!!! Estimating the EIS spatial resolution using observed features

If an observed solar feature has a size much smaller than 1 arcsec and is also much brighter than the surrounding plasma then it will be seen by EIS as a point source, and thus will reveal the point spread function of the instrument. Such brightenings are quite common at transition region temperatures in active regions. 

This method was used by [Young (2004)|http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004soho...13..257Y] for estimating the PSF of SOHO/CDS.

This page lists examples of small, intense brightenings, with links given to figures showing the variation of line intensity along the EIS slit. The spatial resolution can be considered to be the full width at half maximum of the brightening.

__12-Dec-2006, 19:07__ [O V 192|http://solar.bnsc.rl.ac.uk/~young/solarb_eis/psf_examples/bpt_12dec06.ps]

__20-Jan-2007, 22:32__ [O V 192|http://solar.bnsc.rl.ac.uk/~young/solarb_eis/psf_examples/bpt_20jan07_ov.ps], [Mg V 276|http://solar.bnsc.rl.ac.uk/~young/solarb_eis/psf_examples/bpt_20jan07.ps]

The examples above suggest to me that the spatial resolution is about 3-4 arcsec in the Solar-Y direction.

! Caveats

* This method leads to an upper limit to the spatial resolution. If other brightenings are found with narrower widths, then users are encouraged to post details here.

* The spatial resolution will vary along the EIS slit, so the location of the brightening on the detector is important.

Peter Young, 25-Mar-2008



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Comparisons of EIS slot (40") and 1" slit rasters with AIA images confirm the estimates 
in spatial resolution (3 - 4 arcsec) given above.

The plots below show a comparison of:

a) A 195.119 A slot raster with a AIA/SDO 193 A image convolved with several Gaussians of different FWHM in AIA pixels.
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[{Image src='http://msslxr.mssl.ucl.ac.uk:8080/eiswiki/attach/TRbrightenings/aia193_eis195_slot.png' width=500  align='center'}]
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b) a 195.119 A slit raster with a composite of several AIA/SDO 193 A images to mimic the EIS rastering. The AIA final composite image has been convolved with several Gaussians of different FWHM in AIA pixels.
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[{Image src='http://msslxr.mssl.ucl.ac.uk:8080/eiswiki/attach/TRbrightenings/aia193_eis195_slit.png' width=500 align='center'}]
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Visually the closest match seems to be around a FWHM of 6 AIA pixels. Given that 1 pixel = 0.6 arcsec, this means a FWHM of 3 - 4 arcsecs.


--[Ignacio Ugarte-Urra|http://null], 09-Nov-2010