Lenstool version 8.9.0: Change of coordinates transformation
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benjamin beauchesne
Nov 20, 2025, 10:41:07 AM11/20/25
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Dear Lenstool-ers,
In the new version 8.9.0 released today, lenstool will use more accurate coordinate transformations when converting RA/DEC values (in degrees) from multiple image files and catalogues into its internal reference frame.
Currently, lenstool uses the following approximate relations:
Relative RA = -(RA-RA_{ref})*cos(DEC_{ref})/3600
Relative DEC = (DEC-DEC_{ref})/3600
These work well near the reference point but lose accuracy at greater distances. While the errors are usually small, they can become noticeable for large clusters or when high precision is needed for source reconstruction.
The new version uses wcslib, which provides proper geometric transformations for the WCS standard for all the conversions to the lenstool internal frame. This change will likely impact existing models. For example, their RMS will change by a small amount of ~0.1 arcsec or less, depending on the cluster size/image configuration. A re-optimisation of those models should yield results similar to the previous version.
You will now be able to generate a FITS file containing the WCS information, allowing you to transform between lenstool's internal frame and standard astronomical coordinates yourself. To do this, there is a new keyword for the runmode section (wcsfile) and a new function in the Python wrapper (Lenstool.create_internal_wcs).
In the new version 8.9.0 released today, lenstool will use more accurate coordinate transformations when converting RA/DEC values (in degrees) from multiple image files and catalogues into its internal reference frame.
Currently, lenstool uses the following approximate relations:
Relative RA = -(RA-RA_{ref})*cos(DEC_{ref})/3600
Relative DEC = (DEC-DEC_{ref})/3600
These work well near the reference point but lose accuracy at greater distances. While the errors are usually small, they can become noticeable for large clusters or when high precision is needed for source reconstruction.
The new version uses wcslib, which provides proper geometric transformations for the WCS standard for all the conversions to the lenstool internal frame. This change will likely impact existing models. For example, their RMS will change by a small amount of ~0.1 arcsec or less, depending on the cluster size/image configuration. A re-optimisation of those models should yield results similar to the previous version.
You will now be able to generate a FITS file containing the WCS information, allowing you to transform between lenstool's internal frame and standard astronomical coordinates yourself. To do this, there is a new keyword for the runmode section (wcsfile) and a new function in the Python wrapper (Lenstool.create_internal_wcs).
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