Finding the First Star

1. Ask the telescope operator to point the telescope at a bright star, preferably one of the standard mid-infrared calibrators that is 0 mag or brighter. You can activate the Object Coordinates utility from the SCACQ main window, select an entry, and send the coordinates to the operator manually, or you can tell him or her to move to a certain Yale Bright Star number. The Object Coordinates command will automatically enter the object's name in the SCDISP window and FITS headers of subsequent scans. If you don't use this, remember to enter the object name manually using the Log Entry button and the object keyword.

2. Find the star first in the Centerfield TV camera. You should see two stellar images corresponding to the two chop beams. Move the telescope until the upper image star is falling on the SC-10 pickoff mirror. You should still see the lower image below the mirror silouette if the throw is about 20''.

3. Turn on Cont. Acq and select Show Diff. The star should be visible on the detector.

4. In SCDISP, turn AutoScale on, put the mouse in SBox mode, and place the scaling box over the star. Make the box big enough so that the whole star is clearly visible inside it.

5. Watching the infrared image with Cont. Acq. on, carefully focus the telescope. We recommend moving to lower focus numbers until the star just begins to look out of focus. Then move to higher numbers, through the best focus, until the star again goes out of focus. Then set the focus at the mean position. In the range +/- 0.30 mm from best focus, it is very difficult to see significant changes in focus quality, but you can usually see a sudden change at about these positions. You may see significant astigmatism, at the out-of-focus positions, and of course the accuracy of focusing will depend on the seeing. Note that if you are using an autoguider, you should select a focus that is closer to the best optical focus (as noted in the centerfield video display), but still acceptable to you on the detector.

   Significant focus changes during a night often occur because of temperature changes. Be sure to record the focus setting, time and temperature (preferably in the log file) every time you focus. The current focus value is saved in the FITS header of each scan. The prime focus temperature has been found to be very closely related to focus change.

6. Move one of the stellar images from the north end of the detector to the south by commanding the telescope to move north and south. Adjust the Cass. Ring angle until the star moves parallel to the rows of the detector, if that is what is desired. Then reduce the chopper throw to about 12 arcsec, and rotate the chopper angle until the two stellar images are on the same row. Set the chopper throw back to the desired value.

7. Place the star on a good position on the detector, then set one of the centerfield TV monitor crosshairs on the optical image of the star visible below the silouette of the pickoff mirror.

   If using the Shepherd Video System, select ADD under the FIDUCIAL MARKER FUNCTIONS. Using the Shepherd's mouse, place the marker over the star and anchor it there.