EIT Data Analysis

Coordinates transformations (by Guy Artzner)

The astrometric properties of EIT 1024x1024 four channels images at 304Å, 171Å, 284Å and 195Å are investigated in order to obtain reliable tansformation formulas from x-y detector coordinates to solar coordinates. This is is a prerequisite for studying the rotation of solar structures.
Measured solar radii


at 171Å, 195Å and 284Å (increasing inflexion point) and at 304Å (decreasing inflexion point) from all IAS available clean 1024x1024 images between January, 15 and June, 21, 1996, i.e. from 118 images at 171Å , 223 images at 195Å , 123 images at 284Å and 122 images at 304Å,

are corrected to an average 147 910 000 km distance from SOHO to Sun, all data points included.

Results are:

	Channel	Value	RMS residuals
	171Å	371.74 pixel	0.085 pixel
	195Å	371.91 pixel	0.105 pixel
	284Å	371.86 pixel	0.108 pixel
	304Å	374.09 pixel	0.182 pixel


RMS value of residuals is of the order of a tenth of a pixel, i.e. 0"26 or 96s travelling time at 2km/s at disk center.


Residual miscentering between different channel images has some influence on line ratios determination. We investigated residual misalignment before and after mid-april 1996. It fits what is expected from the Richey Chretien optical presciption for EIT's telescope:

January, 15 to April,11, 1996:



	Channel		X	RMS deviation		Y	RMS deviation
	171Å		505.91	.22		588.23	.24
	195Å		505.62	.28		587.88	.21
	284Å		505.47	.25		588.21	.33
	304Å		505.94	.34		587.68	.34




April, 17 to June, 21, 1996:



	Channel		X	RMS deviation		Y	RMS deviation
	171Å		505.77	.19		514.32	.18
	195Å		505.20	.23		513.88	.22
	284Å		505.29	.21		514.25	.19
	304Å		505.72	.24		513.79	.41

We presently investigate the angle of position of EIT's detector with respect to classical Sun's axis of rotation in order to obtain reliable transformation formulas from x-y detector coordinates to solar coordinates.

All/ latest update : April 7, 1997