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Direct Imaging

Afternoon:

  • Make sure that the Instrument Specialist has loaded any special filters that you may have brought with you. If you will be using special filters, make sure that you specify this in the "Special Requirements" section of the Instrument Setup Request Form that should be 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.

 

  • Take flat field exposures with the filters that you will be using. To do this, first 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, have the Instrument Specialist turn on the quartz lamp and set the voltage to a value less than 5 V (for broad band filters, a voltage of 2-3 V is usually adequate). 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 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 the desired position. Now, in the IMACS CamGUI, set ExpTime to "5", 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. Repeat these instructions for each filter.

 

  • 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.


Twilight:

  • Take twilight sky flat fields if desired. In the IMACS MechGUI, set the Filter to the desired position, and in the CamGUI, and set ExpTime to a small value (e.g., 2 seconds). Click on the "Start" button to take a test exposure, and repeat until the brightness of the sky has decreased to a reasonable value (e.g., 30,000-40,000 cts/pixel). For the next exposure, have the telescope operator (TO) move the telescope by 20-30 arcsec, increase the exposure time (a factor of 2 should be close to the correct amount), and take another exposure. Repeat until you have obtained at least 5 exposures. Switch to a different filter and repeat the sequence.


Night:

  • Have the TO slew to the object that you wish to observe. Generally speaking, you will want the TO to operate the telescope with the rotator mode set to "EQU" with an offset of 90 + 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). Note that the observer can (and should, if possible) prepare his/her Observing Catalog in advance with the desired rotator mode and offsets.

 

  • As soon as the telescope has slewed to the position of the object, 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, set the Filter to the desired position, and in the CamGUI, set ExpTime and Loops to the desired values, and then click on the "Start" button to begin the exposure.
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