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FITS REGION Binary Table Design
ASC­FITS­REGION­1.0
Jonathan McDowell and Arnold Rots
1998­03­09
1 Definition
This document contains the specification of the REGION table which is the spatial equivalent of
the (temporal) GTI table. Like the GTI table, the REGION table is a FITS binary table. It is
recognized as a REGION table by the presence of HDUCLAS1='REGION'. The REGION table
defines regions of an image that are to be included or excluded from certain operations, specified
by means of geometrical shapes. Each row specifies an element of a certain shape that traces a
specified area in the image; elements are grouped in components; and one or more components make
up a region. As in soccer, the boundary lines are considered part of the element they circumscribe.
The logic of combining elements and components is governed by three rules:
ffl A region consists of the union of one or more components (logical OR operation).
ffl A component consists of the intersection of one or more elements (logical AND operation).
ffl Elements for which the value in the SHAPE column is preceded by an exclamation mark
(``!'') are interpreted as: the entire image, excluding the region traced by the element and its
boundary (i.e., logical NOT operation).
2 Columns
At the present time, the REGION table is defined for a two­dimensional space. The following
columns are mandatory.
ffl Two spatial coordinate columns, which we shall designate here as X and Y, each containing
a vector of length sufficient to describe the geometrical shapes specified in the table. The
1

FITS REGION Binary Table Design 2
numbers may be floating point, double, or integer. The names of the columns (TTYPEi)
indicate the coordinate system in use, in the usual way, and these coordinate names are to be
identified in the MFORM1 keyword (MFORM1='X,Y'); the mandatory keyword MTYPE1
provides a name for the coordinate pair. It is recommended that the (X,Y) pixel coordinate
system be tied to absolute (WCS) coordinates following the convention for pixel lists in
OGIP/94­006.
The following columns are optional, although they are recommended and may be required depend­
ing on the contents of other columns.
ffl SHAPE (rA): specifies the geometrical shape of the element. At the present time the fol­
lowing shapes are defined: point (default), circle, ellipse, annulus, elliptannulus, box, rotbox,
rectangle, rotrectangle, polygon, pie=sector, diamond=rhombus, rotdiamond=rotrhombus.
The recommended value for r is 16. For shape identification, only the first 15 characters will
be significant while case will not be significant. The shape name may be preceded by an
exclamation mark (e.g., ``!Circle''), which shall be interpreted as ``include the entire image,
except for the specified element and its boundary''.
ffl R (radius): for elements that require a radius parameter for their definition; like X and Y,
this column needs to contain vectors of sufficient length.
ffl ROTANG (rotation angle): for rotated elements; like X and Y, this column needs to contain
vectors of sufficient length.
The following column is optional, although it is recommended for more sophisticated use of regions:
ffl COMPONENT (I): component number that the element is to be associated with; default: 1.
Additional columns may be included to allow selective use of regions. In the example below, we
have added the columns SOURCE and GRATING as useful selection criteria for ACIS.
3 Header Keywords
In the following example header, all keywords are mandatory except for the ones marked ``r''
(recommended or required by certain shapes) or ``o'' (truly optional). As noted above, it is rec­
ommended that the (X,Y) pixel coordinate system used be tied to absolute coordinates, e.g.,
(RA­­­TAN,DEC­­TAN) following the convention for pixel lists in OGIP/94­006.
XTENSION= 'BINTABLE' / HDU 1==========================================
BITPIX = 8 / 8­bit bytes

FITS REGION Binary Table Design 3
NAXIS = 2 / 2­dimensional Binary Table
NAXIS1 = 228 / Number of bytes per row
NAXIS2 = 1 / Count the rows!
PCOUNT = 0 / No group parameters (required keyword)
GCOUNT = 1 / One data group (required keyword)
TFIELDS = 8 / Number of columns
COMMENT This table contains an inclusion/exclusion region spec
EXTNAME = 'REGION ' / Region specification table
EXTVER = 1
EXTLEVEL= 1 / Level in DB hierarchy: Data table
CHECKSUM= '0000000000000000' / ASCII encoded HDU checksum
DATASUM = ' 0' / Data unit checksum written in ASCII
ORIGIN = 'ASC '
CREATOR = 'me '
CONTENT = 'REGION '
HDUDOC = 'ASC­FITS­1.0: McDowell, Rots: ASC FITS File Designers Guide'
HDUVERS = '1.0.0 '
HDUCLASS= 'ASC '
HDUCLAS1= 'REGION '
HDUCLAS2= 'STANDARD'
o LONGSTRN= 'OGIP 1.0' / The OGIP long string convention may be used.
o COMMENT This FITS file may contain long string keyword values that are
o COMMENT continued over multiple keywords. This convention uses the '&'
o COMMENT character at the end of a string which is then continued
o COMMENT on subsequent keywords whose name = 'CONTINUE'.
DATE = '1998­02­05' / Time information block­­­­­­­­­­­­­­­­­­­­­­­­­
o DATE­OBS= '1998­01­01T00:00:00'/ TT, with clock correction
o DATE­END= '1998­01­01T00:00:01'/ TT, with clock correction
o TIMESYS = 'TT ' / AXAF time will be TT (Terrestrial Time)
o MJDREF = 50814 / 1998.0(TT)
o / MJD = JD ­ 2400000.5
o TIMEZERO= 0.0 / Clock correction
o TIMEUNIT= 's '
o TIMEREF = 'LOCAL ' / No pathlength corrections
o TASSIGN = 'ASC ' / ASC
o CLOCKAPP= T / Clock correction applied
o TIERRELA= 1.0E­9 / Short­term clock stability
o TIERABSO= 1.0E­6 / Absolute precision of clock correction
o TIMVERSN= 'ASC­FITS­1.0' / AXAF FITS design document
o TSTART = 0.0 / As in the ''TIME'' column: raw space craft clock;
o TSTOP = 1.0 / add TIMEZERO and MJDREF for absolute TT
MTYPE1 = 'pos '

FITS REGION Binary Table Design 4
MFORM1 = 'X,Y '
r RADECSYS= 'ICRS ' / The following keywords tie the REGION
r EQUINOX = 2000.0 / coordinates to WCS
r TCTYP2 = 'RA­­­TAN' / RA
r TCRPX2 = 512
r TCRPX2 = 123.4567
r TCDLT2 = 0.0001
r TCUNI2 = 'deg '
r TCTYP3 = 'DEC­­TAN' / Dec
r TCRPX3 = 512
r TCRPX3 = 45.6789
r TCDLT3 = 0.0001
r TCUNI3 = 'deg '
r TCROT3 = 30.0
r TTYPE1 = 'SHAPE ' / Shape of element
r TFORM1 = '16A '
r TUNIT1 = ' '
TTYPE2 = 'X ' / X­coordinate vector
TFORM2 = '12E '
TUNIT2 = 'pixel '
TTYPE3 = 'Y ' / Y­coordinate vector
TFORM3 = '12E '
TUNIT3 = 'pixel '
r TTYPE4 = 'R ' / Radius vector
r TFORM4 = '12E '
r TUNIT4 = 'pixel '
r TTYPE5 = 'ROTANG ' / Rotation angle vector
r TFORM5 = '12E '
r TUNIT5 = 'deg '
r TTYPE6 = 'COMPONENT' / Component number that shape belongs to
r TFORM6 = 'I ' / default: 1
r TUNIT6 = ' '
o TTYPE7 = 'SOURCE ' / Source number
o TFORM7 = 'I '
o TUNIT7 = ' '
o TTYPE8 = 'GRATING ' / Applicable grating (default 'NONE')
o TFORM8 = '16A '

FITS REGION Binary Table Design 5
o TUNIT8 = ' '
END
To summarize, only the X, and Y columns are required, although a SHAPE column is likely indis­
pensible; whether R and ROTANG are needed depends on the contents of the SHAPE column. The
COMPONENT column is recommended for more sophisticated use. The SOURCE and GRAT­
ING columns are implementation­dependent: any columns, like these, may be added for filtering
purposes.
4 SHAPEs
The value of SHAPE (default: point) defines the precise meaning of the X, Y, R (radius), ROTANG
columns and the minimum number of elements required in their vectors. The following table
enumerates the currently defined values for SHAPE and the meaning of the related columns. Note
the optional use of an exclamation mark preceding the SHAPE value, as described above.
Table 1: REGION Table SHAPEs
SHAPE X Y R ROTANG
Point X Y
Circle Xcen Ycen R
Ellipse Xcen Ycen Rmaj ; Rmin Angle
Annulus Xcen Ycen Rin ; Rout
Elliptannulus Xcen Ycen Rinmaj ; Rinmin ; Angin ;
Routmaj ; Routmin Angout
Box Xcen Ycen Xsize ; Ysize
Rotbox Xcen Ycen Xsize ; Ysize Angle
Rectangle Xmin ; Xmax Ymin ; Ymax
Rotrectangle Xmin ; Xmax Ymin ; Ymax Angle
Polygon X0 ; X1 ; :::Xn Y0 ; Y1 ; :::Yn
Pie Xcen Ycen Angmin ; Angmax
Sector (=pie) Xcen Ycen Angmin ; Angmax
Diamond Xcen Ycen Xsize ; Ysize
Rhombus (=diamond) Xcen Ycen Xsize ; Ysize
Rotdiamond Xcen Ycen Xsize ; Ysize Angle
Rotrhombus (=rotdiamond) Xcen Ycen Xsize ; Ysize Angle
The value of SHAPE (point, circle, ellipse, rectangle, rotbox, polygon; default: point) defines the
precise meaning of the X, Y, R (radius), ROTANG columns and the minimum number of elements
required in their vectors; e.g.
ffl point: first element of X, Y vectors specifies point to be included.

FITS REGION Binary Table Design 6
ffl circle: first element of X, Y, R vectors specifies center and radius.
ffl ellipse: first X, Y (center), ROTANG (counter­clockwise rotation of major axis with repect
to X axis), first two R (semi axes).
ffl annulus: first X, Y (center), first two R (inner and outer radius).
ffl elliptannulus: (area between two concentric ellipses: first X, Y (center), first four R (major,
minor semi­axes inner ellipse, major, minor semi­axes outer ellipse), first two ROTANG (inner
and outer ellipse rotation).
ffl box: first X, Y (center), first two R (size in X, Y).
ffl rotbox: first X, Y (center), first two R (size in X, Y), first ROTANG (counter­clockwise
rotation).
ffl rectangle: first two elements of X, Y (bottom left and top right corners).
ffl rotrectangle: first two elements of X, Y (bottom left and top right corners of rectangle), first
element of ROTANG (counter­clockwise rotation of the rectangle with respect to X and Y
axes).
ffl polygon: m X and Y elements (vertices), where m is the smallest positive number ! n for
which ( ( X[m] == X[0] ) && ( Y[m] == Y[0] ) ) or, if none satisfies, n; in the above example,
n = 12.
ffl pie or sector: first X, Y (center), first two elements of ROTANG (sector from first to second,
counted counter­clockwise from positive X­axis).
ffl diamond or rhombus: first X, Y (center), first two R (size in X, Y, between vertices in X, Y
axis).
ffl rotdiamond or rotrhombus: first X, Y (center), first two R (size in X, Y, between vertices in
X, Y axis), first ROTANG (counter­clockwise rotation).
The (X,Y) coordinate system is specified in the usual way. The length of the X, Y, R, and ROTANG
vectors is arbitrary, as long as it is long enough to accommodate the geometric shapes used in the
table. The R and ROTANG columns are optional if not required by the shapes used in the table.
The names of the coordinate axes on which the regions are defined need to be given in the keyword
MFORM1.