IFU: Data Format and Location

IFU: Data Format and Location

Raw files

Raw, unreduced, data files are in /ukirtdata/raw/uist/YYYYMMDD (where YYYYMMDD is the numeric UT date); at the telscope this is equivalent to $ORAC_DATA_IN. The files are stored as starlink HDS containers (a file with multiple data arrays). Each file is equivalent to 1 observation, and as such contains a header component and 1 or more (actually NINT) integrations. Each integration is stored as an NDF component (single data array) of the HDS file. The raw filenames are uYYYYMMDD_NNNNN.sdf where NNNNN is the observation number, padded with leading zeros when necessary.

Reduced files

Reduced data files are in /ukirtdata/reduced/uist/YYYYMMDD; at the telescope this is equivalent to $ORAC_DATA_OUT.

The filename structure is: (PREFIX)(UTDATE)_(FRAME NUMBER)_(EXTENSION).sdf. (PREFIX) is the letter ‘u’ if the file contains data from a single observation. It is ‘gu’ if the file contains data from a number of observations – i.e. a group. The suffix _(EXTENSION) defines the file in terms of the reduction process (see the table below). The DR produces many “intermediate” frames. Useful frames are left on disk after ORAC-DR has finished so that you can look at these if you wish (these are marked with ** in the table below).

As an example; u20000410_00123_ff.sdf would be data from a single observation, number 123, that has been flat fielded. u20000410_00123_scr.sdf would then be the difference of frames 123 (object) and 124 (sky); this frame has also been “scrunched”, i.e. wavelength corrected and the individual slices re-arranged in a logical order across the array. As a consequence, this frame is arguably more useful to you than the _ff frame.

File Extensions for INDIVIDUAL IFU files
(all recipes)

**_rawThe raw frame
_bpBad pixels masked
_rnvReadnoise variance added
_sbfBias frame subtracted (only present for STARE observations)
_povPoisson variance added
_extSlices extracted and approximately aligned
**_ffFlat fielded
**_ssSky subtracted (result from difference of 2 _ff frames)
**_scrAll rows in _ss frame scrunched and wavelength calibrated
**_quad2 _scr frames averaged (1 _quad per repeat of “obj-sky-sky-obj” sequence)

File Extensions for GROUPED/CO-ADDED IFU files

**no extensionCoadded “scrunched” spectral image
**_cubeScrunched spec image converted to a datacube
**_im_cube collapsed into a 2D image
**_sp_im collapsed into a 1D spectrum
**_std_sp spectrum normalised and divided by BB function

NOTE: For the standard star, the processed spectrum (the _std file) is copied to an std_(num) file. This is grown into an image and then a data-cube (the std_(num)_im and _std_(num)_cube files), for use with the science-target datacube.

File Extensions for GROUPED/CO-ADDED IFU files

**no extensionCoadded “scrunched” spectral image (EXTENDED_SOURCE only)
**_cubeScrunched spec image converted to a datacube
**_cube_dbs_cube divided by standard star cube (the std_(num)_cube file)
**_fc_cube_dbs file flux-calibrated
**_fc_sp_fc cube collapsed into a 1D spectrum
**_im_cube collapsed into a 2D white-light image
**_im_fc_fc collapsed into a 2D white-light image

NOTE: All files except those marked with ** in the above tables are deleted by the DR after reduction (these are intermediate files). Reduced files can have either HDS (multiple data arrays) or NDF (single data array) format as appropriate (see above).

The philosophy behind the MAP_EXTENDED_SOURCE recipe and associated mode of observing is to nod between the target and blank sky, but also to offset the IFU when on the target to map a more extended region. Because the “on-source” frames are offset with respect to each other, obviously the _scrspectral images cannot be (trivially) coadded into a summed spectral image. Consequently, MAP_EXTENDED_SOURCE does not produce a coadded, scrunched spectral image for all data on a given target.

So what should I take home with me…?

If you don’t have access to ORAC at your home institute, then reducing completely raw data frames may be a challenge, to say the least. Instead, it may be preferable to work with some of the products of the DR. I’ve highlighted (in red) the data products that I think may be of most use to users, though at the end of the day, this decision is up to you! In terms of “fully reduced data”:

  • From EXTENDED_SOURCE, the final “scrunched” spectral image, which is the co-addition of all the object-minus-sky spectral images, flat-fielded and scrunched to a common wavelength scale (i.e. the sum of all the u(UTdate)_(num)_scr frames), is one of the most useful product of the DR. This file has no suffix .
  • The white-light image displayed by the DR has the suffix _im. This may also be useful, though images across individual emission lines are probably what you’re really after. These can be produced by re-reducing the data with ORAC-DR using an “extract.images” file.
  • The data cube, which of course contains all the data on your target and may be manipulated with other 3-D data analysis software. This file has the suffix _cube. The flux-calibrated version has the suffix _fc.

Non-Starlink users may convert their data to fits format (before they leave) with the Starlink routines:

     > convert
     > ndf2fits "*" "*"

or, to get .fits as the file extension (instead of .fit)

     > ndf2fits "*" "*|.fit|.fits|"

Note that the complete Starlink software library, which includes ORAC-DR, is available free (speak with your support scientist). You can then simply work with the raw “.sdf” files available from the OMP web pages.

Orientation of IFU field on the sky

Below we list the orientation of the IFU cube and white-light image on the sky for various position angles.

AngleTop of cube/extracted imageRight side of cube/extracted image
-90 degsEastNorth
-45 degsSouthEastNorthEast
0 degsSouthEast
+45 degsSouthWestSouthEast
+90 degsWestSouth

To view with the correct orientation, images need flipping along BOTH axes. In Gaia, click on and .

Note that the top of each slice in an IFU scrunched image should correspond to the top of the white-light image and cube. Similarly, moving up the array – from one slice to the next in a scrunched image – corresponds to moving from left to right across a white-light image/data cube. The direction this corresponds to on the sky is given in the above table.

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