Quad matching in DIPOL polarimetry images

%autoawait off
%matplotlib inline
%run 01_notebook_configuration.py

# To reduce disk usage, only a central cut of of the full field of the DIPOL 
# camera is actually saved.

# polarimetry (cut)
from IPython.display import display
from iop4lib.db import RawFit, ReducedFit
from iop4lib.utils.plotting import imshow_w_sources
rf_pol = RawFit.by_fileloc(f"OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts")
display(rf_pol)
imshow_w_sources(rf_pol.mdata)

RawFit(id=6):
- telescope: OSN-T090
- night: 2023-11-06
- filename: BLLAC_R_IAR-0760.fts
- instrument: DIPOL
- imgtype: LIGHT
- size: 1104x896
- obsmode: POLARIMETRY
- band: R
- exptime: 20.0
- flags: DOWNLOADED,CLASSIFIED,BUILT_REDUCED

The position of the cut is defined in the header of the raw file

rf_pol.header['XORGSUBF'], rf_pol.header['YORGSUBF']

(1496, 1000)

We can compare it with the full field image of the photometry field

# photometry filed (full field)
rf_phot = RawFit.by_fileloc(f"OSN-T090/2023-11-06/BLLac_IAR-0001R.fit")
display(rf_phot)
imshow_w_sources(rf_phot.mdata)

RawFit(id=8):
- telescope: OSN-T090
- night: 2023-11-06
- filename: BLLac_IAR-0001R.fit
- instrument: DIPOL
- imgtype: LIGHT
- size: 4144x2822
- obsmode: PHOTOMETRY
- band: R
- exptime: 300.0
- flags: DOWNLOADED,CLASSIFIED,BUILT_REDUCED

Let’s build the photometry field first

redf_phot = ReducedFit.create(rf_phot)
redf_phot.build_file()

2026-04-18 15:02 - pid 7432 - [reducedfit.py:121] - DEBUG - DB entry of ReducedFit for OSN-T090/2023-11-06/BLLac_IAR-0001R.fit already exists, it will be used instead.
2026-04-18 15:02 - pid 7432 - [instrument.py:391] - WARNING - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: masterflat in this epoch could not be found, attemptying adjacent epochs.
2026-04-18 15:02 - pid 7432 - [instrument.py:369] - WARNING - Master Flat from epoch <Epoch 1 | OSN-T090/2023-12-13> is more than 1 week away from epoch <Epoch 3 | OSN-T090/2023-11-06>.
2026-04-18 15:02 - pid 7432 - [instrument.py:498] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: building file
2026-04-18 15:02 - pid 7432 - [dipol.py:235] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: applying masters
2026-04-18 15:02 - pid 7432 - [instrument.py:504] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: performing astrometric calibration
2026-04-18 15:02 - pid 7432 - [astrometry.py:214] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: 9 different combinations of parameters to try.
2026-04-18 15:02 - pid 7432 - [astrometry.py:217] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: attempt 1 / 9, ({'output_logodds_threshold': 14, 'position_hint': PositionHint(ra_deg=330.6804125, dec_deg=42.27766361111111, radius_deg=0.23137333333333335), 'size_hint': SizeHint(lower_arcsec_per_pixel=0.1273, upper_arcsec_per_pixel=0.14070000000000002), 'bkg_filter_size': 7, 'bkg_box_size': 64, 'seg_kernel_size': None, 'npixels': 128, 'seg_fwhm': 3.0, 'n_rms_seg': 0.33, 'keep_n_seg': 200, 'border_margin_px': 20, 'disp_sign_mean': array([-209.,   13.]), 'disp_sign_err': array([12., 16.])}) ...
2026-04-18 15:02 - pid 7432 - [astrometry.py:299] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: len(pos_seg)=354
2026-04-18 15:02 - pid 7432 - [astrometry.py:302] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: Keeping only 200 brightest segments.
2026-04-18 15:02 - pid 7432 - [astrometry.py:306] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: Removing segments within 20 px from border.
2026-04-18 15:02 - pid 7432 - [astrometry.py:316] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: seg pairs xy -> 94, disp_sign_xy=[-209.   13.]
2026-04-18 15:02 - pid 7432 - [astrometry.py:318] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: seg pairs xy best -> 83 (43.2%), seg_disp_sign_xy_best=[-209.51711583   12.94073723]
2026-04-18 15:02 - pid 7432 - [astrometry.py:349] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: trying to solve astrometry with Seg Best XY Pairs (n=83) (output_logodds_threshold=14).
2026-04-18 15:02 - pid 7466 - [__init__.py:215] - INFO - loaded 240 index files
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: start
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[0, 25) (1 / 3), index="5200/index-5200-14.fits" (1 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[0, 25) (1 / 3), index="5200/index-5201-14.fits" (2 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[0, 25) (1 / 3), index="5200/index-5202-14.fits" (3 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[0, 25) (1 / 3), index="5200/index-5203-14.fits" (4 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[0, 25) (1 / 3), index="5200/index-5204-14.fits" (5 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: slice=[25, 50) (2 / 3), index="5200/index-5200-14.fits" (1 / 5)
2026-04-18 15:02 - pid 7466 - [__init__.py:297] - INFO - solve 1: logodds=388.455, matches=69, conflicts=0, distractors=9, ra=330.684, dec=42.2758, scale=0.133503, index="5200/index-5200-14.fits"
2026-04-18 15:02 - pid 7432 - [astrometry.py:364] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: Seg Best XY Pairs (n=83) worked.
2026-04-18 15:02 - pid 7432 - [astrometry.py:365] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: bm.index_path=PosixPath('/mnt/astrometry_cache/5200/index-5200-14.fits')
2026-04-18 15:02 - pid 7432 - [astrometry.py:366] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: bm.center_ra_deg=330.68364760181345
2026-04-18 15:02 - pid 7432 - [astrometry.py:367] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: bm.center_dec_deg=42.27578281844256
2026-04-18 15:02 - pid 7432 - [astrometry.py:368] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: bm.scale_arcsec_per_pixel=0.13350302923020646
2026-04-18 15:02 - pid 7432 - [astrometry.py:369] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: bm.logodds=388.45477294921875
2026-04-18 15:02 - pid 7432 - [astrometry.py:222] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: WCS built with attempt 1 / 9 ({'output_logodds_threshold': 14, 'position_hint': PositionHint(ra_deg=330.6804125, dec_deg=42.27766361111111, radius_deg=0.23137333333333335), 'size_hint': SizeHint(lower_arcsec_per_pixel=0.1273, upper_arcsec_per_pixel=0.14070000000000002), 'bkg_filter_size': 7, 'bkg_box_size': 64, 'seg_kernel_size': None, 'npixels': 128, 'seg_fwhm': 3.0, 'n_rms_seg': 0.33, 'keep_n_seg': 200, 'border_margin_px': 20, 'disp_sign_mean': array([-209.,   13.]), 'disp_sign_err': array([12., 16.])}).
2026-04-18 15:02 - pid 7432 - [astrometry.py:234] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: building summary images.
2026-04-18 15:02 - pid 7432 - [plotting.py:477] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: plotting astrometry summary image of background substraction results
2026-04-18 15:02 - pid 7432 - [plotting.py:489] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: plotting astrometry summary image of segmentation results
2026-04-18 15:02 - pid 7432 - [plotting.py:589] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: plotting astrometry summary image of astrometry results
2026-04-18 15:02 - pid 7432 - [plotting.py:215] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: found 4 catalog sources in field: [AstroSource.objects.get(name='2200+420'), AstroSource.objects.get(name='BL Lacertae C'), AstroSource.objects.get(name='BL Lacertae test 2'), AstroSource.objects.get(name='BL Lacertae test 4')]
2026-04-18 15:02 - pid 7432 - [connectionpool.py:1049] - DEBUG - Starting new HTTPS connection (1): simbad.cds.unistra.fr:443
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "GET /simbad/sim-tap/capabilities HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [logger.py:235] - WARNING - AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version. 
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [instrument.py:443] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: saving WCSs to FITS header.
2026-04-18 15:02 - pid 7432 - [instrument.py:515] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: astrometric calibration was successful.
2026-04-18 15:02 - pid 7432 - [instrument.py:518] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: searching for sources in field...
2026-04-18 15:02 - pid 7432 - [instrument.py:521] - DEBUG - <ReducedFit 8 | OSN-T090/2023-11-06/BLLac_IAR-0001R.fit>: found 4 sources in field.

WARNING: AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version.  [iop4lib.utils]

As we can see, the astrometric calibration was done with the blind astrometry.net solver as before.

Next, if we go to the polarimetry field, we see that there are not enough stars in the image to do the blind astrometric calibration.

The calibration of this file will use the previously calibrated photometry field and compare quads of stars in both images to find the transformation between them, as we can see in the log:

redf_pol = ReducedFit.create(rf_pol)
redf_pol.build_file()

2026-04-18 15:02 - pid 7432 - [reducedfit.py:121] - DEBUG - DB entry of ReducedFit for OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts already exists, it will be used instead.
2026-04-18 15:02 - pid 7432 - [instrument.py:391] - WARNING - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: masterflat in this epoch could not be found, attemptying adjacent epochs.
2026-04-18 15:02 - pid 7432 - [instrument.py:498] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: building file
2026-04-18 15:02 - pid 7432 - [dipol.py:235] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: applying masters
2026-04-18 15:02 - pid 7432 - [instrument.py:504] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: performing astrometric calibration
2026-04-18 15:02 - pid 7432 - [logger.py:235] - WARNING - AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version. 
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [dipol.py:465] - DEBUG - target_src.srctype='blazar'
2026-04-18 15:02 - pid 7432 - [dipol.py:466] - DEBUG - n_estimate=11
2026-04-18 15:02 - pid 7432 - [dipol.py:467] - DEBUG - n_estimate_centered=5
2026-04-18 15:02 - pid 7432 - [dipol.py:468] - DEBUG - redf_phot=ReducedFit.objects.get(id=8)
2026-04-18 15:02 - pid 7432 - [dipol.py:469] - DEBUG - n_expected_simbad_sources=0
2026-04-18 15:02 - pid 7432 - [dipol.py:470] - DEBUG - n_expected_calibrators=4
2026-04-18 15:02 - pid 7432 - [dipol.py:524] - DEBUG - Trying attempt: {'method': '_build_wcs_for_polarimetry_images_photo_quads', 'conds': {'redf_phot__ne': None}, 'args': {'n_seg_threshold': [1.1, 1.0, 0.9], 'npixels': [64, 32], 'min_quad_distance': [4.0, 8.0]}} for <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>.
2026-04-18 15:02 - pid 7432 - [dipol.py:536] - INFO - Attempt: {'method': '_build_wcs_for_polarimetry_images_photo_quads', 'conds': {'redf_phot__ne': None}, 'args': {'n_seg_threshold': [1.1, 1.0, 0.9], 'npixels': [64, 32], 'min_quad_distance': [4.0, 8.0]}}: _build_wcs_for_polarimetry_images_photo_quads with {'n_seg_threshold': 1.1, 'npixels': 64, 'min_quad_distance': 4.0} for <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>.
2026-04-18 15:02 - pid 7432 - [dipol.py:677] - DEBUG - Invoked with n_seg_threshold=1.1, npixels=64
2026-04-18 15:02 - pid 7432 - [dipol.py:701] - DEBUG - Using 10 sources in polarimetry field and 10 in photometry field.
2026-04-18 15:02 - pid 7432 - [dipol.py:704] - DEBUG - Building summary image for astrometry detected sources.
2026-04-18 15:02 - pid 7432 - [dipol.py:793] - DEBUG - Selected 5 quads with distance < 4.0. I will get the one with less deviation from the median linear transform.
2026-04-18 15:02 - pid 7432 - [dipol.py:801] - DEBUG - Filtering out big translations (<1400 px)
2026-04-18 15:02 - pid 7432 - [dipol.py:805] - DEBUG - Filtering large transformations
2026-04-18 15:02 - pid 7432 - [dipol.py:813] - DEBUG - Filtered to 5 quads with distance < 4.0 and translation < 1400 px.
2026-04-18 15:02 - pid 7432 - [dipol.py:854] - DEBUG - median_t=array([100.24747473,  27.39696465])
2026-04-18 15:02 - pid 7432 - [dipol.py:860] - DEBUG - Selected the quads [7,1]
2026-04-18 15:02 - pid 7432 - [dipol.py:864] - DEBUG - t = [100.18744966  27.39696465], R = [[ 9.99999897e-01 -4.53157715e-04]
 [ 4.53157715e-04  9.99999897e-01]]
2026-04-18 15:02 - pid 7432 - [dipol.py:865] - DEBUG - det R = 1.0000000000000002
2026-04-18 15:02 - pid 7432 - [dipol.py:869] - DEBUG - Building summary image for quad matching.
2026-04-18 15:02 - pid 7432 - [__init__.py:186] - WARNING - Less than 5 calibrated fits for DIPOL 2200+420, using all of them
2026-04-18 15:02 - pid 7432 - [dipol.py:912] - DEBUG - Using angle=181.17162179631404 for pre wcs.
2026-04-18 15:02 - pid 7432 - [dipol.py:934] - DEBUG - Building summary image for astrometry.
2026-04-18 15:02 - pid 7432 - [plotting.py:215] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: found 3 catalog sources in field: [AstroSource.objects.get(name='2200+420'), AstroSource.objects.get(name='BL Lacertae C'), AstroSource.objects.get(name='BL Lacertae test 4')]
2026-04-18 15:02 - pid 7432 - [logger.py:235] - WARNING - AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version. 
2026-04-18 15:02 - pid 7432 - [connectionpool.py:544] - DEBUG - https://simbad.cds.unistra.fr:443 "POST /simbad/sim-tap/sync HTTP/1.1" 200 None
2026-04-18 15:02 - pid 7432 - [instrument.py:443] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: saving WCSs to FITS header.
2026-04-18 15:02 - pid 7432 - [instrument.py:515] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: astrometric calibration was successful.
2026-04-18 15:02 - pid 7432 - [instrument.py:518] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: searching for sources in field...
2026-04-18 15:02 - pid 7432 - [instrument.py:521] - DEBUG - <ReducedFit 6 | OSN-T090/2023-11-06/BLLAC_R_IAR-0760.fts>: found 3 sources in field.

WARNING: AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version.  [iop4lib.utils]
WARNING: AstropyDeprecationWarning: "epoch" was deprecated in version 0.4.8 and will be removed in a future version.  [iop4lib.utils]

!ls {redf_pol.filedpropdir}

from IPython.display import Image
Image(filename=f"{redf_pol.filedpropdir}/astrometry_matched_quads.png")

BLLAC_R_IAR-0760.fts		 astrometry_matched_quads.png
astrometry_detected_sources.png  astrometry_summary.png
astrometry_info

The image shows the quads of stars that were matched in both images, their distance in the hash space and the distance in the image space.

This quad matching is necessary because the number of stars in the polarimetry field is not enough for the astrometry.net solver to work, plus the scale of the images is too small for the default index files. This quad matching needs at least 4 (ideally a few more) stars in the image, and a previously calibrated photometry field. If any of these conditions are not met, other methods must be used to calibrate the image (if there are only two bright sources at the right distance, can be assumed that they are the target star, and if there are between 3 and 4-6 stars, we can try to match to known sources in the field).

The process is implemented in the DIPOL._build_wcs_for_polarimetry_images_photo_quads method, which is called by the ReducedFit.astrometric_calibration method. We could also manually follow the process:

import numpy as np
import itertools
from iop4lib.instruments import DIPOL
from iop4lib.utils.astrometry import BuildWCSResult
from photutils.aperture import CircularAperture
from pathlib import Path
import matplotlib as mplt
import matplotlib.pyplot as plt

# get the subframe of the photometry field that corresponds to this polarimetry field, (approx)
x_start = redf_pol.rawfit.header['XORGSUBF']
y_start = redf_pol.rawfit.header['YORGSUBF']

x_end = x_start + redf_pol.rawfit.header['NAXIS1']
y_end = y_start + redf_pol.rawfit.header['NAXIS2']

idx = np.s_[y_start:y_end, x_start:x_end]

photdata_subframe = redf_phot.mdata[idx]

# find 10 brightest sources in each field

sets_L = list()

for redf, data in zip([redf_pol, redf_phot], [redf_pol.mdata, photdata_subframe]):

    positions = DIPOL._estimate_positions_from_segments(redf=redf, data=data, n_seg_threshold=1.5, npixels=32, centering=None, fwhm=1.0)
    positions = positions[:10]

    sets_L.append(positions)

fig = plt.figure(figsize=(12,6), dpi=iop4conf.mplt_default_dpi)
axs = fig.subplots(nrows=1, ncols=2)

for ax, data, positions in zip(axs, [redf_pol.mdata, photdata_subframe], sets_L):
    imshow_w_sources(data, pos1=positions, ax=ax)
    candidates_aps = CircularAperture(positions[:2], r=10.0)
    candidates_aps.plot(ax, color="b")
    for i, (x,y) in enumerate(positions):
        ax.text(x, y, f"{i}", color="orange", fontdict={"size":14, "weight":"bold"})#, verticalalignment="center", horizontalalignment="center") 
    ax.plot([data.shape[1]//2], [data.shape[0]//2], '+', color='y', markersize=10)
    
axs[0].set_title("Polarimetry field")
axs[1].set_title("Photometry field")
fig.suptitle("astrometry detected_sources")
plt.show()

# Build the quads for each field
quads_1 = np.array(list(itertools.combinations(sets_L[0], 4)))
quads_2 = np.array(list(itertools.combinations(sets_L[1], 4)))

from iop4lib.utils.quadmatching import hash_ish, distance, order, qorder_ish, find_linear_transformation
hash_func, qorder = hash_ish, qorder_ish

hashes_1 = np.array([hash_func(quad) for quad in quads_1])
hashes_2 = np.array([hash_func(quad) for quad in quads_2])

all_indices = np.array(list(itertools.product(range(len(quads_1)),range(len(quads_2)))))
all_distances = np.array([distance(hashes_1[i], hashes_2[j]) for i,j in all_indices])

idx = np.argsort(all_distances)
all_indices = all_indices[idx]
all_distances = all_distances[idx]

indices_selected = all_indices[:6]
colors = ["r", "g", "b", "c", "m", "y"]

figs, axs = zip(*[plt.subplots(figsize=(6,4), dpi=iop4conf.mplt_default_dpi) for _ in range(2)])

for (i,j), color in list(zip(indices_selected, colors)): 
    
    tij = find_linear_transformation(qorder(quads_1[i]), qorder(quads_2[j]))[1]

    for ax, fig, data, quad, positions in zip(axs, figs, [redf_pol.mdata, photdata_subframe], [quads_1[i], quads_2[j]], sets_L):
        imshow_w_sources(data, pos1=positions, ax=ax)
        x, y = np.array(order(quad)).T
        ax.fill(x, y, edgecolor='k', fill=True, facecolor=mplt.colors.to_rgba(color, alpha=0.2))

plt.show()