EDUCATION: URBANMAAP

Aerosols Study: Sunphotometer Alignment

Practice making sunphotometer measurements prior to the IOP.

Procedure:

1. Pick a day when the sun casts a readily discernible shadow. Pick a location where you have an unrestricted view of the sun. Connect a digital voltmeter to the output jacks of the sunphotometer and turn the voltmeter on. Select the DC volts setting for the voltmeter.



2. Switch on your sunphotometer. Without pointing your sunphotometer at the sun, note the voltage reading. It should be about 13 or 14 mV.



3. There is a small round hole on one end of your sunphotometer. Point this end at the sun. When you do this, the sun shines through the straw in the front alignment bracket and makes a small spot on the paper attached to the rear alignment bracket. Adjust the pointing of your sunphotometer until the sunlight spot shines on the paper face of the rear alignment bracket. Watch the voltage output and continue to adjust the pointing until the voltage is maximum.



4. When you have found the position where the voltage is maximum, this means that the sun shining through the hole in the case is centered on the LED detector inside. Lightly mark a dot in the center of the sunlight spot on the rear alignment bracket. The alignment for the maximum voltage will be easier to determine if the sky is clear near the sun. The sun may cast an obvious shadow even when it is partially obscured by thin clouds, but the voltage output from your sunphotometer will vary from moment to moment. This will make the position of the maximum voltage more difficult to determine. If the sky near the sun is not completely clear, do the best you can to determine where to put the pencil mark. The dot will not necessarily be in the middle of the rear alignment bracket. This is OK.



5. Move the sunphotometer away from the sun and then toward the sun again, using the dot you marked previously as a guide. If the position you have marked is approximately correct, the maximum voltage should be observed with only minor adjustments to the pointing direction. If you believe the position of the dot needs to be changed, erase it and mark it again. Note that the position of this dot will always be JUST an approximate guide to the pointing of your sunphotometer to get the maximum voltage when you make a measurement.

After you have used your sunphotometer several times to make measurements, as described in the measurement protocols, you will have a better sense of whether the dot is in the right place. If it is not, repeat this activity and move the dot as required. If the dot appears to be in the proper place, go over the pencil dot with a pen or permanent marker.

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