III. NEOWISE Single-exposure Images

III.1. General Properties

III.1.b. Single-exposure Image Header Descriptions

The NEOWISE single-exposure images are formatted in the Flexible Image Transport System (FITS). For information on file naming, see section III.1.a.

An extensive image header is added to the raw fits image data during the ingest process (IV.1.a.i). The header consists of two main sections: keywords extracted directly from the engineering housekeeping (H/K) data, and non-H/K keywords. H/K data are sampled at various rates and not synched with the acquisition of the image data. The H/K data file is downlinked and transferred to IPAC where all measurements are stored in a database. During the ingest process, this database is searched for the value of a particular keyword closest in time to the DATE_OBS mid-exposure image data acquisition time. Hence there is a time difference between DATE_OBS and the time the keyword was sampled. The header keywords HK_MINDT and HK_MAXDT (see table below) indicate the minimum and maximum time difference for all H/K-derived header values.

Times in the non-H/K section of the header are all computed from various ancillary files. The times saved in the image source packet headers are from the spacecraft (S/C) clock at the time the first data reach the MUB, which is within a few msecs of the end of the exposure. It will take another 1.1 sec for the last pixels to reach the MUB. During ingest, a constant offset of -4.853 sec (indicated by the header keyword FRMTOFF) is added to this time and saved as the frame time. This timestamp corresponds to the midpoint of the track followed by the boresight during the exposure.

Example of a NEOWISE single-exposure image header.

Notes to Table 1

[1] BUNIT is 'DN' for the intensity image (int) and uncertainty (unc) files, and 'null' for the mask (msk) files.

[2] Various event keywords only appear in the header if the data were taken during one of these possible events:

 'SAA'= within pre-defined SAA boundary
 'MOON'= within moon keepout zone
 'MOM'= momentum dumping
 'OCCULTED'= Sun is occulted
 'IMCAL'= image calibration
 'DESC'= we are in a descending scan
 'SCAN'= yes, we are in a science scan (you should always see this)
 'ASCE'= we are in an ascending scan
 'NEP' = North ecliptic pole crossing
 'SEP' = South ecliptic pole crossing
 '12PM', '6AM', '6PM', '12AM' = Orbit points

The events are listed in the header keyword INEVENTS. For each event, an additional header entry is created, starting with 'EV'. The keyword value corresponds to the time since the event began. For example, the events ASCE (or DESC) and SCAN occur as listed under INEVENTS. Hence, the additional keywords EVDESC and EVSCAN are added to the header of this particular image:

INEVENTS= 'SCAN   DESC'   / Active orbit events
EVDESC  =     2009.17127251625 / [sec] Time since orbit event start
EVSCAN  =     2061.64127248526 / [sec] Time since orbit event start

[3] -9999 or -999 indicate "no value" is available or could be computed.

[4] The payload has two redundant scan mirror assemblies, indicated by [A] and [B].

[5] This keyword contains the WISE Science Data System software release version.

[6] MAGZPUNC always = -999 in the NEOWISE single-exposure FITS headers. Software to compute this was not implemented.

[7] The temperatures reported by these keywords are not valid for NEOWISE. The Focal Plane Array sensors stopped operating following cryogen exhaustion. For approximate FPA temperatures during the NEOWISE survey, see the BSPLMOUT keyword.

Example of a NEOWISE single-exposure image header.

Last update: 25 September 2024