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EISPointing
EISPointing

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At line 3 changed one line
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.
Deriving precise pointing coordinates for EIS data can be a little complicated due to a number of issues:
At line 5 changed one line
The best way of extracting the EIS pointing is by loading the data object into IDL and doing:
# there are spatial offsets between different wavelengths in the Y-direction;
# raster images are obtained over a time period during which the instrument pointing is affected by satellite jitter and instrument jitter.
At line 7 changed 3 lines
IDL> obj=obj_new('eis_data',filename)\\
IDL> xpos=obj->getxpos()\\
IDL> ypos=obj->getypos()\\
This document provides a guide for users seeking to understand the pointing for their data-set.
At line 11 changed one line
For each EIS exposure there will be a single xpos value and an array of ypos values corresponding to each pixel along the slit.
[EIS Software Note #9|https://hyperion.nascom.nasa.gov/svn/eis/release/doc/eis_notes/09_POINTING/eis_swnote_09.pdf] has more detailed information about technical issues relating to EIS pointing.
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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.
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!!Some facts about XPOS and YPOS
!!First approximation
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* XPOS includes the satellite jitter
After reading your data file into an IDL object, you can obtain the approximate position of the center of your raster by doing:
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* XPOS does not include the 2" slit offset. If you use the 2" slit you should manually subtract 8" from XPOS.
{{{
IDL> xcen=data->getxcen(/raster)
IDL> ycen=data->getycen(/raster)
}}}
At line 21 changed one line
* YPOS values are currently 13.7 arcsec larger than they should be so users should manually perform this correction.
The values XCEN and YCEN are obtained by taking the pointing of the center of the slit (in both X and Y) of the first exposure of the raster (the exposure at the right-hand side of the raster) and correcting this for satellite jitter. For the X-direction, the value of half of the field-of-view is then subtracted to yield the center of the raster. E.g., if a raster was obtained with the 1" slit with 100 slit positions, then the field-of-view in X is 100" and so 50" is subtracted from the position of the first exposure.
At line 23 changed one line
* YPOS does not include the satellite jitter - use the routine eis_jitter to find the jitter values
''Why is this an approximate pointing?'' Rasters can take several minutes to several hours to complete and, during this time, the Sun is rotating opposite to the raster direction. This has the consequence that the field-of-view in X is effectively reduced. For observations at disk center the reduction will be 10" per hour of observation. An additional factor is that each individual exposure is affected by satellite and instrument jitter, which may have a systematic trend over the course of the raster although this effect will affect the raster field of view by a few arcseconds at most.
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* 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.
A further point to remember is that all coordinates are assumed to be correct for He II 256.32 only. To obtain the Y-pointing for a line at another wavelength, do:
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{{{
IDL> ycen = ycen - eis_ccd_offset(195.12)
}}}
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!!What about XCEN and YCEN?
for the example of the 195.12 line. The offset can be as large as 18" for lines in the short wavelength band of EIS.
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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.
!!More detailed information
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!!Checking pointing with eis_image_tool
A pointing coordinate can be obtained for each Y-pixel of each exposure by using the following object methods:
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{{{
IDL> xpos=data->getxpos()
IDL> ypos=data->getypos()
}}}
XPOS has the same number of elements as there are exposures in the raster, while YPOS has the same number of elements as there are pixels along the slit. XPOS includes the satellite jitter, but YPOS does not.
The time-dependent behavior of the Y-jitter is obtained as follows:
{{{
IDL> ycen=data->getycen()
IDL> y_jitter=ycen-ycen[0]
}}}
Therefore if you want the coordinates for a particular Y-pixel, j, of a particular exposure, i, to be corrected for satellite jitter, then they are:
{{{
IDL> x=xpos[i]
IDL> y=ypos[j] + y_jitter[i]
}}}
!!Instrument jitter
In addition to the satellite jitter there also appears to be jitter internal to the EIS instrument. This can be measured by, e.g., looking a time series of slot images and comparing the jitter obtained by co-aligning the images to the satellite jitter: the residuals represent the instrument jitter. This jitter has not been characterized yet and no software tools exist to yield the jitter values. However the magnitude of the instrument jitter is believed to be smaller than the satellite jitter.
!!What about XCEN and YCEN in the file header?
As with most solar data, the EIS data have a header that contains XCEN and YCEN values. These can be obtained by doing:
{{{
IDL> xcen=data->getinfo('XCEN')
IDL> ycen=data->getinfo('YCEN')
}}}
In general the user should ''not'' use these values as they are set when the FITS file is created, whereas the values obtained via the object methods are computed at that time and so contain the most up-to-date pointing information.
!!Checking what the EIS Chief Observer intended
It can sometimes be useful to look at what the EIS Chief Observer intended to observe when he/she created the EIS observing plan, in order to compare with what the raster image actually looks like.
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IDL> s=fix_zdbase(/eis)\\
{{{
IDL> s=fix_zdbase(/eis)
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}}}
This page (revision-13) was last changed on 07-Dec-2016 14:14 by Peter YoungTop