Observing with a long slit mask

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Observing with a long slit mask

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Daytime Preparation:

  • Make sure that the Instrument Specialist has loaded the masks that you requested in the Instrument Setup Request Form that you submitted at least a month before your IMACS observing run.
  • If the IMACS GUI has not already been started by the Instrument Specialist, open an xterm, type "imacs ", and follow the instructions in the "Getting Started" section of the IMACS Manual.
  • At night, the procedure for centering an object in the long slit is done in three steps. The first step is to slew the telescope to the coordinates of the object, which will place the object approximately at the center of the array of 8 CCDs. The second step is to offset the telescope approximately to the desired postion along the slit. The final step is to carry out a fine-centering of the object in the slit. The procedure for the second step of moving the object from the center of the CCD array to the approximate position along the slit can be set up during the day.

To do this, you will need to take an image of the slit mask in order to determine the position along the long slit where you wish to place your objects. 

Ask the Instrument Specialist to open the telescope mirror covers. Start the Flat Field Screen gui by typing "ffs" in an xterm. Once the gui appears, click the mouse on the "IN" button to put the screen into the telescope beam. Next, turn on the neon lamps by clicking on the two "NE" buttons. Now, in the IMACS MechGUI, make sure that the Hatch is in the "open" position, the calibration lamps are all turned off, and the CF-Guider is in the "out" position. The Slit Mask must be set to the position with the long slit mask that you intend to use, and Disperser should be set to either "f/2-Imaging" or "f/4-Imaging", depending on which camera is being used. Finally, set the Filter to "Spectroscopic". Now, in the appropriate IMACS CamGUI, set ExpTime to "5", Loops to "1", ExpType to "Object", Binning X to "1", Binning Y to "1", and then click on the "Start" button to take an exposure.

  • Once the image of the slit mask has been taken, use the QL-Tool to measure the xy CCD coordinates of the position along the slit at which you wish to place your objects. You will need to calculate the offsets from the optical axis to your desired chip coordinate.  To first order, the centre of rotation or the optical axis is basically at the centre of the array and you need to count pixels from the centre to your chip target coordinate with X going positive to the right and Y going positive up.  [Update as of 150917 the centre of rotation for the f/4 camera is located in the upper right corner of CCD03 near [172,3993] rather than in the central CCD gap].  It is safer to offset by 100-200 pixels extra to cleanly place your target near your desired chip target location. You may use the equations below to calculate rough parameters for "xoffset" and "yoffset" for the "toslit" task. Note that these depend on which CCD chip the slit position falls. Here you may find the offsets for the f/2 camera (Apr. 2019) and here the offsets for the f/4 camera (Nov. 2019) 
  • Open an IRAF xgterm, and type "imacs" to load the imacs package. Edit the parameters of the "toslit" task:

       query1 = no                 Send RA/DEC offset to TCS?
      (camera = "long")         Camera ('long' or 'short')
        (xbin = 1)                   Binning in x direction
        (ybin = 1)                   Binning in y direction
     (xoffset = -312.)            Offset in x (pixels)
     (yoffset = -609.)            Offset in y (pixels)
        (mode = "ql") Make sure to enter the correct parameters for the camera configuration (f/4 is "long" - f/2 is "short" ), and set "xbin" and "ybin" equal to the binning you are using. Finally, enter the values of "xoffset" and "yoffset" calculated in the previous step.

As a last step, make sure to return the Binning X and Binning Y values in the IMACS MechGUI to the values you will use take your science images during the night.

  • Take a sequence of flat field exposures with the slit that you will be using. With the telescope mirror covers still open, and the Flat Field Screen in the "IN" position (and the HE, Ne, and AR lamps all turned off), have the Instrument Specialist turn on the quartz lamp and set the voltage to a value less than 5 V. In the IMACS MechGUI, make sure that Slit Mask is set to the position with the long slit mask that you will use, Disperser to the grating or grism that you will use, and Filter to "Spectroscopic". Now, in the IMACS CamGUI, set ExpTime to "20", Loops to "1", ExpType to "Object", and the Binning X and Binning Y values to whichever values you plan to use at night, and then click on the "Start" button to take an exposure.
  • Adjust either the exposure time and/or the quartz lamp voltage to achieve the desired count level. Now take a sequence of flats by setting Loops in the IMACS CamGUI to the desired number of exposures, and then click on the "Start" button to begin the sequence.
  • Turn off the quartz lamp, click the mouse on the "OUT" button to take the flat field screen out of the telescope beam, and take a sequence of bias images. In the IMACS CamGUI, set Loops to the number of exposures you wish to take, ExpType to "Bias", and the Binning X and Binning Y values to whichever values you plan to use at night, and then click on the "Start" button to start the loop sequence. Although IMACS is reasonably light-tight, bias images should be taken with the dome as dark as possible.


Night:

Note: Follow this sequence completely for every object that you wish to observe

  • Have the Telescope Operator (TO) slew to the object that you wish to observe.
    • For slit oriented along the parallactic angle, make sure that the TO has the rotator mode set to "HRZ" with the offset set to 180 + X, where X is the IMACS rotator offset angle which is a number like -46.15 (the Instrument Specialist can give you the latest value).
  • For slit aligned along a specific position angle, PA(slit), then make sure that the TO has the rotator in the "EQU" mode with the offset set to X + PA(slit) - 180 (+ 180 if the target is between the zenith and the pole).

Note: observers should prepare their Observing Catalog in advance with the desired rotator mode and offsets.

  • As soon as the telescope has slewed to the position of the object, run the "toslit" task by typing "toslit ". This will offset the telescope so that the object should be positioned within ~5-10 arcsec of the desired position on the slit. It is important to get the object to be on the same chip as the part of the slit you wish to use!
  • Have the TO set up on a guide star and an off-axis S-H star.
  • Make sure that flexure control is ON and properly applied (no flexure control error messages in the IMACS message window). Then, take an image of the slit mask. To do this, go to the IMACS MechGUI and make sure that the Hatch is in the "open" position, the calibration lamps are all turned off, and the CF-Guider is in the "out" position. The Slit Mask must be set to the position with the long slit mask that you intend to use, and Disperser should be set to either "f/2-Imaging" or "f/4-Imaging", depending on which camera is being used. Finally, set the Filter to "V" (or whatever filter is closest to the center of the wavelength region to be observed spectroscopically). Now, in the IMACS CamGUI, set ExpTime to "10", Loops to "1", ExpType to "Object", Binning X to "1", Binning Y to "1", and then click on the "Start" button to take an exposure. After the exposure has ended, display the CCD image which contains the the part of the slit where you want to center your object, and use the "imexam" task to measure precise xy coordinates for this position.
  • Next take an image of the object. To do this, go to the IMACS MechGUI and set the Slit Mask to "f/2-Imaging" or "f/4-Imaging", depending on which camera is being used. Now, in the IMACS CamGUI, set ExpTime to an appropriate value so that the object will be visible, and then click on the "Start" button to take an exposure. Note that the orientation of N and E will be indicated in the QL-Tool when the exposure has read out. (A plot of field orientations versus the rotator offset angle may also be found here.)
  • Now run the IRAF imacs task "icobject", the parameters of which are:

        image =                    Image
        xslit =                       x coord of slit position
        yslit =                       y coord of slit position
       query1 = no               Send RA/DEC offset to TCS?
       query3 = yes              Do coordinated offset?
      (camera = "short")      Camera ('long' or 'short')
      (orienta = "normal")         Dewar orientation ('normal' or 'N&S')
        (line = "")
      (cursor = "")
      (mode = "ql") The "image" parameter should be set to the name of the object image just taken. The CCD chip which contains the object must be specified through the image name (e.g., "ccd0034c5"). The parameters "xslit" and "yslit" must be set to the xy coordinates of the slit position that you just measured. Lastly, make sure to specify the camera ("short" = f/2, "long" = f/4) that you are using.

  • Now run "icobject": The image will be displayed on the "ds9" display, and you will be asked to mark the object. If the object is stellar, or is a galaxy with a well defined nucleus, use the "space bar" to mark the position; otherwise, use the "m" key. If you wish to change the minimum and maximum data values mapped into ds9, hit the "d" key. If you wish to abort, hit the "I" key. Once you've marked the object, an offset is calculated and printed to the screen. You will then be asked if you wish to offset the telescope this amount. Usually the correct answer is "yes". Make sure you also reply "yes" to the question of whether you want to do a coordinated offset (which moves both the telescope and the guide probes), but let the TO know before actually executing it.
  • Now take another image of the object, and run "icobject" a second time. The offset calculated should now be small, but it is usually worth executing it.
  • You should now be ready to observe your object. In the IMACS MechGUI, re-insert the appropriate slit mask, set the Disperser to the grating or grism that you will use, and set the Filter to "Spectroscopic". In the IMACS CamGUI, set ExpTime and Loops to the desired values, and then click on the "Start" button to begin the exposure.
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