Output Images

A VOTable containing a list of image URLs will be returned to VOSpace. The table also gives basic information like position, size and pixel size. You can view the table in TopCat and copy the URLs to download the FITS files, or you can load the VOTable into Aladin and view the images directly, or you can send to URLs to any service which understands them, e.g. the AstroGrid-wrapped SExtractor (with suitable configuration files). Getting and Viewing Data describes this more fully. The screenshot below shows the use of Aladin. Use the PLASTIC menu to register the service and then LOAD the VOTable produced by the MERLIN Imager from your MySpace. In this example the top two images were produce by the Imager at 4993 and 1658 GHz respectively; the bottom images are a false colour composite and a higher resolution (VLBI) image, all with the 4993-GHz contours overlaid.

Aladin.png

Usage Notes:

Troubleshooting

If you choose a very large image size (over about 20 (80) arcsec at around 5 (1.4) GHz) it may take a long time. Smaller images (e.g. 10 arcsec) should only take minutes to tens of minutes before the output table appears in MySpace The most likely reason if no images are returned after several hours is that there are no suitable data in the region selected; check MERLIN Archive Search Form. See the Development section below for known issues.

Occasionally images may fail or be poor quality due to bad observing conditions, interference or other problems (such as the presence of a bright source on the edge of the selected image area). Please report any problems to merlin_archive@jb.man.ac.uk.

How it works

The imaging service applies the calibration, Fourier transforms and CLEANs the visibility data. If the requested position is far from the original pointing centre, that is also CLEANed out to avoid confusion.

Resolution (restoring beam size) is adjusted by data weighting to give a resolution as close to the user preference as possible. It will be elongated for observations at low Dec. At minimum resolution, noise may be increased and sensitivity to extended structures is reduced. The natural beamsize usually gives a good balance of low noise and sensitivity to (few) hundred-mas-scale structures. The upper limit is the maximum spatial scale of imageable data (determined by the shortest baseline).

There may be artefacts/sidelobes, especially at low Dec. or for observations with reduced coverage (short duration and/or antennas missing). This will be more pronounced at the highest resolutions allowed for the frequency, and extended emission may be broken up. If your image is many arcmin from the original centre it may be distorted.

This service uses the AstroGrid CEA and the ParselTongue scripting language developed by RadioNet to control the AIPS data reduction package. The output VOTable is consistent with the Simple Image Access Protocol, so that it is recognised by Aladin and other VO services.

Development

These are some of the issues which we hope to address (either via AstroGrid or via MERLIN):

  1. Link to SIMBAD to allow the use of standard source names as well as positions and MERLIN names.
  2. Return a list of all fields observed by MERLIN covering the requested position, not just images of the top 5.
  3. Improve the description of the coverage of the MERLIN archive so that it is easier to see if your region of interest has been observed and at what frequencies.
  4. Return useful error messages.
  5. Increase the amount of imaging data available
  6. Add services to return visibility data and other data products such as spectra, time series and data cubes, and polarization images.

Author: Anita Richards, amsr@jb.man.ac.uk
Last revised : 28 Apr 2006

you are here: [[ParentLinkage]] = MERLIN Imager = The [http:www.merlin.ac.uk MERLIN] radio interferometry archive contains visibility data. If an observation has been suitably calibrated, an image can be extracted on demand from anywhere in the field of view. See [http://www.merlin.ac.uk/user_guide/OnlineMUG The MERLIN User Guide] for more information about MERLIN data reduction. Please credit AstroGrid and use the [http:www.merlin.ac.uk/archive/acknowledge.html acknowledgement] in any publications. At present (January 2008) the MERLIN archive contains processed data taken between 1991-2005 in the 1.3-1.7 and 4-7 GHz bands (22-17 and 6-5 cm wavelength, or L- and C-band; more data are to be added for post-2000). Up to 5 images per position will be returned (prioritised by the likely quality) in the form of a list of URLs pointing to FITS files, which you can manipulate or download. == How to run the MERLIN Imager == === Minimum inputs === Use the [wiki:VODesktop2008beta-doc/VOExplorer VOExplorer] to select the MERLIN Imager and click on Execute Task. This gives you the Task Runner interface. [[Image(M273MERLINImager.png, align=central, width=100%)]] You must enter either a position or the exact MERLIN name. Other inputs e.g. to restrict by frequency, are optional; the software will seek the closest match if your exact requirements can't be met. The Information field in the Task Runner gives the format required for each entry (e.g. the units of the Resolution are Arcsec). Use the tab under output to enter a VOSpace destination and name for your output file. Click on Execute (top right) to send the query. It may take minutes or hours depending on the image sizes. Once complete, start Aladin and send the output file as a table. You can then select the images to visualise. Alternatively, view the table in !TopCat and use the URLs given to download image FITS files to your desktop. [[Image(M273Aladin.png, align=central, width=100%)]] === Available ranges === * Click on "Browse MERLIN Archive List" in the [http://www.merlin.ac.uk/archive/archive_form.html MERLIN Archive Search Form] to see a list of all objects/positions covered in the archive, or search for individual objects. If a source is of type `Target` or `Phase ref.` then it should be possible to extract an image. The field of view of an individual pointing is usually 1-10 arcmin radius, around an object of interest, depending on the frequency and observing mode. * Image size (arcsec) of square field to be imaged. If this is outside the range corresponding to 512-8192 pixels it will be adjusted to lie in this range. The default is 512 pixels square. The pixel size will be 0.012 arcsec at C-band and 0.04 arcsec at L-band, so that the available ranges are approx.: * 22.5-368 arcsec at 1370-1550 MHz * 20-327 arcsec at 1550-1730 MHz * 6-98 arcsec at 4500-5200 MHz * 5-81 arcsec at 6000-7000 MHz * Values outside these ranges will be rounded to the nearest possible value. At low Declination the highest resolution is only attainable E-W and the beam will be elongated N-S by a factor of up to ~2 * Resolution: Leave blank for the natural synthesised beam or use the default 0.1 arcsec for matching resolution at any frequency (may produce artefacts from faint extended emission). Allowed ranges are ||Band (MHz)||(1370-1430)||(1550-1730)||(4500-5200)||(6000-7000)|| ||Min. resolution||0.13||0.11||0.035||0.030|| ||Natural beam||0.20||0.17||0.060||0.050|| ||Max. resolution||2.40||2.00||0.720||0.600|| * Frequencies available (actual image bandwidth normally 13 MHz): * 4500-5200 MHz or 6000-7000 MHz (C-band, approx. 5-6 cm wavelength) * 1370-1430 MHz or 1550-1730 MHz (L-band, approx 18-21 cm wavelength) * The archive is complete for continuum data in these ranges in normal observing modes between 19920101 and 19991212 and for 2005; it is fairly complete for 1991 and 2000-2004. * The data are averaged for up to 1 month prior to calibration and imaging. Shorter periods, if available, can be extracted via the [http://www.merlin.ac.uk/archive/uvdata_form.html visibility data service].