P11: Night Sky Brightness

Observing Goals:

Your goal is to measure the sky brightness above your observatory, and investigate how it is connected to any sources of light pollution. Useful Background and References:

  • Polar Graph Paper is attached which may be helpful in keeping track of your observations.

Observation Planning:

  • Observation Time

    • You will be observing HALF a night.

  • Target Selection & Observations

    • A suggested observing strategy is to observe in each location 3 sets of observations in the B, V, R (and possibly I) filters with a telescope dither between each set so that you can

      1. combine the images so as to remove the stars and be left with just the sky background or

      2. shift and combine the images to get magnitude measurements of the stars in the field. You then repeat this around the sky.

    • You should make sure to take some measurements vey close to the Zenith, to compare to other measurement tools (such as SQMs)

    • It is also very important to observe several standard star fields at different altitudes/air-masses. These fields, with stars of known magnitudes allow you to calibrate the other images, and get accurate and meaningful measurements of the sky brightness.

    • You should also go outside(!) and we can make some panorama images (e.g. with a smart phone) around the horizon, and also map the angles and positions of significant sources of light pollution in the area.If you are observing from lauwersmeer, we will provide some panoramic images for you.

    • Also, Importantly if the Moon comes in the sky then you should also mark the path of the moon, and the time, on the images.

    • It is also worth noting the time of Astronomical Twilight, which is when the sky gets as dark as it gets, and before that time you should also take into account how illuminated the sky still can be by left over sunlight – and whether that is even important compared to the level of light pollution that we have… maybe a good idea to start your observations in the opposite direction from sunset!

Data Analysis:

  • Data Reduction Basic Steps

  • Further for this project

    • As desribed above you should produce two sets of combined images for each location

      1. median images without aligning the dithered frames, to get “blank sky” images

      2. images where you align the frames and combine them to measure brightnesses of stars in the frame

  • Further Analysis for this project

    • Using the standard star fields, and then star field images from each frame – calibrate the magnitude scale for the observations that you make

    • Remember that the stars need to be corrected per airmass for extinction – but the sky magnitudes should NOT be corrected for airmass

    • Calculate the sky brightess for your observed locations and create a map of the sky above your observing site in terms of brightness

Results:

Can you answer these questions with your work?

  • Q1: What is the aveerage sky brightess above your observing site? and how does it vary with position?

  • Q2: Can you identify sources of light pollution that contribute to the sky brightness?

  • Q3: If you measured the sky brightess on two different nights, what differences did you note, and do you have explanations?

  • Q4: You can also obstain satellite and SQM (Sky Quality Meter) measurements of brightness for the nights that you observed (ask Jake), how do those results compare to the measurements that you made with the telescope.

Presenting your results: