Observation campaign description

Detailled description of specific observing sessions are given here.

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Focus z campaign

Aim: characterizing the variations of the best focus across NIKA2 field of view. Several "focus z sessions" were done in NIKA2 Run3. However, the outermost regions of the array, where the axial focus is expected to show the largest variations w.r.t. the center of the array, were mainly probed. Here, the objective is to characterize the curvature of the focal plane and to confirm its flatness in the inner part of the arrays.

Method: We suggest to explore 2 perpendicular diameters in Nasmyth offset bins from 30 arcsec to 1 arcmin. The list of Nasmyth offsets is given at the end of the page (jump to list). For each Nasmyth offset location, the optimal axial focus is estimated from 5 x 1 arcmin square OTF scans at 5 different focus around the initial focus set in PAKO using the command "focusOTF". In each map produced using NK_RTA, the main beam is fitted using a 2D Gaussian function. Two optimal z focus estimates are fitted using NK_FOCUS_LISS_OLD, which resorts to a polynomial POLY_FIT function: the focus z that maximises the flux at the beam peak and the focus z that minimizes the main beam FWHM, the former being usually more precise.

Analysis: Several focus z campaigns were done during NIKA2 run 3, and were analysed. For information, a compilation of the results is available in the NIKA2 private page in http://www.iram.fr/wiki/nika2/index.php/Focus_variation_across_the_focal_plane.

referent person: Laurence Perotto (laurence.perotto@lpsc.in2p3.fr), Jean-François Lestrade

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Commands sequence

Below the detailled sequence of commands to be run is given.

1./ source selection

Choose a bright point source of several Jy (e.g. 3C84, 3C273, 3C279, ...) NB: Although 3C286 has also been used during Run 3, it is not bright enough (0.3 Jy) to yield very good focus measurements.

      PAKO> source 3C273
      PAKO> track
      PAKO> start

2./ pointing correction

Reminder: There are 2 methods, the pointing lissajous resorts to a small lissajous around the source, whereas the pointing cross does 2 perpendicular scans across the source. The pointing lissajous is often more reliable when the reference pixel is off-centered in Nasmyth coordinates (whenever the pointing model is not implemented after a proper pointing session).

      PAKO>@nkpoint l

or

      PAKO>@nkpoint b

in case of poor fit with a lissajous, try resorting to the 'common_mode_kids_out' method" by running IDL> nk_rta, 'scan', /mask

      PAKO>set pointing xx yy

3./ best focus at the array center f_0

     PAKO>offset 0. 0. /sys nasmyth
     PAKO>@focusOTF
enter 0 as the center focus f.

It will run 5 scans at focus f-0.8; f-0.4; f; f+0.4; f+0.8

Analyse each scan individualy using:

     IDL> nk_rta, 'scan', /nas

once the 5 scans are reduced, run (for ex.):

     IDL> nk_focus_liss_old, '20160125s'+strtrim([1,2,3,4,5], 2), /nas

4./ best focus across the field-of-view f_{off-center}

      PAKO>offset xoff yoff \sys nasmyth
      PAKO>@focusOTF
enter 0 as the center focus f.

Analyse each scan individualy using:

      IDL> nk_rta, 'scan', /nas, /xyguess, /largemap

once the 5 scans are reduced, run (for ex.):

      IDL> nk_focus_liss_old, '20160125s'+strtrim([1,2,3,4,5], 2), /nas, /xyguess

Please keep track of the (xoff, yoff) in TAPAS for easier analysis of the campaign results the list of nasmyth coordinates to be explored can be read on the A2 printed sheet.

NB: In case the focus@1mm and the focus@2mm are very discrepant, the focus exploration bin (-0.8, +0.8) may be not sufficient to fit simultaneously f_1 and f_2. Relaunch using f=<f_2>, the focus@2mm value from the previous focus campaign measurements:"

       PAKO>@focusOTF

enter <f_2> as the center focus f.

5./ monitoring of the focus temporal drift

Since we are interested in the difference between f_0 and various f_{off-center}, f_0 should be regularly checked (every few points depending on its stability).

re-iterate 3/ after 4 focus z measurements at non-zero Nasmyth offsets.

List of Nasmyth offsets

nasmyth_offset_focus_session_march2016.eps