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Lab 2: Sky Brightness Lab 2: Sky Brightness As we discussed in class, the brightness of the night sky during an observation depends upon a number of factors. These factors include the wavelength of the observations, airmass, cloud-cover, the location and phase of the moon, and terrestrial light pollution. In this lab your objective is to directly investigate the sky brightness at Rosemary Hill Observatory using a sky quality meter during our RHO trip. As always, the lab report that you turn in should provide clear explanations of your work, and include the requested numbers, figures, and tables. Objectives: Your basic objectives for the lab are the following. 1. Quantify the dependence of sky brightness upon time after sunset. 2. Quantify the dependence of sky brightness upon zenith angle. 3. Compare the minimum sky brightness at Rosemary Hill Observatory with numbers for at least one other research observatory at a dark site (be sure to cite your reference for these numbers). Your report should include a table with the darkest sky levels at each site. 4. Discuss and attempt to explain any anomalous data. Procedure: During the trip to RHO, we will have on hand a sky quality meter. The unit is point-and- shoot, providing the sky brightness in visual magnitudes per square arcsec. The Full Width Half Maximum (FWHM) of the angular sensitivity is 20 degrees, so if you for example point at zenith the meter is sensitive to the sky brightness within ~10 degrees. We will share the meter as a class. All measurements should be made from a single location. There is a circle painted on the ground outside the RHO 30” that provides a good spot for taking these measurements. 1. The first objective listed above is to quantify the dependence of sky brightness upon time after sunset. Every 15 minutes, starting at sunset (which is at 6:03pm), two members of the class will be expected to obtain and record a series of 4 measurements of the sky brightness at zenith. Everyone should take a turn measuring the brightness at some point during the evening. We will maintain one master record of all measurements from the night, which should also include information on cloud cover at zenith at the time of each observation. 2. The second objective listed above is to quantify the dependence of sky brightness (in mag arcsec-2) upon zenith angle (angle from zenith). As some point after twilight, each person should take a turn using the sky meter to record the sky brightness as a function of angle above the horizon, starting on the western horizon and taking measurements every ~30 degrees ending on the eastern horizon. Repeat going from south to north. To preserve the directional information, label the zenith angle with both the angle and direction (e.g. 80W or 80N) as opposed to the normal definition, which is simply the angle. Note any factors that may impact the measured sky brightness in a given direction. 3. Record these observations in your notebook (again including information about weather conditions), and before you turn in the result you should include both these numbers and the numbers from step 1 in a clear, concise table in your notebook. 4. After lab, you should generate a plot that shows the mean surface brightness (with error bars) at zenith as a function of the angle of the sun below the horizon (note that you can calculate this directly from the time if you know the time at sunset). Discuss your results. Is this what you expected? Do you think that the sky would have gotten appreciably darker if we had stayed out longer. Were there any factors other than the sun angle that may be impacting your results? 5. Plot the sky brightness as a function of zenith angle (angle from zenith) for both your east-west and north-south data. Graph both sets of data on the same plot, using different data points for clarity. Start with West and South on the left hand side of the plot. Compare the sky brightnesses in the different directions. Do you notice any significant differences? If so, hypothesize on why the sky may be brighter in one direction than the other (be specific and justify your conjectures). 6. Based upon all available data, estimate the direction (angle) in which the sky is darkest. Bonus: Fit a curve to the data and overlay this curve on the plot above, recording the best-fit parameters. 7. For the darkest position, record the sky brightnesses and calculate the airmass. Compare this sky brightness with comparable data for another observatory (this data can be found online), clearly noting the source of your comparison data and being sure to compare with the appropriate filter (V). Calculate the difference in sky brightness relative to this observatory. If we were to clone the RHO 30” and place it at this observatory, how much longer/shorter would be need to observe to reach comparable signal-to-noise for an extended object like the Crab Nebula, assuming that the noise is dominated by sky brightness (i.e. dark current and read noise are zero)? The sky brightness at CTO is approximately 18.2 mag arcsec-2 in V on a good night. How much longer/shorter would be needed relative to RHO to again reach comparable signal-to-noise for the same extended object? .
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