Supernova remnants: the crab in five different visualizations

[ptc]

Those that know me know that I like to do new things and push the envelope of amateur astronomy into new areas. One of the best examples is the tricolor emission line method that I introduced to the amateur community beginning in 2001. It took off sort of slow but now it is one of the major techniques in wide use.

In reading about the crab nebula one thing I learned is that the signature of the pulsar is the synchrotron radiation it emits. This radiation is non-thermal, and arises from the acceleration of charged particles (angular acceleration in this case). It is a form of Bremsstrahlung (braking radiation: http://en.wikipedia.org/wiki/Bremsstrahlung ) and one of its fingerprints is the fact that it is broadband (from radio waves through Xrays) and exhibits strong linear polarization that serves as a fingerprint.

Reading Simon Mitton's book "The Crab Nebula" he outlines the history of the discovery of the mechanism of the Crab. At the heart was identifying the linear polarization.

The crab has two major components; the core that is broadband non-thermal synchrotron radiation surrounded by an exquisite network of emissive filaments that exhibit line emission and no polarization.

So to really get the most out of an image of the Crab it would be nice to separately image the polarized region using polarizing filters and then to image the filaments using emission line filters.

I did just that in early 2007 in my backyard.

http://www.narrowbandimaging.com/images/crisp_crab_5_ways_to_sunday_vga.jpg

starting at the upper left and moving clockwise we have an ordinary RGB image, The next is a tricolor emission line image, below that is a Stokes Polarigram (polarization difference image) followed by an ordinary polarigram.

The central image is the one I like the best and is a composite of the Tricolor Emission Line image and the Stokes Polarigram. Notice how the pulsar is prominent in the image as well as the filaments. I don't know of any other way to do that.

I wrote a paper on this method that appeared in the fall 2007 issue of "Amateur Astronomy" that describes this work as well as other objects in which I have found linear polarization and are not synchrotron sources.

http://www.narrowbandimaging.com/images/Crisp_polarization_article.pdf


I additionally prepared and gave a talk at a local astronomy club explaining this work: http://www.narrowbandimaging.com/images/polarize2008_excerpt.pdf
It also describes other classes of objects in which linearly polarized objects can be found that do not include synchrotron sources.

Note that the outburst from Comet Holmes in late 2007 featured linear polarization. http://www.narrowbandimaging.com/comet_17_p_holmes_stokes_page.htm

This is not a newly discovered phenomenon, linear polarization arising from silicate ejecta from comets has been discussed in the professional literature

http://www.springerlink.com/content/p25k608154667745/ (http://www.springerlink.com/content/p25k608154667745/)


I hope you folks find this polarization work as interesting as I did.

[luigi]

Richard,

Thanks a lot for all this info, it's super-interesting.
The images are really amazing.
I'm reading and learning, ty!

[mjc]

Thanks for posting that Richard

I have been curious for some time about the value of using polarization filters in astronomical imaging.

What sort of polarization filters do you use and how does one choose the angle of rotation? Do you use the Polarization filter pairs used for reducing lunar glare and remove one of the pair?

Mark C.

[ptc]

First off, you ought to read the paper I wrote and review the slideset because it speaks to what exactly I am doing and in reading your questions it appears to me that you may not be understanding the goals and methods very well.

Your questions actually relate more to a potential use to eliminate sky glare and so on. This is NOT what I am doing. I am seeking polarized objects and then showing how to detect the polarization and to use it for visualization of the polarization of the object by using color.

I am using LINEAR polarizing analyzers. For use in eliminating glare circular polarizers are commonly used in Terrestrial photography. That is a very different application that what I am doing.

So in summary I am detecting polarization rather than using polarizers to eliminate glare. The moon doesn't feature polarized light but getting its light through the atmosphere it may be beneficial to use a circular polarizing filter. I haven't tried it partly because I have little interest in lunar imaging.

What interests me is using filters on terra firma to help me see latent structure or features that are commonly missed in conventional imaging. It is a powerful tool for visualization and that is how I am using it.

Please read the paper and look over the slideset and see if your questions about angles and types are answered.....

[multiweb]

Great shots and very interesting article. :thumbsup:
PS: how do you rotate the threaded polarizing filter in its cell? Do you have a custom holder like a camera rotator type?

[renormalised]

Very interesting post Richard and great images:)

Downloaded your articles and the article from SSR for the polarisation in comets:)

Will keep me interested reading those for awhile!!!:)

[ptc]

Marc

there are several ways to do the rotation
1) rotate the entire camera/filter as a unit
2) rotate a filter wheel relative to the camera
3) or do like I did: put a set of four linear polarizing filters in the wheel, each at a different orientation. In my case I set one at 0deg, 45 deg, 90deg and 135deg

[seeker372011]

very interesting approach..have you found any other targets since for this?

[ptc]

yes I have found several classes of targets: sounds as if you did not look at my PDF of the presentation I gave at a local astronomy club. Go back and have a look toward the end as I outline these many differernt types of targets. It was linked in my original posting.

1) supernova remnants
2) jets
3) circumstellar disks
4) silicate-rich comets
5) star forming regions with significant reflection nebulosity

[mjc]

Richard - I did read your paper and I understood what you are doing. If I've gone off track anywhere it is in not understanding that the concept of circular polization existed - I thought that there was only one type and linear polarization is the only polarization conept that I know and understand.

My question was basically the question of sourcing the filter and mechanism of rotation. My reference to lunar glare was with reference to the devices which are commercially sold to allow one polarization filter of a pair to be rotated with respect to a fixed filter to reduce lunar brightness to varying amounts. I thought, therefore, that removing the fixed oriented filter may allow the modifed device to be used in the sort of imaging that you have done (as they are readily and cheaply sourced and they provide a mechanism of changing rotation). If it is the case that this is not suitable because the filters are of circular polarisation type then any info on that (from any anyone) would be welcome. I will be, of course, be doing more research myself.

Edit:
Here's an example of the device I was thinking about
http://www.rothervalleyoptics.co.uk/lumicon-1-25inch-polarising-filter_d2741.html

I think however using three filters in a filterwheel, as you have suggested, is probably the ideal solution.

Keep up the good work.

Mark C.

[ptc]

that is used to provide a variable attenuator for viewing the moon visually

when you have two linear polarizers and then rotate one relative to the other you go from maximum transmission when the grooves are in alignment to 100% extinction when they are orthogonal to each other

in this case the polarizers are not used to detect polarization, their usage properties are being exploited to give you a variable neutral density filter.

to see how that works, grab a pair of polarized sunglasses and look at your LCD panel while rotating them..... should be pretty dramatic

a circular polarized filter will be useful for terrestrial photography to cut sky glare. That is very different from what I am doing. I am detecting polarized light from objects in space