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;