!!!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. 

!!Checking pointing with eis_image_tool

eis_image_tool is a widget-based routine used by the EIS Chief Observer (CO) to perform pointing for EIS studies. When the CO performs the pointing he/she is typically using solar images obtained 1-3 days before the observation takes place. For the scientist studying EIS data, eis_image_tool can be used to see where EIS actually pointed using a solar image taken at the same time as the EIS data. The procedure is as follows:

IDL> s=fix_zdbase(/eis)\\
IDL> eis_mk_plan

These commands set up the EIS databases and start the EIS planning tool. Two windows pop up with names 'EIS Make Plan Tool' and 'EIS Plan Timeline'. For the latter window, go to the 'Display Start' text box and type in the date your observation was taken. The timeline will change to show the studies on this date. Find the study for which you want pointing information by clicking on the timeline entry. If you can't find the study easily, go to the Make Plan Tool widget, click on 'Tools' and then 'Plan summary...'. A text list of the studies for the day will be shown and you can click on your study to select it.

After the study has been selected, within the Make Plan Tool widget the bottom half has a list of rasters for that study (the headings are 'ID', 'Acronym', etc.). Click on your raster (usually there will only be one listed). Below the raster list there are several tabs. Click on 'Pointing' and then 'Send to image_tool'. The eis_image_tool widget pops up.

For this widget you need to load an image. Click on 'File' and 'Import Images'. A new widget called show_synop appears. Type in start and end times that bracket your observation. Next to 'remote sites' there is a drop-down list. Select 'SOHO/EIT (prepped)' and then hit the search button.

From the list of files that appear find the filter/time that is most appropriate for your data-set and click the 'Download' button. After it is downloaded to a local directory, select the file from the list at the bottom of the widget and then click 'Display'. This sends the image to eis_image_tool. You can now close show_synop.

From the eis_image_tool widget, click on 'Plot all rasters' and the location of your raster on the EIT image will be shown.

If EIT images are not available for your date, try looking for a TRACE mosaic at:

[http://trace.lmsal.com/mosaic_archive/fits/|http://trace.lmsal.com/mosaic_archive/fits/]