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Re: wcscoordsys to eq-j2000 does not work
From: |
Mohammad Akhlaghi |
Subject: |
Re: wcscoordsys to eq-j2000 does not work |
Date: |
Fri, 11 Nov 2022 20:10:24 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.2 |
Hi Alex,
Thanks a lot for reporting this problem, it has been fixed (by updating
the 'EQUINOX' keyword, see below) in the following commit:
https://git.savannah.gnu.org/cgit/gnuastro.git/commit/?id=470d4434ab
You can get the latest Gnuastro version with this bug fix from
http://akhlaghi.org/gnuastro-latest.tar.lz
The keywords of the output files had indeed changed (here I am running
'astfits' on the two files that you had uploaded). To visually see this
change better, we'll first write the header keywords in plain-text files:
$ astfits NGC6946_scaled.fits -h1 > NGC6946_scaled.txt
$ astfits NGC6946_scaled-eq-j2000.fits -h1 > NGC6946_scaled-eq-j2000.txt
Then we can look at the differing lines between the two files. You see
that the 'CRVAL1' (RA of reference point), 'CRVAL2' (Dec of reference
point), 'LONPOLE' (longitude of the pole) and 'LATPOLE' (latitute of
pole) have indeed changed significantly (on the scale of half a degree
in some cases!). You also see that 'RADESYS' (RA/Dec System) has also
been changed from 'FK4' to 'FK5'.
$ diff NGC6946_scaled.txt NGC6946_scaled-eq-j2000.txt
22,25c22,25
< CRVAL1 = 308.45 / [deg] Coordinate value at reference
point
< CRVAL2 = 59.98333333 / [deg] Coordinate value at reference
point
< LONPOLE = 180.0 / [deg] Native longitude of celestial
pole
< LATPOLE = 59.98333333 / [deg] Native latitude of celestial
pole
---
> CRVAL1 = 308.702781271596 / [deg] Coordinate value at reference
point
> CRVAL2 = 60.1568349215312 / [deg] Coordinate value at reference
point
> LONPOLE = 179.564003507 / [deg] Native longitude of celestial
pole
> LATPOLE = 60.1568349215 / [deg] Native latitude of celestial
pole
29c29
< RADESYS = 'FK4' / Equatorial coordinate system
---
> RADESYS = 'FK5' / Equatorial coordinate system
The confusion was caused by the 'EQUINOX' keyword not being changed (the
output still had a vale of '1950.0'). So with the commit above, Gnuastro
now also updates that (as it should have).
Once you install the latest version you will notice that the only
difference is in the value of 'EQUINOX' (all the other numbers will be
the same as the output you sent). This is because the main keyword in
this aspect is the 'RADESYS' one (which was correctly set before also:
so in case of conflict 'RADESYS' has priority over 'EQUINOX').
Cheers,
Mohammad