Last modified: 15 August 2019


Full-Field Field of View File (fov3.fits)

The field of view file consists of a single FITS format file for each observation that records the region of the sky that was imaged by the detector during the observation, including the effects of spacecraft dither.

The field of view FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_fov3.fits

where 〈i〉 is the instrument designation, 〈s〉 is the data source, 〈obs〉 is the observation identification, 〈obi〉 is the observation interval identification, 〈v〉 is the data product version number, and 〈c〉 is the cycle. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.


Since virtually-all Chandra observations are made with a dither pattern, the field-of-view file is created to be conservative, in that it creates regions that are slightly larger than the actual observed data. It does not include any of the specialized spatial filtering - for chip edges and read-out streaks - that are applied during the catalog processing. If you need to find out if a particular position on the sky is included in the catalog, then the observation pixmask3 file should be used.

The fov3.fits file may be slightly different than the fov1.fits from the Chandra Data Archive, due to the different GTIs from background flare filtering and possible changes to the aspect solution.


An example fov3 file from release 2.0 is:

unix% dmlist acisf09335_000N021_fov3.fits blocks
Dataset: acisf09335_000N021_fov3.fits
     Block Name                          Type         Dimensions
Block    1: PRIMARY                        Null        
Block    2: FOV                            Table         6 cols x 5        rows

For ACIS observations the number of rows depends on the number of chips that were used in the observation. For HRC-I data there will be one region.

The columns are:

unix% dmlist acisf09335_000N021_fov3.fits cols
Columns for Table Block FOV
ColNo  Name                 Unit        Type             Range
   1   POS(X,Y)[12]         pixel        Real8(12)      -Inf:+Inf            Position
   2   SHAPE                             String[16]                          Region shape type
   3   R[2]                 pixel        Real8(2)       -Inf:+Inf            Radius
   4   ROTANG[2]            pixel        Real8(2)       -Inf:+Inf            Angle
   5   COMPONENT                         Int2           -                    Component number
   6   CCD_ID                            Int2           -                    CCD ID
World Coord Transforms for Columns in Table Block FOV
ColNo    Name
1:    EQPOS(RA ) = (+0.1470 )[degree] +TAN[(-0.000136667)* (POS(X)-(+4096.50))]
           (DEC)   (-57.8065)              (+0.000136667)  (   (Y) (+4096.50)) 

This file can be used as a standard CIAO region file, to filter event files and images, or displayed with ds9. For example, the following command displays the regions in ds9:

unix% ds9 -bin factor 8 -smooth yes -scale sqrt -rgb \
  -red acisf09335_000N021_evt3.fits -bin filter "energy=500:1200" \
  -green acisf09335_000N021_evt3.fits -bin filter "energy=1200:2000" \
  -blue acisf09335_000N021_evt3.fits -bin filter "energy=2000:7000" \
  -region acisf09335_000N021_fov3.fits 


[The observation data is shown (binned and heavily smoothed) and the edges of each ACIS chip is outlined by a green polygon, which comes from the FOV file.]
[Print media version: The observation data is shown (binned and heavily smoothed) and the edges of each ACIS chip is outlined by a green polygon, which comes from the FOV file.]


The data is shown as a heavily-smoothed "three-color" image (where red corresponds to the CSC soft band, green the medium band, and blue the hard band), and the five separate polygons outline each chip that was used in this observation.

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