STEREO HI data processing documentation

The camera has no shutter, therefore the image is smeared along one axis both as the CCD is cleared prior to an exposure and as the CCD is read out. A small number of images are sent to the ground with no other processing but most images are scrubbed for cosmic rays and are then summed on board the spacecraft [normally 30 images for HI-1 and 99 images for HI-2]. In order to optimise the communication to the ground these summed images are then split into two - high word and low word. These data are transmitted using 'rice' compression - a lossless technique. Image and spacecraft status data is also transmitted. Some images are transmitted using a highly compressed "beacon mode", these are primarily intended for space weather alerts.

• HI Level 0 data
• HI Level 1 data
• HI Level 2 data

HI Level 0 data

The first stage production of user data is conducted at NRL. The split summed images are re-integrated and coded and tabular values from the spacecraft status fields are turned into physical values and fits files are generated. Images from the behind spacecraft are rotated 180° so that solar north is towards the top of the images.

yyyymmdd_hhmmss

The nominal start of the exposure

tt

n4 - unscrubbed, unsummed image

s4 - scrubbed, summed images [normal science image]

s7 - scrubbed, summed, highly compressed image

hc

h1 - hi_1 camera

h2 - hi_2 camera

s

A - ahead spacecraft

B - behind spacecraft

The s4 images also use fits extended headers to give the header information for all images in the sum.

In the s4 data the last pixels in the image are replaced with a report from the cosmic ray scrubbing routine. This typically affects 31 pixels in HI-1 and 100 pixels in HI-2 images - in conventional plots these pixels are in the lowest row at the far left on the ahead images and in the highest row at the far right for behind images.

As better determination of the spacecraft orbit become available the level 0 data is occasionally refreshed.

HI Level 1 data

1. Reject corrupted images - a list is maintained of images that were badly corrupted in processing and transmission - these images are not processed to level 1.
2. Extract cosmic ray report - [only present in s4 images]. The pixels are replaced by a duplicate of the line below to allow subsequent processing.
3. Remove columns where saturation has occurred on the CCD - due to there being no shutter on the camera it is not in general possible to recover columns where saturation has occurred. Bright stars and planets often saturate.
4. De-smear the image - by applying a matrix multiplication algorithmically remove the smear caused in clearing and readout of the CCD. This process automatically normalises the exposure time to 1 second and removes the summing effect in the s4 images.
5. Remove pixels marked as having missing telemetry from the image.
6. Apply a calibration factor to the image - currently this allows for the geometric effect in the lenses and for a linear production feature in the CCD.