I have sources that I extracted from The Tiered Radio Extragalactic Continuum Simulation (T-RECS) catalogue for a deep field. I use these sources to generate a sky model at different fields within a given field-of-view (fov). Imaging the sky model results in an 'elongated' fov, which suggests that I did not compute the RA correctly using the cosine declination rule to account for the angular separation.
With my little understanding, I have correctly applied the cosine(declination) rule but it seems that during the transformation, the angular separation between the original sources to a new field was not preserved, hence the distortion seen on the second image.
Below is a code snippet of how I am calculating the ra and dec values for the new field given the longitudes and latitudes extracted from the catalogue. I seek help in understanding what I am doing wrong. Is the problem my logic or I am missing some fundamentals?
from astropy import units as u from astropy.table import Table from astropy.coordinates import SkyCoord, Angle fov = Angle(60, unit=u.arcmin) field_centre = SkyCoord('15 04 00 -60 00 00', unit=(u.hourangle, u.deg)) cat = Table.read('/path/to/catalogue.txt', format='ascii') cat_centre = SkyCoord(0.0, 0.0, unit=u.deg) cat_source_coords = SkyCoord(cat['longitudes'], cat['latitudes'], unit=u.deg) separations = cat_centre.separation(cat_sources) position_angles = cat_centre.position_angle(cat_sources) source_offsets = field_centre.directional_offset_by(pos_angles, separations) ra_deltas = source_offsets.ra - field_centre.ra dec_deltas = source_offsets.dec - field_centre.dec model_dec = field_centre.dec.deg + dec_deltas.deg model_ra = (field_centre.ra.deg + ra_deltas.deg * np.cos(np.radians(model_dec))) % 360