et al., 2004. ApJ 600, 107. Fusco, T., Conan, J., Michau, V., et al., 1999. Propagation and imaging through the atmosphere III. In: Roggemann, M.C., Bissonnette, L.R. (Eds.), Proc. SPIE, 3763, 125. Hammer, F., Saye`de, F., Gendron, E., et al., 2001. Scientific drivers for ESO future VLT/VLTI instrumentation. Proc. ESO workshop held in Garching, 139. Hammer, F., Puech, M., Asse´mat, F. et al., 2004. Second workshop on ELT. Proc. SPIE, 5382, 727. Le Louarn, M., Foy, R., Hubin, N., et al., 1998. MNRAS 295, 756. Le Louarn, M., 2002. MNRAS 334, 865. Nicolle, M., et al., 2004. Advancements in adaptive optics. In: Domenico, B., Calia, Brent, L., Ellerbroek, Roberto Ragazzoni (Eds.), Proc. SPIE, vol. 5490, pp. 858–869. Puech, M., Saye`de, F., 2004. Ground based instrumentation for astronomy. Proc. SPIE 5492, 303. Puech, M., Chemla, F., Laporte, P., et al., 2005. Astronomical adaptive optics systems and applications II. Proc. SPIE 5903, in press. Ragazzoni, R., Marchetti, E., Rigaut, R., 1999. A& A 342, L53. Rigaut, F., Gendron, E., 1992. A& A 261, 677. Rigaut, F., 2002. In: Vernet, E. et al. (Eds.), ESO Conf. Proc. 58, Beyond Conventional Adaptive Optics. ESO, Garching, p. 11. Tallon, M., Foy, R., 1990. Adaptive telescope with laser probe – isoplanatism and cone effect. A& A 235, 549. Tokovinin, A., Le Louarn, M., Viard, E., et al., 2001. A& A 378, 710. Tokovinin, A., Viard, E., 2001. JOSA-A 18, 873. Tokovinin, A., 2004. PASP 116 (824), 941–951. Vdovin, G.V., 1998. Adaptive optical system technologies. Proc. SPIE 3353, 902.