4.6 Locating the ``Hot Spot''

The `hot spot' is the location on the guide T.V. which corresponds to the entrance aperture of the instrument. The exact procedure for locating the `hot spot' depends on whether the FTS is installed with the guide pellicle or guide mirror installed. An outline of the technique for each circumstance is given below. The `hot spot' location is not fixed, and can drift slightly during the night. Consequently, it is important to check the `hot spot' location periodically.

4.7.1 Guide Pellicle in Place

  1. Slew to a relatively bright source (V ). Insert the K band filter, even if the instrument is configured for work in the thermal infrared to provide a measurable signal above the background.

  2. Select a preamp setting appropriate for the source. Typically, setting `1' should be used for 2, Setting `10' should be used for 2 8, and setting `100' should be used for all fainter sources.

  3. Ask the telescope operator to position the Bonnette guide probe to (x,y) = (0,0).

  4. Position the rectangular guide box on the acquisition T.V. screen to the current `hot spot' location. If the `hot spot' location is not known, a good default preliminary setting is (X,Y) = (119,43) - note that these coordinates refer to those displayed in the lower left hand corner of the acquisition T.V. screen, and not to the position of the Bonnette guide probe.

  5. Center the source in the preliminary `hot spot' location.

  6. Display the S1 signal on the main oscilloscope.

  7. Use the hand paddle to gradually move the star away from the `hot spot' - the S1 signal displayed on the oscilloscope should decrease. If not, hunt around the `hot spot' location until a signal is detected. If a signal is not detected see Chapter 6.

  8. Move the star around until the S1 signal is maximized and then relocate the guide box to the location of the star on the acquisition T.V. (Note: if this procedure does not work you can also visual inspect the entrance field of the FTS as discussed in Section 3.1.1)

  9. Using the appropriate buttons on the Main Control panel, close the iris to its smallest opening. (This step is not necessary if you are using a small cold stop.)

  10. Readjust the position of the star to maximize the S1 signal, and move the guide box to the new position of the star on the acquisition T.V. It is also a good idea to fine-tune the focusing at this point as well, adjusting the focus and position of the star in an iterative manner until the maximum signal is realized. The final position of the guide box then corresponds to the `hot spot' location. You should record the value for future reference and also inform the telescope operator.

  11. Reopen the iris to its maximum setting.

4.7.2 Guide Mirror in Place

  1. Slew to a relatively bright source (V ). Insert the K band filter, even if the instrument is configured for work in the thermal infrared to provide a measurable signal above the background.

  2. Select a preamp setting appropriate for the source. Typically, setting `1' should be used for 2, Setting `10' should be used for 2 8, and setting `100' should be used for all fainter sources.

  3. Ask the telescope operator to position the Bonnette guide probe to (x,y) = (0,0).

  4. Position the rectangular guide box on the acquisition T.V. screen to the current `hot spot' location. If the `hot spot' location is not known, a good default preliminary setting is (X,Y) = (119,43) - note that these coordinates refer to those displayed in the lower left hand corner of the acquisition T.V. screen, and not to the position of the Bonnette guide probe.

  5. Center the source in the preliminary `hot spot' location.

  6. Display the S1 signal on the main oscilloscope.

  7. Ask the telescope operator to find a suitable guide star.

  8. Use the hand paddle to gradually move the guide star - the S1 signal displayed on the oscilloscope should decrease. If not, hunt around the `hot spot' location until a signal is detected. If a signal is not detected see Chapter 6.

  9. Move the guide star around until the S1 signal is maximized and then relocate the guide box to the location of the star on the acquisition T.V. (Note: if this procedure does not work you can also visual inspect the entrance field of the FTS as discussed in Section 3.1.1)

  10. Using the appropriate buttons on the Main Control panel, close the iris to its smallest opening. (This step is not necessary if you are using a small cold stop.)

  11. Readjust the position of the guide star to maximize the S1 signal, and move the guide box to the new position of the star on the acquisition T.V. It is also a good idea to fine-tune the focusing at this point as well, adjusting the focus and position of the star in an iterative manner until the maximum signal is realized. Ask the telescope to reposition the Bonnette guide probe to (x,y) = (0,0) and then relocate the guide box to the new location of the source on the acquisition T.V. The final position of the guide box then corresponds to the `hot spot' location. You should record the value for future reference and also inform the telescope operator.

  12. Reopen the iris to its maximum setting.





Please send comments and suggestions to: veillet@cfht.hawaii.edu