Overview

This checklist describes the process of setting up LRIS for longslit spectroscopy when the target is too faint to be seen on the slit-viewing guider. The procedure for setting up for spectroscopy on very faint objects involves taking a direct image of the object and offsetting it to the location of the LRIS long slit. The telescope focus is different for the two modes of direct imaging and longslit; therefore, if high-quality data are desired in both modes using this procedure, the telescope must be focused twice, or the telfocus values must be pre-determined. The procedure is slightly different for the red and blue sides. The blue side is faster, but if the target is very red, the red side procedure should be used instead.

Faint target acquisition for Longslit spectroscopy

Prerequisites

The telescope focus for direct imaging mode should be determined via MIRA.
The telescope focus for longslit spectroscopy mode should be determined via AUTOFOC.

Procedure

Determine the longslit location

Slew to the target field.
Have the OA point the telescope to the science target, using the SLITB pointing origin and the desired position angle for your longslit.
Have the OA set the telescope secondary focus to the value derived from MIRA (for direct imaging).
Configure LRIS for direct imaging on the preferred side, but leave the longslit in place.
Acquire a 30 s direct image of the long slit with the preferred side.
Determine the row center of the longslit:
- Move the cursor into the ds9 window and place it onto the slit at the location you want the target to end up (preferred location is halfway between the slit center and the "top" of the slit in order to avoid the CCD gaps).
- Measure the center column using the cursor.
- Call the coordinates for the slit location x2, y2.

Determine the target location

Have the OA Start guiding using the offset guider.
Select the direct slitmask position.
Acquire a direct exposure of the field of the desired duration (suggested exposure time 30 sec) on the preferred size.
When the image is read out, identify the object on the ds9 image display (Note: if your coordinates were accurate, the object should lie very close to the slit center you measured above).
Estimate the object centroid by placing the cursor on the object and reading the PANE coordinates off ds9; call these (x1,y1).

Offset the telescope to center the slit on the target.

In an xterm window on the LRIS host computer, enter the appropriate command to offset the telescope:
- Blue side: movb x1 y1 x2 y2
- Red side: movr x1 y1 x2 y2
The telescope should move and the target should be placed at the slit location.
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).

Switch 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, then you may want to switch to the slit-viewing guider. Otherwise, proceed to the next step.

Insert the desired longslit.
Ask the OA to check for a suitable guide star on the slit guider while continuing to guide on the offset guider. If none is available, then continue to the next phase.
Ask the OA to switch to the slit-viewing guider.

Reconfigure LRIS and start science observations

Reconfigure LRIS for spectroscopy either manually or by using the restore_state command.
Remind the OA to reset the telescope focus to the longslit value!.
If desired, use the CMD... menu on the Xpose widget to 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.