| .. include:: references.rst | |
| .. doctest-skip-all | |
| .. _note_sip: :orphan: | |
| Note about SIP and WCS | |
| ********************** | |
| `astropy.wcs` supports the Simple Imaging Polynomial (`SIP`_) convention. | |
| The SIP distortion is defined in FITS headers by the presence of the | |
| SIP specific keywords **and** a ``-SIP`` suffix in ``CTYPE``, for example | |
| ``RA---TAN-SIP``, ``DEC--TAN-SIP``. | |
| This has not been a strict convention in the past and the default in | |
| `astropy.wcs` is to always include the SIP distortion if the SIP coefficients | |
| are present, even if ``-SIP`` is not included in CTYPE. | |
| The presence of a ``-SIP`` suffix in CTYPE is not used as a trigger | |
| to initialize the SIP distortion. | |
| It is important that headers implement correctly the SIP convention. | |
| If the intention is to use the SIP distortion, a header should have | |
| the SIP coefficients and the ``-SIP`` suffix in CTYPE. | |
| `astropy.wcs` prints INFO messages when inconsistent headers are detected, | |
| for example when SIP coefficients are present but CTYPE is missing a ``-SIP`` suffix, | |
| see examples below. | |
| `astropy.wcs` will print a message about the inconsistent header | |
| but will create and use the SIP distortion and it will be used in | |
| calls to `~astropy.wcs.wcs.WCS.all_pix2world`. If this was not the intended use | |
| (e.g. it's a drizzled image and has no distortions) it is best to remove the SIP | |
| coefficients from the header. They can be removed temporarily from a WCS object by | |
| >>> wcsobj.sip = None | |
| In addition, if SIP is the only distortion in the header, the two methods, | |
| `~astropy.wcs.wcs.WCS.wcs_pix2world` and `~astropy.wcs.wcs.WCS.wcs_world2pix`, | |
| may be used to transform from pixels to world coordinate system while omitting distortions. | |
| Another consequence of the inconsistent header is that if | |
| `~astropy.wcs.wcs.WCS.to_header()` is called with ``relax=True`` it will return a header | |
| with SIP coefficients and a ``-SIP`` suffix in CTYPE and will not reproduce the original header. | |
| **In conclusion, when astropy.wcs detects inconsistent headers, the recommendation | |
| is that the header is inspected and corrected to match the data.** | |
| Below is an example of a header with SIP coefficients when ``-SIP`` is missing from CTYPE. | |
| The data is drizzled, i.e. distortion free, so the intention is **not** to include the | |
| SIP distortion. | |
| >>> wcsobj = wcs.WCS(header) | |
| INFO:: | |
| Inconsistent SIP distortion information is present in the FITS header and the WCS object: | |
| SIP coefficients were detected, but CTYPE is missing a "-SIP" suffix. | |
| astropy.wcs is using the SIP distortion coefficients, | |
| therefore the coordinates calculated here might be incorrect. | |
| If you do not want to apply the SIP distortion coefficients, | |
| please remove the SIP coefficients from the FITS header or the | |
| WCS object. As an example, if the image is already distortion-corrected | |
| (e.g., drizzled) then distortion components should not apply and the SIP | |
| coefficients should be removed. | |
| While the SIP distortion coefficients are being applied here, if that was indeed the intent, | |
| for consistency please append "-SIP" to the CTYPE in the FITS header or the WCS object. | |
| >>> hdr = wcsobj.to_header(relax=True) | |
| INFO:: | |
| Inconsistent SIP distortion information is present in the current WCS: | |
| SIP coefficients were detected, but CTYPE is missing "-SIP" suffix, | |
| therefore the current WCS is internally inconsistent. | |
| Because relax has been set to True, the resulting output WCS will have | |
| "-SIP" appended to CTYPE in order to make the header internally consistent. | |
| However, this may produce incorrect astrometry in the output WCS, if | |
| in fact the current WCS is already distortion-corrected. | |
| Therefore, if current WCS is already distortion-corrected (eg, drizzled) | |
| then SIP distortion components should not apply. In that case, for a WCS | |
| that is already distortion-corrected, please remove the SIP coefficients | |
| from the header. | |