Photometry in Exoplanet Detection

Last month I discussed that one of the most popular methods of exoplanet detection was the transit method. This is the process of looking at the “brightness” of the star, and seeing how it varies when the planet passes over the face of the host star, as seen from Earth. The transit method is the easiest for amateurs to do, and can be done with very modest equipment. I’ve seen people using a telephoto lens and digital SLR.

When I say easiest, that’s really a relative statement. I once heard the transit method described as “Detecting the change in brightness of a 100 watt light bulb as a fly passes by…..seen from 10 miles away” This is a pretty tall order, but it can be, and is, done!

Photometry basically boils down to counting photons. Looking at the number of photons that fall onto the CCD and converting that into a numeric value. It’s not just confined to exoplanets. Photometry has many powerful and far reaching applications, a few of which I have been dabbling in over the last few weeks.

Using my subscription to the iTelescope service gives me access to many wonderful telescopes in exotic locations (Australia, New Mexico to name just two) I did a bit of photometry during my degree, to form colour-magnitude diagrams (basically a Hertzsprung – Russell diagram) so this is what I decided to do using my iTelescope Time.

Using iTelescope’s T24 (a 0.6m Telescope – image below) I took a series of 10 images each of 60s integration of M45 the Pleiades. (5 with a blue filter, and 5 in a ‘visual’ filter).

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Using blue and visual enables a process of photometry called B-V (B minus V). B-V is basically a measure of the colour of a star. The smaller the B-V value, the more blue the star is. Conversely the larger the value, the more red. The images are auto calibrated, so the FITS files downloaded from the server are ready to go. I use a website called astrometry.net which is very clever and identifies stars in your image, this enables me to find my target, and also find a suitable reference star (the reference is of known magnitude and so forms the calibration against which the object magnitude is measured)

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Once the target and reference star have been identified, I use a bit of software called MaximDL to perform the photometry. It’s super easy to use, and spits out the results in graph or CSV format for both the Blue and Visual filter bands.

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The B-V value is literally that, you subtract the V magnitude from the B. Simple! Plotting the V magnitude against the B-V gets you the colour magnitude diagram.

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Once the B-V value is obtained there are many fun things that can be calculated, effective temperature, distance, even age. A lot of really interesting science can be done, just by taking a small number of images, and doing a little processing magic.

For my target in this case, I only went as far as calculating the effective temperature, for which I got a value of 8282.6K. The currently accepted value is 9200K. I was pretty darn happy with this result. Now, I know that a lot of photometry purists would be screaming right now “you’ve not considered airmass” or “your values don’t compensate for interstellar dust extinction” – and they would be right. This was my first dabbling into the area, something which I had initially only intended to use for my exoplanet project, but actually, it proved to be really interested, and a lot of my values (yes, un-corrected) were very close to some stated in a paper published by a group at Keele University (which can be found here; http://www.astro.keele.ac.uk/jkt/pubs/Southworth-proc-Preston.pdf )

The point is, there is a lot of really interesting science, really accessible to amateurs on a budget. You’ve just got to get out there, and get stuck in. Who knows, you might actually find something new!

Feature image by: http://gamma-sci.com/photometry/

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