Index of Air

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Index of Air

Post by colinsk » Sun Jul 22, 2012 11:15 pm

I wanted to leave this more searchable than the thread it is all in now.




From NIST where:

n = refractive index
p = kPa
t = C
RH = Relative Humidity in %

from:


Look here: http://emtoolbox.nist.gov/Wavelength/Documentation.asp for a discussion of the refractive index of air, scroll down to Appendix B (at the bottom) for the abbreviated (useful) version.


And,

So for air spaced etalons:

* increasing n (pressure) is red shifted

* increasing l (spacer thickness, ie heating) is red shifted
(This assumes the expansion coefficient of the spacers is greater than the change of pressure caused by heating air... )

* increasing theta (tilt) is blue shifted
Thank you, Here are some references I have collected.



Etalon Article Part I

Etalon Article Part II

Solar Patents of interest

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Re: Index of Air

Post by swisswalter » Mon Jul 23, 2012 4:24 am

Hi Colin

thank you very much for the formula. Makes it more easy to understand
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Re: Index of Air

Post by robert » Mon Jul 23, 2012 7:30 am



[quote]So for air spaced etalons:

* increasing n (pressure) is red shifted

* increasing l (spacer thickness, ie heating) is red shifted
(This assumes the expansion coefficient of the spacers is greater than the change of pressure caused by heating air... )

* increasing theta (tilt) is blue shifted

Does that mean an etalon designed for tilting will be wrongly setup to be pressure tuned with positive pressure increase?

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Re: Index of Air

Post by Valery » Sat Feb 07, 2015 6:32 pm

colinsk wrote:I wanted to leave this more searchable than the thread it is all in now.




From NIST where:

n = refractive index
p = kPa
t = C
RH = Relative Humidity in %

from:


Look here: http://emtoolbox.nist.gov/Wavelength/Documentation.asp for a discussion of the refractive index of air, scroll down to Appendix B (at the bottom) for the abbreviated (useful) version.


And,

So for air spaced etalons:

* increasing n (pressure) is red shifted

* increasing l (spacer thickness, ie heating) is red shifted
(This assumes the expansion coefficient of the spacers is greater than the change of pressure caused by heating air... )

* increasing theta (tilt) is blue shifted
Hi Colin,

Sorry for re-animate a very old thread.

You wrote: "increasing n (pressure) is red shifted "

This contradicts with my own observations and what Andy Lunt wrote here: "By removing the need for tilt we have placed the etalon in the most optimized position possible.
We install a very accurately tuned etalon. This etalon is tuned to the red side of the CWL. Given that it is already tuned to the red, the user has the ability to shift the tune of the CWL to the Hydrogen-alpha line and then Doppler tune to the blue or back thru to the red.

Due to the fact that there is no tilt involved, the image field remains flat and very precise."


So, should we finally conclude that increasing n (pressure) is BLUE shifting?


Thanks,

Valery.
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Re: Index of Air

Post by marktownley » Sat Feb 07, 2015 10:25 pm

Valery wrote:Hi Colin,

Sorry for re-animate a very old thread.

You wrote: "increasing n (pressure) is red shifted "

This contradicts with my own observations and what Andy Lunt wrote here: "By removing the need for tilt we have placed the etalon in the most optimized position possible.
We install a very accurately tuned etalon. This etalon is tuned to the red side of the CWL. Given that it is already tuned to the red, the user has the ability to shift the tune of the CWL to the Hydrogen-alpha line and then Doppler tune to the blue or back thru to the red.

Due to the fact that there is no tilt involved, the image field remains flat and very precise."


So, should we finally conclude that increasing n (pressure) is BLUE shifting?


Thanks,

Valery.
You've lost me here Valery, why would we conclude that?
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Re: Index of Air

Post by mdwmark » Sun Feb 08, 2015 3:50 am

easy way to find out, increase the pressure and see which way it moves.
The disk looks different in the blue wing then the red wing.
To check, If you tilt and it looks better you are in the red wing. If you are in the blue wing tilting will not improve the disk.
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Re: Index of Air

Post by Valery » Sun Feb 08, 2015 7:56 am

marktownley wrote:
Valery wrote:Hi Colin,

Sorry for re-animate a very old thread.

You wrote: "increasing n (pressure) is red shifted "

This contradicts with my own observations and what Andy Lunt wrote here: "By removing the need for tilt we have placed the etalon in the most optimized position possible.
We install a very accurately tuned etalon. This etalon is tuned to the red side of the CWL. Given that it is already tuned to the red, the user has the ability to shift the tune of the CWL to the Hydrogen-alpha line and then Doppler tune to the blue or back thru to the red.

Due to the fact that there is no tilt involved, the image field remains flat and very precise."


So, should we finally conclude that increasing n (pressure) is BLUE shifting?


Thanks,

Valery.
You've lost me here Valery, why would we conclude that?
My SM40 etalon has a CWL (when not tilted) very slightly shifted to the red vs H-a CWL. When I tilt it (very little!) the picture becomes maximally contrasty.
This is at 760mm Hg barometric pressure. When the pressure is 765mm Hg, the picture is maximally contrasty without a tilt. When the barometric pressure becomes 770mm Hg the picture still is Ha, but even slightest tilt makes H-a details start to disappear.
When the barometric pressure is about 755mm Hg then I need a bit more tilt to come to Ha CWL than at 760mm Hg.

So, the higher pressure the more etalons CWL shifts to the blue.

The same with LS35F Ha etalon I own.


Valery.
Last edited by Valery on Sun Feb 08, 2015 1:50 pm, edited 1 time in total.
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Re: Index of Air

Post by marktownley » Sun Feb 08, 2015 10:57 am

Interesting Valery. At what sort of temperature are you talking here?

In winter in high pressure (1030mB / 773mm Hg) for me with the cold temperatures (0deg celcius) I really cannot get my DS40 onband http://brierleyhillsolar.blogspot.co.uk ... nuary.html - however in warmer conditions (20deg celcius) at the same atmospheric pressure I get a lovely disk http://brierleyhillsolar.blogspot.co.uk ... april.html

I'm no good at excel, but it would be interesting if someone who is could put the above formula into it and generate a model where it allows us to change the variables and see the effect of them changing has on the refractive index and as such how our etalons perform.
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Re: Index of Air

Post by Valery » Sun Feb 08, 2015 1:35 pm

marktownley wrote:Interesting Valery. At what sort of temperature are you talking here?

In winter in high pressure (1030mB / 773mm Hg) for me with the cold temperatures (0deg celcius) I really cannot get my DS40 onband http://brierleyhillsolar.blogspot.co.uk ... nuary.html - however in warmer conditions (20deg celcius) at the same atmospheric pressure I get a lovely disk http://brierleyhillsolar.blogspot.co.uk ... april.html

I'm no good at excel, but it would be interesting if someone who is could put the above formula into it and generate a model where it allows us to change the variables and see the effect of them changing has on the refractive index and as such how our etalons perform.
September - October season. And then also (when higher than 768mm Hg) December somewhere +2-3C.


Your observations are well according to what I say: the dense the air (when it cold) - the larger it's refractive index - the more etalon's CWL shifts to blue and when out of the H-a CWL, it, naturally, can't come back to h-a CWL by tilting. ???


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Re: Index of Air

Post by grimble_cornet » Sun Feb 08, 2015 2:55 pm

I'm no good at Excel either Mark :oops:

But I've had a play anyway:
Effects of temperature pressure and humidity on refractive index of air.jpg
These graphs were produced with the 'simplified' equation given above and checked using the on-line calculator also referenced above.
The values in my graphs don't quite match the on-line calculator (not surprising as other factors such as wavelength and carbon dioxide concentration are involved in the complex equation) but the trends are correct.


Being a 'thicko' I'm still not certain that I know what this means in terms of red/blue shifting ?

My simplistic understanding would suggest that as the refractive index INCREASES then the wavelength would SHORTEN giving a BLUE SHIFT?
IF this is correct ( :roll: ) then:
INCREASING PRESSURE -> INCREASING REFRACTIVE INDEX -> BLUE SHIFT ?
INCREASING TEMPERATURE -> DECREASING REFRACTIVE INDEX -> RED SHIFT (but the effect is small)
INCREASING HUMIDITY -> DECREASING REFRACTIVE INDEX -> RED SHIFT (but the effect is VERY small)

But I'm quite happy for others to show me the error of my ways :lol: :? :lol:
.

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Re: Index of Air

Post by Valery » Sun Feb 08, 2015 7:00 pm

"But I'm quite happy for others to show me the error of my ways"

I see no errors.

These cold days ( 0 to +3C) my SM40 is partly within an Ha CWL. Any even smallest tilt makes images less contrasty and some more tilt - the H-a details
disappear completely, while during summer hot at the same barometric pressure, I have a very small etalon's shift to red. The smallest tilt and it is in the CWL.
At very high pressure even at summer it is right at CWL.

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Re: Index of Air

Post by marktownley » Mon Feb 09, 2015 10:07 am

Yesterdays full disk - poor seeing, temperatures couple of degrees above freezing, 1040mB, most definitely off band...
ha-full-disk-bw.jpg
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Re: Index of Air

Post by Valery » Mon Feb 09, 2015 11:42 am

marktownley wrote:Yesterdays full disk - poor seeing, temperatures couple of degrees above freezing, 1040mB, most definitely off band...
ha-full-disk-bw.jpg
Hi Mark,

Is it a DS or SS?

If it is SS, then I see no evidence of the off-band.


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Re: Index of Air

Post by marktownley » Mon Feb 09, 2015 2:56 pm

double stack
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Re: Index of Air

Post by Bob Yoesle » Mon Feb 09, 2015 6:47 pm

So, should we finally conclude that increasing n (pressure) is BLUE shifting?
Hmm... Right now I'd have to agree Valery. I could be wrong of course (I have neither a physics nor optics degree), but tilting increases the optical length traveled by a light ray. Increased air pressure (air density) increases the gap refractive index, and as far as the photon is concerned does the same thing (e.g. slows it down and it takes 'longer' to traverse the etalon gap)...

My very simple way of looking at it: CWL of un-tilted filter is a little red shifted off the H alpha line to accommodate changes in altitude and therefore decreased air pressure. Tilting will only blue shift an etalon to bring it on band, and more tilting (blue shifting) is needed in order to compensate for increasing altitude (decreasing pressure). This infers decreased atmospheric pressure results in a CWL red shift until we reach the "absolute" CWL of the filter in a vacuum (n = 1.0).

Same thing for Pressure tuning -- CWL to the red, increasing gap pressure (increasing the refractive index n) blue shifts the etalon to be on-band with increased altitude (decreased atmospheric pressure.) Decreasing the gap pressure red-shifts the etalon. Another reason why pressure tuning is superior to tilt tuning -- you can both blue and red shift the etalon, whereas with tilting it is only possible to blue shift.

Lunt apparently designs it's pressure tuned etalons to be on-band at a higher atmospheric (gap) pressure than found at sea level:
We have enclosed the etalon system within a sealed cavity.
The sealing of the cavity is done via the collimating and refocus lens so that the etalon itself is isolated from differential pressure.
The piston applies from zero to a pressure that is equivalent to taking an etalon from -500ft to +12,000ft above sea level.
This essentially makes the etalon system altitude insensitive.
In addition the etalon can be used from -50 to +200 degrees Celsius due to the fact that the tuning can compensate for the very small changes that heat would have on the "feet" of the etalon. (Emphasis added)
See: http://www.cloudynights.com/topic/21666 ... from-lunt/

Note this is a far greater adjustment range than can be compensated for by tilting - which is generally stated to be only about 2000 ft (~600 M) or so.

So the filter is "naturally" on-band at a high atmospheric pressure found at - 150 m. As we increase altitude, the atmospheric pressure drops, and the filter becomes more red-shifted off the H alpha line. We need to increase the gap pressure (blue-shift the filter) in order to keep the filter on-band.

Therefore we can again see that the etalon has a "red" shifted CWL at sea level and above, and increasing the gap pressure shifts the CWL to the blue to keep it on-band from - 500 ft (~ -150m) to sea level and on up to the maximum pressure allowed by the unit ~ 12,000 ft (~3660 m). I don't have much experience with pressure tuned etalons, but this seems to imply at least some pressure is needed for sea level observation, and increased pressure for increasing altitude or approaching low pressure weather systems.
Last edited by Bob Yoesle on Mon Feb 16, 2015 6:54 am, edited 2 times in total.
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Re: Index of Air

Post by Valery » Mon Feb 09, 2015 10:55 pm

Hi Bob,

You perfectly described the nature of the light behavior in a transparent medium with different refractive index.
Unfortunately, my english is really bad for such detailed descriptions.

Thanks.


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Re: Index of Air

Post by Merlin66 » Mon Feb 09, 2015 11:14 pm

Bob,
You mention:
""blue shifts the etalon to be on-band with increased altitude (decreased atmospheric pressure)""
then say the ""tilt can only compensate for a altitude of 600m"".....
I don't have the data to hand to verify the change in CWL with altitude, but generally there is far more tilt available into the blue than the red.....I would have thought more than sufficient to cover >600m
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Re: Index of Air

Post by Bob Yoesle » Mon Feb 09, 2015 11:49 pm

Valery - your English is way better than my Ukrainian!

Hi Ken,

Ooops, my bad. You're right - I was recalling the range for an internal etalon given by Andy Lunt awhile back (Lunt Blog, December 14, 2010 Internal Etalon Performance):
Lunt have learnt that internal Etalons are highly sensitive to tilt. I would guess that internal Etalons are about 5-6 times more sensitive to tilt than external Etalons. The reason comes down to the inability (physics) to PERFECTLY re-collimate the internal rays of the light path after the objective. Even though we are monochromatic, the compromises of light cone, image rays etc. cannot be overcome. Only optimized...

So, Lunt are producing internal tilt Etalons that are highly specified for internal systems. While a given system will work for a +/-2k feet change in altitude, people living at higher elevation need systems that are specific to their location.
This is the field angle magnification issue again. So a front (externally) mounted etalon will likely have more than enough tilt adjustment to compensate for much higher altitudes. If we take 5 times as the figure: 600 m x 5 = 3000 m, or about 10,000 ft, and roughly what can be achieved by the equivalent pressure tuned internal etalon - haven't tried it though!
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Re: Index of Air

Post by swisswalter » Tue Feb 10, 2015 5:07 am

Hi folks

fantastic discussions, thank you so much
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Re: Index of Air

Post by marktownley » Tue Feb 10, 2015 6:32 am

Yes, an excellent discussion indeed. I will put it in the library, but doing this locks it for repsonses, and I reckon we've got a bit more chatter left in this one yet...
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Re: Index of Air

Post by swisswalter » Tue Feb 10, 2015 4:53 pm

Yes please Mark, don't lock it at the moment, thanks in advance
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Re: Index of Air

Post by Bob Yoesle » Wed Feb 11, 2015 6:11 am

OK - here's confirming Valery's quote about the PT etalon CWL being on the red side, and postulations about the CWL shifting blue-ward with increased etalon gap pressure to effect the tuning range described above: http://luntsolarsystems.blogspot.com/20 ... chive.html

This is illustrated (shown below) as the CWL is going from the RED, to on-band, to BLUE with increasing the gap pressure (e.g. increased refractive index n). At pressure "0 PSI" the etalon is off-band RED (where it would be on-band at - 500 ft. elevation). At pressure "2.5 PSI" the etalon is now blue shifted to be on-band at 5000 ft., and at the max pressure of "5.0 PSI" it is BLUE shifted to be on-band at 10,000 feet elevation (from Lunt Solar):
Lunt Pressure Tuning sm.jpg
So we can see that an air-spaced etalon can be on-band at only a very specific value of n -- e.g. a refractive index dependent on air pressure (and to a lesser extent temperature and humidity). For the Lunt internal PT etalons this is stated to be the pressure at about - 500 feet elevation. Since sea level air pressure is 14.7 PSI, and Lunt states the the on band air pressure is + 2.5 PSI, we can infer the actual on band pressure is 14.7 + 2.5 = ~ 17.2 PSI. As atmospheric pressure drops to the sea level value of 14.7 PSI, the etalon is red-shifted requires 2.5 PSI of pressure applied by the pressure tuner to stay on band. If we take the scope to 10,000 feet, atmospheric pressure drops further, and the etalon is red-shifted even more, and requires 5.0 PSI to remain on-band.
Last edited by Bob Yoesle on Wed Feb 11, 2015 4:23 pm, edited 3 times in total.
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Re: Index of Air

Post by marktownley » Wed Feb 11, 2015 6:42 am

Great visual representation of this Bob!
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Re: Index of Air

Post by Bob Yoesle » Wed Feb 11, 2015 6:51 am

Hi Mark, I just cut and pasted it together form the Lunt Blog ;-)
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Re: Index of Air

Post by Merlin66 » Wed Feb 11, 2015 7:49 am

Great stuff!
Now, I need to calculate tilt angle equivalence to pressure difference....
I think we know 1 degree tilt gives a CWL shift of 0.4A from previous discussions....
How far off-band is the +2.5psi difference? 0.2A? or more.....
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