Research

Impact of Space Weather On The Natural Night Sky

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SQM – LU – DL Testing and Data Reduction

A. D. Grauer

Preliminary Report of July 29, 2018

Summary

This is a preliminary project report to measure the natural night sky being conducted by A. D. Grauer and P.A. Grauer at Cosmic Campground International Dark Sky Sanctuary (CCIDSS) in New Mexico and Nayalini Davies and Gareth Davies at Aotea/Great Barrier Island International Dark Sky Sanctuary (AGBIIDSS) in New Zealand. All tests indicate that the Unihedron SQM-LU-DL produces stable, reliable, scientific measurements. The purpose of the data reduction program is to extract the SQM-LU-DL measurements made during astronomical dark under a cloud free sky when the moon was not above the horizon and to tag each measurement with the place in the sky from which it was obtained. Results will be placed into a data base to track the brightness of the natural night sky over time and to calibrate contributions of the Milky Way and other natural sources.

Equipment

The measurements are made using a SQM-LU-DL narrow field of view sky quality meter manufactured by Unihedron.com in a weather proof housing.

The equipment pictured above, in its testing phase in Silver City, NM, was installed at the CCIDSS on 27 July 2018 under a Special Use Permit from the Gila National Forest, Glenwood Ranger District. The SQM housing and solar power unit are near the top of the pole to prevent the incoming data being shadowed by the mounting pole. (Hopefully, the pole will mitigate lightning.)

Testing and Calibration

The SQM-LU-DL Serial Number 4074 to be installed at the CCIDSS was calibrated at Unihedron on 2018-04-23 and had a light calibration offset of 19.94 magnitudes per arc second squared (mpsas). To further establish its accuracy Anthony Tekatch of Unihedron, extensively retested this unit July 2018. The first calibration light offset was 0.01 magnitudes brighter than in April, however, the subsequent July retesting repeatedly produced exactly the same light calibration offset of 19.94 magnitudes per arc second squared (mpsas) previously obtained. These tests indicate that the instrument's zero point is very stable. To further test this unit, Tekatch placed it in Unihedron's dark room illuminated by a test lamp placed at a low setting. The results are plotted below:

This test has 6272 data points. Tekatch reports spikes 21.45 (brightest) and 21.49 (darkest) (currently unexplained), but that it is quite difficult for him to exclude all possible extraneous light sources. For more than 6 hours the readings were stable between 21.47 and 21.48 mpsas. This stability has been confirmed by measurements made on the night sky as will be shown later in this report. All of our tests indicate that the SQM-LU-DL produces stable reliable scientific measurements at or below it's resolution element of 0.01 magnitudes.

The SQM-LU-DL is an accurate piece of scientific equipment and comes with a complete set of software tools.

Data Reduction Program 

The purpose of the data reduction program is to extract the SQM-LU-DL measurements taken during astronomical dark, under a cloud free sky when the moon is below the horizon and to tag each measurement with the place in the sky from which it was obtained. Results will be placed into a data base to track the brightness of the natural night sky over time and to calibrate the contributions of the Milky Way and other natural sources to it.

The SQM-LU-DL's capacity of over a million readings allows it to take data, unattended, for a month or more. In our series of experiments it was set to take a measurement every 5 minutes when the sky is fainter than 10 mpsas in evening twilight until dawn twilight. The instrument produces a nightly succession of measurements until the data are read out into a laptop. A typical data set consists of a header and a succession of adjoined nights (see plot above). The program written in FORTRAN is called SQM_Data_Read.f. It reads in the data line by line and places the values into program arrays. Next, it calculates the Julian Date (JD) and the Local Sidereal Time (LST) for each sky measurement. Then the program determines the start and end of each each night and uses this information to find the beginning and end of astronomical twilight for each night. 

The SQM-LU-DL has a 20 degree full width at half maximum field of view. 

As the Earth rotates, pointed directly upward, it scans the Sky in a strip.

Next, the LST calculated for each measurement is used to assign a number from 1 to 48. These numbered positions in the sky are indicated on the following map by Dominic Ford:

The three plots, below, are of a night with no clouds, a completely cloudy night, and a night with a trace of clouds. 

On all of the plots the red line is the SQM data, green line is the temperature, blue line the solar battery voltage, tan line solar altitude, and black line the lunar altitude. 

On all of the plots the red line is the SQM data, green line is the temperature, blue line the solar battery voltage, tan line solar altitude, and black line the lunar altitude. 

On all of the plots the red line is the SQM data, green line is the temperature, blue line the solar battery voltage, tan line solar altitude, and black line the lunar altitude. 

Using the visual impression, on a clear night, the SQM data are very smooth and short segments can be represented by straight lines, the program then determines if clouds appear to be influencing the data as follows.

For each data point from evening to morning astronomical twilight the program selects points on either side of the point in question and fits a straight line to the SQM data versus time data. In the examples included in this report, a total of 11 points are selected for each line segment, 5 ahead and 5 behind in time of the central point. Each line segment is made of 11 data points obtained over a 55 minute time period. These parameters can be changed but this set seems to work to identify cloudy periods of time. A least squares routine calculates a chi squared in mpsas for each fitted line segment. Thanks to the repeatability and accuracy of the SQM data cloudy periods can be eliminated by rejecting all fitted line segments with a chi squared greater than 0.015 magnitudes. Foggy nights and those completely overcast may be rejected using the SQM Values. Weather information provides reality checks.

The plot above, is the best night, of a 7 night run, 2 miles from the center of Silver City, NM which is at an altitude of 6,000 feet above sea level.

Below is spread sheet of these data:

From left to right the columns are the Julian Date, LST, cloud flag (0=no clouds), Moon (0 if no Moon – calculation yet to be programmed), Sun (0= astronomical twilight), place in the sky (1-48), site # (1 = CCIDSS, 2 = AGBIIDSS, 3 = Silver City), battery voltage, SQM reading, record type. The accuracy of the SQM-LU-DL is readily seen. The average chi squared for the line segments is 0.000839 magnitudes. At 2 miles from the center of Silver City, NM at an altitude of 6,000 feet there appears to be about 0.35 mpsas of light pollution.

Calculating when the Moon is above the horizon is a work in process. The moonrise/set is a challenging issue since either Moon rise or set can occur the day before, day after or not at all.

The accuracy and repeatability of the SQM-LU-DL is impressive.