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Last modified: 15 August 2019

URL: https://cxc.cfa.harvard.edu/csc/data_products/stack/fov3.html
/ csc / data_products / stack / fov3.html

Stacked Observation Detections Field of View File (fov3.fits)

The field of view file consists of a single FITS format file that records the region of the sky that was imaged by the detector, including the effects of spacecraft dither, from the combined set of observations in the stack. It provides an approximate outline of the field, and is created by combining all the obsid-level fov3 files in the stack. For detailed comparison the obsid-level pixel mask (pixmask3) files or the limiting-sensitivity service should be used.

The field of view FITS file is named: 〈i〉〈s〉〈stkpos〉_〈stkver〉N〈v〉_fov3.fits

where 〈i〉 is the instrument designation; 〈s〉 is the data source; 〈stkpos〉 is the position component of the stack name, formatted as "Jhhmmsss{p|m}ddmmss"; 〈stkver〉 is the 3-digit version component of the stack name, formatted with leading zeros; and 〈v〉 is the data product version number.

There are two main blocks in the file:

  1. FOV - which contains a "merged" outline of all the individual obsid-level polygons, and so can contain "exclude" regions (as shown in the example below);
  2. and CCD_FOV - which contains all the per-chip polygons from the obsid-level FOV3 file (and so does not contain any exclude regions).

There may also be additional GTI blocks, which should be ignored.

Example

An example fov3 file from release 2.0 is: 

unix% dmlist acisfJ0035508m433654_001N026_fov3.fits.gz blocks
 
--------------------------------------------------------------------------------
Dataset: acisfJ0035508m433654_001N026_fov3.fits.gz
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
Block    1: PRIMARY                        Null        
Block    2: FOV                            Table         5 cols x 2        rows
Block    3: CCD_FOV                        Table         8 cols x 12       rows
Block    4: GTI                            Table         2 cols x 2        rows

The FOV block contains the "combined" polygons, and so has no information on ObsId or CCD_ID: 

unix% dmlist "acisfJ0035508m433654_001N026_fov3.fits.gz[FOV]" cols
 
--------------------------------------------------------------------------------
Columns for Table Block FOV
--------------------------------------------------------------------------------
 
ColNo  Name                 Unit        Type             Range
   1   COMPONENT                         Int4           -                    Component number
   2   SHAPE                             String[8]                           Region shape type
   3   POS(X,Y)[53]         pixel        Real8(53)      -Inf:+Inf            Position
   4   R[2]                              Real8(2)       -Inf:+Inf            Radius
   5   ROTANG[2]                         Real8(2)       -Inf:+Inf            Angle
 
--------------------------------------------------------------------------------
World Coord Transforms for Columns in Table Block FOV
--------------------------------------------------------------------------------
 
ColNo    Name
3:    EQPOS(RA ) = (+8.9619 )[degree] +TAN[(-0.000136667)* (POS(X)-(+4096.50))]
           (DEC)   (-43.6151)              (+0.000136667)  (   (Y) (+4096.50))

The CCD_FOV block contains the per-chip polygons from the per-observation fov3 file, and so has columns to indicate the observation (OBSID and OBI) and chip (CCD_ID): 

unix% dmlist "acisfJ0035508m433654_001N026_fov3.fits.gz[CCD_FOV]" cols
 
--------------------------------------------------------------------------------
Columns for Table Block CCD_FOV
--------------------------------------------------------------------------------
 
ColNo  Name                 Unit        Type             Range
   1   POS(X,Y)[13]         pixel        Real8(13)      -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
   7   OBSID                             Int4           -                    
   8   OBI                               Int4           -                    
 
--------------------------------------------------------------------------------
World Coord Transforms for Columns in Table Block CCD_FOV
--------------------------------------------------------------------------------
 
ColNo    Name
1:    EQPOS(RA ) = (+8.9619 )[degree] +TAN[(-0.000136667)* (POS(X)-(+4096.50))]
           (DEC)   (-43.6151)              (+0.000136667)  (   (Y) (+4096.50))

The data in the two blocks represent the same region, but display differently, as shown in the figures below:

The FOV block

[detector field-of-view for stack, which outlines the image data]
[Print media version: detector field-of-view for stack, which outlines the image data]

The FOV block

For this observation, the roll difference was large enough between the individual observations that the ACIS-S chips of one observation overlap the ACIS-I chips of the other observation. This results in the combined polygon having an outer include region (blue line) and an inner excluded region (orange line). The points that are shaded blue are included within this region, and those orange are not.

The CCD_FOV block

[detector field-of-view for stack, which outlines the image data]
[Print media version: detector field-of-view for stack, which outlines the image data]

The CCD_FOV block

The colored lines and fills indicate the individual rows of the the CCD_FOV block, and they are labelled by the OBSID and CCD_ID value from the block (in this stack there are no observations with different OBI values, so we do not include the value in the label).

The solid black line shows the outer polygon from the FOV block, which matches the outer edge of all the individual polygons. The inner edge of the FOV block (the orange polygon in the previous figure) is not explicitly drawn in this figure, but corresponds to the white space between the colored regions.

Related Science Documents

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