How to get the most accurate EIS pointing information#

There is some confusion over EIS pointing information and so this document attempts to summarize current knowledge so as to be useful for EIS scientists in practical applications.

The best way of extracting the EIS pointing is by loading the data object into IDL and doing:

IDL> obj=obj_new('eis_data',filename)
IDL> xpos=obj->getxpos()
IDL> ypos=obj->getypos()

For each EIS exposure there will be a single xpos value and an array of ypos values corresponding to each pixel along the slit.

If you use the eis_getwindata routine to extract a data window into a structure then you will find the XPOS and YPOS values in the SOLAR_X and SOLAR_Y arrays.

Some facts about XPOS and YPOS#

• XPOS includes the satellite jitter

• XPOS does not include the 2" slit offset. If you use the 2" slit you should manually subtract 8" from XPOS.

• YPOS values are currently 13.7 arcsec larger than they should be so users should manually perform this correction.

• YPOS does not include the satellite jitter - use the routine eis_jitter to find the jitter values

• The pointings apply to He II 256.32. The CCD Y-offset, the grating tilt and the CCD X-offset will change the pointings for other wavelengths. The user will need to manually correct for these offsets. The routine eis_ccd_offset() gives the combined offset value for a specified wavelength.

What about XCEN and YCEN?#

Most solar observers will look to the FITS keywords XCEN and YCEN to determine the center of their instrument's field of view, however XCEN and YCEN often contain incorrect values for EIS so generally users should not use these values.