AIPS HELP file for IMAGRPRM in 31DEC24
As of Thu Apr 25 3:43:15 2024
HELP SECTION
IMAGRPRM
Type: Adverb (array(20))
Use: To specify correction and other "enhancement" parameters to
IMAGR . See EXPLAIN IMAGR for further discussion.
IMAGRPRM(1) If > 0 then make frequency dependent primary beam
corrections assuming an antenna diameter of IMAGRPRM(1) meters.
Can change with TELL which only makes sense if you are going to
repeat the subtraction with a filtered set of components (see
IMAGRPRM(8)). Note that VLA and ATCA arrays (TELESCOPE header
parameter) use the default primary beam parameters defined
elsewhere in AIPS, while other antennas actually use IMAGRPRM(1)
as the diameter of a "standard" telescope. Usage affected by
FQTOL adverb as well.
(2) Visibility amplitudes will be corrected to the average
frequency assuming a spectral index of IMAGRPRM(2). Note: the
typical optically thin synchrotron spectral index is about -0.7.
(3) If > 0, then the u,v and w terms are scaled by IMAGRPRM(3)
before imaging.
(4) If > 0, then SDI Clean will be used when the fractionof
residual pixels in the Clean boxes stronger than half the maximum
residual exceeds IMAGRPRM(4). <= 0 -> never use or allow SDI
Clean. Can change with TELL. If SDI Clean is enabled, the output
CC files will have been merged.
(5) If > 0 then scaling of residuals is requested and
(6) Half-width in x of box to determine the dirty beam area
(default = 5)
(7) Half-width in y of box to determine the dirty beam area
(default = 5) Can change (5)-(7) with TELL.
(8) If non-zero, select only those Clean components having >
ABS(IMAGRPRM(8)) Jy within a radius of IMAGRPRM(9) cells of the
component. If IMAGRPRM(8) < 0, the abs value of the flux near
the component is used. This is an optional filter to remove weak
isolated components which can cause a significant bias. Can
change with TELL but only if it was non-zero to begin with. A
copy of the input data has to be made for this option and it is
only made if IMAGRPRM(8) is non-zero. If this option is
selected, the output CC files will have been merged. Note that
IMAGRPRM(8) should always be <= 0 for images of Q, U, and V
Stokes parameters since negative brightnesses are valid.
Filtering is done on restarts, when requested from the TV, on
certain Cleaning failures, on normal completion (after which the
task resumes Cleaning until the completion points such as NITER
and FLUX are reached a second time) and on final exit. If
ALLOKAY >= 2, the filter is not applied on the restart. If the
filtering option was selected at the start (IMAGRPRM(8) non
zero), it may be turned off by setting IMAGRPRM(8) exactly 0 and
running TELL. To delete all negative regions, set IMAGRPRM(8) to
a tiny positive number.
(9) abs (IMAGRPRM(9)) is the radius in cells for the area in
which fluxes are computed. If IMAGRPRM(9) < 0, the Clean will
restart following the "final" filtering on the assumption that
the filter will have changed things enough that the residual will
be > FLUX for example. abs (IMAGRPRM(9)) < 1.1 => 3.1.
Can change with TELL.
(10) = multiplier of max image size to set beam size. Values
of 2, 1, 0.5, and 0.25 are allowed. 0 => 2. Smaller beam images
are a bit faster, but less accurate in the early Clean cycles.
The largest beam image used is 2048 on a side except when
IMAGRPRM(1) > 0.75. When IMAGRPRM(10) is 1, the limit is 4096
and when IMAGRPRM(10) > 1.5, the beam is twice the image size
limited by 32768.
Multi-scale experimental controls are based on
BeamRatio = (field beam area) / (min beam area)
See below for more discussion.
(11) Multi-scale experimental control: select which field
to Clean using peak fluxes (in Jy/beam) weighted by
1 / (BeamRatio)**IMAGRPRM(11). This is the most important
multi-scale control. Set it so that all scales have about an
equal chance of being selected.
(12) Multi-scale experimental control: decrement the value
of IMAGRPRM(11) used above by IMAGRPRM(12) each time an non-point
resolution field is Cleaned until it is 0.
(13) Multi-scale experimental control: use gain =
GAIN * [ {1/BeamRatio} ** IMAGRPRM(13) ]
(14,15) Multi-scale experimental control: use
factor = FACTOR + IMAGRPRM(14) * [ 1 - {BeamRatio} ** IMAGRPRM(15) ]
(16) Multi-scale experimental control: use maxpixel
= MAXPIXEL + IMAGRPRM(16) * (BeamRatio)
(17) 1 => use a spectral-index image represented inIN3NAME,
IN3CLASS, IN3SEQ, IN3DISK below to correct the Clean component
model for each channel. IN4NAME et al will also be used as a
curvature image iff IN3NAME are specified. IMAGRPRM(17)-0.5 is
used as a radius in pixels over which the spectral index image is
averaged. When it is small (0 < IMAGRPRM(17) <~ 1), the spectral
index is interpolated rather than averaged. See FQTOL below as
well. When doing spectral index, the primary beam correction
(IMAGRPRM(1)) costs very little extra.
(18) In OVERLAP>=2 mode, when imaging multiple fields, IMAGR
grid and FFT multiple fields in an attempt to determine the next
one to Clean. Multiple fields are done to reduce I/O in this
search which may otherwise have to re-read the work file several
times to find the next field to Clean. The limit on the number of
fields done depends on the maximum size of the AP, the size of the
images, etc - trying to guess when I/O will be expensive in time.
Sometimes, IMAGR will make more images than are needed at a
subsequent excess cost. To limit the number of fields imaged at
any one try, set IMAGRPRM(18) to the maximum number you want to
allow. The task will now reduce the maximum number when the
multiple fields all have similar maxima - i.e. after the wide
dynamic range early cleans are done.
(19) In OVERLAP>= 2 mode, when Cleaning a field with a small
bright source, it is possible to Clean too deeply. Then weak
lumps due to sidelobes of strong sources in other fields are
treated as sources in the present field. By the time the present
field is Cleaned again, these errors become very apparent. This
parameter is used to limit the weakest source Cleaned in this
major cycle to IMAGRPRM(19) times the strongest source in this
cycle. The default is the sum of the maximum sidelobe outside a
radius of 5 pixels and the maximum sidelobe outside a radius of
MINPATCH pixels.
(20) In OVERLAP >= 2 mode, the objective function of the
selected field after it is re-imaged is compared to the objective
function of the second best field (without re-imaging). If the
second best now appears better than the selected field by a
factor greater than IMAGRPRM(20), then the task will try another
field. 0 = 1.005. (Values < 1 are converted to 1/IMAGRPRM(20)
and, finally, values > 5 => 1.005.)
The MS Clean is an experimental algorithm and has been provided with a
number of "knobs" to adjust its behavior. All the knobs use the ratio
of the current beam area (BMAJ(field)*BMIN(field)) to the minimum beam
area (R). These knobs are:
1. Show some preference to select higher resolution images (lower
R) for the next image to Clean. Otherwise a strong point with
some extended emission will be over Cleaned at low resolution,
forcing higher resolutions to correct many pixels:
IMAGRPRM(11) select which field to Clean using peak fluxes
(in Jy/beam) weighted by 1 / R**IMAGRPRM(11).
2. Reduce this preference to zero as one Cleans more and more of
the R > 1 fields:
IMAGRPRM(12) decrement the initial value of IMAGRPRM(11)
used above by IMAGRPRM(12) each time an R > 1 scale
field is Cleaned until it is 0.
3. The lower resolution fields may easily over Clean creating zero
net flux from a mix of negative and positive areas. These then
have to be corrected with numerous high resolution Clean
steps. To use a lower loop gain for lower resolution:
IMAGRPRM(13) use gain = GAIN / R ** IMAGRPRM(13)
4. To avoid over Cleaning with lower resolution, one may also
Clean each major cycle less deeply with the FACTOR parameter.
To control FACTOR:
IMAGRPRM(14,15) use
factor = FACTOR + IMAGRPRM(14) * (1 - R ** IMAGRPRM(15))
Multi-scale experimental control limits:
0 <= IMAGRPRM(11) <= 1.0 not changed by TELL Try 0.5
0 <= IMAGRPRM(12) <= 0.1 changed by TELL Try 0.03
0 <= IMAGRPRM(13) <= 1.0 changed by TELL Try 0.5
0 <= IMAGRPRM(14) <= 1.0 changed by TELL Try 0.1
0 <= IMAGRPRM(15) <= 1.0 changed by TELL Try 0.5
Note that IMAGRPRM(11-15) = 0 causes each Clean to be done on the
field with the highest Jy/point-source-beam with the same GAIN and
FACTOR.
IMAGRPRM(11) is important, (12)-(16) can be left 0.0 usually.
Tasks:
IMAGR......Wide-field and/or wide-frequency Cleaning / imaging task.
Verbs:
OBITIMAG...Access to OBIT task Imager without self-cal or peeling.
IMAGRPRM(1,2,11,12,13) as above (3) min flux to invoke
auto-centering.
OBITPEEL...Access to OBIT task Imager with self-cal and peeling.
IMAGRPRM as in OBITIMAG.
OBITSCAL...Access to OBIT task Imager with self-cal, NOT peeling.
IMAGRPRM as in OBITIMAG.
Procedures: (IMAGRPRM used as in IMAGR)
LINIMAGE...Build image cube from multi-IF data set.
TD_STEP3...Time-dependent imaging procedure "step 3".
TD_STEP5...Time-dependent imaging procedure sequence: later steps
EXPLAIN SECTION