Overview

NaCo provides adaptive optics assisted imaging, imaging polarimetry, coronography, sparse aperture masking and spectroscopy, in addition to a new "no AO" mode.  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.  The magnitude limit for correction depends on the spectral type of the Natural Guide Star; for reference partial correction can be obtained for targets as faint as V=16.7 (for F0V or later), or K=15.9 (for F0V or earlier).  The AO reference star can be either the science object itself or a close by star (within 55").

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 and estimate the achievable Strehl ratio of their observations.  The preparation software (NAOS-PS), a NaCo specific tool, is dedicated to the performance estimation of the AO system under user's given conditions, and can also be used to predict the Strehl ratio (note that the ETC now internally calls the NAOS-PS).

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

Starting in P95 and for on-axis bright targets and high Strehl ratios (>50% in K, > 30% in H), users should consider using the extreme AO instrument SPHERE (in particular it's NIR imager IRDIS, with 11" field of view).

Instrument Mode Offered observing mode [S/V] 1
Period P90 P91 P92

P93,P94,P95

Imaging, all filters (except Mp2) S/V S/V S/V S/V
Imaging + Cube mode S/V S/V S/V S/V
No AO ("speckle imaging") S/V S/V S/V S/V
Grism Spectroscopy S/V S/V S/V V
Apodizing Phase Plate (APP) Imaging S/V S/V S/V S/V
Apodizing Phase Plate (APP) Spectroscopy3 V V S/V -
Prism Spectroscopy V V V V
SDI+3 S/V S/V S/V -
Lyot Coronagraphy3 S/V S/V S/V -
AGPM Coronagraphy4 - V**** S/V V
4QPM Coronagraphy3 S/V S/V S/V -
SDI+43 - - - -
Wollaston Polarimetry S/V S/V S/V V
Any LGS observations5 S/V S/V S/V -
LGS Seeing Enhancement (SE)5 S/V S/V S/V -
SAM/SAMPol V V V V
Pupil tracking6 S/V
(except SAM/SAMPOL)
S/V (except SAM/SAMPOL) S/V (except SAM/SAMPOL) S/V (except SAM/SAMPOL)

1 Changes appear in color.

2 Mp imaging can be requested in VM only for bright stars using extremely short integration times and dithering (no chopping) at the expense of SNR.

Starting in P90 the SDI+4 is decommissioned due to a lack of demand and some technical difficulties. SDI+, APP Spectroscopy, Lyot/4QPM coronagraphy are discontinued since P93.

In November 2012 an annular groove phase mask (AGPM; commonly known as "vortex coronagraph") was inserted into NACO. It is optimized for 4 microns and was tested/commissioned successfully in December 2012 and January 2013. It is available for use since Period 91 in visitor mode only. 

5 LGS operations are discontinued since P93.

The PT drift was fixed in october 2011 allowing focal plane coronagraphy.

CONICA limiting magnitudes with the visual dichroic

Band
J
H
Ks
L'
M'[2]
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.
[2] Calculated with chopping (no longer offered since P86).

Imaging, polarimetric and coronagraphic modes

Wavelength range
Scale (mas/pix)
Field of view (arcsec)
SW filters [1]
54.3
56X56
SW filters
27.0
28X28
SW filters
13.3
14X14
NB 3.74, NB 4.05
54.7
56X56
NB 3.74, NB 4.05, L'
27.1
28X28
M'
27.1
14X14

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

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 the annular groove phase mask (AGPM)) which is optimized for use at 4 microns.  To observe very bright objects in imaging (without masking), one can insert a neutral density filter, reducing the flux by a factor of 80 (λ < 2.5μm) or 50 (λ > 3 μm).

To observe very bright objects in imaging (without coronographicmask), it is possible to insert a neutral density filter,reducing the intensity by a factor of 80 (λ < 2.5μm) or50 (λ > 3 μm).

Apodizing Phase Plate (APP) coronograph Imaging

Since Period 86, there is a new coronagraphic Apodizing Phase Plate (APP) placed in a pupil plane and designed to work in the 3 to 5 µm range.  It should be used together with the NB_4.05 filter.  It allows direct imaging (no masking) with an improved companion detection limit between 0.2 and 0.7” on one side of the PSF.. More information is available here.

Prism Spectroscopy

Since Period 87, Prism Spectroscopy is offered again in VM only.

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+).
Since P88, SDI+ is offered in service mode and can be combined with pupil tracking if no mask is inserted (SDI+ alone).

SAM/SAMPol: sparse aperture interferometry without and with Polarimetry

SAM uses special aperture masks in the pupil wheel toobtain the very highest angular resolution at the diffraction limit. There are currently 4 masks available in CONICA, with different characteristics (i.e. number of holes and hole 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. Interested users are advised to consult sources form the open literature concerning aperture masking.

SAMPol is equivalent to SAM with the additional insertion of the Wollaston_00 in the optical path.

Cube mode

Cube mode is a variant of the burst mode already offered with other ESO instruments.  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 lose frames, depending on the setup.  Some setups are safe for time resolved applications, if one assumes that the rate of frame generation is accurate.  Time resolved observations with a 1sec resolution have been proven successful.  Users interested in this mode must contact the Instrument Scientist (naco@eso.org) to confirm the feasibility of their program.

There are stringent limitations to the use of this mode.  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 approximately 500 MB.  Please see the User Manual for details of achievable DITs and possible frame losses.  This mode is offered in both SM and VM.

no AO

As of Period 86, an open-loop (no AO) imaging mode is offered with NAOS-CONICA. Associated with hardware windowing and fast readouts (cube), it allows the user to apply various speckle interferometry techniques. Some tests were carried out recently with NaCo and gave nice results using the following “poor man’s AO” techniques:

  • simple shift and add (SSA) or weighted shift and add (WSA) methods allowing to get Strehl  of ~10-40%.
  • speckle masking image reconstruction a la Weigelt (1977) allowing a very high strehl ratio but  on a small field (adapted to not so extended sources, multiple systems, etc.). It has given promising results on close binaries (high strehl ratios), please see Rengaswamy et al., 2014 (The Messenger, vol. 155, p. 12-16).
  • speckle holography (Petr 1998) allowing very precise astrometry over a somewhat large field of view, particularly adapted to crowded fields. Please see Schödel et al., 2013 (MNRAS, 429, 1367) and Schödel et al., 2012 (The Messenger, vol. 150, p. 26-29).
  • Etc.

Pupil tracking

Pupil tracking has been implemented for SAM mode and is also offered in combination with imaging since Period 82.

Starting in P90 - and since the PT drift was fixed in october 2011 -  Pupil tracking is offered for imaging (including APP) and AGPM coronagraphy in both SM and VM.

For observations that require a maximum of field rotation using pupil tracking (angular differential imaging), users should consult the NACO-specifics on the Service Mode Rules and Recommendations for Observation Blocks page, the OB naming Conventions page, and the Observing Constraints and Classification Rules page. 

Spectroscopic modes

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

Two slit widths are available: 86 and 172mas. Slitless spectroscopy is no longer offered.

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).