The NLC observers handbook states that the light from NLCs is Horizontally polarised and this can be used to help identify them and increase the contrast.
Last night there was some high Cirrus probably a partly dissipated aircraft con trail visible from my home in the Eastern Sky. This was about opposite the Moon and opposite where I would expect NLCs to be which should be in a NW direction.
Testing my polarising filter on the 'cirrus' clouds showed that they polarising filter increased the contrast of these as well ! Therefore this may not be a sure way of assisting in identifying NLCs. It is not really clear if the polarising filter was allowing more light through from the clouds or reducing the light from the sky. I need to do a bit of analysis on the images I took to see if I can determine what the effect is.
Has anyone tried a polarising filter on real NLCs and if so what did you observe ?
Hello, To observe the polarisation of the light actually scattered by the NLC requires a proper polarimeter. It is unlikely that a normal photographic polarising filter on it's own will assist in identifying NLC (at least visually). The percentage polarisation is small and to detect it the images (data) would need to be well calibrated. As you've seen, what is happening is that the polariser is removing the background light. I've used various polarisers and other polarising elements in the past and have always ended up taking them off (when taking pretty pictures). They are good however, as also shown in the observers handbook, for enhancing all sky pictures as the background light scatters in a particular manner which when removed makes the NLC stand out well. If you can get your hands on a copy of Gasdens and Schroders book on NLC there is everything you'll need to know about NLC polarimetry in it. It's not for the faint hearted though...! I would be interested to hear if you do find some effect. Digital camera's offer a huge potential for analysis if care is taken. cheers, Bill.
During my observing session of the NLC on the morning of 23rd June 2018 I tried taking images of the NLC with a polarising filter. This was rotated by around 45 degrees between images though difficult to judge as the 'zero' position is not marked. I was viewing to the East around 45 degrees from the Sun Azimuth.
I can see no difference in the images which seems to support what BillW said in the post above.
Plan for the future is to identify where the 'zero' position is and then take some images with calibrated positions
I did a bit more experimenting during the NLC display this morning. Having discovered the 'zero' position is marked - there is a big white mark on the rotating part of the filter !!! I took several images rotating the filter about 15 degrees between each. No significant difference noted but as the NLC was above some thin cirrus cloud and there was relative movement it is difficult to tell.
There is an interesting pair of images in the NLC Observing Handbook that shows an all sky fish eye image with and without a polarising filter - the NLC are much clearer with the filter. Few details are available so it is not clear if the effect is just the result of the increased exposure to allow for the polarising filter. (Edit) I am not sure how they used used a polarising filter with a fish eye lens - the front element is so curved that it is not possible to put a filter in front of it without significant vignetting. The only other option would be to put it behind the lens ?? Does thei have any impact on the effect ? (end of edit)
It is not quite clear how the white mark relates to the position of the linear polariser - need to do a bit more research or experiment on that.
Incidentally the polariser I used is described as a circular polariser - I was not sure how that worked as the light from the sky and water reflections is linearly polarised. There is an interesting Wikipedia article here that describes circular polarisers. Apparently they were created specially for digital cameras as the autofocus and the anti-alias filter cannot cope with linear polarised light. A 'circular' polariser has a linear polariser on the front and then has a quarter wave plate behind it to convert the linear polarised light to circular polarised which allows the auto focus and anti-alias filters to work. So though described as circular polarisers they are in fact linear polarisers. Apparently you can tell if they are circular polarisers by looking through the wrong way - they do not work if the light goes through the filter the wrong way !
Last Edit: Jul 8, 2018 12:41:19 GMT by johnm: Added a bit more on polarising filters and fish-eye lenses
I have done some experiments using horizontally polarised light from a saucepan of water. This shows when the white mark is vertical horizontally polarised light is blocked and allowed through when the white mark is ‘horizontal’.
I have also found a quick way of testing if a filter is a circular polariser. If you hold it in front of an LCD display when it is the correct way round with the linear polariser at the front the display will go dark as the filter is rotated.
If you reverse the filter so that the male thread is nearer the screen a circular polarised filter will behave differently to a plain polarised filter.
When reversed the plain polarised filter will block the light from the LCD display at one orientation ( on my displays this seems to be 45 degrees). A reversed circular polariser will show little change as it is rotated, When I tried this with my Hoya Circular polarised filter there was some colour change from a slight yellow to slight blue tinge as it was rotated but the display remained visible at all orientations.
This provides a quick check to allow you to determine if unknown filters are linear or circular.