Known Issues

This page documents commonly known issues. “Issues” here is defined broadly and refers to oddities or specifics of how irispy or the Python ecosystem works that can and will catch users off guard.

Per-exposure WCS metadata

The IRIS FITS files contain the per-exposure WCS metadata (reference coordinate and PCij matrix) in an extension, while the primary header has only values averaged over the observation. For example, OBS 4204700138 SJI file, CRVAL in the primary header is (1.93840, 1.96290), but the reference coordinate is actually (-3.56638378, 1.69258388), which is retrieved from the appropriate index in the XCENIX/YCENIX arrays in the extension.

This was (and partially still is) a problem for the irispy package, which assumed that the reference coordinate is the same for all exposures. This has been addressed by the SJI reader but not fully by the spectrograph reader.

In the future, the goal is to create a gWCS to account for this.

Spectrogram WCS

For different programs that are run for the slit spectrograph, a sit-and-stare has the CDELT of 0 arcseconds in the X direction. This is not allowed by the WCS standard, so we set its value to 1e-10 arcsec in the X direction, which essentially tricks the WCS calculation to get it to work if there is no rotation. However, the PCij matrix used is derived from the SJI, with square pixels, so the PCij matrix is a pure rotation. This means that one gets the correct answer only if one does the matrix multiplication in the wrong order: first by PCij and then by CDELTs.

We work around this by modifying the PC_ij matrix to have the correct skew. Since the X CDELT is 1e-10 arcsec, the inverse is thankfully not infinity. Using equation 187 in Calabretta & Greisen 2002, we correct for this.

Note that since these pixels are extremely rectangular, with an aspect ratio of ~3e-10, the cross terms in the PCij matrix are quite small: -3.4e-12 and -3.8e-7. Hopefully, 64-bit floats have enough precision to enable this to work all of the time.