Taking Exposures: Observing Your Targets

Observer support will open the dome 1 hour before sunset to clear the air out of the dome. Important: Vsub must be off and filter mechanism blocked while dome is open until 30 min after sunset.

POINTING

Once ready to observe, ask the night assistant to move to a bright star to zero point the telescope.  Take a 1-second object exposure.  To calculate the offset needed to bring the star to the center, go to the observer2 console (or ssh to observer2).  Open up a terminal and type:

> observer

to run the observer tool (aka "Kent Tools"). Type <commands> to see a list of commands, several of which are described below:

prompt> load <exp num> [ccd-id]

loads the image given by unique exp number, e.g., 149001  and ccd, e.g. "S4" (do not include []'s in cmmand). If no image is given, defaults to the last image delivered.
 

prompt> center <exp num>

center is a useful command that will calculate a RA and DEC offset from the current telescope position as measured from a catalog query and positions of objects on the image "exp num."  You may execute these offsets from the exposure control tab in sispi or ask the operator to execute them. You may ask the operator to make a telescope zero point after making the offset.

prompt> seeing

This will allow an interactive measurement of the FWHM of a single star on the frame.
 

FOCUS

 

To verify your images are in focus the telescope, display one of the focus and alignment sensor CCDs and check that the zernike terms is appropriate.  To do this, go to a terminal and type:

> observer

> load "" FS1

Note that you can type two double-quotes to load the latest image.  Otherwise replace the two double-quotes with the image number you are interested in.

> plotInit

This makes sure that a plotting window is open

> donutfit [saoDonutGet]

This command will pause - you position the cursor in the center of a donut and left-click.  It should cut out the donut, do a quick fit, and print out some numbers.  The first number of the last line should be something in the range 10-14 and is the radius of the donut in pixels. Images when the telescope is in-focus will have a value around 12.
 

To manually focus the telescope, first turn the automatic focus finder off by clicking on the "none" button in the Hexapod gui.

[1]

Then go back to the Observer Console gui, click on the Exposure Control tab, and start the focus procedure by selecting "focus" from the exposure type selection.

[2]

Choose an exposure time of 5-15 seconds and your desired filter (for example, r).  Choose a shift of 30 rows, your desired number of focus steps and your desired focus step size.  An image will be taken of N expsosures (number of focus steps specified), stepping the focus and shifting the charge by a number of rows between each one.  A good focus increment is 75 microns and a good number of focus steps is 7.  Then click the "Add" button.

After the exposure is done reading out, you will see an image containing lines of stars like this. 

[3]

Open a terminal on observer2 and load iraf and mscred by typing:

> cl
vocl> mscred

display the image and run the mscfocus command by typing:

mscred> cd /data_local/images/fits/2012B-9999/
(change 2012B-9999 to your proposal ID)
mscred>!ds9 &
mscred>!funpack DECam_00123456.fits.fz
mscred> mscdisp DECam_00123456.fits 1
vocl> mscfocus DECam_00123456.fits
(change DECam_00123456.fits to the focus image you just took)

You will now select stars to use to calucalate the best focus.  Place the cursor on the top star in a cleanly separated and unsaturated sequence and type the "g" key (always use the top star, regardless of whether or not there is a double step between it and its neighbor).  A window will appear displaying the FWHM as a function of focus for that star.  Type "q", place the cursor on the top star of another column, and type "g" again.  Repeat this sequence for a number of different stars.  With the cursor in the ximtool, finally type "q".  You will now see a window overlaying the focus sequences for all the stars you just measured.  If there are large outlying points, place the cursor on them and type "d", which deletes the whole star associated with that point.  Once you are satisfied, type "q", and the program calculates the best focus for you.

You can also use the command:

mscred> mscexam DECam_00123456.fits

Move the mouse over a star and type "r" and/or "e" to see the radial profise and ellipticity of the star.
 

TAKING SCIENCE OBSERVATIONS

The most efficient way to take your observations is by loading a pre-written script of your desired observations.  Writing scripts for DECam uses a friendly ScriptsEditor tool.

To use this tool, on the observer2 or observer1 console, type:

>setup ScriptsEditor
>ScriptsEditor

Then you can fill out the gui as explained here [4] and save your script. 

You may download the stand alone ScriptsEditor code [4] and install it on your computer.  Python (with Tkinter support) is the only requirement.