Next: Overview of the NDF_ Routines
Up: OVERVIEW
Previous: OVERVIEW
The simplest way of regarding an NDF is to view it as a collection of those
items which might typically be required in an astronomical image or
spectrum.
The main part is an N-dimensional array of data (where N is 1 for a
spectrum, 2 for an image, etc.), but this may also be accompanied by a
number of other items which are conveniently categorised as follows:
Character components: |
TITLE |
-- |
NDF title |
|
LABEL |
-- |
Data label |
|
UNITS |
-- |
Data units |
Array components: |
DATA |
-- |
Data pixel values |
|
VARIANCE |
-- |
Pixel variance estimates |
|
QUALITY |
-- |
Pixel quality values |
Miscellaneous components: |
AXIS |
-- |
Coordinate axes |
|
WCS |
-- |
World coordinate systems |
|
HISTORY |
-- |
Processing history |
Extensions: |
EXTENSION |
-- |
Provides extensibility |
The names of these components are significant, since they are used by the
NDF access routines to identify the component(s) to which certain operations
should be applied.
The following describes the purpose and interpretation of each component in
slightly more detail.
Character components:
- TITLE
- - This is a character string, whose value is
intended for general use as a heading for such things as graphical output;
e.g. `M51 in good seeing'.
- LABEL
- - This is a character string, whose value is
intended to be used on the axis of graphs to describe the quantity in the
NDF's data component; e.g. `Surface brightness'.
- UNITS
- - This is a character string, whose value
describes the physical units of the quantity stored in the NDF's data
component; e.g. `J/(m**2*Ang*s)'.
Array components:
- DATA
- - This is an N-dimensional array of pixel values
representing the spectrum, image, etc. stored in the NDF.
This is the only NDF component which must always be present.
All the others are optional.
- VARIANCE
- - This is an array of the same shape and size
as the data array, and represents the measurement errors or
uncertainties associated with the individual data values.
If present, these are always stored as variance estimates for each
pixel.
- QUALITY
- - This is an array of the same shape and size as
the data array, and holds a set of unsigned byte values.
These are used to assign additional ``quality'' attributes to each pixel
(for instance, whether it is part of the active area of a detector).
Quality values may be used to influence the way in which the NDF's
data and variance components are processed, both by
general-purpose
software and by specialised applications.
Miscellaneous components:
- AXIS
- - This component name represents a group of axis
components which may be used to describe the shape and position of the NDF's
pixels in a rectangular coordinate system.
The physical units and a label for each axis of this coordinate system may
also be stored.
(Note that the ability to associate extensions with an NDF's
axis coordinate system, although described in
SGP/38, is not yet
available via the NDF access routines described here.)
- WCS
- - This component may be used to hold information
about any ``world coordinate systems'' associated with the NDF. These
may include celestial coordinate systems, such as right ascension and
declination (in various flavours), but may also represent other
coordinates, including wavelength.
Multiple
coordinate systems may be present.
The WCS component is a rather more complex entity than most other NDF
components and a full description is currently beyond the scope of
this document. It stores world coordinate information in a format
defined by the AST library (see SUN/210) and known as
a ``FrameSet''. You should consult SUN/210
for a full description of the facilities which a FrameSet
provides. The NDF_ library simply provides routines for reading and
writing this information (see NDF_GTWCS and
NDF_PTWCS).
- HISTORY
- - This component may be used to keep a record of
the processing history which the NDF undergoes.
If present, this component should be updated by any applications which
modify the data structure.
Extensions:
- EXTENSIONs
- are user-defined HDS
structures associated with the NDF, and are used
to give the data format flexibility by allowing it to be extended.
Their formulation is not covered by the NDF definition, but a few simple
routines are provided for accessing and manipulating named extensions, and
for reading and writing the values of components stored within them.
Next: Overview of the NDF_ Routines
Up: OVERVIEW
Previous: OVERVIEW
Starlink User Note 33
R.F. Warren-Smith
11th January 2000
E-mail:rfws@star.rl.ac.uk
Copyright © 2000 Council for the Central Laboratory of the Research Councils