This lab project is part of one of my 4th-year modules at the University of St. Andrews, AS4025 Observational Astrophysics. To aid in my understanding of the module and coursework, and to give non-scientists and scientists alike an idea of what a 4th year astrophysicist does, I will be attempting to explain my lab/course work in a manner which is accessible to all (which as I see it is the aim of my 'Science teaching and communication course)
The lab runs over a span of several weeks and so the post below will not be complete for a while, however I will update this post as often as I can detailing the work done towards the completion of the lab, as well as informative and interesting bits of information concerning its' content along the way.
PURPOSE
The aim of this lab is to obtain CCD images (CCD stands for 'charge coupled device' and it is used to convert incoming light into a digital signal, the same piece of kit used in your digital camera) of several galaxies in order to examine their light and colour profiles (more easily understood as their size and colour) , the central surface brightness and their scale-lengths (the distance from the centre of the galaxy where the brightness has decreased by a factor of e, or 2.718.....). All of this research to be conducted using the James Gregory telescope operated by the University of St. Andrews.
You may be asking why in the world we need to know these quantities as they seem rather boring and not very useful. You have to remind yourself that all of the information from galaxies arrives at our telescope, and ultimately in our data logs, as electromagnetic radiation. The same form of radiation that carries our radio signals, visible light, x-rays, microwaves et al. So astronomers have to be very good at drawing conclusions based solely on incoming electromagnetic radiation, here after referred to as 'EM rad.'.
When we observe one of these galaxies, and gather information about the visible light being emitted from it, we can draw some conclusions about the structural properties of the galaxy. By simply looking at the incoming light (Light Profile) we can define the size of our galaxy. How bright the centre of the galaxy is (Central surface brightness) and how quickly that brightness fades (Scale length) as you move away from the centre both allow us to estimate how many stars are in certain regions of the galaxy, and how quickly that density of stars falls away as you move towards the boundaries of the galaxy. The type of light (Colour profile) we receive can tell us what size or type of star dominate a region.
Think of the stars as burning candles, more light per unit area means more candles per unit area. And stars, just like candles, burn in different colours depending on the temperature. So more blue light means very hot high-mass stars, while more red light means more cool low-mass stars. For more on stellar classification see here
Selecting Galaxies
So we now know why were are imaging galaxies, but which galaxies do we image? This lab is designed to permit the imaging of 2-3 galaxies and subsequent calculations over a month long period but there are upwards of 170 billion galaxies in the known universe -- which do we pick?
Location --
Thankfully there are a few limiting factors in galaxy selection so that we may narrow this extremely large pool down to a few hundred candidates. One of the most important limiting factors is the galaxies location in the sky. If you are working from an observatory in the northern hemisphere you will have access to a different portion of the sky than astronomer in the southern hemisphere. And since on any night you can only see a portion of the sky you want to make sure the galaxy you are trying to observe happens to be in your range of view during night time hours. Many astronomers still calculate if a galaxy will be in the correct location on the date of observation by using spherical trigonometry, which can prove cumbersome if you are not well acquainted with the method.
Magnitude --
You want to make sure that your galaxy is bright enough to be easily detected by your equipment (for more information on an objects magnitude click here). In this lab, because of our telescopes limitations, we want objects brighter than 12 magnitude.
Size --
You also want to make sure that your galaxy is the right size to be imaged by your telescope. The field of view for the James Gregory telescope is a window 15 minutes square (15’ x 15’). A full 360 degrees is divided into just that, 360 degrees. Each degree is divided into 60 equal arc minutes. If your object is on the order of 6’ x 6’ than you will have too large a portion of your image filled with background stars and it will be difficult to determine the brightness of JUST your galaxy. If your galaxy is any larger than 15’ x 15’ you will not be able to fit the entire galaxy in your viewing window and hence will not be able to collect all the light for analysis.
How we find candidate galaxies for observation in this lab --
There are several data bases on-line containing all of the information detailed above for many galaxies in then night sky. However, to make the search easier, there is also a very handy program called Stellarium (http://www.stellarium.org/) which is a free downloadable virtual planetarium. In the program you can set your location of observation using latitude and longitude, change the observing conditions, point your 'telescope' in the correct direction and search for candidate galaxies. I suggest you download the program and see if you can find any galaxies that match the criteria detailed above. Use the search feature to find NGC0185 and you will easily find the correct portion of the sky visible from St. Andrews around October 25th 2011.
Extracting Data from the Galaxy Images
Observation will be performed sometime before the end of October 2011, where the exact date depends on cloud cover and the availability of the James Gregory telescope. I should mention that I drew a parallel earlier between the CCD used in astronomy imaging and the CCD used by your digital camera, but there are some differences. The CCD we are using for our galaxy observation needs to have a filter placed in front of it to filter only certain types of photo-electrons. i.e. if you wanted to 'take a colour picture' you would have to take a CCD reading in the red, green and blue bands and overlay them to produce a colour image in the visual spectrum.
Taking a 'picture' of a galaxy also presents some difficulties not present or relevant in day to day photography. To better understand why these problems are problems and exactly what adjustments need to be made I need to talk a little more about how a CCD works.
Our images will consist of 1024x1024 pixels (ie a mega-pixel) and each of these pixels in a CCD can be thought of as a bucket to catch incoming photo-electrons. When you take an exposure you allow each of these buckets to catch a number of electrons for a certain period of time, then the number of electrons in each bucket is tallied and the computer turns this into a visual representation of how much light came from each portion of the sky. It all sounds simple enough. However to effectively image your galaxy you need to subtract any random 'noise' caused by statistical variation in readouts, and subtract the background sky from your image along with any dead pixels that are not working.
Once these steps are carried out we are left with an image that has been corrected (to the best of our ability) and represents a very good indication of the light coming form our target galaxy. Unfortunately most galaxies in the sky tend to be in regions where there are also stars present in the foreground and background. resulting in an image similar to the one below
Photo credit: http://www.calvin.edu/~dhaarsma/research.html
In order to determine how much light is coming from the galaxy you want to analyse, say the elliptical galaxy quite near the centre of this image, you much 'patch' all stars and anomalies that may interfere with your calculations. This is done using a program call 'gaia' where stars are erased from the image by doing statistical analysis of the surrounding sky and setting the brightness of a star to the background brightness in that particular region.
After you have patched your image and your galaxy is not polluted by any surrounding light you are then ready to perform surface photometry on your image. This is done by selecting an elliptical region you deem to contain all light from the galaxy, 'gaia' will then draw a number of increasingly smaller ellipses inside the one you selected and return an average number of photo electron counts (per pixel) inside each ellipse. This text file, containing the radius of each ellipse and the corresponding average count can be exported to a Microsoft Excel file.
This data, once in Excel, can be plotted in a graph of 'Radius vs. Average Pixel Count' and will produce a graph called the 'light profile' of your galaxy.
How we find candidate galaxies for observation in this lab --
There are several data bases on-line containing all of the information detailed above for many galaxies in then night sky. However, to make the search easier, there is also a very handy program called Stellarium (http://www.stellarium.org/) which is a free downloadable virtual planetarium. In the program you can set your location of observation using latitude and longitude, change the observing conditions, point your 'telescope' in the correct direction and search for candidate galaxies. I suggest you download the program and see if you can find any galaxies that match the criteria detailed above. Use the search feature to find NGC0185 and you will easily find the correct portion of the sky visible from St. Andrews around October 25th 2011.
Extracting Data from the Galaxy Images
Observation will be performed sometime before the end of October 2011, where the exact date depends on cloud cover and the availability of the James Gregory telescope. I should mention that I drew a parallel earlier between the CCD used in astronomy imaging and the CCD used by your digital camera, but there are some differences. The CCD we are using for our galaxy observation needs to have a filter placed in front of it to filter only certain types of photo-electrons. i.e. if you wanted to 'take a colour picture' you would have to take a CCD reading in the red, green and blue bands and overlay them to produce a colour image in the visual spectrum.
Taking a 'picture' of a galaxy also presents some difficulties not present or relevant in day to day photography. To better understand why these problems are problems and exactly what adjustments need to be made I need to talk a little more about how a CCD works.
Our images will consist of 1024x1024 pixels (ie a mega-pixel) and each of these pixels in a CCD can be thought of as a bucket to catch incoming photo-electrons. When you take an exposure you allow each of these buckets to catch a number of electrons for a certain period of time, then the number of electrons in each bucket is tallied and the computer turns this into a visual representation of how much light came from each portion of the sky. It all sounds simple enough. However to effectively image your galaxy you need to subtract any random 'noise' caused by statistical variation in readouts, and subtract the background sky from your image along with any dead pixels that are not working.
Once these steps are carried out we are left with an image that has been corrected (to the best of our ability) and represents a very good indication of the light coming form our target galaxy. Unfortunately most galaxies in the sky tend to be in regions where there are also stars present in the foreground and background. resulting in an image similar to the one below
Photo credit: http://www.calvin.edu/~dhaarsma/research.html
In order to determine how much light is coming from the galaxy you want to analyse, say the elliptical galaxy quite near the centre of this image, you much 'patch' all stars and anomalies that may interfere with your calculations. This is done using a program call 'gaia' where stars are erased from the image by doing statistical analysis of the surrounding sky and setting the brightness of a star to the background brightness in that particular region.
After you have patched your image and your galaxy is not polluted by any surrounding light you are then ready to perform surface photometry on your image. This is done by selecting an elliptical region you deem to contain all light from the galaxy, 'gaia' will then draw a number of increasingly smaller ellipses inside the one you selected and return an average number of photo electron counts (per pixel) inside each ellipse. This text file, containing the radius of each ellipse and the corresponding average count can be exported to a Microsoft Excel file.
This data, once in Excel, can be plotted in a graph of 'Radius vs. Average Pixel Count' and will produce a graph called the 'light profile' of your galaxy.
