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This activity for CliC-PoLiT students and teachers looks at faint stars in astronomical images, allowing you to measure their brightness and determine some properties of a particular telescope site. Have a look at the image below - it's of a field of stars known as a 'standard field'. Although not commonly used in the 21st century, astronomers used these standard fields to understand the behaviour of their equipment (their telescopes and cameras) and the skies that they were observing under.
This standard field is known as SA109 (SA = 'selected area') and, within this image, there are a few stars that are numbered (e.g. 954, 956) that we know the brightness of very precisely - and we know, from repeat observations, that these particular stars do not vary.
This activity allows you to look at several images taken of this same star field on different days and from different telescopes within the LCO (Las Cumbres Observatory) network to identify and measure the faintest star in each image. From those values, it may be possible to make some conclusions about the various sites. You could go further and even take more images of this star field yourselves to add even more data - alternatively, there are lots of other standard star fields that would work just as well with this method.
When we look at different astronomical images of the same object, we sometimes see that they have different numbers of stars in them. Sometimes we can make qualitative statements by comparing images but as scientists, we should aim to produce more quantitative measurements. One measurement we can make involves searching the image to determine which is the faintest star we can make out using our software (Salsa J; see below) and using this value from different images for comparison allowing us to make some hypotheses on the conditions under which the images were taken.
By measuring the faintest star you can see in the image and comparing it with the brightness of the known stars (231 or 537 are the nearest measured stars) , we can determine a value for the brightness of this faint star. This is done most easily with this spreadsheet.
You can use the free image processing program, Salsa J, to perform photometry on lots of images from LCO telescopes. Salsa J can be downloaded from here and is available free-of-charge for multiple platforms (Windows, Mac, Linux) and in several languages.
Instructions on basic image processing are provided here and further instructions show what photometry is (it's the science or art of measuring stars' brightness) and how to measure the brightness of stars.
In this Google Drive folder, you will find three files. They are ...
SA109 Test file.fits (24 MB) - this FITS file is the standard format for astronomical images. You will need a package such as Salsa J to open this file.
SA109 finder.gif (180 KB) - this is the finder chart provided as a 'map' to find your way around the star field. It's the same image as we show above.
CliC-PoLiT Magnitude Calculator (1 KB) - this is a Google Sheet which you can download to help you calculate the values that you need.
The steps for the activity are as follows ...
Open Salsa J and use the instructions on image processing to display the image so that you can see lots of stars.
Open the finder GIF and try to identify the patterns of stars (Hint: you will find your image includes stars 231 and 537).
You are now ready to perform photometry on your image. First, you need to measure a value for the number of counts (it's proportional to the number of photons hitting the camera from a particular star) for both the known star (you need to put this value into Cell B1 of the spreadsheet) and the faint star (Cell B2). You will also need to type the magnitude of the known star into Cell B8 (the values you'll need are in B16 and B17). The spreadsheet calculates the faint star's magnitude for you and displays it in Cell B10.
The value in B10 represents the magnitude of the faintest star in this particular image. Note that there are a couple of odd things about the magnitude system that astronomers use. Firstly, it's inverted so lower numbers represent bright stars and higher numbers represent faint stars. Secondly, the system is logarithmic so a magnitude 1 star is not twice as bright as a magnitude 2 star (in fact, its set to base 2.512 but that's all done for you by the spreadsheet !).
The value you have calculated represents the faintest star in your image and can be used in comparison studies. Contact us <insert contact email here> and we can share more images of this star field with you. Do you expect the faintest star to be the same brightness in each image ? If so, why ? And if not, why not ?
You may be able to think about what changes at a given telescope site. Think about the night sky from where you live - what factors do you think might affect what you can see over the course of a night. And what about over the course of a few days ? Or even a full year ?
Researchers are keen to put telescopes where conditions are best to observe the night sky but of course, there are different locations throughout the World for telescopes. For example, the three best telescope sites in the World are Chile (mountain tops within a desert), Hawai'i (tropical islands around 4000 km from mainland USA) and the Canary Islands (another island group but only 300 km off the African coast). So it's reasonable to expect different image qualities from these places and also from the telescopes in the LCO network.
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