Argus FLD file

The Phase II requires you to create an FLD file from which the FLAMES fibres are allocated. This can be quite a tricky stage.

What you need to do is the following:

  1. Install FPOSS (currently only for Linux).
  2. Create an ".fld" file which is input for FPOSS.
  3. You load this fld file in FPOSS and following the steps in the main menu. You can choose to assign sky fibres (15 for FLAMES) in an ordered fashion (some circular shape) or input the coordinates manually.

Creating the fld file:
The coordinates of your fibre positions specified in the fld file must be accurate to <0.2" compared to the UCAC2 astrometric system (if you are using the IFU, it is not necessary to be so precise) The best way to measure your coordinates is from an HST (preferably an ACS or WFC3) image with WCS coordinates cross-matched to the UCAC2 system. How do you do this?

  • check if you have imwcs installed (e.g. with scisoft look for "/scisoft/bin/imwcs"):
  • if not, install the package from the website I sent you before
  • attached to this page (click on the Files button below) is a script "" that you can run on an HST image. It calls another scripts (imsex, which are also attached. Make sure to change the coordinates at the top of the script to your IFU coordinates, and "J9EUA5020.fits" to the name of your fits image file. The script will return UCAC2 sources in a field centred on your coords. The final command updates the WCS of your image by applying a correction based on your UCAC2 cross-correlated extracted sources.
  • The residuals (dx and dy) should be smaller than a pixel or so. You can check your WCS by overplotting in ds9 the UCAC2 catalogue. Using circles with 2 pix radius, the coords should be spot on.

Now you should make a magnitude selection. The VLT guide stars should be between 11 and 13 and the FACB between 13 and 15 mag in R. The UCAC2 mag is close enough to R to use that as a cut. How do I do this?
Attached is a little IDL script. It should be pretty self-explanatory. It will make a magnitude cut and produce the fld file. It still needs an input list with central coordinates (pointings.radec), and it reads in a list of all the available UCAC2 stars (ucac2.format).

Specifying the sky positions:
Depends on what you want from the sky. For example for a very red setup, you will mainly be interested in the sky emission lines. Try to put the fibres as much as possible away from any nebulae emission. You can manually select position that you like and feed them in the fld file (don't forget to remove the ones you do not like). For selecting sky coords, you can open up the image with IRAF/DS9 and use "imexamine" with the cursor "A" and save the coordinates. You then need to convert xy to ra and dec with xy2rd which goes well if you have the correct WCS (which you now have!).

What is the difference between the GEN_SKY and ARG_SKY types?
hmm, I believe GEN_SKY was for UVES and MEDUSA sky fibres, but I am not sure. ARGUS has his own set of sky fibres, because the resolution and wavelength ranges are slightly different from the MEDUSA ones. So you need only ARG_SKY.

(Created from an e-mail exchange in Jan 2008 between Mark Westmoquette and Mark Gieles, Mark G being the expert and Mark W being the hopeless beginner!!)

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