MERLIN Imager Help

• Summary
  • Available Data
• How to run the MERLIN Imager
  • Using the Workbench
  • Inputs
• Output Images
• Usage Notes
  • Troubleshooting
  • How it works
• Development

1. Summary

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  MERLIN acknowledgement in any publications.

1.1  Available Datasets

At present (Spring 2006) the MERLIN archive contains processed data taken between 1992-2000, 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 being added. The field of view of an individual pointing is usually 1-10 arcmin radius, around an object of interest. Click on "Browse MERLIN Archive List" in 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

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.

2. How to run the MERLIN Imager

2.1 Using the Workbench

• Launch the AstroGrid Workbench
• From the Workbench, select Data Discovery > Task Launcher
• Enter MERLIN as a search term (see ) and select the MERLIN Imager
• This will give you a dialogue box (see screenshot).

2.2 MERLIN Imager inputs

You must enter either the Position or the Source name, and the Output name for the list of images which will be saved to your MySpace area. All other inputs are optional .

1. Position: RA and Dec, in their respective boxes
   • Allowed formats:
     • sexagesimal 23:55:03.3820 +28:38:01.050
     • decimal degrees 358.7640917 28.633625

2. Equinox: J for J2000 (default) or B for B1950

3. Source name can alternatively be entered; this must be the exact MERLIN observing name (not necessarily SIMBAD).

4. Frequency and tolerance (MHz) in their respective boxes, e.g. 5000 100.
   • Available ranges:
     • 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)

5. 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

6. Resolution (arcsec)
   • The allowed ranges, in arcsec, at most Declinations, 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

   • 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.

7. Start and end times, in their respective boxes, in the format YYYYMMDD (earliest data is approx. 19920101).
   • The data are averaged for up to 1 month prior to calibration and imaging. Shorter periods, if available, can be obtained on request as visibility data.

8. Output VOTable name e.g. (VO path +) MKN273.vot.  The best way to do this is to click on the Ref? button, select the destination folder in the MySpace browser which appears, click on the 'new file' icon and enter the output file name in the dialogue box that appears.
   

3. Output Images

A VOTable containing a list of image URLs will be returned to MySpace. 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 (see the Data Access worksheet), or you can send to URLs to any service which understands them, e.g. the AstroGrid-wrapped SExtractor (with suitable configuration files). The screenshot below shows the use of Aladin.  If there is a PLASTIC menu, use this 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.

Right-click on any image to open it full-size in a new window

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 (now available inside VOTable).
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