This page (revision-13) was last changed on 07-Dec-2016 14:14 by Peter Young

This page was created on 12-Nov-2009 15:08 by PeterYoung

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EISPointing
EISPointing

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At line 1 changed one line
!!!How to get the most accurate EIS pointing information
[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
!!!Obtaining pointing information from EIS data
At line 15 added 20 lines
!!Improved EIS pointing information __%%(color:red;)Update%%__
From 2014 a new EIS routine yields the spatial offset between EIS and AIA. It is called as
{{{
IDL> xy=eis_aia_offsets(date)
}}}
At present the correction needs to be applied manually to the pointing values described in the sections below. So, for example,
{{{
IDL> xcen=xcen+xy[0]
IDL> ycen=ycen+xy[1]
}}}
The EIS-AIA offsets are determined regularly by co-aligning EIS slot images with AIA images. They should lead to much-improved pointing accuracy, to the level of 5" or better. In particular, the large variations seen during the eclipse season are corrected for.
Note that the EIS-AIA offsets do not correct the short-term (orbit period) jitter of EIS, only the longer-term (~ days) pointing variation.
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IDL> ycen=data->getycen(/raster)
IDL> ycen=data->getycen(wvl,/raster)
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where WVL is the wavelength you are interested in. If WVL is not specified then 195.12 (the wavelength of the strong Fe XII line) is assumed. Note that the raster center in solar-Y varies with wavelength by up to 18" due to the tilt of the EIS grating relative to the detector and a spatial offset between the two CCDs.
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''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.
''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 will affect the raster field of view by a few arcseconds at most.
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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:
!!Field of view
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The field of view (FOV) of an EIS raster can be obtained by doing:
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IDL> ycen = ycen - eis_ccd_offset(195.12)
IDL> fovx=data->getfovx()
IDL> fovy=data->getfovy()
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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.
or by reading the values from the FITS header via
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{{{
IDL> fovx=data->getinfo('FOVX')
IDL> fovy=data->getinfo('FOVY')
}}}
so if you want to overplot a box on an AIA image (for example) to show the EIS FOV for a raster, you can do:
{{{
IDL> x0=xcen-fovx/2
IDL> x1=xcen+fovx/2
IDL> y0=ycen-fovy/2
IDL> y1=ycen+fovy/2
IDL> oplot,[x0,x1,x1,x0,x0],[y0,y0,y1,y1,y0]
}}}
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IDL> ypos=data->getypos()
IDL> ypos=data->getypos(wvl)
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IDL> ycen=data->getycen()
IDL> ycen=data->getycen(wvl)
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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.
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 at 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.
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As with most solar data, the EIS data have a header that contains XCEN and YCEN values. These can be obtained by doing:
As with most solar data, an EIS data-set has a header that contains XCEN and YCEN values. These can be obtained by doing:
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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.
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 when the method is called and so will always contain the most up-to-date pointing information.
This page (revision-13) was last changed on 07-Dec-2016 14:14 by Peter YoungTop