Test of STIX compression using HXT data

J.McTiernan

SSL/UCB

Proposed STIX Compression:

• Goal: To take the data from 64 sub-collimators (which will result in one image) and compress it down to less than 40 bytes.

• Compression Algorithm: Maximum value is compressed into a 2 byte integer (between 0 and 65535) using square root compression.

• The remaining numbers are divided by the max. value and the fractional values are scaled into 4 bit numbers (from 0 to 15).

• Total number of bytes per image: 2 + 64*(1/2) = 34

• IDL> max_data = max(data)• peak_value = uint(max_data+0.5) ;rounds• fraction = byte(15.0d0*data/max_data+0.5)• ;idl does not do 4bit integers, but we can test

with this.

We can test this using Yohkoh HXT

• Procedure: Accumulate HXT data for a given time interval.• Data consists of counts for 64 sub-collimators.• Compress data – then uncompress data:

The white curve shows the observed counts for an interval during a flare on 2-nov-1991.

The red curve has been compressed-and uncompressed.

The difference between the count rates is 1 to 5% for most collimators, but larger for “fan-beams” (0-15)

Top: Pixon images of 2-nov-1991 flare: LO=13-23 keV, M1=23-33 keV, M2=33-53 keV

Bottom: Same images, but with compression-decompression

The two sets of images look similar, but are not identical.

To look more closely, lets do some primitive imaging spectroscopy, using the images from all 4 HXT channels.

We can get count and photon spectra for the two sources with the boxes drawn around them.

This plot shows count spectra for the full image, and for the two sources – red denotes the original data, and blue shows the compressed-uncompressed data. There is a systematic difference in counts of 2 to 3%, except for the LO sources.

(Note that the 5 other flares studied do not show this LO channel behavior.)

This shows the photon spectra for Source A, just to show that it can be done.

Photon spectra for Source B.

Conclusion:

• From comparisons using HXT data, it seems entirely reasonable that the information from 64 collimators can be compressed into less than 40 bytes.