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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At line 1 changed 3 lines
[{ALLOW edit EISMainUsers}]
[{ALLOW view Anonymous}]
!!!The tilts of the EIS slits
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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.
Anyone think they know what the tilt of the EIS slit is?
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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:
Ken Dere
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--[129.174.112.201|http://null], 25-Apr-2007
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And how linear is it (and where, if at all)?
--[David R Williams|DavidRWilliams], 03-May-2007
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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. [TiltFiles | TiltOfEISslit/spectral_tilt.pdf]
--[Louise Harra|http://null], 14-May-2007
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I find the tilt of the SW slit to be 1.6 x 10^-6 Angstroms / pixel
Peters number correspond to a value of 1.2 x 10^-6 Angstroms / pixel
--[Ken Dere|http://null], 15-May-2007
I've made another attempt at measuring the slit tilt. For the SW band I get a number close to Peter's value. I've written a routine called eis_slit_tilt that calculates the offset as a function of position along the slit. I'm sending the routine to be included in SSW. Some details on my calculations are given in the attached pdf. There is still a lot of scatter in the data and more analysis is needed. [TiltFiles | TiltOfEISslit/eis_slit_tilt.pdf]
--[Harry Warren|http://null], 06-JUL-2007
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The slit tilt is different for the 2" slit. Maria Madjarska has measured a tilt of 1.14 x 10^-4 Angstroms/pixel for the SW band. Further work should be performed to check this value.
--[Peter Young|http://null], 10-AUG-2007
Harry also has a routine which is preliminary called eis_wave_corr - this corrects for the tilt for the 1" slit and the orbital variation.
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I've measured the tilt for the 2 arcsec slit for the SW band. I used a quiet Sun raster made on 20th April 2007. This raster uses the full 512 arcsec slit height and is 120 arcseconds in the x-direction. I measure a tilt of 1.12 x 10^-4 Angstroms/pixel. This seems to be in reasonably good agreement with the value determined by Maria Madjarska of 1.14 x10^-4 Angstroms/pixel, although I do not know which raster(s) Maria used.
--[Gemma Attrill|http://null], 05-Oct-2007
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I made statistics of slit tilt from all the EIS data.
Average slit tilt determined from Fe XII 195 are:\\
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[ TiltOfEISslit/eis_slit_tilt_195.png ]
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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.
--[SKamio|http://null], 23-Nov-2007
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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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A key point to note is that the 2" slit shows a much greater tilt than the 1" slit.
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I don't understand what the negative values means in the statistics plot about the values of the tilt done by SKamio. Does it mean that the 1" slit tilt sometimes is to left (negatives values)?
Thanks.
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[ TiltOfEISslit/eis_slit_tilt_195.png ]
--[David Pérez-Suárez|http://null], 23-Nov-2007
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!!Impact of slit tilt on velocity errors
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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.
The slit tilt calculations are based on the assumption that the Doppler shifts at a given slit position should average to 0 with a large enough sample size. There is considerable noise in the measurements so a lot of averaging needs to be done. It is the mean values that are important.
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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.
Kamio's results are consistent with what I've calculated with smaller samples. I need to put them on the Wiki. They are also consistent with the number for the 2" slit posted by Gemma Attrill. At the recent EIS meeting Peter Young also noted some odd behavior near the "bottom" of the slit. This need to be investigated more.
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!!Automatically correcting for the slit tilt
--[Harry Warren|http://null], 26-NOV-2007
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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.
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!!Are the slits straight?
Is the tilt well corrected by eis_tilt_correction routine ?
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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.
--[Celine Boutry|http://null], 23-Sep-2008
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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.
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The routine eis_tilt_correction is for performing the tilt correction after the data have been fit with eis_auto_fit, and it uses tilt values of 1.22e-5 and 1.14e-4 for the 1" and 2" slits, respectively. The former is from Harry's document, while the latter is that measured by Maria Madjarska - see the various comments above. I've found eis_tilt_correction does a reasonable job of correcting the slit tilt, but if you find any problems please post them to this wiki or send me an e-mail.
-- [Peter Young|http://solar.bnsc.rl.ac.uk/~young], 23-Sep-2008
This page (revision-18) was last changed on 18-Apr-2021 19:22 by PeterYoungTop