Discussion on NIKA-2 beams

Page created by FXD, CK
Last updated by CK 2016-02-02, SL 2015-11-02, JFL 2015-11-07, NB 2015-11-11

Back to NIKA2 Run1 Main page

If not said otherwise, we measure the beam by using the middle scan of the 3-scans sequence launched with @beammap . These are 20'x20' maps done on-the-fly with 55"/sec scanning speed. Here, we show the best-of on 3 sources.

Astigmatism

JP is constantly monitoring the temperature structure of the telescope via many sensors placed at different parts of the antenna. This allows him to predict the amplitude of astigmatism. Attached is a plot showing this predicted amplitude and its variation during last month February. There is a good correlation with the temperature difference between the backstructure and the reference sensors placed in the top part of the yoke. Notice also the often marked day/night variation.

If we suspect that the 10% shoulder seen in the beam profiles of NIKA2 stem from astigmatism, we should often notice a diurnal variation. That is something to be checked.

resumeAstigm-Feb2016.pdf Predicted astigmatism amplitude of the primary mirror from temperature sensor values for February 2016 by JP.

Comparison of NIKA2 beam with NIKA1, GISMO, EMIR (CK, 02-Feb-2016)

NIKA1 and NIKA2

GISMO 2mm

GISMOrun8medFHWMallRPs_YES_YES.png

Conclusions

Primordial problem: synchronization (RZ, 2016-01-11)

The synchronization problems btw the receiver and the telescope show up in the standard maps az a zig-zag of the source position from one row to the next. The time difference with respect to the master telescope clock is given by deltaT = 0.5*zigZag/scanningVel

In the example shown below the zigZag in the scanning direction is ~3.5arcsec for scanningVel ~55arcsec/sec. This gives deltaT~30msec btw A_t_utc and the master telescope clock. (I chose as example an out-of-focus map because due to the astigmatism the Main Beam becomes elliptical, with the minor axis even smaller than the nominal value and the increased major axis allows to compare more rows on source, when the elongation is perpendicular to the scanning direction.)

compCTB_NIKA-3-20151130s7.png NIKA-3-20151130s7.png NIKA-3-20151130s7_021.png

left: in red - the source distribution in two adjacent rows using A_t_utc, in black - using A_t_utc+30msec; centre: beam without, right: with A_t_utc correction. As in this map all usable beams show identical zigZag anomalous refraction as a reason for this zigZag can be ruled out.

However, as the time difference btw the two NIKA2 clocks shows a considerable variation the time difference btw NIKA2 A_t_utc clock and the master telescope clock is most probably not constant. This is very hard to demonstrate using on-sky data as the atmosphere limits the determination of deltaT from the zigZags to ~5-10msec at best (depending on scanning_vel).

UTCdiffAll_records.png

above: A_t_utc - B_t_utc for 52 scans

For completeness another synchro-problem has to be mentioned, though it does not really affect the calculation of the Main Beam size: the synchronization between the commanded action of the telescope (subscan structure) and the NIKA2 data acquisition. The switch on and off of NIKA2 does not correspond to the subscan starts/ends. The time difference might be even 1min ! Observations which require tuning per subscan (e.g. skydips) were therefore completely desynchronized.

Last not least: the described synchro-problems are not new, i.e. special for NIKA2; they are documented since the first run with NIKA.

More on NIKA2 beams change across the FoV (JFMP, 2016-01-03)

As discussed below by RZ we observe large variations on the shape of the NIKA2 beams across the FOV. RZ on one hand and BC on the other and have shown with independent analysis that the NIKA2 beams get wider and more distorted as we move away from the center of the FOV, while homogeneous beam shapes are expected from optics design (SL). To check this in more details we have checked the geometry beam maps KID per KID and identify the following cat├ętories: not distorted, distorted beams, double KIDs, very close double KIDs, and identical KIDs:

The distribution of those categories across the FOV are represented in the figures below.

beam_fov_array1.jpeg beam_fov_array2.jpeg

We observe that as identified by RZ, BC and SL, most of the distorted beams are in the edges of the FOV, however this is not the case for all. We observe a relatively important number of double or very close double (distorted beams showing two main beam like features). Finally we have identfied two couples of identical KIDs (exactly same properties).

Given the observed of the NIKA2 beams we need most probably to perform the caracterisation of the NIKA2 beams per KID. This might be one of the priorities of next observation campaign. We probably need to combine various beammap scans and/or improve the observation step.

NIKA2 beams change strongly across the FoV (RZ, 2015-12-03)

The NIKA2 beams become very elliptical to the edges of the arrays, even with a ghost secondary beam @2mm (can hardly call it a side lobe). Therefore the analysis of the average beam cannot help much to understand the physics behind.

Further problem is the stability of the data. All 3 arrays show large (i.e. at a couple of Jy level) different kind instabilities, with even >50% of pixels affected. This strongly affects the concerned beams, thus also the average.

NIKA-2-20151128s315_rp114.png NIKA-2-20151128s315_rp65.png NIKA-2-20151128s315_rp433.png

Examples of 2mm beams in the central region and at the edge of the array. For the 1mm beams see DailyReportsNika2Run2, Nov 26.

Beam maps on Mars

Beam maps on Uranus

Maps on 3C84

Series of small maps on 3C84 by JFL under poor conditions

Preliminary findings

Maps seem too small ! Comment by RZ

It seems to me that one important problem was not taken into account while analysing the beams: the incorrect coordinates during roughly 2 sec at beginning of each subscan (see e.g. daily reports, Oct 29-30 and my other reports during the past runs). More recent examples on 3C84 are shown below.

antenna-20151106s17_azErrSu4.png antenna-20151106s17_azErrSu5.png

In blue are shown the available tracking errors, in white the correct values. How large is the error of the coordinates depends on the elevation and the scanning speed. For usable obs. parameters the values may reach even 30arcsec ! Though this scan was not used for the beam analysis the shown examples (i.e. 20151029s95 & 96, 20151106s17) are representative for all OTF observations.

In such a case the general condition for map size in the scanning direction in case of not perfectly stable receiver:

mapSize = sizeOfSource + sizeOfFOV + base

must be changed to:

mapSize = sizeOfSource + sizeOfFOV + base + 2*sizeOfErrorZone

sizeOfSource is the diameter of the to be analysed error beam (read from the attached figures 1 to 2arcmin), sizeOfFOV is 6.5arcmin, base should be at least 3*HPBW, sizeOfErrorZone is ~2sec*scanning velocity, i.e. ~2arcmin.

This shows that only the 20arcmin maps satisfy the above condition. For smaller maps the pixels which appear at map edges must be excluded from the analysis. As the array rotates with elev in (azim,elev) different pixels are affected. (RZ, 6-Nov-2015)

Theoretical beams from Zemax simulations (for comparison, SL 2015-11-02 and 04)

By design of the optics the image plane is as flat and aberration-less as possible, but for such a big FoV and given the constraint on the number of lenses we can't avoid that the optimal focus surface on the image plane has a residual bowl shape, which correspond to 0.4 mm amplitude of M2 along Z between the central pixel and an edge pixel (6.5'diameter ring). But the distance along the M2 Z axis between the best central and the best average is 0.2mm (= best at the 4' diameter ring). As a consequence there's a Strehl ratio (~beam peak amplitude) variation of 10% at 1.2mm and 4% at 2mm along the FoV if the focus is on the central pixel, but these values are reduced to less than 3% at 1.2mm and less than 1% at 2mm if the focus is on the 4' diameter ring.

The images below show the 1mm band beam shape in false color and logarithmic scale (top), and a cross section in linear scale (bottom), for the central pixel (left), an edge pixel at Y=6.5' (center) and an edge pixel at X=6.5' (right) for various position of the focus (note the 2mm band beam shape is identical to the 1mm beam, but it is larger and twice less sensitive to the focus variation). I also added plots of the Strehl ratio along the FOV on the X axis of the image plane from 0 to 3.25' radius. For both the focus on the central pixel, or the best average, we are still significantly above the diffraction limit; in the worst case of 10% decrease of the Strehl ratio (e.i. focus on the central pixel), we convolve a 0.2 mm radius aberration spot with a 1.7 mm radius Airy pattern for the central pixel and a 0.8 mm radius aberration spot with a 1.7 mm radius Airy pattern for the edge pixel, while the aberration spot variation is at most +/- 0.1 mm on the image plane for the best average focus. Thus, the beams are always acceptable to maintain the angular resolution, but less optimal in terms of contrast on the image (e.i. dynamic, e.i. gain) if we focus on the central pixel versus the best average focus.

Table of the HPBW for the central and an edge pixel, at central focus an best average, based in Zemax simulation (linear plots below) and pixel physical size (illustrate well that the effect of focus inhomogeneity on HPBW is much smaller than on the Strehl ratio, and more importantly much smaller than the variations on real measurements due to other factors):

One question is the size of the main beam (resolution) the other is the efficiency of the main beam (contrast). The main beam efficiency apparently changes quite a lot across the FoV depending on the z focus. This is of course a problem for interpretation of the data (-> astrophysics). Another problem however appears before this, it concerns the data processing: as the SNR of the data increases during the processing by factors even >100 (mainly because the correlated signal (mostly the sky noise) can be filtered out) the change of the main beam efficiency across the FoV sets in general very severe limits for the quality of the processing !

Further, in practice it will not be easy to put the focus to the "best value at the 4' diameter ring" using the focus on the central pixels + offset (every measurement has an error, for the focus it is hard to get better than ~0.1mm) and - of course - the focus DOES change with time. The normal situation therefore will be that most pixels will have much larger error beams than in the optimal case. (RZ 9-Nov)

To improve the overall beam efficiency for all pixels, we could add a constant offset to each focus, i.e. to the measured best focus value of the central pixels at a given time. This constant offsets could come from the optical simulations and the predicted variation of the Strehl ratio. (CK 10-Nov)

Figures below: 1mm band beam in logarithmic scale (top), and a cross section in linear scale (bottom), for the central pixel (left), an edge pixel at Y=6.5' (center) and an edge pixel at X=6.5' (right). Strehl ratio along the FOV on the X axis of the image plane from 0 to 3.25' radius.

  1. Focus optimized on the central pixel:

    Beams_NIKA2_0_Y6p5am_X6p5am_OptimCentral_M2-1p14.PNG Beams_NIKA2_Strehl_alongX_central_pixel_focus_1mm_band.PNG

  2. Focus optimized on the 4'FOV ring = best overall focus on the array = best central + 0.2 mm:

    Beams_NIKA2_0_Y6p5am_X6p5am_OptimGlobal_M2-1p0.PNG Beams_NIKA2_Strehl_alongX_best_average_focus_1mm_band.PNG

  3. Defocus: best central - 0.5 mm:

    Beams_NIKA2_0_Y6p5am_X6p5am_Defoc_M2-1p64.PNG Beams_NIKA2_Strehl_alongX_defocus_M2-1p64_1mm_band.PNG

  4. Defocus: best overall - 0.5 mm:

    Beams_NIKA2_0_Y6p5am_X6p5am_Defoc_M2-1p5.PNG Beams_NIKA2_Strehl_alongX_defocus_M2-1p5_1mm_band.PNG

  5. Defocus: best central + 0.5 mm:

    Beams_NIKA2_0_Y6p5am_X6p5am_Defoc_M2-0p64.PNG Beams_NIKA2_Strehl_alongX_defocus_M2-0p64_1mm_band.PNG

  6. Defocus: best overall + 0.5 mm:

    Beams_NIKA2_0_Y6p5am_X6p5am_Defoc_M2-0p5.PNG Beams_NIKA2_Strehl_alongX_defocus_M2-0p5_1mm_band.PNG Beams_NIKA2_0_Y6p5am_X6p5am_Defoc_M2-0p5.PNG Beams_NIKA2_Strehl_alongX_defocus_M2-0p5_1mm_band.PNG

Beam Profiles (NB)

Fits for Beam Maps, radial profiles, integrals, NIKA2-EMIR (October-2015, FXD-NP, CK)

OffProcNika2Run1Beam (last edited 2017-08-08 08:26:51 by NikaBolometer)