Sky Brightness 3

As we saw in part 1 and part 2 of this series the typical measurements of sky brightness in Providence are between about 4.1 - 4.3 nelm (naked eye limiting magnitude) on clear nights. Here is a graph that shows a typical hazy summer night. The readings were taken on the night of July 1st into the morning of July 2nd of 2014 and are in the range that we commonly see. The dashed horizontal line is a somewhat arbitrary divider between typical and darker nights. When the sky brightness is below about 4.3 the observing is much better.

Looking at a graph of the sky brightness doesn't give an intuitive idea of what the sky actually looked like for observing. We can see this by looking at the wide angle views of the sky using the camera mounted on the roof. Here is a time lapse movie from the same night as the above graph.

Sky Brightness 2

In my previous post I began to analyze the data from the sky brightness meter at Ladd Observatory. Now we'll take a closer look at the broader trends. Here is a scatter plot showing the data from the summer and fall of 2013. The plot is a little busy but we're really only interested in the "bottom line" where the data points are at the lowest values. All of the nights are superimposed on one another with the x axis showing hours UTC. This graph summarizes how the sky brightness changes during the course of the night. The many values between 3.7 and 4.3 are due to nights that are more or less hazy. There moisture in the atmosphere scatters light from the city back down to us and causes the overall sky to look brighter.

If we follow the lowest readings there is a definite trend where the clearest nights start off at about 4.2 at the end of twilight and slowly, steadily, decrease to about 4.45 at 4 hours UTC. There is then a small but rather sudden drop to 4.55 after which the slow decrease continues until we are at about 4.6 in the early morning. I'm not sure what is causing the drop at 4 hours but it may be due to city lights that are on a timer. The takeaway here is that the sky is slightly, but significantly, brighter in the early evening. The best time to observe is after midnight local time through the early morning.

Sky Brightness

"The sky above the port was the color of television, tuned to a dead channel."
- Neuromancer, William Gibson, 1984.

At the Ladd Observatory we operate a weather station and a number of other rooftop instruments to monitor the environment. One of them is a sky brightness meter. On a regular basis we use the live data to judge the quality of the sky for observing. It is also used to document long term changes such as the increase in light pollution.

Sky brightness meter
and camera on the roof.

The meter is contained in a weather proof housing next to a wide field sky camera. The camera takes a low resolution image of nearly the entire sky every 10 seconds and these images can then be compared to the brightness readings. I can then verify what the sky looked like when a measurement was taken. When the sky is very cloudy it scatters light from the city and the readings are very bright. Haze or high humidity can also cause elevated readings.

The sensor is too sensitive to take a measurement during the daytime. It starts collecting data shortly after sunset when the sky begins to darken and stops during morning twilight just before sunrise. Last summer I calibrated the meter and we've now collected 300,000 data points in about one year. I thought this would be a good time to analyze what we have so far.

As the Bubbl Bursts

The invention of magnetic bubble memory was once seen as a revolutionary computer development - the wave of the future. It is now a nearly forgotten technology.

"Many persons expect that the most dramatic changes in digital systems will result from magnetic-bubble chips that could well hold a million or more bits in the not-too-distant future. Along with charge-coupled devices, these memories show promise of replacing magnetic tape and disks for small systems." [emphasis in the original]

- Understanding Digital Electronics, Texas Instruments, 1978

One of the more unusual computer objects that I've collected over the years uses this memory. It is called the QSB-11A Bubbl-Board. I was told by the person that sold it to me that it had been used in a system at Los Alamos National Laboratory. I have no idea how it was used. Given the nuclear research conducted there I sometimes wonder if I should check to see if it is "hot."

W9GYR

As a young child I can remember my late grandfather operating a ham radio station in Chicago using surplus military equipment that he obtained at the end of World War II. When I received my own amateur radio license about a year ago I began to wonder when he first became involved in the hobby. I suspected that he started before the war so I started to dig through old FCC publications which listed newly issued licenses. I couldn't find a single mention of his name or the call sign that he was assigned: W9GYR.

My first clue to narrow down the search was a website called Old QSL Cards which has a large collection of the postcards that amateur operators send each other to confirm that they had made a radio contact. QSL is early radiotelegraph shorthand for "I am acknowledging receipt" of a wireless message. They had a card from my grandfather that was dated 1939.

QSL card from my granfather from Jan. 26, 1939.
Scan courtesy of Old QSL Cards.