Observation date FITS Header

If i’m correct the ‘Date FITS file was generated’ is the exact moment of observation in the DECaLS FITS header?



I’m not sure which file(s) you’re looking at. The DECaLS coadded images are created from multiple individual exposures that may have been taken years apart. For the individual exposures, there should be DATE-OBS header cards giving the date of observation.


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Hmm I just found out i’m wrong, the date is given when you check in the viewer with DECaLS DR 3 CCD’s button, as you explained earlier.

But strangely I cannot find DATE-OBS in the Metadata or Image Headers in the individual exposures; Metadata is barely filled with any information and no info on ‘Date’.

Am I maybe doing something wrong, would be strange if this isn’t included in the file?

An example file which I used is image-decam-425096-N8-r.fits


What program are you using to look at these files?

When I use ‘listhead’ (from the cfitsio package), I get:

Header listing for HDU #1:
SIMPLE = T / file does conform to FITS standard
BITPIX = 16 / number of bits per data pixel
NAXIS = 0 / number of data axes
EXTEND = T / FITS dataset may contain extensions
COMMENT FITS (Flexible Image Transport System) format is defined in ‘Astronomy
COMMENT and Astrophysics’, volume 376, page 359; bibcode: 2001A&A…376…359H
DATE = ‘2015-03-30T23:27:14’ / Date FITS file was generated
IRAF-TLM= ‘2015-03-31T03:22:14’ / Time of last modification
ORIGIN = ‘NOAO-IRAF FITS Image Kernel July 2003’ / FITS file originator
OBJECT = ‘DECaLS_32897_r’ / Name of the object observed
PROCTYPE= 'InstCal ’ / Data processing level
PRODTYPE= 'image ’ / Data product type
PIXSCAL1= 0.27 / [arcsec/pixel] Pixel scale, axis 1
PIXSCAL2= 0.27 / [arcsec/pixel] Pixel scale, axis 2
OBS-LONG= 70.81489 / [deg] Observatory east longitude
TELESCOP= ‘CTIO 4.0-m telescope’ / Telescope name
OBSERVAT= 'CTIO ’ / Observatory name
OBS-LAT = -30.16606 / [deg] Observatory latitude
OBS-ELEV= 2215. / [m] Observatory elevation
INSTRUME= 'DECam ’ / Instrument used to obtain these data
EXPREQ = 62. / [s] Requested exposure duration
EXPTIME = 62. / [s] Exposure duration
DARKTIME= 63.1466999 / [s] Dark time
OBSID = ‘ct4m20150327t072848’ / Unique Observation ID
DATE-OBS= ‘2015-03-27T07:28:48.588788’ / UTC epoch
TIME-OBS= ‘07:28:48.588788’ / Time of observation start (UTC)
MJD-OBS = 57108.31167348 / MJD of observation start
OPENSHUT= ‘2015-03-27T07:28:48.683340’ / Time when shutter opened (UTC)
TIMESYS = 'UTC ’ / Time system
EXPNUM = 425096 / DECam exposure number

and many more pages of headers.


ps, since these are FITS files, the headers are actually in text format (80 characters per line), so (after unzipping them) you can just look at them with a text editor or ‘less’. The actual pixels are stored in binary so will show up as gibbering in your editor, but the headers are plain text.


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Thanks I will try that, hope it helps. Most info you list I can see but not observation date.

The programs I used were FITS liberator and Aladin desktop. I still have to try DS9 to check for DATE-OBS in the data.

Great response and feedback :slight_smile: