I'm working with some NEOWISE images that seem to have incomplete header tags. I've been advised by the SAO (because I'm using DS9 to view them) that I have

"a partial PC matrix defined, yet it does not conform to the FITS WCS standard. With a combo of CDELT and PC keywords, the CDELT keywords hold the scaling factor, the PC keywords, the rotation matrix, which should be normalized. If you want the rotation matrix to also contain the scale factor, use CD keywords".

Here's one of the headers I need to fix:

SIMPLE  =                    T / conforms to FITS standard
BITPIX  =                  -32 / array data type
NAXIS   =                    2 / number of array dimensions
NAXIS1  =                 1459
NAXIS2  =                  903
WCSAXES =                    2 / Number of coordinate axes
CRPIX1  =                729.5 / Reference pixel for axis 1
CRPIX2  =                451.5 / Reference pixel for axis 2
PC1_1   =     -0.0333333333333 / Coordinate transformation matrix element
PC2_2   =      0.0333333333333 / Coordinate transformation matrix element
CDELT1  =                   1. / [deg] Coordinate increment at reference point
CDELT2  =                   1. / [deg] Coordinate increment at reference point
CUNIT1  = 'deg     '           / Units of coordinate increment and value
CUNIT2  = 'deg     '           / Units of coordinate increment and value
CTYPE1  = 'GLON-AIT'           / Projection for axis1
CTYPE2  = 'GLAT-AIT'           / Projection for axis1
CRVAL1  =                   0. / [deg] Coordinate value at reference point
CRVAL2  =                   0. / [deg] Coordinate value at reference point
LONPOLE =                   0. / [deg] Native longitude of celestial pole
LATPOLE =                  90. / [deg] Native latitude of celestial pole
RADESYS = 'ICRS    '           / Equatorial coordinate system
BAND    = 'W1      '           / Wise Band
END


I've already managed to fix a couple of the invalid tags, but apart from these minor edits, this is unaltered from the file that my research supervisor gave me. The FITS standard here gives descriptions of each of the tags, but I'm not sure how to implement the SAO's advice. Can anyone help me fix the tags?

• Do you have Fits View (FV)? heasarc.gsfc.nasa.gov/ftools/fv Sep 24 '17 at 22:38
• Oh, I didn't know about that! I've downloaded it now, thank you. My main issue is that I'm not sure how to implement the SAO's suggestions. I've been using astropy to edit the headers, so that's not the problem, although FV looks like it will make that more convenient. Sep 24 '17 at 22:44

You are missing the PC1_2 and PC2_1 elements of your pixel transformation matrix. If your coordinate transformation is a rotation, you would use

PC1_1 = cos(t)
PC1_2 = sin(t)
PC2_1 = -sin(t)
PC2_2 = cos(t)


for the PC matrix. The pixel scale (in degree/pixel, according to CUNITi) goes into CDELTi.

As pointed out, you can combine the PC and CDELT steps into a single CD matrix.

See here and here for more details.

Using Fits Fits View (FV)View (FV), you can either modify the header by terminal commands, or using the GUI which makes you visualize all the keywords and values.