[fitswcs] Distortions

[Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC]]

Folks:

It’s been a while since I’ve heard any chatter about WCS.  What’s the status of Paper IV on distortions?  Are distortions handled in any of the software packages, such as WCSLIB?

It appears that we may need to use distortions for an upcoming mission.  Ray tracing for a spectrometer for the Solar Orbiter mission shows that the relationship between the pixel positions I,J and the real-world coordinates have significant second-order terms.  Our plans are to provide data in two formats, one in the original detector pixel space, and one where the data have been interpolated to linearize and rectify the data.  The latter is no problem, but I would like to accurately describe the coordinates in the former.

Second order effects show up in two places.  The most significant is that the plate scale in arc seconds increases linearly with wavelength.  In other words

S = a0 + a1 * J * (1 + a2*I)

where S is the spatial dimension along the slit, J is the pixel dimension associated with the spatial dimension, and I is the pixel dimension associated with the spectral dimension.  Here, the second order effect comes from the J*I term.

The other second order effect comes from the fact that the slit images are slightly bowed, so that the wavelength has a dependence on J^2.

Thanks,

Bill Thompson
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