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NACO Science Data:
Sparse Aperture Interferometric Masks (SAM)

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QC links:
  • Frame Type SCIENCE_SAM
  • INS.OPTI3.NAME 18Holes; 7Holes; 9Holes; BB_9holes

description: The Sparse Aperture interferometric Masks (SAM) mode was commissioned in March 2008 and offered to the public as of P82. The purpose of this mode is to obtain the very highest angular resolution at the diffraction limit. Four masks were manufactured to fit NACO specification - 18Holes, 7Holes, 9Holes, and BB_9holes. They were installed in the OPTI3 filter wheel of CONICA. For detailed description of the masks see the NACO SAM web pages. Sparse Aperture Masking is useful for very narrow fields of view (the outer limit is set by the resolution of the shortest baseline in the mask). Its supertiority over conventional full-pupil imaging applies only to typically several resolution elements of the PSF core. In the infrared, this typically means that SAM is useful for objects where the entire field of interest lies within several hundred milli-arcsecs from a bright star. (Although there may be ways to mosaic larger fields together, these have never been successfully demonstrated). In general, the more holes in the mask, the smaller the holes must be (to preserve non-redundancy) and consequently the less light is passed by the mask. To choose which masks is appropriate for scientific applications consult the NACO User Manual. To take a full advantage of the SAM technique it was estimated that for NACO the useful magnitude limit could be as faint as 10-12 mag.

In observations, the dominating source of background noise is turbulent atmosphere. Rapid readout of the detector array tends to freeze the motion of the interference fringes, reducing the impact of turbulances on the measured coherence of the incoming wavefront. Thus, preferably the SAM observations are taken in the cube mode, where the Double_ReadResetRead/HighDensity (DCR/HD) readout detector mode is used. Also, given the very small useful science field-of-view, only selected window of the array is read (e.g. 256 x 258 pixels), which further speeds up the read out time.

In addition, SAM must be combined with pupil tracking mode because the occultation of one of the mask holes by the telescope spiders would compromise the calibration properties of the experiment.

The NACO image of the pupil showing the telescope spiders.
An example of the SAM observations with the 7Holes mask. The frame is windowed to 256 x 258 pixels.

Given its complexity, SAM is offered only in the visitor mode. It is not supported by the pipeline.

calibrations:The calibration plan includes

  • standard detector darks with readout mode and DIT matching the one used for science;
  • standard sky (twilight) and/or internal flat fields without the mask;

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