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Placing an Object Onto the Slit

An IRAF script called TELOFFSET has been written to assist in placing an object onto the slit using the file created in the previous subsection.

Focus the telescope with FILTER=CLEAR, since you will not be using a filter for the spectroscopy.

Go to your field and set the COSMIC base position angle to your desired angle (where COSMIC BASE POSITION ANGLE = 270 - TRUE PA; and COSMIC BASE POSITION ANGLE must be between 70 and 300).

(1) Take an image (either unbinned or binned) with FILTER=CLEAR, APERTURE=CLEAR, and GRISM=OPEN.

(2) Display the image by flipping the y-axis, i.e. display ccd123[*,-*] 1.

(3) Edit the parameters for TELOFFSET, i.e. epar teloffset. Enter the appropriate filename created in the previous subsection, the current COSMIC BASE POSITION ANGLE, and identify if this is a unbinned image (SNAP=no) or a binned image (SNAP=yes). Type :go in order to run the task. Find the object in the SAOIMAGE display, type ``a'' to identify it, and then ``q'' to quit. The appropriate telescope offsets needed to move this object onto the chosen slit position will be shown.

(4) Tell the night assistant the offsets to execute.

(5) Find a guide star and start guiding.

(6) Take another image and repeat steps 1-3. If your object is very faint, or if it is a crowded field, you might have to use the unbinned mode. If the offsets are < 0.3'', then this is sufficient. If not, put the guider in ``follow'' mode, ask the night assistant to execute the offsets, then enable guiding.

(7) You should be done. If you are unsure, then repeat step 6 until you are fully satisfied.


next up previous
Next: Making Multislit Masks Up: Observing with Longslits Previous: Determining the Position of
Roy Gal
4/25/1998