This page (revision-18) was last changed on 18-Apr-2021 19:22 by PeterYoung

This page was created on 03-May-2007 12:49 by JianSun

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Version Date Modified Size Author Changes ... Change note
18 18-Apr-2021 19:22 3 KB PeterYoung to previous Tidied up this page, and assumed that Kamio-san's result is the definitive one.
17 23-Sep-2008 14:10 3 KB PeterYoung to previous | to last
16 23-Sep-2008 10:47 3 KB CelineBoutry to previous | to last question
15 26-Nov-2007 12:19 3 KB Harry Warren to previous | to last
14 23-Nov-2007 17:21 2 KB David Pérez-Suárez to previous | to last Comment by David Pérez-Suárez
13 23-Nov-2007 09:01 2 KB SKamio to previous | to last
12 23-Nov-2007 08:56 2 KB SKamio to previous | to last Comment by SKamio
11 05-Oct-2007 15:22 2 KB GemmaAttrill to previous | to last
10 05-Oct-2007 15:21 2 KB GemmaAttrill to previous | to last Comment by GemmaAttrill
9 31-Aug-2007 14:17 1 KB LouiseHarra to previous | to last
8 10-Aug-2007 11:38 1 KB PeterYoung to previous | to last
7 06-Jul-2007 13:24 1 KB Harry Warren to previous | to last
6 06-Jul-2007 13:22 1 KB Harry Warren to previous | to last
5 15-May-2007 13:13 722 bytes Ken Dere to previous | to last Comment by Ken Dere
4 14-May-2007 14:06 529 bytes Louise Harra to previous | to last
3 14-May-2007 11:21 528 bytes Louise Harra to previous | to last
2 14-May-2007 11:03 484 bytes Louise Harra to previous | to last Comment by Louise Harra
1 03-May-2007 12:49 234 bytes JianSun to last

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TiltOfEISslit
TiltOfEISslit

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[{ALLOW edit EISMainUsers}]
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!!!The tilts of the EIS slits
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Anyone think they know what the tilt of the EIS slit is?
There are four EIS slits (1", 2", 40" and 266") and they are designed to be perpendicular to the dispersion axes of the EIS CCDs. Due to the difficulties in aligning optical elements of spectrometers, none of the four EIS slits are perfectly perpendicular to the CCD axes, and the small tilts that are present can affect data analysis. Of most concern for data analysis are the tilts of the narrow slits since these can affect velocity determinations from the instrument.
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Ken Dere
The most complete study of the tilts of the 1" and 2" slits was performed by Suguru Kamio using several months' worth of EIS data. The tilts he found are:
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--[129.174.112.201|http://null], 25-Apr-2007
1" slit tilt: 1.18E-05 (angstroms/pixel), standard deviation 1.43E-05\\
2" slit tilt: 1.09E-04 (angstroms/pixel), standard deviation 1.03E-05\\
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these were derived by determining the centroid of the Fe XII 195 line along the slit, and then fitting a straight line to the variation. Plots summarising the results are shown below.
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----
To use the slit tilt values above consider the following example. Suppose a data-set takes 200 pixels in the Solar-Y direction. If a measured centroid at pixel 0 is 195.120, then the expected centroid at pixel 150 will be 195.120 + 1.18e-5*150 = 195.122.
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And how linear is it (and where, if at all)?
A key point to note is that the 2" slit shows a much greater tilt than the 1" slit.
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--[David R Williams|DavidRWilliams], 03-May-2007
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[ TiltOfEISslit/eis_slit_tilt_195.png ]
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!!Impact of slit tilt on velocity errors
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The slit tilt is not fully understood and some further calibration studies need to be run. Preliminary work has been done, so for example Peter Young has produced a short document on this.
A common technique for determining velocities in active regions is to measure the velocity in a loop (for example), and then calibrate this against a quiet Sun measurement taken at a different location along the slit. E.g., if the quiet Sun is at pixel 0 and has a measured velocity v_1 and the loop at pixel 150 with measured velocity v_2, then we assume that the loop has an absolute velocity of v_2-v_1-dv, where dv is the velocity difference due to the slit tilt.
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Following Kamio-san's analysis above dv has an associated error which, for the 1" slit and considering two locations separated by 150 pixels, is 1.43e-5*150=0.0021 Ang. For large separations, this error can become quite large: e.g., for 400 pixels it corresponds to 9 km/s for the 195.12 line. This error should be included in error analyses for velocity measurements.
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--[Louise Harra|http://null], 14-May-2007
!!Automatically correcting for the slit tilt
Two IDL routines are available for correcting measured line centroids for the slit tilt. These are eis_wave_corr and eis_tilt_correction. Please check out the [EIS tutorial|http://solar.bnsc.rl.ac.uk/~young/solarb_eis/paris_tutorial/index.html], Worksheets 7a and 7b, for more details on these routines.
!!Are the slits straight?
Inspection of plots of line centroids vs. slit location often show structure beyond the simple linear slope. This raises the question of whether the slits are straight, or whether they are curved, or even whether there could be 'notches' at certain locations. Study of this problem is hampered by the Sun itself which shows significant velocity structure even off-limb in the quiet Sun, and so a definitive answer is not available yet.
Another factor the user has to bear in mind is that any slit structure is fixed relative to the CCD pixel positions, however the user is generally unaware of exactly where his/her raster occurs on the detector. E.g., if the raster is 200 pixels high, it could lie between pixels 1-200 on the detector, or between pixels 801-1000, depending on the satellite pointing that day. For a straight slit this isn't a problem, but if there's any other structure then it could significantly impact slit tilt corrections.
This page (revision-18) was last changed on 18-Apr-2021 19:22 by PeterYoungTop