Post by Bill W on Jul 2, 2011 13:01:03 GMT
NLC's like you've never seen them before.
It is fours years since johnm asked the question about using polarisers in nlc observing. (post on 22/6/07).
Where does the time go!!!
Anyone who has read the standard NLC text by Gadsden and Schroder or even the abreviated notes in the observers handbook will be aware that polarisation measurements are not easy to make nor interpret.
It took me two years to fully understand what these books were saying and get my head around how to explore this with modern equipment. Previously individual photographs were taken with rotating polarisers etc, etc as described in the books.
Theoretically the model is straightforward and the various graphs presented show the change of polaisation with increasing angle from the sun and differing for various wavelengths.
It took me another year to scrounge the bits and get to grips with the software and hardware. The Mk1 was ready last year but good weather never coincided with a good nlc!
However last night I finally got to try out the Mk2!
If somebody can point to another paper or article I'd like to see it but this might be the first time ever that digital video imaging techniques have been used to produce "proper" polarimetric images of nlc.
So....
A frame grab of one of the sequence needed.
The brightest "star" is Jupiter. Image taken 01.42UT.
I'm using a deep blue filter centered on 400nm. By taking a series of images and determing the intensity differences across the image whats called the Stokes vectors can be calculated (It's all in the book). Applying these values to the pixels gives an "image" of the polarisation at every point in the frame, giving
What this shows are the degrees of polarisation per pixel, thus this is a polarimetric map of the nlc cloud. It can also be seen that there is a general gradient left to right, darker on the left, nearer to the sun less polarisation and brighter to the right, further from the sun and more polarised. (almost the reverse of the brightness levels in the light frame) A completely unique insight into the physical nature of the nlc. There are a few more developments which will make it useful rather than just instructive but that's for later....
Now, from my other post, remember the time of day. Because you have control over the retarder you can take out the background and tweak to get best contrast to give remarkable views of the nlc in very low contrast conditions. This video was taken when, almost overhead the nlc was visible but not particularly well due the brightening sky (after 3am local)
Here's a frame grab.
and the video....
www.youtube.com/watch?v=I26xnuxUtVE&feature=player_profilepage
Using the polarimeter in video mode almost invisible NLC jumps out the sky.
Remember where you saw it first!
Cheers,
Bill.
It is fours years since johnm asked the question about using polarisers in nlc observing. (post on 22/6/07).
Where does the time go!!!
Anyone who has read the standard NLC text by Gadsden and Schroder or even the abreviated notes in the observers handbook will be aware that polarisation measurements are not easy to make nor interpret.
It took me two years to fully understand what these books were saying and get my head around how to explore this with modern equipment. Previously individual photographs were taken with rotating polarisers etc, etc as described in the books.
Theoretically the model is straightforward and the various graphs presented show the change of polaisation with increasing angle from the sun and differing for various wavelengths.
It took me another year to scrounge the bits and get to grips with the software and hardware. The Mk1 was ready last year but good weather never coincided with a good nlc!
However last night I finally got to try out the Mk2!
If somebody can point to another paper or article I'd like to see it but this might be the first time ever that digital video imaging techniques have been used to produce "proper" polarimetric images of nlc.
So....
A frame grab of one of the sequence needed.
The brightest "star" is Jupiter. Image taken 01.42UT.
I'm using a deep blue filter centered on 400nm. By taking a series of images and determing the intensity differences across the image whats called the Stokes vectors can be calculated (It's all in the book). Applying these values to the pixels gives an "image" of the polarisation at every point in the frame, giving
What this shows are the degrees of polarisation per pixel, thus this is a polarimetric map of the nlc cloud. It can also be seen that there is a general gradient left to right, darker on the left, nearer to the sun less polarisation and brighter to the right, further from the sun and more polarised. (almost the reverse of the brightness levels in the light frame) A completely unique insight into the physical nature of the nlc. There are a few more developments which will make it useful rather than just instructive but that's for later....
Now, from my other post, remember the time of day. Because you have control over the retarder you can take out the background and tweak to get best contrast to give remarkable views of the nlc in very low contrast conditions. This video was taken when, almost overhead the nlc was visible but not particularly well due the brightening sky (after 3am local)
Here's a frame grab.
and the video....
www.youtube.com/watch?v=I26xnuxUtVE&feature=player_profilepage
Using the polarimeter in video mode almost invisible NLC jumps out the sky.
Remember where you saw it first!
Cheers,
Bill.
