Overview

NaCo provides adaptive optics assisted imaging, imaging polarimetry, coronography, sparse aperture masking and  "no AO" imaging.  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).

News (P94-P95): Since the installation of the new Aladdin2 detector at the end of 2014, CONICA features an additional background noise of about 80e-/pix/sec. This noise can be modeled as an increased sky background contribution, corresponding to an increase of 0.65 mag/arcsec in Ks and 0.75 magnitude in H. The ETC has been updated accordingly. The problem is still present after the intervention in October and November 2015, see the news page for an example 300s dark. At the time of writing (February 2016), the lower left quadrant also has no signal in every eigth column.

 

Instrument Mode        
Period               P98       P99(1)    P100   P101
Imaging, all filters               S/V except Mp(2)       S/V(3)    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
AGPM Coronagraphy          V (LW only)       S/V    S/V     S/V
Wollaston Polarimetry                V       S/V    S/V   S/V
SAM/SAMPol                V       S/V    S/V   S/V
Pupil tracking S/V (except SAM/SAMPOL)
      S/V    S/V   S/V

(1) Changes appear in colour.

(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.

(3) Starting from P99, Mp imaging can be done in SM, following the acceptence of a waiver to discuss the observing strategy, i.e. bright stars, extremely short integration times (windowing necessary) and dithering (no chopping).

 

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 (only AGPM at L-band)

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).

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 is offered for imaging, coronography (AGPM), and  SAM mode.

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.