Overview

Faint target acquisition for Longslit spectroscopy

1. Determine Longslit Focus
   • If the telescope (not LRIS) has not yet been focused for long-slit mode, then tell the OA to determine the forucs for long-slit spectroscopy (Autofoc or Fast-Focus).
   • In XLRIS, select the longslit of choice using the Slitmask selector menu.
2. Determine the location of the longslit
   • While the OA is focusing the telescope for longslit spectroscopy, take a 30-second direct image of the long slit.
   • Determine the column center of the slit:
     • Move the cursor into the FIGDISP window and place it onto the slit, roughly equidistant between the top and bottom of the slit.
     • If using the blue side, do not select the center of the long axis of the slit. Offset from center to avoid putting your object in the gap between the two blue side detectors.
     • Measure the centroid by pressing the s key. A window will pop up, displaying the measured centroid and FWHM. Note that only one of the centroid values is accurate and which one will depend on whether you are using the red or blue side.
     • call the coordinates for the slit location x2, y2.
3. Configure LRIS for imaging
   • For maximum efficiency, create a file to configure LRIS for your particular spectroscopy setup. Either the WRITE button on XLRIS to save the current configureation to a file, or use the Scripter program to generate the setup file.
   • Set up either the blue or red side for direct imaging mode
4. Determine Direct Focus (Optional)
If optimal image quality in the direct image is not required, then simply have the OA decrease the current value of telfocus (just determined in the previous step via Autofoc) by 60 microns and proceed to the next step. Otherwise, instruct the OA to run MIRA in order to redetermine the direct imaging focus.
5. Image Target
• Slew to target field
  Have the OA point the telescope to the science target, using the SLIT pointing origin and the desired position angle for your longslit.
• Start guiding using the Offset Guider.
  Ask the OA to locate a guide star as close to the center of the guider field-of-view as possible and begin guiding. If no guide star is seen, have the OA move the guider until a suitable guide star is located.
• Image the target field
  Take a direct exposure of the field of the desired duration (suggested exposure time 30 sec) using LRIS.
6. Determine target location
   • When the image is read out, identify the object on the FIGDISP image display (Note: if your coordinates were accurate, the object should lie very close to the slit center you measured above).
   • Measure the object centroid by placing the cursor on the object and pressing the s key. A window will pop up, displaying the measured centroid and FWHM. Note the column and row centers; call these (x1,y1).
7. Offset the telescope by issuing a move command.

   Display tool	move command	where
   FIGDISP blue side	movb x1 y1 x2 y2 movb_f2d x1 y1 x2 y2 NOTE: x2 y2 are measured coordinates using IRAF/DS9 when using movb_f2d	punaluu window
   FIGDISP red side	mov x1 y1 x2 y2	punaluu window
   SAOInage ds9 blue side	movb_iraf x1 y1 x2 y2 mshiftb x1 y1 xref=x2 yref=y2	punaluu window IRAF window
   SAOInage ds9 red side	mov x1 y1 x2 y2 mshift x1 y1 xref=x2 yref=y2	punaluu window IRAF window

   • The telescope should move and the target should be placed at the desired destination.
   • If the telescope move caused the guide star to be lost, acquire a new guide star and repeat the alignment.
   • If the required telescope move was larger than about 10 arcseconds, you should take a new direct image to confirm the centering on the desired coordinate (the telescope move may not have been sufficiently accurate).
8. Switch to Slit-Viewing Guider (optional)
If you will be observing this field for a long time and are concerned that differential flexure between the offset guider and the slit will move your object out of the slit, and you think that a guide star may be available there, then you may want to switch to the slit-viewing guider. Otherwise, proceed to the next step.
• Ask the OA to note the current pixel position of the guide star on the offset guider.
• Insert a longslit
  • using the GO button on the XLRIS widget to recall your spectroscopy setup script, or
  • manually selecting the desired longslit from the Slitmask selector menu on XLRIS.
• Ask the OA to switch to the slit-viewing guider and select a suitable guide star.
• If no suitable guide star can be found, then you will need to return to offset guiding. Ask the OA to switch back the offset guider and re-center the guide star on its previous position.
9. Reconfigure LRIS and start Science Observations
• If you previously created an LRIS setup script for your spectroscopy configuration, then use the GO button in the XLRIS window to execute the script now. Otherwise, manually select the desired long slit, grating, filter, and focus.
• Remind the OA to reset the telescope focus to the longslit value!.
• Using the CMD... menu on the Xpose widget, reset the CCD readout window to longslit. Also select the desired readout mode (single or dual-amp, as desired) using this same menu.
• When the setup is complete, enter the desired integration time in the Xpose widget and start your spectroscopic exposure.