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MERLIN Imager

The 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 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 VOExplorer to select the MERLIN Imager and click on Execute Task. This gives you the Task Runner interface.

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. See Helper Applications for more information on tools which link directely to AstroGrid.

In this example the top two images were produce by the MERLINImager 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.

Available ranges

• Click on "Browse MERLIN Archive List" in the 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 visibility data service.

Output

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 (e.g. radio experts can manipulate these in AIPS), 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).