Nature of project: experimental, data analysis
Available to students on full-time physics degree schemes only.
To detect polarization on the lunar surface, and to attempt to produce maps of the percentage of polarization for different areas of the Moon at different lunar phases. You will base your work on the methodology set out by Fearnside and Masding...
https://web.archive.org/web/20150613185915/http://www.baalunarsection.org.uk/tmnr2_december2012.pdf (see p55-75)
This is quite a new remote sensing technique and can provide information on grain size, and other optical properties.
1) The altitude of the Moon should be at least 30 degrees at the time of observation, and the sky must be completely transparent, as any thin high cloud or haze will affect the image brightness.
2) The objective or corrector plate must be free of dew throughout; again, dew reduces the image brightness and distorts the curves that are generated from the polarisation images.
3) Ensure all the images cover the same region of the Moon to obtain maximum overlap in the data.
4) Some webcams have different sensitivity to different orientations of polarisation. This effect is typically at the 5% level, which gives large errors in the measured polarisation degree, as this is at most around 15%. The effect can be allowed for in processing provided the camera has been calibrated using the same polariser and a source that is known to be unpolarised.
5) If the camera is known to have such a varying sensitivity, it is important to orientate the image frame so that its sides are parallel and perpendicular to the scattering plane. This is because the camera correction factor is the ratio of sensitivities to polarisations parallel to the long and short sides of the sensor. If the frame is rotated, a different correction factor will be needed for that set of data.
N.B. we have some archive data from a previous project attempt.
A successful project will develop beyond the above in one/some of the following directions:
1) Acquire a sequence of telescopic images of the chosen region of the Moon through a polarising (and bandpass) filter, each image with a different angle of the filter transmission axis. The orientation of the axis should cover a range of at least 180 degrees in roughly equal steps. A change of 10 degrees between images is about right, but the exact values of the angles do not matter as long as they are known to the nearest degree. Ideally the entire set of images should be obtained in no longer than 20-30 minutes. Some observing precautions are noted above.
2) Use image stacking software such as Registax to reduce the S/N ratio, but do not sharpen.
3) Prepare a reference image by stacking the polarisation images and sharpening the resulting stack.
4) Write an analysis program that is capable of holding images in a data cube, and finding minima and maxima brightness levels, in the image stack, at each pixel.
5) On the reference image, identify and mark with crosses at least five small craters with known selenographic latitude and longitude. The selenographic coordinates and the pixel coordinates of the craters on the image are entered in a data file that will be input to the the above analysis program. The coordinates of the sub-solar point on the Moon and the angles of libration, the date and time of the observations are also put into the data file. The inputs to the analysis program are the data file, the reference image and the set of images in different polarisations. The output will be a polarization map image.
When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.
This project is only available as a Y3 project.
Please speak to Tony Cook (atc) if you consider doing this project.
Initial literature for students:
Polarimetry is challenging to do well, but laboratory equipment will be available to help understand and try out the technique on test surfaces and soil. Telescope training will be supplied by your supervisor.
|milestone||to be completed by|
|Plan when you will need to observe in the new year||end of November|
|Complete image capture||mid-March|
|Compare polariztion maps obtained with other remote sensing data about the lunar surface and discuss results||Easter|
Students taking this project will have to submit a full risk assessment form