Observing in the Center Field Slit-Viewing Mode

Observing in the Center Field Slit-Viewing Mode

The purpose of the Centerfield Slit-Viewing mode is to set up and take spectra of objects that are bright enough to be seen in “real time” in the guide camera, instead of using the IMACS Mosaic Cameras to do the acquisition.  This is a time saving feature for programs of relatively bright, single objects, including radial velocity or photometric standard stars.  The Centerfield Guider can be positioned in both at the center of the IMACS field and at a position withdrawn from the center, enough that spectra do not fall in the gaps between the CCDs!  Since Slit-viewing spectroscopy is not done at the very center of the IMACS field, an extra step is required to move the target object to the slit-view position.

Daytime Preparation:

  • Confirm with the Instrument Specialist that IMACS has been setup with the masks, filters and any other “Special Requirements” noted in your Instrument Setup Request Form that was submitted a month in advance of 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.
  • When working at the f/4 camera, the spectra produced using the center field slit will fall on CCD chips 1-4, so it is useful to set up a subraster that will read only the appropriate portion of these chips. If you would like to adopt a subraster readout of the appropriate section of only these CCDs, please click on the Subraster button of the IMACS CamGUI. This will pop up a Subraster Definition GUI. Click on the “File” button at the lower left, and browse for the subraster file (there should be one named slitviewf4.sub in the instrument default directory). Once you have found it, click on the “Load” button to load the settings, and then select “minimal” for the SaveMode. Finally, click on the “Apply” button to apply these changes, and then click on “Done” to close the GUI. When working at the f/2 camera, the spectra produced using the Slitview mode will fall on only two CCDs – you may select to read only these CCDs by selecting the cartoon of the CCD mosaic in the CamGUI and unchecking all other CCDs for readout.
  • Take a sequence of flat field exposures with the center field slit viewer inserted. 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 low quartz lamps by clicking on the “Ql” button. Now, in the IMACS MechGUI, make sure that the Hatch is in the “open” position, and the calibration lamps are all turned off. Click on the CF-Guider button and select the “Slitview” position. Doing this should also automatically send the Slit-Mask into the mask position behind the CF-Guider — if it doesn’t, then set the Slit-Mask manually to “Slitview”. Finally, set the Filter to “Spectroscopic” or to one of the order blocking filters, and select the Disperser that you will be using. Now, in the IMACS CamGUI, set ExpTime to an appropriate value (30 seconds works fine for the f/2 camera and 300 l/mm grism in “slow” readout mode). Set Loops to “1”, ExpType to “Flat”, the x and y binning to the values you will use at night, and Speed to either “Slow” or “Fast”. Click on the “Start” button to take a test exposure. Adjust the exposure time as required, and then take a sequence of flats by setting the Loops parameter to the desired number of exposures, and then clicking on the “Start” button to begin the sequence.  
  • Turn off the quartz lamp by clicking again on “Ql” in the Flat Field Screen GUI. Now 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 the values you plan to use at night. 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.
  • 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 = 2) Binning in x direction (ybin = 2) Binning in y direction (xoffset = 0.) Offset in x (pixels) (yoffset = -550.) Offset in y (pixels) (mode = “ql”) The above setup for tolist will move the telescope such that the target is now located at the central crosshairs of the CF-Guider when at the Slitview position. Note, these parameters will work independent of which observing configuration you are actually using (f/4 N&S or f/2 normal – since toslit is just aligning the telescope with the Slitview Position of the Center Field Camera and is completely insensitive to how you are observing). The operator can now interactively move the telescope to place your target on the desired slitlet of the image on GCAM #3.
  • Once you’ve finished editing the “toslit” parameters, type ctrl-D to save them.  


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.  You have two choices for the position angle of the slit: (1) If you want to have the slit oriented along the parallactic angle, make sure that the TO has the rotator mode set to “HRZ” with the offset set to 134 (180 + the IMACS rotator offset angle). (2) If, alternatively, you want the slit aligned to a specific position angle, PA(slit), then make sure that the TO has the rotator in the “OFF” mode with the offset set to PA(slit) – 226 (add 180 if the target is between the zenith and the pole). It is strongly recommended that this is done ahead of time by preparing an Observing Catalog in advance, with the object coordinates, the desired rotator mode, and offsets.  
  • As soon as the telescope has finished slewing to the position of the object, the observer runs the “toslit” task by typing this in the iraf window. This will offset the telescope properly so that the object appears roughly centered (within ~5 arcsec) of the Slitview field. (Note that in this mode the Centerfield Guider never actually moved to its centerfield position — the procedure simply simulates doing so in order to get the proper offset with the desired orientation.) The object should now be visible in the Centerfield guider’s slit-viewing camera. If it is not, try having the TO integrate with the camera for a few seconds. If your object is too faint to be seen with the slit-viewing camera and a short integration, you will be better off using the IMACS Mosaic CCD cameras for setup and using one of the long slit masks (or a slit on one of your mulit-slit masks) to observe your object. For instructions on how to do this, click here.
  • Once you have identified your object in the Centerfield camera (slit-view position), have the TO use the track ball to offset the object to the slit position (or the 7″ aperture for flux standards). Finally, have the TO set up on a guide star and an off-axis S-H star.  
  • You should now be ready to observe your object. In the IMACS MechGUI configure the instrument to your desired filter and grating combination. 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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