New data formats and FITS headers

Latest news: Description of FITS headers at the NOT, v1.0.1., 2014 (Pursimo et al. pdf file).

General page on FITS headers

For all NOT instruments we are gradually implementing new FITS data formats and headers. As of Fall 2005, MOSCA and FIES data are recorded in the new formats, and NOTCAM and ALFOSC are expected to follow suit soon! Highlights include:

Multi-extension data formats for all instruments
Primary WCS information for all imaging data (ALFOSC, NOTCAM, STANCAM, MOSCA)
Mosaic data reduction made easy (ALFOSC dual-readout, MOSCA)
ISO8601-compliant date and time information for start and midpoint of exposures
Improved timing information for NOTCAM
Same orientation for all data (North up, East left for appropriate instrument mount angle offset)
Detector/amplifier characteristics included in header

All FITS headers are now recorded in the recommended formats. Several new keywords have been introduced.

Apart from the change to MEF and the new keywords which apply to all instruments, the most important changes in the data format and header information are for:

ALFOSC
- Dual-amplifier data can now be reduced in a standard way using IRAF mscred
NOTCAM
- Change of data format from cube to multiple 2-dimensional data extensions
- Data orientation changed (North up, East left)
- Possibility of amplifier-dependent data reduction
- Timing information improved: the individual extensions have different DATE-OBS, DATE-AVG, and EXPTIME cards
- New keyword NCOMBINE in the reset-read-read average frame
MOSCA
- Change of data format from combined image to four data extensions
- Data reduction in a standard way using IRAF mscred
STANCAM
- Data orientation changed (North up, East left)
FIES
- MEF format

Multi-extension FITS format (MEF)

All NOT observational data will be recorded in the so-called multi-extension FITS format. A MEF file is comprised of several segments called Header/Data Units (HDUs). Every HDU consists of an Header Unit (the well known FITS headers) in ASCII format followed by an optional Data Unit. The first HDU is called the primary, and any number of additional HDUs may follow. These additional HDUs are referred to as FITS extensions.

In the new NOT formats, the primary HDU only contains ASCII header cards describing the observation, but no data. The astronomical data arrays are stored in additional image extensions. For FIES and ALFOSC in single-amplifier mode there is 1 image extension, for ALFOSC in dual-amplifier mode there are 2 image extensions (1 per amplifier), for MOSCA there are 4 image extensions (1 for each detector in the mosaic), and for NOTCAM there are a variable number of extensions depending on the read-out mode (1 for each readout, plus 1 for the reset frame). We have compiled some practical examples on the use of MEF.

WCS information for all imaging data

Primary WCS information for all imaging data (ALFOSC, NOTCAM, STANCAM, MOSCA) has been included in the data extension headers. By simply loading your image data using stand-alone ds9, you will see the equatorial coordinates RA and Dec as well as the pixel coordinates X and Y. IRAF in combination ds9 with will not display RA and Dec on your screen, but interprets the WCS information silently. See the example on finding astrometric reference stars.