Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/people/Matthew.Whiting/ASKAPsimTrans2/README Дата изменения: Tue May 3 05:32:19 2011 Дата индексирования: Mon Feb 4 04:20:24 2013 Кодировка: Поисковые слова: http www.astronomy.ru forum index.php topic 4644.0.html

This ASKAP data release contains simulated observations of a radio continuum sky affected by refractive scintillation. A number of additional sources exhibiting Extreme Scattering Event (ESE) behaviour are also present.

The simulation starts with a continuum model from the SKADS S3-SEX catalogue. This is the same model as used for the previously-released continuum & transient simulations. A recipe provided by the VAST team permits us to assign to each unresolved core component in the model a lightcurve of variability due to refractive scintillation. This is possible by assuming a physical size of the component, calculated by using the 1400MHz flux and assuming a 10^12K brightness temperature. (Note that while the SKADS catalogue provides sizes for the lobe components, these core components actually have zero size.).

This allows us to create a model image of the sky at each desired timestep. Only core components (structure=1 in the SKADS catalogue ) with fluxes >0.1mJy had lightcurves computed. At each timestep, the relevant flux value from the lightcurve is read and used to add that point source to the static model (being the faint point sources and the extended sources).

Additionally, the VAST team provided lightcurves for ESEs that were improved on those used in the previous simulation. These were initially provided with 10-second sampling over a 24hr period, but were re-binned to match the desired 1-minute, and a representative 8-hour section was chosen (for ese_0.5 and ese_2.0, this was just the first 8 hours, but ese_4.0 exhibits an inflection point, so the 8-hours were chosen to overlap that). These were assigned to sources that were added at random locations within the field. There were three types of lightcurves provided, showing the relative variation over a 12hour period, and these were scaled in different ways for each of the five sources. The ESE source locations, types and scaling is summarised here:
Num RA Dec Flux ESE-lightcurve
1 186.617489604 -43.0905662668 0.1 ese_0.5_4.0_31_32.txt
2 189.323048287 -45.0605156305 0.01 ese_2.0_1.0_31_32.txt
3 189.728281533 -43.4720191768 0.01 ese_4.0_4.0_31_32.txt
4 187.052182023 -43.4683517066 0.05 ese_0.5_4.0_31_32.txt
5 185.088672318 -46.4421102764 0.001 ese_2.0_1.0_31_32.txt


These models were then fed into the telescope simulator. This featured:
* An exposure time per model of 1 minute, made up of 5sec integrations. The total exposure time was 8 hours, requiring 480 timesteps in the lightcurves.
* Each exposure ("snapshot") resulted in its own measurement set.
* The 2km core of the ASKAP array was used, with the phased-array feed simulated by 32 independent beams, spaced at 1 degree separation in a square pattern minus the corners.
* The simulation used 32 channels across the 256MHz bandwidth, although no spectral information was provided with the input catalogue (all sources assumed to have a flat spectrum).
* Thermal noise, assuming Tsys=50K and efficiency of 0.8 was added to the visibilities.

The resulting visibilities were then imaged. We provide a number of different image products:
* A dirty image for each 1min snapshot
* A cleaned image for the full 8hr exposure. This was done with multi-scale clean, using scales of [0,3,10] pix.

We have also simulated the effect of using a sky model, in order to control the response of bright, variable sources. We take the SKADS image, set everything below 1mJy to zero, then subtract the resulting sky model from each of the snapshot models. The simulating and imaging then proceeds as before, with a convolved version of the sky model added back in at the end. This simulates subtracting a previously-obtained sky model from the data at the start of the observation, without updating it during the observation (albeit somewhat idealised, as the subtraction is done perfectly in the sky image plane).

Data provided include:
* The SKADS input catalogue.
* A file containing the lightcurves for all varying sources. Each line corresponds to one source, and has the RA, DEC, component ID and log10(i_1400) from SKADS, followed by 480 flux points.
* A description, provided by J.-P. Maquart and R. Wayth of VAST, of the theory behind the refractive scintillation, demonstrating how to calculate the lightcurve of a source
* A sky catalogue, suitable for use with casaviewer, showing the locations of the five ESEs
* The static model, being extended sources and faint (>0.1mJy) point sources
* A set of snapshot model images, with the varying sources added to the static model. Each is a gzipped FITS file, 5120x5120pix @ 5arcsec/pix, 94 MB.
* The "sky model" image - the full SKADS image truncated at 1mJy.
* The visibilities from each snapshot, in a tar-gzipped measurement set (each 155MB)
* A dirty image made from each timestep (each a gzipped FITS file, 2560x2560pix, 10arcsec/pix, 24MB)
* A cleaned image made from the full 8hr exposure, along with preconditioned psf and weights.
* A dirty image made from the sky-model-subtracted observation at each timestep
* A cleaned image made from the sky-model-subtracted visibilites, with the convolved sky-model added back in.
* A shell script to allow bulk downloading of the snapshot data (models, visibilities & dirty images)

Processing was done on the NCI National Facility machine vayu - see http://nf.nci.org.au

The data are located in the directory
giant:/DATA/DELPHINUS_2/whi550/ASKAPsimulations/Transients/Oct2010
which can be accessed from most machines on the ATNF network. The directory and its sub-directories contains the following files (the large subdirectories are indicative listings only):

> ls -lh
total 1.4G
drwxrwxr-x 2 whi550 at-astro 4.0K 2010-10-21 22:17 Lightcurves/
drwxrwxr-x 2 whi550 at-astro 20K 2010-10-25 11:15 ModelSnapshots/
-rw-r--r-- 1 whi550 at-astro 8.3K 2010-10-25 11:26 README
-rw-r--r-- 1 whi550 at-astro 127K 2010-10-21 22:27 RefractivePowerSpectrum.pdf
-rw-rw-r-- 1 whi550 at-astro 1003M 2010-10-21 22:08 SKADS_S3SEX_1uJy_input_catalogue.dat
drwxrwxr-x 2 whi550 at-astro 20K 2010-10-21 23:19 SnapshotImages/
drwxrwxr-x 2 whi550 at-astro 28K 2010-10-22 00:09 SnapshotImages-modelRemoved/
drwxrwxr-x 2 whi550 at-astro 20K 2010-10-21 23:39 Visibilities/
-rw-rw-r-- 1 whi550 at-astro 1.4K 2010-10-25 11:30 getFiles.sh
-rw-rw-r-- 1 whi550 at-astro 101M 2010-10-21 22:57 trans2.SKADS.static.model.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.image.clean.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.image.dirty.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.image.usingSkyModel.clean.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.image.usingSkyModel.dirty.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.psf.fits
-rw-r--r-- 1 whi550 at-astro 101M 2010-10-21 22:32 trans2.skymodel.fits
-rw-rw-r-- 1 whi550 at-astro 26M 2010-10-21 22:20 trans2.weights.fits

> ls -lh Lightcurves
total 170M
-rw-r--r-- 1 whi550 at-astro 9.7K 2010-10-21 22:04 ESE-0.5.txt
-rw-r--r-- 1 whi550 at-astro 9.7K 2010-10-21 22:04 ESE-2.0.txt
-rw-r--r-- 1 whi550 at-astro 9.7K 2010-10-21 22:04 ESE-4.0.txt
-rw-r--r-- 1 whi550 at-astro 261K 2010-10-21 22:05 ese_0.5_4.0_31_32.txt
-rw-r--r-- 1 whi550 at-astro 261K 2010-10-21 22:05 ese_2.0_1.0_31_32.txt
-rw-r--r-- 1 whi550 at-astro 261K 2010-10-21 22:05 ese_4.0_4.0_31_32.txt
-rw-r--r-- 1 whi550 at-astro 169M 2010-10-21 22:02 trans2_SKADS_lightcurves_0.1mJy.dat

> ls -lh ModelSnapshots | head -3
total 45G
-rw-rw-r-- 1 whi550 at-astro 94M 2010-10-21 10:36 trans2.snapshot0.model.fits.gz
-rw-rw-r-- 1 whi550 at-astro 94M 2010-10-21 10:36 trans2.snapshot1.model.fits.gz

> ls -lh SnapshotImages | head -3
total 11G
-rw-rw-r-- 1 whi550 at-astro 24M 2010-10-21 22:26 trans2.image.snapshot0.fits.gz
-rw-rw-r-- 1 whi550 at-astro 24M 2010-10-21 22:26 trans2.image.snapshot1.fits.gz

> ls -lh SnapshotImages-modelRemoved | head -3
total 11G
-rw-rw-r-- 1 whi550 at-astro 24M 2010-10-21 23:24 trans2.image.snapshot0.noskymodel.fits.gz
-rw-rw-r-- 1 whi550 at-astro 24M 2010-10-21 23:25 trans2.image.snapshot1.noskymodel.fits.gz

> ls -lh Visibilities | head -3
total 73G
-rw-rw-r-- 1 whi550 at-astro 155M 2010-10-21 22:36 trans2.snapshot0.ms.tgz
-rw-rw-r-- 1 whi550 at-astro 155M 2010-10-21 22:36 trans2.snapshot1.ms.tgz