EMMI

ESO Multi Mode Instrument

HOME

INDEX

HELP

NEWS

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>:

Login as mosntt@wlsmos (ask your support astronomer for password).
mkdir <yourname> ; cd <yourname>
ftp the fits image to this directory
export DISPLAY=<your machine:0.0>
do xhost +wlsmos on your local machine.
xm this opens the tool with which one defines the slitlets (important: do not use the command xmos)

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

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:

Object distance from Slit edge . This defines how close to the edge of a slitlet an object can be placed. Put in a value of choice
Interslit gap This defines how far apart in pixels adjacent slits have to be. If 2 slits are defined with their extremities closer than this value the first slit will be erased. This is to make sure that the spectra of 2 adjacent slits to do not overlap. However one can deliberately overlap the slitlets of the reference stars since their spectra are not used. For this one will have to specify a negative value here. Beware! You will have to take care to ensure by eye that the slitlets of the target sources do not overlap!

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:

slit length minimum - On object Select the object using the rectangular cursor described under Slits->On object. A slit of the minimum length will be drawn centred on the object selected. The cursor will then be ready for the next object until the function is terminated by double clicking the central mouse button.
slit length minimum - On cursor Select the pixel using the crosswire cursor and a slit of the minimum length will be drawn centred on it. The cursor will then be ready for the next object until the function is terminated as described before.
slit length variable - On object Selecting the object with the rectangular will define the x-pixel of the slit. The tool will then provide a crosswire cursor for marking (left mouse button) the upper and lower (y-pixel) limits of the slit. On completing one slit the tool will immediately provide a rectangular cursor to repeat the process with another object. The function can be terminated as described before.
slit length variable - On cursor The tool provides a crosswire cursor for marking 2 points with a click of the left mouse button. The upper pixel will define the upper slitlet limit while the lower pixel will define both the lower slitlet limit as well as its x-pixel. On completing one slit the tool will immediately provide a rectangular cursor to repeat the process with another object. The function can be terminated as described before.

Some of the commonly used functions include:

Slits->Delete
Tools->Zoom and Tools->Unzoom
Colour->Clear and Masks->Show All
Masks->Save Mask
Masks->Load Mask

Finally after everything is done:

Save the mask one last time and note down the mask name from the message window.
Get a hardcopy of the image display including the slit graphics. Print postscript file immediately - it will be overwritten. Mark the reference stars on it - this will be useful while identifying them at the time of MOS acquisition during the night.
Send the .mask file to the punching machine manually ftp the .mask file to wemmi:/vltdata/tmp/EMOS-MASKS/.
Punch them (SciOps only)

Some Considerations

The 3 reference stars

should be bright (does not have to be very bright but should stand out in a 20 second exposure)
should form a spread out triangle
should not be located close to the margins of the CCD
should always be centred in a slitlet of minimum length using Slits->On Object option

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.