Spectroscopy with NIRC2

NIRC2 is rather unusual in how it performs spectroscopy. It is similar in style to NIRC, in which different central wavelengths can be chosen by moving the slit in the field, but since NIRC has only low-resolution grisms normally the slit is used in one particular position, column 180.

Another peculiarity is that for some spectroscopic selections the image of the slit may not even be in the field of view of the narrow-field camera. Hence, the observer must center in either the wide- or medium-field camera and then switch to the narrow-field camera once the target is aligned.

The following summarizes the steps and explains each one:

1. Calculate the slit position
   slitcol grism cenwave [camera]
   
   In NIRC2, three elements determine the central wavelength of the spectrum: the slit position in the field, the grism, and the camera. In order to center the target at the appropriate position in the field the user must know which column to center on. Hence the first step is to tell the software all three relevant parameters, using the ``slitcol'' command. This takes three parameters in the following order: grism name, central wavelength in microns, and camera name. If the camera name is omitted, the current camera is assumed. (Note that in some circumstances the camera to be used for the spectrum may not even be able to image the field position at which the target must be centered. Hence a different camera is used for centering.)
   
   The slitcol command then sets some internal keywords so that the observer does not need to retype some of this information, and prints out the column number of the appropriate field position in each camera. One of the keywords set is SLITMM, which is the position in millimeters in the AO focal plane. This is camera-independent, and forms the basis for some later calculations.

2. Center the target
   scent x y
   
   The next step is to take an image of the field and measure the position of the target. The (x,y) values are used as arguments to the scent command, which will move the object from that position to (COL,512), where COL is calculated from the field position and loaded into SLITMM by slitcol. The column value is calculated using the camera currently in place.
   
   The observer normally takes another image or two to fine tune and double check the target's position.

3. Insert the slit
   slit name
   
   The ``slit name'' command inserts the slit called ``name'' to the appropriate AO focal plane position, again given by SLITMM. The "name" is usually given as the slit width in milliarcsec, e.g. "slit 80" to insert the 0.080 arcsec slit.

4. Check centering in slit
   
   The observer will now often take an image through the slit to confirm centering. To fine tune centering, the px and py commands can be used to specify small moves in pixels.

5. Insert the grism, camera, and blocking filter
   spec [filt]
   
   The spec command typed by itself will read the stored names of the desired grism and camera and insert these into the beam. Optionally, a blocking filter can be specified. If no filter is specified, the filter currently in the beam will be used.
   
   Additionally, the preslits will be moved into their spectroscopic positions, tied to the position of the slit within the AO focal plane.

6. Take spectrum
   
   The observer is now ready to take a spectrum, using either goi or one of the dithering scripts designed for spectroscopy.

Changing Your Mind

What if you change your mind about your spectroscopic or imaging setup during this process?

• Grism, central wavelength
  
  If you decide to use a different grism or central wavelength, you should start over again at step 1.

• Camera for centering
  
  If you decide to use a different camera to center your target in the slit, insert the camera using the ``camera name'' command, then start either at step 2, or step 4 if you already have the target in the slit. Remember that changing the camera will not change the positions of either the slit or the target in the AO focal plane, only where they show up on the detector.

• Camera for spectroscopy
  
  If you decide to use a different camera for the spectroscopy, you have two choices. You can return to step 1 and input all of the new information, or you can type ``speccam name'' to load a new value for the spectroscopic camera. If you do the latter, your central wavelength will shift slightly from what was originally requested, because the cameras have slight offsets of their axes.