The tilts of the EIS slits#
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.
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:
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
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.
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.
A key point to note is that the 2" slit shows a much greater tilt than the 1" slit.
Impact of slit tilt on velocity errors#
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.
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.
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, 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.