AIPS HELP file for IMLIN in 31DEC24
As of Fri Apr 26 20:13:44 2024
IMLIN: Fits and removes continuum emission from cube
INPUTS
INNAME Input image name (name)
INCLASS Input image name (class)
INSEQ 0.0 9999.0 Input image name (seq. #)
INDISK 0.0 9.0 Input image disk unit #
INTEXT
File with channel weights
OUTNAME Output image name (name)
OUTCLASS Output image name (class)
OUTSEQ -1.0 9999.0 Output image name (seq. #)
OUTDISK 0.0 9.0 Output image disk unit #.
BLC Bottom left corner of input
TRC Top right corner of input
YINC 0.0 128.0 Do every YINCth row
ZINC 0.0 128.0 Do every ZINCth plane
DOOUTPUT -1.0 1.0 Write images of parms
ORDER 0.0 4.0 Max order of polynomial: 1.0
is recommended (see EXPLAIN)
NBOXES 0.0 40.0 Number of spectral regions to
be used in fitting
BOX 0.0 4096.0 Pairs of begin and end
channels for spectral regions
BADDISK Disk(s) to avoid for scratch
HELP SECTION
IMLIN
Task: Fits and removes continuum emission from spectral cubes.
This is a very good way of removing continuum emission
from spectral-line data. The input cube must be
transposed to 312 order. The UV-plane analog of IMLIN
is UVLIN. The principal advantage of IMLIN over UVLIN
is that the window can be varied spatially. This can
only be done, however, by using windows and then
reconstructing the data cube later with MCUBE. For more
detailed control of the baseline regions, the interactive
task XBASL should be tried.
Adverbs:
INNAME.....Input image name (name). Standard defaults.
INCLASS....Input image name (class). Standard defaults.
INSEQ......Input image name (seq. #). 0 => highest.
INDISK.....Disk drive # of input image. 0 => any.
INTEXT.....File with channel weights. Specify path in usual way
e.g. LOGICAL:FILE. The format of the file is one row
per channel containing channel number and a weight
which has to be 1 if the channel is to be used for
fitting. Use at least one blank at the
beginning of the line and one in between the channel
number and the weight (for an example, see below).
If this is blank then NBOXES and BOX are used
OUTNAME....Output image name (name). Standard defaults.
OUTCLASS...Output image name (class). Standard defaults.
Used only for the corrected image.
OUTSEQ.....Output image name (seq. #). 0 => highest unique.
OUTDISK....Disk drive # of output image. 0 => highest
number with sufficient space.
BLC........Bottom right corner in input image of desired
subimage. Default is entire image.
TRC........Top right corner in input image of desired
subimage. Default is entire image.
YINC.......Do only every YINC'th row (beginning at BLC(2)).
ZINC.......Do only every ZINC'th plane (beginning at BLC(3)).
DOOUTPUT...True (> 0) requests that the constant, slope and
uncertainty images be catalogued (see OUTCLASS).
ORDER......The maximum allowed order for the polynomial. 0
is a constant, 1 is linear, 4 is a bit much and is
the maximum allowed. 1 should be fine in most cases
NBOXES.....The number of regions in the spectrum to be
used in fitting the baseline.
BOX........NBOXES pairs of channel numbers specifying the
beginning and end channel numbers of each of the
regions to be used in the fitting (see EXPLAIN
file for an example)
BADDISK....Disk drives to avoid for scratch files.
EXPLAIN SECTION
IMLIN: Task which removes continuum emission from images
DOCUMENTOR: T.J. Cornwell (NRAO/VLA)
PURPOSE
IMLIN is designed to remove the continuum emission from spectral
line dirty cubes. The continuum emission is estimated from a
linear (or non-linear if you wish) fit to selected channels and
then subtracted. It is complementary to UVLIN and provides
roughly the same capability but without the flagging allowed by
UVLIN.
ORDER OF FIT: We recommend ORDER=1 since this should be adequate and
does not risk erroneously removing part of the line emission. This case
has been extensively analysed (see references) and its error performance
is well-understood.
DIAGNOSTICS: DOOUTPUT=1 will write out 2D images containing the
parameters of the fit (CONT and SLOPE) and the errors (DCONT and
DSLOPE). This is very useful for diagnostic purposes.
WINDOW: The window within which the fit is performed is
specified by NBOXES and BOX. Note that unlike UVLIN, IMLIN can
be used in cases where the line shifts in frequency with
spatial position. Use BLC and TRC to select parts of the input
image.
WEIGHTs can be specified by either using NBOXES and BOX or
by an input file. A non-zero weight is taken to be unity.
Example of weights file:
1 0
2 1
3 1
4 1
5 1
6 1
7 0
8 0
9 1
10 1
11 1
12 1
14 1
15 0
where channel 13 is missing, so its weight will be zero. To
accomplish the same thing with NBOXES:
NBOXES=3
BOX=2, 6, 9, 12, 14, 14
If INTEXT is blank and NBOXES=0 then unity weights are used.
REFERENCES
This program is best described in the preprint by Cornwell,
Uson and Haddad (1991) entitled "Radio Interferometric imaging
of spectral lines: the problem of continuum subtraction",
submitted September 1991 to Astron. & Astrophys. Here is an
excerpt which summarizes the performance of IMLIN and UVLIN.
- If the continuum emission is spread over a
sufficiently small field of view, either the IMLIN or the UVLIN
image will represent the line emission well. Both may be
deconvolved to produce high dynamic range in the line. The
noise level of IMLIN images varies within position, whereas
that of the UVLIN image is approximately constant. In the
presence of a point source of continuum strength S, the errors
for a small field of view are
IMLIN:
sigma^2_B (theta)^2
-------
(theta_F)^2
UVLIN:
sigma^2_B (theta_0)^2
-------
(theta_F)^2
where theta is the distance from the point source, theta_0 is
the distance of the point source from the phase center and
theta_F = freq/bandwidth synthesized beams. sigma_B is the
sidelobe level of the synthesized beam.
- For these methods to work well, the continuum
emission must therefore lie within a field of view theta_F,
centered on the strongest source for IMLIN, and centered on the
phase tracking center for UVLIN. This field of view may be
extended by using UVSUB to pre-subtract the brightest sources.
- For larger fields of view, both IMLIN and UVLIN will
fail completely.
- When imaging small fields in the presence of
instrumental errors or time-variable sources, both IMLIN and
UVLIN are quite robust.
- Both IMLIN and UVLIN have a considerable speed
advantage over UVSUB, especially for low frequency observations
because of the extent of sidelobe confusion.
There seems little in this list of attributes of IMLIN and
UVLIN to favor conclusively one method over the other. The
clearest advantage occurs in the case of weak line emission in
the same field as a very much brighter point continuum source,
where UVLIN with a phase shift of the point source to the phase
center will be almost error free. In the case of continuum
power spread fairly uniformly over the field, neither will win
out decisively. The difference in failure modes makes the
methods complementary and so both should probably be used, each
as a check on the other. In any event, the error levels can be
estimated using our formulae.