Procedure for co-aligning SJI images

This describes my attempts at co-aligning IRIS SJI images. A key point to note is that when you read a SJI file, the xcen and ycen values of the images do not vary smoothly: they contain jitter. So, before attempting to cross-correlate image, it is best to first align the images by interpolating them using the xcen and ycen values. This is done with:

IDL> outmap=iris_sji_align(filename [, imagen=, xrange=, yrange=])

where the output is an array of IDL map structures. Making a movie from this output (with movie_map) will show that there is still jitter in the sequence, and so it's necessary to cross-correlate between neighboring frames.

Study of SJI sequences - case study

For the 3-Oct-2014 22:00 data-set (4 hours) I took the 1400 SJI images (1152 frames) and created a map using:

IDL> read_iris_l2, filename, index, data
IDL> index2map, index, data, map    ; this takes a long time!
IDL> mapx=sdo_align_map(map, xrange=[130,190],yrange=[-160,-100])

This last routine (which I wrote for SDO) takes a map and interpolates the individual frames using the xcen, ycen coordinates. So it does not perform any co-alignment. The movie produced in this way is fairly smooth, but there's clearly drift and jitter.

The data-set also includes 2832, and I repeated the same steps

  1. Use read_iris_l2 to read the data sequence. Use imagen=[i0,i1] to only read a subset of the images.
  2. Extract a sub-array for region of interest, using interpolation to correct jitter in pointing values (equivalent of sdo_align_map), and using dustbuster to remove dust.
  3. Perform coalignment between pairs of frames using

Data analysis notes

Wavelength calibration

Note that the IRIS spectra are not absolutely wavelength calibrated. The team only accounts for the orbit drift but do not put the spectra on an absolute scale. So, for example, you will often find the O I 1355.60 line showing a significant redshift (5-10 km/s). Similarly the Ni I 2799.474 line (used as a reference line in the NUV) can show systematic Doppler shifts too.