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% inthe 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 starcan be either the science object itself or a closeby star (within 55").

Since 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. In order to achieve the best resolution with the LGS a natural guide star is required to correct for the tip-tilt motions, which are not sensed by the LGS (see below for the "Seeing Enhancer Mode"). 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. LGS is no longer offered as for P93. Sarting at the end of P93, NACO - formally on UT4 since 2001 - will be reinstalled on UT1.

In order to evaluate the feasibility of their project, users shouldfirst use the exposure time calculator (ETC) to estimate the expectedS/N ratio and estimate the achievable Strehl ratio of their observations. The preparation software (NAOS-PS), a NaCo specific tool,isdedicated to the performance estimation of the AO system under user'sgiven 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 carefullyweigh the advantages of using NaCo over other IR instruments, such asISAAC, which, in general, have larger fields of view, lowerbackgrounds, slightly higher throughputs, and lower overheads.

Instrument Mode Offered observing mode [S/V] *
Period P85 P86 P87 P88, P89 P90 P91 P92

P93/P94

Imaging, all filters (except Mp***) S/V S/V S/V S/V S/V S/V S/V S/V
Chopping*** S/V (some issues) - - - - - - -
Imaging + Cube mode S/V S/V S/V S/V S/V S/V S/V S/V
No AO ("speckle imaging") - S/V S/V S/V S/V S/V S/V S/V
Grism Spectroscopy V S/V S/V S/V S/V S/V S/V V
Apodizing Phase Plate (APP) Imaging - S/V S/V S/V S/V S/V S/V S/V
Apodizing Phase Plate (APP) Spectroscopy - - V V V V S/V -
Prism Spectroscopy - - V V V V V V
SDI+ S/V S/V S/V S/V S/V S/V S/V -
Lyot Coronagraphy V V V S or V (if pupil tracking) S/V S/V S/V -
AGPM Coronagraphy**** - - - - - V**** S/V V
4QPM Coronagraphy V V V V S/V S/V S/V -
SDI+4 V V V V - - - -
Wollaston Polarimetry S/V S/V S/V S/V S/V S/V S/V V
Any LGS observations* S/V S/V S/V S/V S/V S/V S/V -
LGS Seeing Enhancement (SE) S/V S/V S/V S/V S/V S/V S/V -
SAM/SAMPol V V V V V V V V
Pupil tracking V
S/V (direct imaging and APP)
V (otherwise)
S/V (direct and APP Imaging)
V (otherwise)
S/V (direct, SDI+ and APP Imaging)
V (otherwise)
S/V
(except SAM/SAMPOL)
S/V (except SAM/SAMPOL) S/V (except SAM/SAMPOL) S/V (except SAM/SAMPOL)

* Changes appear in color.
** Note that LGS will, as a rule, NOT be offered with modes that require VM (i.e complex modes).
*** Since P86 chopping is no longer offered. 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.
- Fabry-Perot imaging has not been since Period 82.
- Polarimetry with wire grids and coronagraphy with the semitransparent mask plus the S13 camera have been discontinued as of Period 83.
- Starting in P90 - and since the PT drift was fixed in october 2011 - PT is offered for all imaging (including APP) and coronagraphic modes (including Lyot and 4QPM) in both SM and VM.
- Starting in P90 the SDI+4 is decommissioned due to a lack of demand and some technical difficulties.
****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 in Period 91 in visitor mode only (via a setup change request prior observing). As part of the finalization of the commissioning the User Manual and online documentation have been updated. Please see the P92 version of the manual section 5.3.

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 coronographic 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. 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(0.3% (0.4%) transmission for Ks (H)) 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 otherfor λ =1.60 μm (reduction factor of ~100 at 2.18μm, FOV of 13" x 13" and 8" x 8", respectively).
Since P88, Lyot Coronagraphy is offered in service mode but only without pupil tracking (conventional field tracking).

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 coronographic Apodizing Phase Plate (APP)placed in a pupil plane and designed to work inthe 3 to 5 µm range. Although it was offered to work with the  NB_4.05filter and a new IB_4.05 filter, commissioning of the  IB_4.05 filter revealed it to be unsuitable and is therefore decommissioned.  It allows direct imaging (nomask) with an improved companion detection limit between 0.2 and 0.7”on one side of the PSF. Further commisioning results will be made available on the NACO web pages as soon as possible. More information is available here as well as the commissioning data (April 2010).

Apodizing Phase Plate (APP) coronograph Spectroscopy

Since Period 87, the APP can be combined with long slit grism spectroscopy and offers resolutions ranging from 350 to 1100 upon limited setups (L27 and L54, Lp or L only). Visitor mode only. More information will be 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).

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/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 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 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 aproximtely 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 withNaCo 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.). This project is
    lead by R. Sridharan (srengasw@eso.org) and gave promising results on close binaries (high strehl ratios).
  • speckle holography (Petr 1998) allowing very precise astrometry over a somewhat large field of view (lead R. Schodel, rainer@iaa.es).
  • Etc.

Pupil tracking

Pupil tracking has been implemented for SAM mode and is also offeredin combination with imaging and 4QPM coronagraphy since Period 82.

In this mode, the telescope, independently from NaCo, tracks the pupil instead of the field. For example, a double star will be seen to rotate in subsequent frames while the pupil spiders will remain fixed. Interested users should mention their need for this particular mode in the proposal form under Sections 8 and 14. Given its complexity and novelty, pupil tracking was only offered in VM until Period 85. Starting with Period 86, pupil tracking is offered in SM as well only for simple imaging and APP (without coronographic or SAM masks). When observations take place close to zenith, the pupil rotation compensation is faster and a drift of the order of ~1 pix/minute is observed on the detector plane (at airmass of ~1.05). It can easily be corrected for if the DIT is small (i.e cube mode) in the case of simple imaging (by shift and add methods). However, it’s not yet understood enough to offer PT associated with coronography or SAM in VM.

Starting in P90 - and since the drift described above was fixed in october 2011 - PT is offered for all imaging (including APP) and coronagraphic modes (including Lyot and 4QPM) in both SM and VM.

For OBs that need a maximum of field rotation using pupil tracking (angular differential imaging), users are encouraged to use the _ in their OB name. That way the operator will know this OB has to be observed around the meridian. Beware that in Paranal, the latitude is -24deg 35'. If the declination of the object is within 4 deg of this value, the telescope the meridian and therefore these objects must be observed either before or after the meridian (LST +/- 10 minutes). In addition, explain explicitly in the ReadMe file at which LST the OB should be started, taking into account the preset and acquisition overheads.

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).

Seeing enhancer mode (LGS SE)

Starting with Period 85, a new mode, the so-called seeing-enhancer mode, is offered with the LGS. This mode does not need a tip-tilt star, and will allow better image quality than simple imaging, but worse than what can be obtained with full adaptive optics correction. For an idea of the expected performances, users are encouraged to either use the NAOS-PS or Table 4-4 of the NACO Users Manual.