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LBT FACILITY INSTRUMENTATION PROPOSAL
Title: Optical and NEar IR Interferometric or Combined Imager and Spectrograph (ONEIRIC I/S)
PI A. Richichi*, Co-PI M. Gai**
Co-authors:, C. Baffa*, G. Brusa*, A. Cimatti*, C. Del Vecchio*, S. Esposito*, L. Fini*, S. Gennari*, F. Lisi, R. Maiolino*, F. Mannucci, R. Ragazzoni***, A. Riccardi*, P. Salinari* (all others to be added in alphabetical order)
*) Osservatorio Astrofisico di Arcetri
**) Osservatorio Astronomico di Torino
***)Osservatorio Astronomico di Padova
Introduction
More than a single and specific instrument we are here proposing to develop a family of instruments devoted to exploit the full potential of LBT for high angular resolution work at high sensitivity. In order to obtain the ultimate performances a considerable development of the adaptive optics system is necessary.
We will discuss here only the conceptual grounds on which we base our proposal. The work to evaluate in greater detail the instrumental options and the potential cost of the proposed interferometer and of the associated advanced Adaptive Optics system is still to be done, and will certainly require a number of years and the involvement of many collaborating Institutes.
In the following we will try to discuss various options for this ambitious program, balancing scientific wishes with our present understanding of technical possibilities, but we are fully aware of the tentative nature of our present preliminary discussion.
The reasons of presenting this proposal at the present time are the following:
- We believe that in the first decade of next century instruments similar to the ones we describe here will be able to provide a unique and essential complement, both in imaging and in spectroscopy, to space born astronomy, including not only HST but also NGST.
- Consequently we think that a considerable part of the LBT instrumentation budget has to be reserved to interferometry at short wavelength, and the present Call for Proposal is therefore the appropriate occasion for discussing a preliminary budget allocation.
- A large fraction of the investment, both in terms of money and of work, required by the high angular resolution instruments consists in developing an adaptive optics system considerably more advanced than the one already included in the telescope budget. Various other instruments can take advantage of this investment on AO, while some of options that can be implemented at the combined focus could also be part of instrument at different foci. This is therefore an appropriate time for optimizing the instrument complement of LBT.
This is not a "private" proposal. We look for the involvement of many of the groups participating in LBT in a coordinated program aiming to make of LBT the powerful instrument it can be. The work we propose is challenging and vast, only a large participation of LBT partners can accomplish it in a reasonable time. Of course we desire to contribute work in this program in areas that will have to be agreed.
As mentioned above, the key point of this proposal is that we believe that a further dramatic step can be done in adaptive optics, pushing this technique to very high quality correction over a large fraction of the sky even at optical wavelength. In order to achieve this ambitious objective a number of techniques have to work together. The first section of this proposal will therefore deal with the conceptual approach to the advanced adaptive optics we need. The second section will discuss instrumental options and priorities, the third will attempt to indicate a possible configuration for the combined focus and for the instruments. Section 4 reports preliminary ideas on an implementation sequence, section 5 attempts a census of required expertise areas, and section 6 reports very rough cost estimates for at least a part of the necessary developments. Section briefly 7 reports scientific priorities and possible areas of involvement of the proponents.
1. An advanced AO system
The LBT baseline AO system is already fairly advanced and includes technology that is still under development, such as Sodium lasers and adaptive secondary mirrors. D. Sandler et Al. [1] have shown that an adaptive system such as the one envisaged for LBT, with a single artificial star and a single corrector, can achieve excellent correction (Strehl ratio of about 0.5 in the K band) over a large fraction of the sky.
Both, quality of correction and sky coverage drop unfortunately going to shorter wavelength if only a single artificial star is used for correction, due to focus anisoplanatism. Although the use of bright natural stars allows excellent correction even at optical wavelength, the resulting sky coverage is extremely low. Removing focus anisoplanatism is therefore one of the key factors for extending AO to shorter wavelength with a significant sky coverage.
In a paper of several years ago Tallon and Foy [2, TF] have shown that by using a modest number of artificial stars the focus anisoplanatism effect can be totally removed. Moreover the Tallon and Foy multi-laser technique makes also possible to obtain turbulence tomography, separating the contributions to the instantaneous wave front phase error that are due to different layers of the atmosphere. Although not yet tested in practice, for the obvious reason that even single laser AO is still in its infancy, the TF method is essentially based on geometry and is therefore fairly solid if laser stars work. Extensive simulations of this method are presently in preparation in the frame of a European TMR program.
With the TF technique it becomes therefore possible not only to correct high orders of the wave front error, but also to apply separate corrections for different atmospheric layers. The consequence is that one can use "multi-conjugate AO", a technique described by J. Beckers [3], that extends considerably the isoplanatic angle. The combination of the two techniques therefore provides the possibility of correcting the wave front not only without the limit posed by focus anisoplanatism, but also over a much larger field of view.
The extension of the corrected field is crucial for assuring sufficient sky coverage at short wavelength. Like a single laser AO system, also multi-laser systems cannot measure the global wave front inclination, which is the largest term of the phase error. Although sophisticated techniques have been proposed to solve this problem, the only reliable way of measuring the wave front inclination proven until now is that of using a natural star. As pointed out in [1], the image of a faint star within the field where the high orders are corrected using the artificial stars is essentially diffraction limited (most photons fall in a circle of ~ /D), except that the entire image moves around because of the non-compensated wave front inclination. In this conditions even a modest signal to noise ratio on the star signal allows to correct the wave front inclination error to the required level of a fraction of ~ /D. A wider corrected field increases therefore the probability of finding a suitable natural star.
We have outlined only very qualitatively the basic features of the advanced AO system we aim to, but it is possible to give a very preliminary evaluation of what we expect from such a system using current technology and reasonable assumptions on turbulence parameters. If we assume that we will use nights with Fried coherence length ro 20 cm (in the V band), that the turbulence is evenly distributed between ground and about 10 Km height and that both correctors have actuator separations corresponding to ~30 cm on the beam, we obtain a Strehl ratios S ~ 0.7 and a corrected field for finding a natural guide star of ~30 arcsec in the R band. If the turbulence is mainly concentrated in two layers, close to the conjugates of the two correctors, the isoplanatic field becomes even larger.
A star of spectral type K or later (about 2/3 of the stars are like that at faint magnitudes) with mV 21 in this isoplanatic field provides enough photons in about 30 ms to correct tip-tilt to /5*D or better in the R or I bands. As the average density of such stars at high galactic latitude is about 0.6 per sq. arcmin, the sky coverage would be ~ 15 % even at high galactic latitude, while it would be essentially 100% for the average sky at lower galactic latitudes.
Table 1 shows the Strehl ratio of the dominant residual error, the "fitting error", in the above assumptions as calculated from a model of the adaptive secondary mirror. Fig. 1 shows a section of the corresponding PSF. Although a significant fraction of the energy is missing from the central peak, the residual wave front error is on small spatial scales, therefore diffracts energy on large angular scales so that the contrast between the peak and the scattered light is extremely high, >103. This is an important feature for interferometry, where the reduction of the fringe contrast caused by scattered light is negligible.
Table 1: Strehl ratios corresponding to the dominant error term in a multi-laser double corrector system, the "fitting error". The assumption is that each one of the two correctors, both with the same resolution, is used to correct turbulence with the same r0 (30 cm at =0.5 m), corresponding to a total r0 of 20 cm. The fitting error Strehl ratio of a single corrector is reported in column 2, while column 3, due to the simplified assumptions, is simply the square of column 2.
lambda[m] / Fitting error SR
of one corrector
r0 =30 cm / Fitting error SR
of both correctors
r0=20 cm
0.5 / 0.723 / 0.523
0.7 / 0.846 / 0.715
0.9 / 0.903 / 0.815
1.2 / 0.944 / 0.891
1.6 / 0.968 / 0.937
2.2 / 0.983 / 0.966
Figure 1: A cut through the calculated PSF in the I band . The assumed total r0 is 20 cm at 0.5 m.
1.1 The Sodium lasers
We will now go through the various components of the multi-laser, multi-conjugate AO system just outlined to understand qualitatively what we need. Concerning the laser system we refer to [2] for a description of the geometry, of the algorithms and of the capabilities of the multi-laser technique, and we only answer a few simple practical questions:
How many lasers do we need per pupil?
Based on the analysis of Tallon and Foy the minimum number of lasers that accommodates our preliminary field requirements is four per pupil. The maximum field angle over which a full correction is possible in this case is about 50 arcsec, while it would be too small (~7 arcsec) with 3 lasers. The four lasers would have to be projected at an angle of 83 arcsec from the telescope axis or from the telescope side. Depending on how the lasers are projected and on the efficiency of stray light rejection the number of lasers could become 5. We will assume the conservative number of 5 lasers in the following.
How powerful lasers do we need?
The requirements on power are somewhat less than for a single laser systems. Lasers currently under development for single laser systems are therefore more than adequate.
How will we project the lasers?
The laser beams can be projected exactly like in the single laser system we have foreseen, from the back of the secondary or from the side of the pupil. This second option has not been foreseen in the telescope design and could therefore be more expensive to implement, although it has the advantage of minimizing the number of lasers and the total amount of light back-scattered by the atmosphere toward the telescope. Projecting the lasers from the back of the secondary is simpler in LBT, but may require an odd number of lasers, therefore not less than 5 in our case, to avoid superposition of the Rayleigh beacon of one of the lasers with the Sodium star of the one on the opposite side.
How will we sense the laser stars?
Each artificial star will be analyzed by a "normal" wave front sensor, for instance a Shack Hartman sensor, with a slightly lower spatial resolution than the one needed for single-laser systems for the same atmospheric conditions and correction requirements.
What new technology is needed?
None, if we assume that the laser technology will be available in the next few years. Certainly the reliability and cost requirements are different from those of a single laser. The current development at University of Arizona seems to fit all requirements including moderate unitary cost (~250 k$ per laser).
1.2 The correctors
The proposed scheme of double-conjugated adaptive correction requires two correctors for each beam, conjugated to suitably different heights in the atmosphere.
1.2.1 The adaptive secondary
The first corrector is obviously the Gregorian adaptive secondary, whose conjugated plain lays about 100 m above the primary. The secondary is undersized to be the pupil, determining an effective telescope aperture of 8.25 m. The actuator spacing will be between 25 and 30 mm, corresponding to 25 to 30 cm on the wave front, as required for use as single corrector during initial phases and for other focal stations where double-conjugate AO will not be implemented. It must be noted, in passing, that the implementation of a multiple laser star system is of advantage also for the foci where only a single corrector is used. Overcoming the focus anisoplanatism problem allows better correction at all wavelengths and opens the possibility of observing at short wavelength, although with small field.
The basic technology of the adaptive secondary has been already developed and is currently adopted to construct the first telescope unit, the MMT F/15 adaptive secondary. An important feature of this device is that an internal position loop controls the position of each actuator. It is therefore possible to operate the corrector by updating "absolute" actuator positions rather than zeroing the residual error on a star. This is essential when using multiple laser stars and multi-conjugated correction, because in this case the control loop cannot be closed directly on a star in the usual way.
In addition to correct the high orders for low atmospheric layers, the adaptive secondary will correct the wave front inclination produced by the entire atmosphere and in the optical path inside the telescope and the instrument. Using a single corrector for tip-tilt causes negligible de-correlation of the higher orders. In our configuration, where the inclination is corrected at the low conjugate, the de-correlation on the highest layers (H~10 km) is <1 cm, while the value of ro for a high layer in reasonably good seeing conditions is likely to be more than 30 cm in V.
1.2.2 The Correcting Beam Combiner option
We have preliminarily explored a few optical layouts for the second stage of correction:
- a spherical corrector illuminated by the telescope F/15 beam (two or three added reflections)
- a double pass off-axis parabolic collimator illuminating a flat corrector (four added reflections).
- a corrector coincident with the beam combiner flat mirror (no extra optics).
Although configuration a) and b) give more freedom in choosing the conjugate position, the optical performances where unacceptable at a first attempt, due to the large off-axis angles required in order to avoid vignetting of the beams. The optical efficiency of the first two configurations is also not ideal at visible wavelength due to the extra reflections. Further work could identify better solutions, what counts for the moment is that at least configuration c) seems to be optically viable, and, in fact, very interesting. We will therefore continue the present qualitative discussion of the corrector requirement with reference to the "correcting combiner" (CC).
Of course placing the second corrector at the beam combining flats introduces some constraints, but fortunately these don't seem to limit seriously the performances. If we don't want to introduce extra reflections, the position, and therefore the size, of the second corrector are determined by the available space (less than 2 m at the central combined focus). Preliminary optical design shows that in this case a simple doublet of ~14 cm diameter can form on the beam combiner flat an image of a layer located about 6 km above the primary (~9 km above sea level). This is an excellent conjugate position for observations within about 40 degrees from zenith, where the height of the conjugate would become ~4.6 Km (~7.6 Km above sea level).
The on-axis image of the layer formed on the flat combiner mirror has a diameter of about 80 mm. The doublet has excellent Strehl ratio over about 2x2 arcmin. Of course the beam combiner must be at 45 degrees inclination, therefore the conjugates of opposite edges of the corrector are at a different height. The conjugate surface is inclined, going from about 5.5 to about 6.5 km at the opposite edges of the beam. This means that even if we had only a single turbulence layer exactly at 6 km, there would be a finite isoplanatic angle due to the change in conjugation height across the beam. In most practical circumstances this effect is likely to be negligible.
The CC needs to correct not only on axis but also over some field. Assuming we want to be able to correct a field of about one arc-minute, provided by the 4 laser system and certainly useful at least for finding natural guide stars in favorable atmospheric circumstances, the corrector minor axis would be about 80+36=116 mm. The actuator density (on sky) should be similar to the one of the secondary or slightly less dense, say about 30 to 40 cm on the primary, corresponding to about 3-4 mm physical separation along the minor axis. The separation could be 1.4 times wider along the major axis. These separations are close to present high density correctors using piezo actuators, while the total number of actuators, ~1000, is at the high end of what has already been done. The correction range can be 10 m PtV, because the tip-tilt term is corrected at the secondary, and is again compatible with current piezo actuated correctors. An attractive alternative to piezo actuated correctors seems to be that of electrostatic correctors, although currently available only in much smaller formats, because of the potential for use in vacuum and cryogenical environment and of the potentially much lower cost.
The only requirement that makes these correctors different from most currently available devices is that we want to close an internal position loop for each actuator with nanometer accuracy as done for the adaptive secondary. The reduced gap (~5 m) between deformable mirror and back plate partially compensates the reduced area per actuator, so that using the same type of capacitive sensors adopted in the adaptive secondary seems to be possible, in particular for the electrostatic devices. An alternative could be that of fast optical sensing with a suitable device (fast interferometer or wave front sensor), using normal reflection directly on the corrector surface. A slow interferometer is needed in any case for periodical calibration even if the position sensors are internal to the device.