Calculate the spatial offset of a line relative to He II 256 Å

Spatial offset of the specified wavelength relative to He II 256 Å. If you see a feature in the He II 256 image at coordinate \(Y\), then the corrected coordinate \(Y^{\prime}\) for any other wavelength \(\lambda\) is,

\[Y^{\prime} = Y - o(\lambda),\]

where \(o(\lambda)\) is the CCD offset. Note that the spatial coordinate system for EIS is evalulated for He II 256 Å by cross-correlating with SOT.

The value of ccd_offset represents the number of pixels that a feature seen in a wavelength sits above the He II 256 Å image on the CCD.

The following goes into the calculation of EIS CCD offset:

  • the tilt of the grating is assumed to be linear with wavelength

  • the tilt for SW was derived by [young09]

  • the tilt for LW was assumed to be the same as for SW

  • the offset between SW and LW was measured by co-aligning images from Fe VIII 185.21 Å and Si VII 275.35 Å


This routine is a (nearly) verbatim translation of the IDL version of eis_ccd_offset written by Peter Young. The above documentation has been adapted from that routine.


wavelength (array_like) – Wavelength of the spectral line(s) of interest in units of [Å]


offset – The spatial offset between the specified wavelength and He II 256.32. The value represents how many pixels above He II 256.32 the specified wavelength sits on the EIS detectors.

Return type:




Young, P.R., et al., 2009, A&A, 495, 587