NIRC2 Spectroscopy Procedure

Afternoon setup:

1. Review spectroscopy section of NIRC2 manual. Note that the resolution is given per pixel. This resolution should be divided by slit width to convert to resolution element. It is not recommended that a slit width less than the diffraction limit of the PSF be used. For example a slit width of 50mas or greater should be used for K band.
2. Startup NIRC2 software
3. Run the slitcol script. This is a wizard style script that queries for various parameters. The script converts user parameters into a machine parameter for slit position known as slitmm, the slit position in mm in the focal plane. slitcol sets the keyword slitmm or it can be set directly with the command slitmm
4. Setup NIRC2 to image the slit. Example:

   	  filt k
   	  camera wide
   	  sampmode 2
   	  tint .2
   	  coadd 1
   	  shutter open
   	  pupil largehex
   	  wait4ao off
   	  grism clear
   	  slitmm 5.0
   	  slit 60
   	

   Note that the slit command reads the value of slitmm and puts the slit selected at this position.

5. Setup of the AO bench for the white light source. First check with the SA that the bench is available for calibrations and run the command:

   	  configAOforLamps
   	

6. Turn on white light source.

   	  lamp white on
   	

7. Take image and measure slit position

   	  goi 1
   	

   Note image number and slit position. Note that the image will likely be saturated but should yield a good x centroid for slit position. it is important to image the slit through the same filter and camera that you will be aligning with later in the night. There may be setups that require alignment in the wide camera but the spectroscopy is done in the narrow or medium camera. It is not recommend that narrow band filters be used since they can create ghost reflections. Images with the white light source off or one of the arc lamps may be more appropriate for certain settings.

8. Take a series of spectral flats, arcs, and darks for calibrations. Confirm spectral setup with arcs and filter transmission cutoff points.

Night time setup:

1. Enable DAR (differential atmospheric refraction) compensation. It should be set to acquisition and tracking during NIRC2 spectroscopy setup and integrations. The B-V value of the guide star is also relatively important for best performance of the DAR compensation. Verify with the OA that the DAR compensation is enabled for acquisition and tracking.

   Note: The array in NIRC2 has significant persistence. The is effectively high dark current in areas where high flux levels have been incident on the array. If there are bright objects in the field and the long integrations on faint objects are planned, great care should be taken to avoid the bright objects from being the same y location on the array prior to the long integrations. The NIRC2 dark slide or shutter should be closed during slews for this reason. For more information read about the NIRC2 persistence characterization.

2. Take image of object in same filter/camera configuration used for step 8. of the afternoon setup above.
3. Offset telescope so that the object is placed on the same x pixel location of the slit. The mov command is recommended to move the object from pixel location x1 y1 to pixel location x2 y2.

   	  mov x1 y1 x2 y2
   	

4. Repeat steps 1 and 2 until object it positioned on the correct pixel.

   Note: a useful technique is to display the difference of the object image and the slit image to confirm the setup. Use the math -> arithmetic menu in quicklook to display the difference of any two frames.

   Other useful commands for making small adjustments include:

   	    xy offsetX offsetY
   	  

   units are arcsec, moves the telescope, thus object moves opposite

   	    pxy offsetX offsetY 
   	  

   units are pixels, moves the telescope, thus object moves opposite

   Note that in NGS mode, the accuracy of the offsets has an uncertainty of +/- 40mas and thus offsetting can take several iterations. This is due to the lack of precision in the FSM control on the AO bench.

5. Configure instrument for spectroscopy. Example:

   	  filt k
   	  camera wide
   	  sampmode 3 16
   	  tint 300
   	  coadd 1
   	  shutter open
   	  pupil largehex
   	  wait4ao on
   	  grism medres
   	  slitmm 5.0
   	  slit 60
   	

6. Take integrations.

   	  goi
   	

   Note that scripts exist for slit dithering (example: sp2) but should be used with caution due to the FSM problem noted above.

   	  sp2 dy [n=]
   	

   Takes n pair of spectra seperated by dy arcsec. The spectra pairs are centered about the current position.

   To guaranty slit alignment in NGS mode, it is recommended that the object be imaged between every slit nod. Again, this is due the FSM inaccuracy problem. For wider slits you may just monitor the spectral intensity and realign if it drops significantly. LGS mode is accurate to < 10mas and thus the slit dithering should not be a problem.