There is an offset between the short and long wavelength CCDs - this is approximately 16" in the y direction. Be aware of this when comparing features.
Just to comment further on this... solar features imaged in the short wavelength band will occur at higher Y-pixel numbers than those imaged in the long wavelength band. Thus if you see a brightening at Y-pixel 100 in the Fe XII 195 line, then it corresponds to Y-pixel 84 in the long wavelength band.
The offset between the bands seems to vary with wavelength, with different people reporting offsets of between 16 and 20 pixels.
Note that the heliocentric coordinates stored in the fits headers and returned by the EIS software apply to the long wavelength band, not the short wavelength band. This is because the He II 256 line was used to co-align with the SOT data.
Doing cross-correlations of Fe VIII 185.21 and Si VII 275.35 data, I find the Y-offset between the two images is around 18.5 pixels. I've checked two quiet Sun rasters in Sep-2007 and two active region rasters in Jan-2007 to get this value.
Just to iterate what I say above, I think the offset varies depending on what wavelengths are being compared, so this value shouldn't be applied for all SW-LW comparisons.
The variation with wavelength mentioned above is due to the tilt of the grating relative to the CCD axes. An estimate of this tilt for the SW band has been made by Young et al. (2008; astro-ph 0805.0958). Combining with the offset between the two CCDs it is possible to estimate the offset relative to the EIS reference line (He II 256) for any wavelength. A routine has been written to give this estimate, and it is called as:
where offset is the offset, in pixels, of the Fe XII 195.12 image from the He II 256 image. I.e., if the same feature is seen in both 256 and 195, then the 195 feature will be 'offset' pixels higher on the CCD than the 256 feature.
The routine does the following:
- assume the tilt measured for the SW band is the same as for the LW band
- the offset between SW and LW has been obtained by co-aligning images in Fe VIII 185.21 and Si VII 275.35
-- PeterYoung, 18-Aug-2008
Wavelength dependent Y-offset has been determined from Mercury transit on November 8, 2006. yoffset.pdf