; IMAGRPRM ;--------------------------------------------------------------- ;! Specifes enhancement parameters for OOOP-based imaging ;# ADVERB IMAGING ;----------------------------------------------------------------------- ;; Copyright (C) 2002-2003, 2008 ;; Associated Universities, Inc. Washington DC, USA. ;; ;; This program is free software; you can redistribute it and/or ;; modify it under the terms of the GNU General Public License as ;; published by the Free Software Foundation; either version 2 of ;; the License, or (at your option) any later version. ;; ;; This program is distributed in the hope that it will be useful, ;; but WITHOUT ANY WARRANTY; without even the implied warranty of ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ;; GNU General Public License for more details. ;; ;; You should have received a copy of the GNU General Public ;; License along with this program; if not, write to the Free ;; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge, ;; MA 02139, USA. ;; ;; Correspondence concerning AIPS should be addressed as follows: ;; Internet email: aipsmail@nrao.edu. ;; Postal address: AIPS Project Office ;; National Radio Astronomy Observatory ;; 520 Edgemont Road ;; Charlottesville, VA 22903-2475 USA ;----------------------------------------------------------------------- IMAGRPRM LLLLLLLLLLLLUUUUUUUUUUUU CCCCCCCCCCCCCCCCCCCCCCCCCCCCC ---------------------------------------------------------------- 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)). (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 ratio of the maximum residual and the level to load into the AP is < (1 + GAIN*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, and on 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) The radius in cells for the area in which fluxes are computed. < 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. 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). (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) (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. Tasks: IMAGR.....Wide-field and/or wide-frequency Cleaning / imaging task. ----------------------------------------------------------------