Creates a channel map from a cube NDF by compressing slices along a nominated axis
For each output pixel, all corresponding input pixel values between the channel bounds of the nominated axis to be collapsed are combined together using one of a selection of estimators, including a mean, mode, or median, to produce the output pixel value.
Its integer index within the current Frame of the input NDF (in the range 1 to the number of axes in the current Frame).
Its Symbol string such as "RA"
or "VRAD"
.
A generic option where "SPEC"
requests the spectral axis, "TIME"
selects the time axis, "SKYLON"
and
"SKYLAT"
picks the sky longitude and latitude axes respectively. Only those axis domains present are
available as options.
A list of acceptable values is displayed if an illegal value is supplied. If the axes of the current Frame are not parallel to the NDF pixel axes, then the pixel axis which is most nearly parallel to the specified current Frame axis will be used.
"Mode"
, "Cmean"
and "Csigma"
statistics (see Parameter
ESTIMATOR). The application first computes statistics using all the available pixels. It then rejects all
those pixels whose values lie beyond CLIP standard deviations from the mean and will then
re-evaluate the statistics. For "Cmean"
and "Csigma"
there is currently only one iteration, but up to
seven for "Mode"
.
The value must be positive. [3.0]
"Mean"
— Mean value
"WMean"
— Weighted mean in which each data value is weighted by the reciprocal of the associated
variance. (2)
"Mode"
— Modal value (4)
"Median"
— Median value. Note that this is extremely memory and CPU intensive for large datasets;
use with care! If strange things happen, use "Mean"
. (3)
"Absdev"
— Mean absolute deviation from the unweighted mean. (2)
"Cmean"
— Sigma-clipped mean. (4)
"Csigma"
— Sigma-clipped standard deviation. (4)
"Comax"
— Co-ordinate of the maximum value.
"Comin"
— Co-ordinate of the minimum value.
"FBad"
— Fraction of bad pixel values.
"FGood"
— Fraction of good pixel values.
"Integ"
— Integrated value, being the sum of the products of the value and pixel width in world
co-ordinates. Note that for sky co-ordinates the width is measured in radians.
"Iwc"
— Intensity-weighted co-ordinate, being the sum of each value times its co-ordinate, all divided
by the integrated value (see the "Integ"
option).
"Iwd"
— Intensity-weighted dispersion of the co-ordinate, normalised like "Iwc"
by the integrated
value. (4)
"Max"
— Maximum value.
"Min"
— Minimum value.
"NBad"
— Count of bad pixel values.
"NGood"
— Count of good pixel values.
"Rms"
— Root-mean-square value. (4)
"Sigma"
— Standard deviation about the unweighted mean. (4)
"Sum"
— The total value.
"Sigma"
and "Iwd"
are meaningless for single-pixel channels.
The minimum number of pixels per channel for each estimator is given in parentheses in the list
above. Where there is no number, there is no restriction. If you supply an unavailable option, you will
be informed, and presented with the available options. ["Mean"]
Note, HIGH and LOW should not be equal. If a null value (!
) is supplied for either HIGH or LOW, the
entire range of the axis fragmented into channels. [!]
Note, HIGH and LOW should not be equal. If a null value (!
) is supplied for either HIGH or
LOW, the entire range of the axis fragmented into channels. [!]
!
) asks the application to select a shape. It will
generate one that gives the most square output NDF possible. The value must be positive
and no more than the value of Parameter NCHAN. !
) propagates the title from the input NDF
to the output NDF. [!]
Each axis can be specified using one of the following options.
Its integer index within the current Frame of the input NDF (in the range 1 to the number of axes in the current Frame).
Its Symbol string such as "RA"
or "VRAD"
.
A generic option where "SPEC"
requests the spectral axis, "TIME"
selects the time axis, "SKYLON"
and
"SKYLAT"
picks the sky longitude and latitude axes respectively. Only those axis domains present are
available as options.
A list of acceptable values is displayed if an illegal value is supplied. If a null (!
) value is supplied, the
axes with the same indices as the three used pixel axes within the NDF are used. [!]
WLIM specifies the minimum fraction of good pixels which must be present in order to generate a good output pixel. If this specified minimum fraction of good input pixels is not present, then a bad output pixel will result, otherwise a good output value will be calculated. The value of this parameter should lie between 0.0 and 1.0 (the actual number used will be rounded up if necessary to correspond to at least one pixel). [0.3]
"RA"
, "DEC"
and "Lambda"
, with the lambda axis being parallel to the third
pixel axis. The above command extracts four slabs of the input cube between wavelengths
4500 and 4550 Ångstroms, and collapses each slab, into a single two-dimensional array
with RA and DEC axes forming a channel image. Each channel image is pasted into a
2$\times $2 grid
within the output NDF called chan4. Each pixel in the output NDF is the mean of the corresponding
input pixels with wavelengths in 12.5-Ångstrom bins. "RA"
, and "DEC"
. It extracts seven velocity slabs of the input cube between
$-30$ and
$+$40 km/s,
and collapses each slab, into a single two-dimensional array with RA and DEC axes
forming a channel image. Each channel image is pasted into a default grid (likely
4$\times $2)
within the output NDF called vel7. Each pixel in the output NDF is the maximum of the
corresponding input pixels with velocities in 10-km/s bins. The collapse is always performed along one of the pixel axes, even if the current Frame in the input NDF is not the PIXEL Frame. Special care should be taken if the current-Frame axes are not parallel to the pixel axes. The algorithm used to choose the pixel axis and the range of values to collapse along this pixel axis proceeds as follows.
The current-Frame co-ordinates of the central pixel in the input NDF are determined (or some other point if the co-ordinates of the central pixel are undefined). Two current-Frame positions are then generated by substituting in turn into this central position each of the HIGH and LOW values for the current-Frame axis specified by Parameter AXIS. These two current-Frame positions are transformed into pixel co-ordinates, and the projections of the vector joining these two pixel positions on to the pixel axes are found. The pixel axis with the largest projection is selected as the collapse axis, and the two end points of the projection define the range of axis values to collapse.
The WCS of the output NDF retains the three-dimensional co-ordinate system of the input cube for every tile, except that each tile has a single representative mean co-ordinate for the collapsed axis.
The slices may have slightly different pixel depths depending where the boundaries of the channels lie
in pixel co-ordinates. Excise care interpreting estimators like "Sum"
or ensure equal numbers of pixels
in each channel.
This routine correctly processes the DATA, VARIANCE, LABEL, TITLE, UNITS, WCS, and HISTORY components of the input NDF; and propagates all extensions. AXIS and QUALITY are not propagated.
Processing of bad pixels and automatic quality masking are supported.
All non-complex numeric data types can be handled.
The origin of the output NDF is at (1,1).
Huge NDFs are supported.