ESO Multi Mode Instrument
XMOS -- The MOS Slitlet Defining Tool

Starting the MOS mask tool
MOS mask tool menu options
Making a mask - Using the MOS mask tool
Some considerations
Starting with the MOS Mask Tool

Given below is a recipe for defining slitlets starting with an image  (EMMI.xxx.#.fits ). The phrases in green represent actual computer commands to be typed in as such apart from the usual substitutions for phrases enclosed within <angular brackets>:

The MOS mask definition tool is basically a panel which provides an interface between the user and MIDAS. Apart from the MOS mask definition tool (click on link to see the panel) 2 other windows are opened, a MIDAS display window and a MIDAS message window where the commands and their output are reflected.
MOS Mask Tool Menu Options

Listed below are the various menu options available and a brief description of the same. After that will follow a continuation of the recipe for defining the slitlets.

Options Punch Tools  Here you select the punch tool you wish to use, from those available on EMMI;

Slit length  Here you select if you wish to use a fixed slit (minimum,i.e. physical length of the punching tool) or one with variable length (variable). Slits longer than 8 arcsecs will be made by punching two shorter slits one after the other.

Distortions  Here you decide if you wish to correct for the geometrical distortion of the detector. With the new CCD this seems to be negligible, so the recommended value is no.

  These are the first actions you must perform after starting the program

Load the image: File or Fits Load and R90 EMMI*.fits Image. This can be done either from File by pressing load (file must be rotated 90 degrees clockwise and in bdf format) or from Fits option. The slits are defined in XMOS along the vertical direction. Thus an EMMI image has to be rotated by 90 deg to make it compatible with XMOS convention. This option pops up a file selection panel and the selected FITS file is rotated and converted into a MIDAS file and loaded on the XMOS display.
Defining the slitlets:Slits On Cursor  With this option the x-pixel location of the slitlet will be defined by the crosswire cursor position. One will want to use this option for very faint/diffuse objects on which the next option will not work. A click of the left button of the mouse will select the pixel under the cursor.  Any number of objects can be chosen one after the other and the function can be terminated by double clicking the middle button of the mouse.

On Object With this option the x-pixel location of the slitlet will be defined by a gaussian centroid fit to an object. The object area can be defined by the user by the help of the rectangular cursor provided when this option is chosen. The size of the rectangular cursor can be changed using the arrow keys to enclose as much of the object as possible while excluding its neighbours. After positioning the rectangle the object can be selected by clicking the left button of the mouse. Any number of objects can be chosen one after the other and the function can be terminated by double clicking the middle button of the mouse.

Delete Delete a previously defined slitlet. Select this option and left click close to an existing slit to erase it -- it will still be displayed until Colour->Clear and Masks->Show All are selected. One can continue to delete slits one after another until terminating the function by double clicking the middle button on the mouse.

Masks Quick Look  Shows the slitlets defined so far.

Show All  Shows the slitlets plus a pair of parallel lines extending all across the CCD defining the upper (yellow) and lower (green) extent of the slit. This is useful to see if there is an overlap between slits which is to be avoided.

Load Mask  Pops up a panel to select a previously defined mask (./Image_R##.msk) for display and further editing

Reset  Erases all the slitlets defined so far. Previously unsaved work will be lost

Edit Mask  Pops up an ascii editor of the mask file

Print Mask  In an ideal world dumps the display (the background image + defined slits) on to the nearest printer. However sometimes communication problems with the printer can occur: in this case a postscript file in the current directory is created (always named screen34.ps - will be overwritten each time this option is selected). This file can be printed with the command  lp screen34.ps. It is important that the image display is in the front of the screen, and not obscured by another window. (In case the printer is not setup on wlsmos -- please ftp the postscript file to another computer for printing. The printer in the users's room is called lptastro1)

Save Mask  This saves the existing slitlet configuration into 2 equivalent files:  *.msk and EMOS-MASKS/*.mask. The first file is comprehensible to the MOS mask defining tool (can be reloaded for further work using Mask->Load Mask) while the second is to be used for actual punching  (i.e. the file which is transferred to wemmi:/vltdata/tmp/EMOS-MASKS/). Every subsequent saving of a mask file for the same image will increment the number in mask name.

Tools Zoom  Use this to zoom in on a subsection of an image - useful in selecting blended objects and careful placing of slits such that they do not overlap. Select this option and then define the zoom area by clicking the left mouse button on the two opposite corners. The zoom will always maintain the aspect ratio.

Unzoom  Select this to display the whole image. Note that when this option is used all the graphics will be cleared and will have to be displayed again.

Scroll Use this to change the centre of display. Select this option and click left mouse button on the desired location to move it to the centre.

Next Slit  Scrolls the display such that the next slit moves into view (in a zoomed display, for example)

Cut  Useful (though rather unwieldy) for changing the intensity cuts of the image display. Select this panel and left-click on two pixels - the intensities of the two pixels will define the lower and upper intensity limits with which the image will be redisplayed.

Colors Colour 1-8   Different colours in which slitlets can be shown.

Clear   Selecting this clears all graphics - the only way to remove dead wood. Deleted slitlets will remain on the display until they are Cleared and the redisplayed using Masks->Show All

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Making a mask -- Using the MOS mask tool

  • Select a valid punch tool option --  Note this has to be the first operation after opening the tool and cannot be changed while working on a mask.
  • Load an image using either the FITS->Load and R90.....  option to select FITS files or File->Load *.bdf  to load rotated before
  • Change the values for the two parameters listed below and then click on Update Midas on the panel:
  • Change the display intensity cuts Tools->Cuts  to something sensible.
  • There are 4 possible slitlet defining operations. First select Options->slit length and then Slit->object/cursor option:
  • Some of the commonly used functions include:
  • Finally after everything is done:
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    Some Considerations

    The 3 reference stars

    Slits to the right of centre will yield a spectrum which covers a range blueward of a central slit while those to the left will cover redder wavelengths.

    There is no spatial restriction on the location of targets on the CCD other than for the reference stars. However if you are using the narrowest punch be prepared to lose 1 or 2 objects close to the margins of the CCD especially if your reference star locations are less than perfect.

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