Can A Digital Camera Measure Skyglow

Using a Digital Camera to Measure Skyglow

Abstract

This is a great projection for someone interested in both stargazing and photography. Vivid city lights and even the light of the full Moon obscure the dimmest stars, which tin can make identifying constellations more hard. In this astronomy scientific discipline projection, you will calibrate a digital camera to measure the skyglow in different locations. This can exist a great tool to comparison the quality of dissimilar star viewing locations.

Summary

Long (2-four weeks)

You should accept already taken an algebra class and sympathize what a part is.

A digital camera with full manual command is essential for this projection. If you cannot manually command the ISO, shutter speed, focus, and lens aperture of your camera, and then your photographic camera volition non piece of work for this project. See the Materials and Equipment list for details.

Very Depression (nether $20)

Take an adult with you when you lot accept skyglow photos at night.

Terik Daly and Andrew Olson, PhD, Scientific discipline Buddies

This project is based on an article in Sky & Telescope magazine: Flemish region, T., 2006. "Measuring Skyglow with Digital Cameras," Sky & Telescope, Feb, 2006: 99–104.

Objective

In this astronomy project, you will calibrate the prototype sensor in your digital camera. Then, you volition use your camera to compare the amounts of skyglow in different locations.

Introduction

If y'all've always seen the clear dark heaven out in the country or away from city lights, you know that many more stars tin exist seen under these darker conditions. In urban and suburban areas, however, skyglow—which is scattered low-cal in the atmosphere—obscures dimmer stars (see Figure 1, beneath).

In urban areas, calorie-free pollution created past bogus calorie-free sources is the biggest correspondent to skyglow. Natural sources, such as zodiacal light and airglow, also contribute. Zodiacal low-cal is created every bit dust in space scatters sunlight. It is typically very faint and appears as a glow near the horizon. Airglow consists of light emitted in Globe's upper temper as a upshot of certain chemical reactions. It is also very faint. These natural sources are the chief source of skyglow outside of urban areas.

Professional astronomers employ sophisticated tools to precisely measure out skyglow. Experienced stargazers can judge the corporeality of skyglow past noting what features of the night sky they tin can and cannot see. Just, there is also some other way to measure out skyglow. If you ain a digital camera with full manual control, you lot can brand your very ain tool for measuring skyglow. While yous will not be able to measure absolute brightness the way more than sophisticated equipment tin can, you will be able to mensurate the relative brightness of skyglow in different locations.

The first stride toward using your digital photographic camera to measure skyglow is to calibrate the paradigm sensor in your camera. The image sensor consists of pixels laid out like the squares of a checkerboard. Each pixel in the epitome sensor converts the lite that falls on the pixel into an electrical bespeak. Software in the photographic camera somewhen converts this electrical signal into a pixel value between 0 and 255. For grayscale images, a pixel value of 0 corresponds to black; a pixel value of 255 is white. Values between 0 and 255 are various shades of gray. The picture you see on your computer screen or digital camera brandish is the checkerboard of pixels on the prototype sensor, where the color of each pixel is adamant past the pixel value.

To calibrate the image sensor in your camera, you will expose the sensor to dissimilar amounts of lite and measure the average pixel value that results. A few features of your camera can affect the paradigm sensor'due south response to light, including discontinuity, ISO, and shutter speed. Shutter speed, sometimes called exposure time, is the amount of fourth dimension the paradigm sensor is exposed to light. In this project, you volition vary the amount of calorie-free hitting the image sensor by changing the exposure time. You volition non be able to measure the exact amount of light that hits your camera's image sensor, but you will be able to compare the relative amounts of calorie-free in different images.

In an platonic world, the relationship betwixt the corporeality of light striking the image sensor and boilerplate pixel value would be a linear part. For example, if twice as much light hit the paradigm sensor (i.e., the exposure time were twice as long), the average pixel value would be twice equally high. For an idealized sensor, we can write a linear equation (the "equation of a line") that relates the amount of low-cal hit the image sensor to the boilerplate pixel value, P:

Equation one:

• P is the boilerplate pixel value (dimensionless).
• Southward is the sensitivity constant (i/sec).
• t is the exposure time (sec).
• D is the detection limit (dimensionless).

The sensitivity constant, Southward, describes how much the average pixel value increases with increasing exposure time. The detection limit, D, relates to how much low-cal has to hit the image sensor before the sensor tin create a meaningful paradigm. The detection limit is sometimes called the "noise flooring". For example, if the detection limit of an paradigm sensor were a pixel value of 10, and a photograph with a 1/one,000 sec exposure gave you an average pixel value of 10, then a one/4,000 sec exposure would also give an average pixel value of about 10. The amount of lite in both cases is so modest that random electrical dissonance in the image sensor would overwhelm the tiny signal generated past the small amount of light that fell on the image sensor.

A similar situation also happens when too much light hits the image sensor. The prototype sensor is saturated when increasing the exposure time does not increment the average pixel value. For example, if the average pixel value in an prototype taken with a two sec exposure were 255 (the highest pixel value possible), then the sensor is saturated. No matter how much longer yous expose the sensor to light, the average pixel value will e'er exist 255.

The sensitivity constant, South, and detection limit, D, vary from image sensor to image sensor, camera to photographic camera, and with photographic camera settings. So, every epitome sensor has to exist tested to find out what S and D are for that particular sensor.

Unfortunately, the relationship between exposure time and boilerplate pixel value normally is not linear in images with pixel values that range from 0 to 255. Instead, the human relationship is non-linear. This is often due to the software inside the camera that converts the electrical signals from the image sensor into pixel values. For a not-linear response, we have to make a change to Equation 1:

Equation 2:

• P is the average pixel value (dimensionless).
• South(t) is the sensitivity function (1/sec).
• t is the exposure time (sec).
• D is the detection limit (dimensionless).

Now, instead of a sensitivity constant, S, we have a sensitivity function, S(t) that depends on exposure time. And, the part S(t) changes depending on whether you are about the detection limit of the image sensor, near the saturation limit of the detector, or somewhere in between. The purpose of calibration is to figure out S(t). Because S(t) can be a complicated role, nosotros will not effort to figure out the exact equation; instead, we will make a calibration curve. A scale curve is a graphical representation of the relationship shown in Equation ii, but we tin can use it without having to actually discover the part S(t) analytically. Once you make your scale curve, you will be able to compare the skyglow at unlike locations (or at different times in the aforementioned location).

Calibration is an essential step for any new telescope, camera, or spectrometer that will be used for enquiry. Calibration for image sensors on spacecraft missions, for example, lasts months or even years and costs millions of dollars. Calibration for such instruments takes such a long time (and costs so much coin) considering those scientists desire an absolute calibration. They want to know exactly how the sensor responds to specific numbers of photons. For this project, you volition exercise a relative calibration. You lot want to investigate how skyglow compares in different locations, but it is non of import to know precisely how much light is in each location.

Terms and Concepts

• Skyglow
• Low-cal pollution
• Image sensor
• Pixels
• Pixel value
• Calibration
• Shutter speed
• Exposure time
• Linear office
• Sensitivity constant
• Detection limit
• Saturation
• Non-linear part
• Sensitivity function
• Calibration curve
• Absolute calibration
• Relative calibration

Questions

• What are the sources of skyglow?
• When and where is skyglow particularly loftier?
• How does the image sensor in a digital photographic camera work?
• What settings on a digital camera command camera exposure? How do they work?
• What is the difference between an absolute and relative calibration?
• What are the benefits and drawbacks of absolute and relative calibrations, respectively?

Bibliography

This project is based on an article in Sky & Telescope magazine:

• Flemish region, T. (2006). "Measuring Skyglow with Digital Cameras," Sky & Telescope, February, 2006: 99–104.
• Cizano, F. (2000). The Nighttime Heaven in the World. Retrieved Baronial 25, 2014.
• Cambridge in Colour. (north.d.). Photographic camera Exposure. Retrieved August 25, 2014.
• Nice, Chiliad., Wilson, T. Five., and Gurevich, G. (n.d.). How Digital Cameras Work. Retrieved August 25, 2014.

Materials and Equipment

• Digital photographic camera that tin can take reasonably long (15–30 sec) exposures under total transmission control
  • Note: Full command over shutter speed, aperture, focus, and ISO is essential to this project. If you cannot manually control these four elements on your photographic camera, your camera volition non work for this project.
• Computer with cyberspace access
• ImageJ, a free software programme for analyzing images; (available for download at http://imagej.nih.gov/ij/download.html)
• Multiple locations from which to take photos of night heaven
• Camera tripod or a sturdy box or table on which to rest the camera to go along information technology still as you take pictures
• Towel or rag; if you do non have a tripod, you lot can apply this to protect your camera when information technology is on the ground
• Sail of white newspaper
• Lab notebook
• Optional: Masking tape
• Optional: Semi-log graph paper. Graph paper can exist downloaded and printed from Printable Newspaper.

Experimental Procedure

Calibrating Your Digital Camera

1. Prepare up a piece of white newspaper so that it is uniformly illuminated by indirect sunlight. Choose a place where the light is brilliant, only you should non see whatsoever shadows on the newspaper. Taping the paper on the wall side by side to a large window during the mean solar day would work well, for example. Yous could also set up the paper in the shade of a edifice outside. The almost important thing is that the paper is illuminated evenly, with no bright spots or shadows. Cull a time and place during which information technology is unlikely there will be whatsoever changes in lighting, such equally the Sun passing behind clouds.
2. Position your digital photographic camera and so that the white paper fills the entire field of view. The precise distance from the camera to the paper is not critical.
   1. Tip:Mountain the camera on a tripod, if yous have one. Otherwise, place the photographic camera on a solid support, such as a sturdy box or table. Many of the exposures will be besides long for you to concur the camera steady in your hands.
3. Put the photographic camera in manual mode and brand the following adjustments:
   1. Manually conform the focus so that the photographic camera is focused on the white slice of paper. Once you ready the focus, practise non alter it during the calibration.
   2. Set the camera'due south sensitivity to ISO 200.
   3. Set the aperture to f/2.eight.
   4. Set the prototype resolution to a depression setting (e.g., 640 × 480).
   5. If your camera has a cocky-timer feature, set information technology then that the camera shutter opens a few seconds after you printing the button to take a picture. This minimizes camera shake.
4. Now that your photographic camera is focused on the white piece of paper, with the settings adapted correctly, you are gear up to take calibration photos. Have a serial of photos at different shutter speeds, varying past a factor of 2 each time. Employ 30, 15, eight, 4, 2, 1, 1/ii, 1/4, ane/viii, 1/15, one/30, i/lx, 1/125, one/250, one/500, and 1/one,000 sec shutter speeds.
   1. Tip: Brand sure that shutter speed (exposure time) is the only thing that changes between pictures. All other photographic camera settings and the position of the camera should stay the aforementioned.
5. Echo step 4. This will give you ii complete series of calibration photos. Y'all will compare the two sets of photos after in this procedure.
6. Follow your camera'due south instructions to download all of the calibration photos onto your calculator.
7. Make 2 information tables in your lab notebook to keep track of the pixel value data for each scale photograph. One data tabular array will be for the first set of calibration photos. The other data table will be for the second set up of scale photos. Ready each data table as shown below.
   1. Tip:The photo browsing software on your computer volition tell yous the exposure time or shutter speed for each prototype file. Call up, exposure fourth dimension and shutter speed are the same affair.

8. Measure the average pixel intensity of each photo using ImageJ, a costless scientific paradigm assay programme.
   1. Yous can download ImageJ here: http://imagej.nih.gov/ij/download.html.
   2. First ImageJ. You volition see a pocket-size window similar to the one in Figure two, below. The colored 'skin' may await slightly different on your system. This window is the ImageJ bill of fare bar.
      
   3. Open the beginning calibration photograph using the "File/Open..." card command.
   4. Click on "Clarify" and select "Histogram" from the driblet-downwards menu.
   5. A histogram of the pixel values in the photo will open in its own window, like to Effigy iii, below. You will utilize this histogram to measure the average pixel greyness value in each image.
      
   6. Record the Hateful, StDev, Min, Max, and Mode in your data tables. StDev is short for "standard deviation". Min and Max are brusque for minimum and maximum, respectively. Mean is another name for the average. The mean of this histogram is the boilerplate pixel value.
   7. Click on "File" and select "Open Next" to open the next prototype file. Yous tin can besides employ the keyboard shortcut Ctrl+O to do this.
   8. Repeat steps 3.d.–iii.thou. until you have analyzed all images in both sets of calibration photos.
9. Brand a calibration curve by graphing the average pixel value (the hateful of the histogram) on the x-axis and exposure time (in seconds) on the y-axis. Employ a logarithmic scale for the x-axis and a normal (linear) scale for the y-axis. Your graph will have two information series, 1 for each set up of calibration photos. Choose different colors or symbols for each series.
   1. This kind of graph is called a "semi-log" plot. You tin can make a semi-log plot by using semi-log graph newspaper or by formatting the ten-axis in your graphing software.
   2. It is helpful to use a logarithmic calibration for the x-axis because exposure times vary by a factor of more than ten,000. If you lot made this graph using a normal, linear 10-centrality, it would exist difficult to see the calibration curve clearly.
10. Wait at your graph and compare the calibration curves from each set of photos. The two curves should overlap or merely be slightly separated. If there is a lot of space between the 2 calibration curves or if one of the curves has a very different shape from the other, y'all will need to repeat steps 1–9, making certain that the lighting conditions are the aforementioned for both sets of photos.

Taking Skyglow Photos

1. At present that you accept finished calibration, you are ready to measure skyglow. Identify iii or four places where you would similar to mensurate skyglow. Choose places you recollect volition have different amounts of skyglow. Write a hypothesis nigh which site yous call back will accept the darkest and brightest skyglow, respectively.
   1. Keep in mind that you will demand to visit these places at night. Plan to take an adult with you for safety.
   2. If you lot will be out photographing under cold weather conditions, check out these 10 tips for cold atmospheric condition photography before you head out.
   3. Plan to measure skyglow at the same time of night in each location you lot visit. If your locations are far from each other, you volition need to visit different locations on dissimilar nights.
   4. The presence or absence of clouds can cause variations in your measurements. Check the weather forecast and merely make measurements on nights with clear skies.
2. Travel to your first location. Don't forget your camera, tripod or sturdy box or table, towel or rag, lab notebook, and something to write with. Your get-go location must exist the ane that yous predict will take the brightest sky then that you tin can gear up exposure time appropriately. Cull a clear night so that clouds do not bear on your measurements.
3. In your lab notebook, write down the address of your start location. You could also include GPS coordinates, if available (you can look up the GPS coordinates with various online tools or mapping software). Write a brief description of the location in your lab notebook. What does it look like? Is it in a city or out in the state? Are there lots of street lights? What is the weather similar?
4. Ready your camera to take skyglow photos. Make sure the camera is in full transmission mode. It is essential that y'all use the exact aforementioned photographic camera settings at each site you visit. With the exception of focus and exposure fourth dimension, these are the same settings you used for scale photos.
   1. Manually arrange the focus and then that the focal length is infinity (or as long as your lens will allow).
   2. Set the camera'due south sensitivity to ISO 200.
   3. Set the aperture to f/2.8.
   4. Set the image resolution to a low setting (due east.g., 640 × 480).
   5. Set the shutter speed (exposure time) to xxx sec.
   6. If your camera has a self-timer feature, ready it so that the photographic camera shutter opens a few seconds after you press the push button to take a picture. This minimizes camera shake.
   7. Lay your towel or rag on the ground, and so lay the photographic camera downwards on information technology, with the lens pointing toward the sky. If yous have a tripod, y'all can mount the camera on the tripod and so point the photographic camera toward the sky.
5. Double-check that your camera'due south field of view does non include the Moon, street lamps, or house lights. If whatsoever of these things are in your photographic camera's field of view, reposition the photographic camera and so that only the night sky fills the field of view.
6. Take skyglow photos.
   1. If large parts of the photo with the 30 sec exposure time appear white, the image is saturated. Decrease the exposure fourth dimension until simply the stars are saturated. Considering the beginning location is the place you predict will accept the brightest skyglow, if the images from the first location are non saturated, and then images from your other sites will besides not be saturated.
   2. If something unusual happens, such equally a car'southward headlights shine on you lot, retake the skyglow photo.
7. Repeat steps 2–5 for each of the remaining locations where y'all plan to measure skyglow. Remember to always use the same exposure time that y'all used at your starting time location, fifty-fifty if the photos appear very night.

Using Your Calibration to Measure Skyglow

1. Use the ImageJ software to measure out the boilerplate pixel value in each skyglow image past following the process in step 8 of the Calibrate Your Digital Photographic camera section. Make a information tabular array, like the i beneath, in your lab notebook, and tape the mean, standard difference, minimum, maximum, and mode of each pixel value histogram.

2. Because all of your skyglow images were taken with the aforementioned camera settings and exposure time, you can utilize the calibration bend to determine an "equivalent exposure fourth dimension" (EET) for each skyglow photograph. The EET is how long the exposure time would have to have been nether calibration conditions to achieve the aforementioned average pixel value equally measured in the skyglow photo. Effigy four, below, shows how this process works.

3. Convert the boilerplate pixel value in each skyglow image to an EET using the process described in the explanation to Figure 4, above. Record the EET for each prototype in your lab notebook.
4. By converting the boilerplate pixel values of each skyglow epitome into an EET, you lot can determine how much brighter or darker i location is compared to another.
5. Decide which of your skyglow locations had the smallest EET. This is the location with the darkest skyglow.
   1. Divide the EET of all other skyglow locations past the EET at the darkest location. Write the resulting values in your lab notebook. These numbers are the ratios between the EET at the two sites. The ratio of EETs is the ratio of the heaven brightness of the ii sites. For example, if the darkest skyglow site had an equivalent exposure fourth dimension of 0.003 and a second site had an EET of 0.03, then the skyglow at the 2nd site is 0.03/0.003 = 10 times brighter than the darkest location.
   2. Notation: This relative brightness comparison only works if you are comparing photos taken with the same exposure time.
6. Which of your locations had the darkest skyglow? How much brighter was the skyglow in the brightest location yous photographed? Did these locations match with your hypothesis?

Ask an Expert

Do you have specific questions almost your science project? Our team of volunteer scientists can aid. Our Experts won't exercise the work for you, merely they will make suggestions, offer guidance, and assistance you troubleshoot.

Variations

• For a more than bones experiment on light pollution, encounter the Scientific discipline Buddies project Where Did All the Stars Get?
• In add-on to comparison skyglow at different places at the aforementioned time of night, you can also effort comparing:
  • Skyglow in the same location on both cloudy and clear nights.
  • Skyglow in the same location at dissimilar times of the dark (east.m., shortly later sunset, midnight, pre-dawn).
  • Skyglow in the same location during unlike seasonal weather condition (e.g., snow on the basis vs. non, leaves on copse vs. not, Galaxy visible vs. not).
  • Skyglow in the same location every bit the phases of the Moon change.
• For more than-avant-garde students, "Starlight, airglow, scattered moonlight and various kinds of artificial lights take different spectral signatures. It should be possible to tease out a huge amount of information almost those by comparing the readings of the carmine, green, and bluish pixels. Anybody interested in the challenge?" (Flemish region, 2006). Can yous use spectral data from your camera (i.east., red, green, and blueish values) to identify the source of the skyglow?

Careers

If you like this project, you might enjoy exploring these related careers:

Career Profile

Astronomers remember large! They want to understand the unabridged universe—the nature of the Dominicus, Moon, planets, stars, galaxies, and everything in between. An astronomer's work tin can be pure science—gathering and analyzing information from instruments and creating theories nigh the nature of cosmic objects—or the work tin be applied to practical bug in space flight and navigation, or satellite communications. Read more

Career Contour

Lights, camera, activity! Information technology takes more than than actors to put an exciting motion picture movie together. It takes film and video editors and cut-border computer technology to brand an exciting pic that people want to see. From the many thousands of minutes of movie and video, the editor has to choose the best shots to tell a cohesive and compelling story. In improver to having an artistic bent, the film and video editor must relish working with complicated estimator equipment. If you would like to… Read more

Career Profile

Statisticians use the ability of math and probability theory to answer questions that touch on the lives of millions of people. They tell educators which instruction method works best, tell policy-makers what levels of pesticides are acceptable in fresh fruit, tell doctors which treatment works best, and tell builders which type of paint is the nearly durable. They are employed in virtually every type of industry imaginable, from engineering, manufacturing, and medicine to animal science, nutrient… Read more than

Career Profile

Physicists have a big goal in mind—to understand the nature of the entire universe and everything in it! To achieve that goal, they observe and measure out natural events seen on Earth and in the universe, and so develop theories, using mathematics, to explain why those phenomena occur. Physicists take on the challenge of explaining events that happen on the grandest scale imaginable to those that happen at the level of the smallest diminutive particles. Their theories are then applied to… Read more

Related Links

• Scientific discipline Fair Project Guide
• Other Ideas Like This
• Astronomy Projection Ideas
• My Favorites

News Feed on This Topic

, ,

Annotation: A computerized matching algorithm suggests the above manufactures. It's non as smart as yous are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more near the News Feed

Cite This Page

Full general commendation data is provided hither. Be certain to check the formatting, including capitalization, for the method you are using and update your citation, every bit needed.

MLA Style

Scientific discipline Buddies Staff. "Using a Digital Camera to Measure out Skyglow." Science Buddies, xx Nov. 2020, https://www.sciencebuddies.org/scientific discipline-fair-projects/project-ideas/Astro_p022/astronomy/using-a-digital-photographic camera-to-measure-skyglow. Accessed 26 May 2022.

APA Style

Science Buddies Staff. (2020, November xx). Using a Digital Camera to Mensurate Skyglow. Retrieved from https://www.sciencebuddies.org/scientific discipline-fair-projects/project-ideas/Astro_p022/astronomy/using-a-digital-camera-to-measure-skyglow

Last edit date: 2020-11-twenty

Explore Our Science Videos

Cricket Wicket Knockdown: 2020 Fluor Engineering Claiming

Make an LED Lightsaber with a Straw

Build a Motorcar to Lift Water - STEM Lesson Plan

Costless scientific discipline fair projects.

Source: https://www.sciencebuddies.org/science-fair-projects/project-ideas/Astro_p022/astronomy/using-a-digital-camera-to-measure-skyglow

Posted by: beaudrydonsfult.blogspot.com

Share this post