Twilight monitoring using the SQM

There may be cases where it is desirable to measure light levels in the transition between night and day. The SQM products are very sensitive to night light, but will saturate in the daylight.

Our meters are mostly used to give an idea of sky brightness at the zenith and are too sensitive to give defined readings once the Sun has risen. For this reason, we have designed an adopter to hold a Neutral Density (ND) filter to subdue the incoming light. ND filters are fairly inexpensive and are supplied at many astronomy equipment dealers.

We have a live unfiltered SQM-LE operation in Grimsby Ontario, Canada (time zone = EDT). You can see the plot here. The plot is cropped at 16mpsas. Valid readings continue up to about 8mpsas and more detail about twilight could be extracted from there. You will be able to see the available resolution that an unfiltered SQM-L series meter provides around dawn/dusk.

Uses

• Aid in the study of Hilal visibility (crescent moon). See report here by Laoucet layari@comcast.net .
• Study bird songs timing at dawn:
  Artificial Night Lighting Affects Dawn Song, Extra-Pair Siring Success, and Lay Date in Songbirds
• Report uses SQM to assist in determining bird chorus timing due to other effects. By Diego Gil et al. Birds advance their dawn chorus near airports

Concerns

• Light levels fluctuate wildly during dawn/dusk because of all the reflections and gradient light source. You may have to take many readings. Our meters with the built-in lens are best suited to more detailed readings because of their ability to point to a more focused area.
• The brightness of the sky also depends largely on the cloud cover and other obstacles. The ground brightness or darkness will affect the reading if the meter is pointed close to the horizon.
• Depending on your application, a filter may not be necessary to gather valuable readings before the meter saturates in bright sunlight.

SQM Resources

• Filter adaptor ordering page.
• SQM-L filter adaptor photos.
• SQM-LU-DL-FA PDF filter adaptor document
• Turtle studies forum posting

External resources

• Wikipedia entry on Twilight
• Radiometry and photometry in astronomy By Paul Schlyter