HESSI Image Guide

This document describes the basics for using the HESSI software data analysis software. It describes how to use the command-line user interface hessi_image. To work with hessi_image, IDL version 5.2 or higher must be installed, as well as the solar soft tree with the HESSI branch of the tree. Please note: this guide is frequently modified.

Last modified: Thu Mar 9 09:30:36 2000 (csillag@soleil) Release 4

First image
Reconstructing images using the test file
Changing image reconstruction parameters
Using the simulator

First Image

When hessi_image is started for the first time in an sswidl session, an image with all default parameters is created:

IDL> hessi_image, image, parameter

image is an IDL 2D array that contains the reconstructed image; parameter is an IDL structure that contains all metadata associated with the image. Further information is given by:

IDL> help, image, parameter, /STRUCT

To create this first image, the following operations are processed:

A model of a point source at <600,200> arcsec off the sun center is created.
An event list for this model is created (4 seconds duration)
The event list is transformed into a calibrated, binned event list
The modulation patterns associated with the calibrated evcent list are created
The count rate is back-projected using the modulation patterns, producing a dirty map

The image can be displayed with

IDL> hessi_image, image, /plot

The result will look like this:

HESSI Reconstructed Image

Reconstructing images from the test file

There is a HESSI FITS test file available: telemetry.fits. Once downloaded on a local computer, the image is reconstructed by typing

IDL> hessi_image, image, parameters, FILENAME = 'telemetry.fits', /plot

Changing image reconstruction parameters

The way the image is reconstructed can be modified by overwriting the defaults of the image reconstruction parameters. The available parameter are listed as tag in the structure parameters. The most important are:

A2D_INDEX_MASK
ENERGY_BAND
IMAGE_ALGORITHM
IMAGE_DIM
PIXEL_SIZE
TIME_RANGE
XYOFFSET

For a description of all available parameters, please refer to the reference manual. Parameters are modified by appending the associated name as keywords to hessi_image (see examples).

A2D_INDEX_MASK

Sets which detectors to use for creating the photon list. Example:

IDL>hessi_image, image, param, A2D_INDEX_MASK = [0,0,1,1,1,0,0,0,0]

will create an image using the 3rd, 4th and 5th detector. Default: all detectors selected.

ENERGY_BAND

Sets the energy band, in keV, for which the image should be reconstructed. Example:

IDL>hessi_image, image, param, ENERGY_BAND =[7., 50.]

will create an image for the energy band 7keV .. 50 keV. Default: [6., 100.]

IMAGE_ALGORITHM

Sets the image algorithm used for cleaning the image. Available algorithms are: CLEAN and MEM SATO. Example:

IDL>hessi_image, image, param, IMAGE_ALGORITHM = 'CLEAN'

will use the clean algorithm to restore the image. Default: none, i.e. the image returned is the back-projection.

IMAGE_DIM

Sets the image dimension. Usually, the dimension is a power of two. Example:

IDL>hessi_image, image, param, IMAGE_DIM=[128,128]

Default: [64,64]

PIXEL_SIZE

Sets the pixel size of the image. Unit: 1 arcsec. Default: PIXEL_SIZE = [1., 1.] arcsec

TIME_RANGE

Sets the start and end time in seconds for the reconstructed image. For example:

IDL>hessi_image, image, param, TIME_RANGE=[0., 2.]

Default: TIME_RANGE = [0.,4.] sec

XYOFFSET

Sets the offset of map center from the sun center. For example:

IDL>hessi_image, image, param, IMAGE_DIM=[64,64], XYOFFSET=[600,200], PIXEL_SIZE = [2.,2.]

Will reconstruct an image with the map center at <600,200> arcecs, with a map size of 256 arcsec. Default: XYOFFSET = [600.,200.] arcsec

Using the HESSI simulator

Simulated data can be used to reconstruct images. The simulated data is created by specifying simulation parameters. The most important are:

SIM_A2D_INDEX_MASK
SIM_MODEL
SIM_PHOTONS_PER_COLL
SIM_PIXEL_SIZE
SIM_TIME_RANGE
SIM_UT_REF
SIM_XYOFFSET

All simulation parameters are described in the reference manual.

SIM_A2D_INDEX_MASK

Sets the detectors used in the simulation. Same format and default as A2D_INDEX_MASK above.

SIM_MODEL

Defines the model used to creeate simulated data. It is usually a 2D square array. Example:

IDL>model = FltArr( 64, 64 )
IDL>model[50,50] = 1
IDL>hessi_image, image, param, SIM_MODEL=model

The default is a 64 by 64 pixel array with two point sources, the first at [32,32], the second at [20,20].

SIM_PHOTONS_PER_COLL

: Number of photons to shoot at each collimator per second, before losses due to grids and time_profile. For example:

IDL>hessi_image, image, param, SIM_MODEL=model, SIM_PHOTON=40000L

SIM_PIXEL_SIZE

Sets the pixel size used by the model. Same format and default as PIXEL_SIZE above.

SIM_TIME_RANGE

Start and end time used by the simulation Same format and default as TIME_RANGE above.

SIM_UT_REF

Sets the UT start time of the simulation.For example:

IDL>hessi_image, image, param, SIM_UT_REF=AnyTim( '

SIM_XYOFFSET

Sets the offset of map center from the sun center. Same format and default as XYOFFSET above.

A. Csillaghy