4.7 Observing an Object on the Sky

4.7.1 Guide Pellicle in Place

Select the appropriate pair of filters.
Select an appropriate preamp gain setting and modulation frequency based on the brightness of the source. Typically, the preamp gain 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. Use an internal modulation frequency of 300 Hz for bright objects, 150 Hz for objects of intermediate brightness, and 20 Hz for very faint objects. Long integration times ( 200 ms) should also be used when the 20 Hz modulation frequency is selected.
Select an appropriate cold stop. A small aperture should be used for thermal IR observations while a large aperture should be used for bright stars and in cases of poor seeing to minimise scintilation effects.
Position the object on the hot spot.
Move target away from the hot spot until the signal level on the voltmeter drops. Turn up the `amplitude' knob on the FTS Data Acquisition Control Panel until you see a signal from the background on the voltmeter.
Adust the `offset' knob until the voltmeter reads 0.
Reposition the object in the hot spot and maximimize the signal by adjusting the position of the object. Reposition the T.V. guide box as needed. Ask the telescope operator to turn the autoguider on.
Adjust the `gain' dial until the voltmeter is reading in the 7 to 8 V range.
Readjust the `amplitude' again so that the voltmeter reads 5 V. This provides a useful voltage level for monitoring the sky conditions and should be checked frequently.
Fill out the Section 5.2.1; remember to select `move carriage' option if the carriage is still positioned at optical ZPD. An integration time should be selected which will produce an easily recognizable signal on the interferogram at ZPD; if this condition is not met it will not be possible to reduce the data.
Once the scan has been started, monitor the interferogram and power spectrum as they are displayed. Two questions which should be asked during data acquisition are (1) `Is ZPD apparent on the interferogram?', and (2) `Does the power spectrum look reasonable?'. Ripples outside of the filter boundaries may indicate that the preamp setting is set too high, so that the detectors are saturated near ZPD. It is also useful to use the Eng Menu to examine the most recent forward and reverse scans and check for possible problems such as missed steps.

4.7.2 Guide Mirror in Place

Select the appropriate pair of filters.
Select an appropriate preamp gain setting and modulation frequency based on the brightness of the source. Typically, the preamp gain 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. Use an internal modulation frequency of 300 Hz for bright objects, 150 Hz for objects of intermediate brightness, and 20 Hz for very faint objects. Long integration times ( 200 ms) should also be used when the 20 Hz modulation frequency is selected.
Select an appropriate cold stop. A small aperture should be used for thermal IR observations while a large aperture should be used for bright stars and in cases of poor seeing to minimise scintilation effects.
Ask the telescope operator to position the Bonnette guide probe to (x,y) = (0,0).
Center the source on the hot spot, after making sure that the guide box is in the proper (x,y) location you recorded while locating the hot spot.
Ask the telescope operator to find a suitable guide star.
Center the T.V. guide box on the guide star.
Select an appropriate preamp gain setting and modulation frequency based on the brightness of the source.
Move the guide star away from the guide box until the signal level on the voltmeter drops. Turn up the `amplitude' knob on the FTS Data Acquisition Control Panel until you see a signal from the background on the voltmeter.
Adust the `offset' knob until the voltmeter reads 0.
Reposition the guide star in the guide box and maximimize the signal by adjusting the position of the object. Reposition the T.V. guide box as needed. Ask the telescope operator to turn the autoguider on.
Adjust the `gain' dial until the voltmeter is reading in the 7 to 8 V range.
Readjust the `amplitude' again so that the voltmeter reads 5 V. This provides a useful voltage level for monitoring the sky conditions and should be checked frequently.
Fill out the Section 5.2.1; remember to select `move carriage' option if the carriage is still positioned at optical ZPD. An integration time should be selected which will produce an easily recognizable signal on the interferogram at ZPD; if this condition is not met it will not be possible to reduce the data.
Once the scan has been started, monitor the interferogram and power spectrum as they are displayed. Two questions which should be asked during data acquisition are (1) `Is ZPD apparent on the interferogram?', and (2) `Does the power spectrum look reasonable?'. Ripples outside of the filter boundaries may indicate that the preamp setting is set too high, so that the detectors are saturated near ZPD. It is also useful to use the Eng Menu to examine the most recent forward and reverse scans and check for possible problems such as missed steps.

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