Standard Star Observations

  1. If you haven't already done so, point the telescope at a standard star and set up the autoguider on an offset star.

  2. Setting Beams: You will now determine the distance and direction between the two chop beams. The -Beam can be in any direction from the +Beam, but N--S nods are preferable because the telescope settles faster than when moving E--W. For this example we will assume a 20'' N--S chopper throw, a phase setting of 0 deg, and a 90 deg Cass. ring angle. Make sure the upper chop image is falling on the SC-10 pickoff mirror; with 0 deg phase angle this will give a positive signal.

  3. Set the adjustable Step to 20''.

  4. Press the Define Beams button in the main SCACQ window. The button will change its label to Here is -Beam.

  5. Press the S button to move the telescope 20'' S. You should see the lower stellar image disappear behind the pickoff mirror silouette, and the upper image appear above the mirror. You should also see the star temporarily disappear from view in the Offset guider TV, then reappear as the camera picks up the other beam. Wait for the guider to lock onto the image. You may have to move the telescope slightly to help get the star into the center of the video cursor.

  6. Press Here is -Beam to record the -Beam position.

  7. Press the N button to move back to the other beam, and wait for the autoguider to lock onto the image.

  8. Press the Here is +Beam button to record the +Beam position. The software now calculates the distance between the two beams and displays it on the screen. A message is also printed to the log file.

  9. Press Nod to switch beams. Watch the stars on the centerfield and autoguider TVs to make sure the nod motion is correct. The motion should bring the other beam into the autoguider cursor very quickly, and the guider should lock on in just a few seconds. Nod back and forth a few times to make sure the motion is consistent.

  10. If the nod is unsatisfactory, you can repeat the above procedure. You must define the -Beam first, then the +Beam. You can also double-click on the nod amplitude fields, and type in their numbers manually to make small adjustments.

  11. Put the +Beam image back on the infrared detector. You must do this before starting a scan, because the first nod is always from the +Beam to the -Beam position.

  12. Set the NBeams to 4, and the Int. Time to 10 sec using the text-entry fields in the main SCACQ window.

  13. In the SCDISP window, set the display to Coadd Diff, Show Diff, turn off Nod Avg, and set the units to mV.

  14. Press SCAN take to begin a scan. There are many things to watch for at this time:

    1. A header will appear in the log file window giving the scan number, the current telescope position, and several instrument parameters.

    2. After the first integration (10 sec), an image will appear in the display window, and the telescope will nod. A new line will appear in the log file giving the new beam number, and the Beam and Sense indicators in the main window will change. You should familiarize yourself with the behaviour of the nod on the telescope status display (the RA and Dec offsets will change), and on the TV monitors, because on occasion the telescope will not nod correctly and the software will loose track of which beam is which.

    3. With Nod Avg off and Show set to Diff, the screen is updated at the end of each beam. The sky will not be well zeroed because of gradients in the telescope emission between the two chop positions. During the scan, you can change Show to CoAdd to monitor the accumulating signal in the CoAdd buffer. On the even beams, the nodding process will cancel out the gradients in the coadd buffer and the sky surrounding the star should look very flat, except for scattered light from the star itself.

  15. At the end of the scan, select Show Diff, put the mouse in photometry mode, and measure the signal-to-noise of the star.

  16. Turn on Nod Avg, select Show Diff, and start another scan. Now the display will be updated after each even beam, and telescope gradients will always be canceled out. You can switch from Show Diff to Show Coadd as before, but be aware that doing this forces a screen update and an odd number of beams may be displayed.

  17. At the end of this scan, select Show Coadd, then click on the SCAN more button (instead of take). This starts a new scan just like take (opening a new FITS file, etc.), but does not clear the CoAdd buffer. Use this to increase the S/N of the CoAdd buffer over several scans.