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02-12-2016, 06:54 AM   #76
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QuoteOriginally posted by BrianR Quote
Is it wrong that I plan to try to cause this Fabry-Perot interference with the next aurora here?
If you plan to test the Fabry-Perot hypothesis, by all means go ahead! As I intend to test it myself, the 550nm Dichrioc filters I have will produce monochromatic light. Collimation might prove to be a bit problematic, though a spot fresnel attached to a studio strobe should do the trick.

02-12-2016, 07:17 AM   #77
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QuoteOriginally posted by Digitalis Quote
If you plan to test the Fabry-Perot hypothesis, by all means go ahead! As I intend to test it myself, the 550nm Dichrioc filters I have will produce monochromatic light. Collimation might prove to be a bit problematic, though a spot fresnel attached to a studio strobe should do the trick.
I'm afraid your dichroic filters may not be sufficient to obtain monochromatic light. Look here https://www.go-ttv.com/optical-filters/ Also, you will have problems obtaining the parallal beams you need. The pattern will be smeared and you most likely will not see it.

Why not just get a small semiconductive laser, spread the beam over a large wall at a distance and take a picture?

Edit: This filter, for example, is not even close: http://opticalfiltershop.com/shop/bandpass-filter/visible-bandpass-filter-550nm-fwhm-100nm/

Last edited by DagT; 02-12-2016 at 07:43 AM.
02-12-2016, 07:48 AM   #78
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QuoteOriginally posted by Digitalis Quote
If you plan to test the Fabry-Perot hypothesis, by all means go ahead! As I intend to test it myself, the 550nm Dichrioc filters I have will produce monochromatic light. Collimation might prove to be a bit problematic, though a spot fresnel attached to a studio strobe should do the trick.
Aurora happen several times per year where I am, that'll be the easiest setup for me and a good reason to go running around in fields after dark.

Do you have the data sheets for your 550nm Dichroic filters?

QuoteOriginally posted by DagT Quote
Why not just get a small semiconductive laser, spread the beam over a large wall at a distance and take a picture?
How about a red led? Still too broad a spectrum?

QuoteOriginally posted by DagT Quote
Sorry, I can't see the picture in the first link, but yes, please try the next time you have a chance. If you find a high quality filter (which would have nice, plane surfaces) without coating (which would secure lots of internal reflections) the chances should be good. In any case, let us know
I've added it as an attachment, I think it's one of the clearest examples of this I've seen. It belongs to a Tom Witte and was taken with a Canon IS 28-135mm @ 100mm w/filter (the link again for reference: Strange Concentric Circles in Northern Lights Photo - Photo.net photo.net Forum).

I would think that since the central area is bright in some examples and dark in others should be a point for F-P and a strike against newton rings? Since the wavelength is fixed for these green aurora displays, whether the central circle is "bright" or "dark" would depend on the thickness of the filter for the Fabry-Perot view - which would determine if the head on rays are in a destructive or constructive interference zone. For Newtons rings, the dark or light centre depends on which side the light is coming from, in other words whether you're viewing the transmitted or reflected image, and I wouldn't think this would change.

How will distance of the filter to the front element affect my chances? e.g. a dfa-100mm has a very deeply recessed front element.
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02-12-2016, 08:04 AM   #79
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Wow, that was good. Very typical pattern and I would have guessed a tele lens. In order to get this with Newton rings you would need two parts very similar in shape and very close to the sensor. otherwise it would not be so sharp.

You are abslutely right about the central disc. It only depends on the interference straight through the filter.

You do not have to be concerned about the distance from the front lens to the filter. As long as we are discussing parallel rays they stay that way.

---------- Post added 02-12-16 at 04:12 PM ----------

QuoteOriginally posted by BrianR Quote
How about a red led? Still too broad a spectrum?
Depends on the LED, but usually not. It has to be sufficiently monochromatic to maintain a certain coherence length, which means that the light has to be able to interfer with light that has passed back and forth in the filter a number of times. There are some superluminence LEDs that have a short coherence length that maybe could be used.

I line emmision light source is a lot better, maybe a sodium lamp? They have been commonly used in streetlights. Looks like thay get interference patterns here:
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodium.html

02-12-2016, 10:36 AM   #80
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QuoteOriginally posted by BrianR Quote
Do you have the data sheets for your 550nm Dichroic filters?
I do - transmission is possibly a bit too broad. 545~560nm @10% threshold bandpass the peak is at 550nm. It is possible to obtain filters with even narrower bandpass. Failing that either a synchotron or a laser should do the trick.
02-12-2016, 10:43 AM   #81
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QuoteOriginally posted by DagT Quote
Not really. Condensation comes from circulating warm air with relatively high humidity over a cold surface. Thereīs not enough humidity in the air enclosed inside the camera and lens to give much condensation.

We often go from -10C into a warm cottage with high humidity around here.
That would be your special Norwegian form of humidity. Where I live, the dewpoint happens regardless of whether the air is inside or outside of the camera.


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02-12-2016, 10:44 AM   #82
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QuoteOriginally posted by Digitalis Quote
If there is a destructive interference occurring anywhere within the optical path it will be visible no matter what focus distance your lens is set to.
Ta! Da! Finally!


Steve
02-12-2016, 11:22 AM - 1 Like   #83
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QuoteOriginally posted by DagT Quote
I line emmision light source is a lot better, maybe a sodium lamp? They have been commonly used in streetlights. Looks like thay get interference patterns here:
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodium.html
I'm 83% certain you posted this link to get me to electrocute a pickle. I'm 72% certain I will sometime this year.


Low-pressure sodium lamps look like fun, but not high on availability. I'll look around

02-12-2016, 01:31 PM   #84
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QuoteOriginally posted by stevebrot Quote
That would be your special Norwegian form of humidity. Where I live, the dewpoint happens regardless of whether the air is inside or outside of the camera.


Steve
Dew point is one thing,the actual amount of available humidity another.

---------- Post added 02-12-16 at 09:39 PM ----------

QuoteOriginally posted by stevebrot Quote
"Originally posted by Digitalis Quote
If there is a destructive interference occurring anywhere within the optical path it will be visible no matter what focus distance your lens is set to."

Ta! Da! Finally!
I didnīt understand that one. It is obviously wrong when light in the optical path interfers with itself, crossing its own path repeatedly
(Sorry, itīs friday around here right now)
02-12-2016, 01:50 PM   #85
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QuoteOriginally posted by DagT Quote
It is obviously wrong when light in the optical path interfers with itself
I think we can all agree it's wrong for anything or anyone to interfere with themselves on a public forum!
02-12-2016, 01:56 PM   #86
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QuoteOriginally posted by BigMackCam Quote
I think we can all agree it's wrong for anything or anyone to interfere with themselves on a public forum!
No, no, no at the risk of destructively interfering with one self at the risk of disappearing completely it is totally OK. It happens every time a star twinkles
02-12-2016, 05:52 PM   #87
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QuoteOriginally posted by stevebrot Quote
Ta! Da! Finally!
With sufficient lack of sleep even I can make a mistake.
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