Discussion on beams

We measure the beam by using the middle scan of the 3-scans sequence launched with @beammap Here are the best-of on 3 sources.

Preliminary findings

The main beam (the Gaussian part) seems with a normal width aka 11", 17" (but only in single-pixel analysis). Blind 2D Gaussian fits to the maps result in broader beams of 13", 19.5" (see Table below).
The quadrupod holding the secondary gives clear diffraction marks as a faint cross at about -30dB, visible in all Mars scans. Uranus and 3C83 are not bright enough to reach -30dB. To go deeper, we need Saturn or Jupiter maps.
In between, there is a large near-side lobe, especially at 2mm (but also visible at 1mm), that shows up as a rectangular pyramid-like shape at the 10% level, the origin of which is unknown. It seems to scale with wavelength. It was not so apparent in NIKA. Therefore the quality of the dichroic is not at the center of this issue. In fact the rectangular shape of the first side lobe is known (by me only, I recognize I didn't communicate about it until now) and expected from optical simulations; see the figure at the bottom of this page. It is due to the combined diffraction on the primary mirror and the tetrapod holding the secondary mirror. -SL-
For comparison, see the NIKA-1 beam maps in Fig.8 of Catalano+2014 and in Fig.8 of Monfardini+2011.

The following table shows the FWHMs derived from 2D-Gaussian fits to the observed emission.

The image shows the beam at 1mm. For the 2mm band the shape is strictly identical, it is just twice bigger. (SL 2015-11-02)

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- * In between, there is a large near-side lobe, especially at 2mm (but also visible at 1mm), that shows up as a rectangular pyramid-like shape at the 10% level, the origin of which is unknown. It seems to scale with wavelength. It was not so apparent in NIKA. Therefore the quality of the dichroic is not at the center of this issue.
+ * In between, there is a large near-side lobe, especially at 2mm (but also visible at 1mm), that shows up as a rectangular pyramid-like shape at the 10% level, the origin of which is unknown. It seems to scale with wavelength. It was not so apparent in NIKA. Therefore the quality of the dichroic is not at the center of this issue. ''In fact the rectangular shape of the first side lobe is known (by me only, I recognize I didn't communicate about it until now) and expected from optical simulations; see the figure at the bottom of this page. It is due to the combined diffraction on the primary mirror and the tetrapod holding the secondary mirror. -SL-''