TRACE Pointing Correction
October 26, 2009 by admin
Filed under Image Processing
TRACE/EIT 19.5 nm Image Alignment using Cross-Correlation
(Peter T. Gallagher, L-3 Com Analytics Corp., NASA/GSFC)
This page describes a simple method to correct the pointing information in TRACE
19.5 nm images by cross-correlating them with EIT 19.5 nm images using IDL
SSW. The data were taken during an X-class flare which occurred on 21-Apr-2002, more details on which can be found here.
An alternative method for correctng TRACE pointing (using MDI continuum images) is described at Tom Metcalf’s page.
Step 1: Read, calibrate and convert data to maps
It is best to select images which have been taken as
close in time as possible. In this example the images are separated by
11 seconds.
IDL> eit_prep, 'efr20020421.011334', eit_index, eit_data IDL> index2map, eit_index, eit_data, eit_map IDL> eit_map = map2earth( eit_map ) ; SOHO @ L1 -> Earth IDL> mreadfits, 'trf20020421_011323_a2.fits', index, data IDL> trace_prep, index, data, trace_index, trace_data, /wave2point IDL> index2map, trace_index, trace_data, trace_map
Further information on mapping software can be found at Dominic
Zarro’s
pages.
Figure 1: TRACE and EIT 19.5 nm images with the same
field-of-view.
Step 2: Extract regions with distinct features and plot
We extract a region where there is a bright loop visible:
800″ < Solar_X < 1000″ and -350″ < Solar_Y < -150″. This
is plotted
in Figure 1 above.
IDL> xrange = [ 800, 1000 ] IDL> yrange = [ -350, -150 ] IDL> sub_map, eit_map, sub_eit, xrange = xrange, yrange = yrange IDL> sub_map, trace_map, sub_trace, xrange = xrange, yrange = yrange IDL> eit_colors, 195 IDL> !p.multi = [ 0, 2, 1 ] IDL> plot_map, sub_eit, /log IDL> plot_map, sub_trace, /log
Step 3: Extract TRACE and EIT images and convert to same size
In order to compute the cross-correlation between two images the
image with the lesser number of pixels (EIT) is rebinned
to the size of the larger (TRACE).
IDL> trace_image = sub_trace.data IDL> sz = size( trace_image, /dim ) IDL> cube = fltarr( sz( 0 ), sz( 1 ), 2 ) IDL> rsub_eit = coreg_map( sub_eit, sub_trace ) IDL> cube( *, *, 0 ) = rsub_eit.data IDL> cube( *, *, 1 ) = sub_trace.data
Step 4: Compute offsets between TRACE and EIT and update TRACE map.
Use cross-correlation to find the pixel offsets between TRACE and EIT
and create a new TRACE map with corrected pointing.
IDL> offsets = get_correl_offsets( cube )
IDL> print, offsets
0.00000 0.00000
5.19975 8.60905
IDL> correct_trace_map = shift_map( trace_map, offsets( 0, 1 ) * trace_map.dx, $
IDL> offsets( 1, 1 ) * trace_map.dy )
Figure 2: Left: TRACE with EIT contours before pointing
correction. Right: The corresponding data after TRACE pointing correction.
Step 5: Plot the uncorrected and corrected TRACE data with EIT contour
overlay
The corrected and uncorrected TRACE 19.5 images together with EIT contour
overlays are given in Figure 2 above.
IDL> plot_map, trace_map, xrange = xrange, yrange = yrange, /log IDL> plot_map, eit_map, /over IDL> plot_map, correct_trace_map, xrange = xrange, yrange = yrange, /log IDL> plot_map, eit_map, /over
Figure 3: Movie which blinks between the coaligned TRACE and EIT images (Hit reload/refresh to run).
Peter T. Gallagher -
ptg@hessi.gsfc.nasa.gov
(Last updated: 9-May-2002)
