[gnuastro-devel] [bug #50073] ImageWarp 'rotate' does not correctly propagate WCS pixel scale information into header

[Mohammad Akhlaghi]

Follow-up Comment #4, bug #50073 (project gnuastro):

Thanks for the check. 

This cannot be due to the missing `3', the missing decimal is thirteen
decimals after the point, so the effect of this change is roughly 10^{-13} of
one pixel, not 10^{-1}!

The missing `3' is probably due to the precision in printing of floating
points. I would guess this is because the version of WCSLIB that you are using
(version 4.24) maybe only prints to 12 decimals. This is the result of the
operation on my system (with WCSLIB version 5.15):


$ astimgwarp --rotate=360 input.fits

$ astheader input.fits | grep CD
CD1_1   =  -9.416666295793E-05 / Linear projection matrix                     
 
CD1_2   =   0.000000000000E+00 / Linear projection matrix                     
 
CD2_1   =   0.000000000000E+00 / Linear projection matrix                     
 
CD2_2   =   9.416666295793E-05 / Linear projection matrix                     
 

$ astheader input_rotated.fits | grep CDELT
CDELT1  =   9.416666295793E-05 / [deg] Coordinate increment at reference point
 
CDELT2  =   9.416666295793E-05 / [deg] Coordinate increment at reference
point


Just to confirm, I manually removed the last `3' digit from the `CDELT'
keywords of `input_rotated.fits' and done as you said: opened the two images
as different frames in DS9 and matched them with WCS. Then clicking on the
"flip" button, I couldn't see any change. Just out of curiosity, what version
of DS9 are you using? (mine is version 7.5, maybe this was a bug in DS9 that
has been corrected).

I would recommend updating your WCSLIB, it has implemented many new
distortions as defined in the WCS paper IV
<http://www.atnf.csiro.au/people/mcalabre/WCS/dcs_20040422.pdf> (which will
now hopefully be officially published in a journal after 13 years!). As a
technical side-note, it also has a function to return the version of WCSLIB.
So probably on your system, Gnuastro's output FITS files don't have the
`WCSLIB' keyword (which reports the version of WCSLIB).

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