Overview

NaCo provides adaptive optics assisted imaging, imaging polarimetry, coronography and spectroscopy. Only the broad characteristics of each modes are presented here; please refer to the user manual for details.

NAOS, the adaptive optics (AO) front end, has been designed to work with natural guide stars and moderately extended objects (<4") and is equipped with one infrared (0.8-2.5 µm) and one visual wavefront sensor (0.45-1.0 µm). For a point-like reference source with a visual brightness of V=12, NAOS can provide Strehl ratios as high as 50% in the K band. It can provide partial correction for targets as faint as V=16.7 or K=12 (13 with the N90C10 dichroic). The AO reference star can be either the science object itself or a closeby star (within 55").

As of Period 78 the NAOS AO module can be fed by an artificial sodium laser guide star (LGS) which is positioned on sky on top of the science target allowing for high-order AO corrections. A natural guide star is still required to correct for the tip-tilt motions, which are not sensed by the LGS. This tip-tilt star (TTS) has to be in the V magnitude range 12-17 and can be as far away as 40" from the science target, however, with decreasing performance with increasing distance. At 40" distance about half the Strehl ratio is achieved as compared to having the TTS on-axis with the LGS (in K band). The TTS must be specified in the target list of the Phase 1 proposal using the ESOFORM proposal template.

The LGS mode of NACO is still limitedly tested and little is known about the actual performance. The use of the LGS mode is for the time being recommended only for science programs taking advantage of moderate Strehl ratios ("seeing enhancements") to achieve their scientific goals. The users are encouraged to frequently check the instrument's webpage for updates on the LGS performances. In Period 81 the LGS mode of NACO is offered only in Service Mode.

In order to evaluate the feasibility of their project, Users should first use the exposure time calculator (ETC), to estimate the expected S/N ratio. The preparation software (PS), a NaCo specific tool, is dedicated to the performance estimation of the AO system under User's given conditions, and should be used to predict the Strehl ratio. Its result can be used in the ETC to provide higher accuracy.

For low Strehl ratios (a few percent or less), users should carefully weigh the advantages of using NaCo over other IR instruments, such as ISAAC, which, in general, have larger fields of view, lower backgrounds, slightly higher throughputs, and lower overheads.

Coronography with the 4QPMs in K and H, SDI+4, combining SDI+ and the 4QPM_H (differential imaging combined with coronagraphy), Sparse aperture Interferometry (SAM, if offered), cube mode (if offered), use of pupil tracking mode (if offered) and spectroscopy with the prism disperser, are offered in Visitor Mode only; all other instrument modes are offered in both Visitor and Service Mode. See table below for a summary.

Instrument Mode	Offered observing mode [S/V]
Imaging, all filters	S/V
Imaging + Cube mode (if offered)	V
Grism Spectroscopy	S/V
Prism Spectroscopy	V
SDI+	S/V
Coronagraphy (Lyot)	S/V
4QPM Coronagraphy	V
SDI+4	V
Wollaston Polarimetry	S/V
Any LGS observations*	S
SAM (if offered)	V
Pupil tracking (if offered)	V

^* Note that LGS will, as a rule, NOT be offered with modes that require VM (i.e complex modes). For example, LGS+SAM will not be a valid option for p82.

In P82 chopping is offered again for imaging applications.

Fabry-Perot imaging, polarimetry with wire grids, coronagraphy with the semitransparent mask and S13 camera, will not be > offered in P82.

CONICA limiting magnitudes with the visual dichroic

Band	J	H	Ks	L'	M'
Diffraction limited FWHM [mas]	32	42	56	98	123
Sky background [mag]	16.0	14.0	13.0	3.0	-0.5
Limiting magnitude for imaging [1]	24.05	24.05	23.35	18.55	15.15
Magnitude limit range for narrow band imaging	21-22	21-22	20-21	15-16	12-13

[1] Calculated as 5 sigma in 1 hour using a V=11.5 mag reference 10" away from the source with a visible seeing of 0.8". Please note that these limits are valid for point sources and have been computed over apertures with a radius of 1.25 times the values listed in the first row.

 

Imaging, polarimetric and coronographic modes

Wavelength range	Scale (mas/pix)	Field of view (arcsec)
SW filters [1]	54.3	56 × 56
SW filters	27.0	28 × 28
SW filters	13.3	14 × 14
NB 3.74, NB 4.07	54.7	56 × 56
NB 3.74, NB 4.07, L'	27.1	28 × 28
M'	27.1	14 × 14

[1] Short Wavelength (SW) filters refer to filters with wavelengths shorter than 2.5 μm. This includes the FP.

Polarimetry can be done with a retarder plate and a Wollaston in Ks and H bands. J-band polarimetry observations are not possible

Coronagraphy can be done with occulting masks of 0.7" or 1.4" in diameter. It can also be done with a partially transparent mask (0.01% transmission) of 0.7" in diameter (only with the S27 and S54 cameras) or with two four quadrant phase masks (4QPM), one optimized for λ =2.18 μm the other for λ =1.60 μm (reduction factor of ~100 at 2.18 µm, FOV of 13" x 13" and 8" x 8", respectively). Both masks also include a central Lyot mask 0.15" in diameter, which correspond to a IWA of ~0.12".

To observe very bright objects in imaging (without coronographic mask), it is possible to insert a neutral density filter, reducing the intensity by a factor of 80 (λ < 2.5μm) or 50 (λ > 3 μm).

Simultaneous differential imager

SDI+, the new simultaneous differential imager, provides high contrast imaging mode, in the H band. Contrasts of 30 000 can be obtained at 0.5" in 40 min at S/N=6 between a bright (H< 7 mag) primary star and a methane rich (Teff < 1000 K) companion. The pixel scale of this mode is 17.25 mas/pixel and the field of view is 8" x 8" (SDI+).

SDI+4: Simultaneous differential imager and 4QPM_H

SDI+ can be used in combination with the 4QPM_H: this mode is expected to improve the search for methane companions like giant extrasolar planets around nearby stars.

SAM: sparse aperture interferometry

This mode will be offered pending successful commissioning. SAM uses special aperture masks in the pupil wheel to obtain the very highest angular resolution at the diffraction limit. There are currently 4 masks available on CONICA, with different characteristics (i.e. number of holes and configuration). When used correctly, these masks transform the single 8-m telescope pupil into a sparse interferometer array, and it is therefore necessary to understand the principles of optical interferometry and in particular the recovery of complex Fourier data (amplitudes and phases) from the Fizeau interference patterns that result. A full explanation of the mathematical techniques necessary to do this task is beyond the scope of the present document. The interested users is advised to consult sources form the open literature concerning aperture masking. More information will be posted on the NaCo web pages once the mode is fully commissioned and put into operations.

Cube mode

The cube mdoeis a variant of the of the burst mode already offered with VISIR and ISAAC. In this mode, a data-cube with each single DIT frame is saved. This mode is particularly interesting for lucky-imaging type of observations, where one wants to select the best frames out of a set before co-adding them. The mode is not suited for time resolved applications, since we have no way to time-stamp each single DIT frame and we occasionally loose frames. There are stringent limitations to the use of this mode, in particular it will only be offered in combination with basic imaging. Users will be allowed to specify different window formats (e.g. 64x64 up to full frame - 1024x1024) with increasing minimum DIT, once the window size gets bigger. The maximum number of DIT frames that can be saved in a cube is limited by the need to have file sizes smaller than 500 Gb. Preliminary tests have shown that in most cases the system is able to deliver 100% of the DIT frames, when one selects the minimum DIT. This is yet not the case for the full frame option with DIT=0.35, which has an efficiency of ~70%, that is still 30 out 100 frames are lost when the cube is written to disk. The number of parameters involved is large and till we can improve the performance of this mode, the feasibility of cube-mode observations will be done on a case by case basis. Given the great interest shown for this mode by the community, we will offer it despite it is not fully implemented. Users should also be aware that the file headers will not complete. Given the need for real-time decisions, this mode will only be offered in VM

Spectroscopic modes

Nineteen different modes are offered for long slit spectroscopy between 0.9 and 4.2 microns.

For short wavelength (λ < 2.5μm), it is possible to perform slitless (14"×14") spectroscopy. Otherwise two slit widths are available: 86 and 172mas. The slits' length is 40" for the S/L54 camera and 28" for the S/L27 camera.

Typical spectral resolution are: 400 in J, 500 or 1500 in H, 700 or 1400 in K, 700 or 1100 in L, 550 in SHK (1.3-2.6 µm).

A special prism disperser covering the entire spectrum from J to M at once, with a spectral resolution from 40 (J band) to 250 (M band) is also offered (in visitor mode only).