Thin sections in polarized light?
Posted 20 September 2013 - 09:44 AM
I looked through the 44-odd pages of earlier topics and didn't see one on this subject ...
Here's an example (Allende through a microscope, using cross-polarized light) ...
Posted 20 September 2013 - 11:06 AM
Those are awesome! Thanks for sharing them.
I personally don't work with thin-sections, but I love looking at the photos. A few years back, I acquired a collection of thin-section photos from a collector in Australia. I have a couple of CD-ROM's filled with them and I have made a few prints (for framing), but otherwise don't know what to do with them.
Here is a shot of Zagami -
Posted 20 September 2013 - 11:08 AM
It mimics a petrographic microscope, except in image scale. You can pretty much do 1:1 macros this way.
To get the cross-polar effect, you sandwich the thin section between two large linear polarizing filters -- the kind that thread onto the end of a large camera lens. You then rotate the top filter until the filters block all the background light coming up through the slide copier.
If you have a good macro lens, you can also use this same polarizing trick using a simple light box, like a tracing table.
Here is what the thin section looks like in macro mode using a standard DSLR "kit lens" held over the filter stack illuminated by a tracing table:
For the image above, the camera was deliberately held far enough from the filter stack to show how the filters block the background light. Normally, the filters are screwed together and the macro is much tighter, more of a closeup:
Posted 20 September 2013 - 11:14 AM
One thing I like to do with them is use the macro photos (rather than the micrographs) to compare the different subtypes of chondrites. After a while, you can get pretty good at recognizing the differences between the LLs and the amount of weathering. Doing the actual classification is much more difficult, but I find it fun and educational to try to sort things out this way.
And sometimes the photos are quite beautiful.
Tom Phillips is the master at this, but I don't have anywhere near his range and quality of equipment ...
Posted 21 September 2013 - 02:12 PM
Nice photos :-) I have a few micrographs on my Flickr feed along with some photos of my microscope setup:
Chelyabinsk thin section photos:
Posted 21 September 2013 - 06:09 PM
You have a much better equipment setup than I do, congrats!
Posted 22 September 2013 - 12:01 PM
I like the look of your Bowens Illumitran setup, i use a light box and 2 x polarisers with a camera on a tripod but your solution is much more elagant :-) On the lookout now for a slide copier!:-)
Posted 22 September 2013 - 01:32 PM
The Illumitran gives very firm control of magnification, via the bellows. You'll need a bellows adapter to fit your camera if you can find one. I found a T-adapter for the bellows and mated it to one for my camera. This works because the bellows allows you to control the location of the image plane and the plane of the chip.
There are a half-dozen or so Illumitrans units on eBay right now, between $200 and $400 and they appear to be in better shape than mine was (I paid much less, but had to repair the Xenon strobe and the translucent "stage").
I do not use the contrast control because I can't figure out how to do it with the size polarizers I need.
Linear (LP), not circular (CP) filters are required, in case I forgot to mention that.
I am also looking for an affordable stereo petrographic microscope, and for one that allows not only a camera light path but also a goniometer and spindle. I would really like to get into measuring index of refraction.
I have some fiber-optic bundles to fiddle with for better lighting a high magnification, and want to fiddle with lasers to try to pick up any mineral fluorescence I might be able to induce in the UV.
I am nearly finished with a mount for my spectrograph. It will be very interesting, I think, to see how much detail I can coax out of the minerals that way.
So little time, so much fun to be had!
I have a handful of thin sections I have not yet photographed, and will probably be doing that after the first couple of weeks in October.
Posted 23 September 2013 - 03:48 PM
I use 2 x polarisers (linear) over a lightbox now which works fine with the camera on a copy stand or tripod.
You sound like you have a few interesting projects on the go there, good luck :-)