Index of Air

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Valery
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Re: Index of Air

Post by Valery » Thu Feb 19, 2015 1:40 am

Bob Yoesle wrote:
So for etalon tuning, item one (1) is air pressure tuning as implemented by Lunt, which effects a change in refractive index n. Item two (2) is where “rich-view” mechanical pressure tuning is used by Coronado to change the actual gap thickness t. For both filter makes, tilt tuning is utilized to change the incidence angle θ for front mounted etalons. Rear mounted etalons use temperature, which affects both the gap thickness t, and the refractive index n.
Today I performed a little experiment. I observed the sun at the same altitude and barometric pressure, but at the different temperatures. I observed the sun from outside at my backyard at 767nm Hg and -5C and from inside (+25C) through a window with good flat glasses surfaces. My estimation of the air refractive index change is from 1.000263 (inside) to 1,000275 (outside).
The CWL shifted red when inside (lower refractive index).
So, we can conclude that with the refractive index goes smaller, the etalon's CWL shifts red.
The only factor of uncertainty is the etalon's spacers thickness change. With it's increasing the CWL should shift red-ward. So, I am not sure what is the reason of this CWL shift - the air refractive index change or the etalon spacers thickness change.


Bob,

As some peoples realized, the Rich View tuner does not change the pressure on the etalon plates and does not change the etalon gap. It just changes the range where one can tilt the etalon. The same is with PST etalons.



Valery.
Last edited by Valery on Thu Feb 19, 2015 9:28 pm, edited 1 time in total.
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Re: Index of Air

Post by Merlin66 » Thu Feb 19, 2015 2:12 am

Some comments:
Valery re-read your message - it states the index was HIGHER indoors - hence moves to the red.
Also, the small spacers used in the air-spaced etalons I believe a quartz crystal flakes - about 0.2mm thick. I haven't check recently the thermal expansion of quartz, but I know it's VERY low.
I'm intrigued by the comment on the Rich View....I had cause recently to pull one apart (SM90 etalon) and was a little surprised to see the small nylon tip pressing directly (well, on the silly circular piece of aluminium tape) on the centre of the external plate which was NOT supported in any way, just sitting on the spacers, no side restraint.
The rear of the etalon (the second plate being held with numerous blobs of silicon around the edge) was supported by a resilient rubber (?) ring. This made me question how much pressure could actually be applied (but then I remember seeing someone who had managed to crack front plate!!)
This Rich View "arrangement" has no direct bearing, in my opinion, on the tilt you can apply via the T-max adaptor, so I don't know how it would affect the "range were one can tilt the etalon" Maybe you have some additional info.
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Re: Index of Air

Post by Valery » Thu Feb 19, 2015 9:31 pm

Merlin66 wrote:Some comments:
Valery re-read your message - it states the index was HIGHER indoors - hence moves to the red.
Also, the small spacers used in the air-spaced etalons I believe a quartz crystal flakes - about 0.2mm thick. I haven't check recently the thermal expansion of quartz, but I know it's VERY low.
I'm intrigued by the comment on the Rich View....I had cause recently to pull one apart (SM90 etalon) and was a little surprised to see the small nylon tip pressing directly (well, on the silly circular piece of aluminium tape) on the centre of the external plate which was NOT supported in any way, just sitting on the spacers, no side restraint.
The rear of the etalon (the second plate being held with numerous blobs of silicon around the edge) was supported by a resilient rubber (?) ring. This made me question how much pressure could actually be applied (but then I remember seeing someone who had managed to crack front plate!!)
This Rich View "arrangement" has no direct bearing, in my opinion, on the tilt you can apply via the T-max adaptor, so I don't know how it would affect the "range were one can tilt the etalon" Maybe you have some additional info.
My fault, Ken. Air refractive index was 1,000263 inside. I edited the post. Thank you for pointing out this my error.
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Re: Index of Air

Post by Valery » Thu Feb 19, 2015 9:41 pm

Bob,

You took too much air index change. In this case the picture map of the peaks changes so, that you see ANOTHER peak near the 6562.8A. You need to take the following
samples: R1= 1,00028, R2=1,00027, R3=1,00026, R4=1,00025 etc and see that peaks slowly moves blue with refractive index decreasing.


Valery.
Last edited by Valery on Fri Feb 20, 2015 11:12 am, edited 1 time in total.
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Re: Index of Air

Post by Bob Yoesle » Fri Feb 20, 2015 7:12 am

Thanks Valery.

OK, I see that: increased refractive index (air pressure) of the etalon gap shifts the transmission peak red-ward.

So to resolve the paradox in my thinking, I have to separate the world of the etalon gap and the world of the atmosphere and elevation with respect to refractive index changes.

In the atmosphere, a decreased air pressure as we gain elevation results in a decreased refractive index, and the H alpha line is shifted red-ward until we get to the vacuum of space, where the wavelength will be defined by: λair = λvac / n. So if we define the sea level index of refraction as 1.00028, λvac = 656.28 nm x 1.00028 = 656.46 nm

So to compensate for this increasing red-shifting of the transmission peak with altitude, we can use tilting, which per λp = 2 n t cos θ, the increased angle of incidence through the etalon gap will decrease the wavelength of the peak transmission. The etalon used for tilt tuning would therefore appear to have to be on-band at higher elevations, and tilted to be shifted blue-ward as we approach and get to sea level elevations. Put another way, the etalon is “tuned” to be on the red side of the H alpha emission when near sea level, and must be tilted to come on band. As we gain elevation, less tilt is required to put the etalon on the H alpha line:
Blue.JPG
Blue.JPG (100.03 KiB) Viewed 1524 times
For pressure tuning, the opposite is the case. We can use increased gap pressure/refractive index, which per λp = 2 n t cos θ, the increased pressure/refractive index of the etalon gap will increase the wavelength of the peak transmission. The etalon would appear to have to be “tuned” to on the blue side of the H alpha emission when near sea level, and must have the pressure increased to shift the transmission peak red-ward as we gain elevation:
Red.JPG
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Re: Index of Air

Post by Merlin66 » Fri Feb 20, 2015 10:08 am

Bob, now we have clarified that, we need to address the sweet spot in tilt systems vs pressure tuned.................
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Re: Index of Air

Post by marktownley » Fri Feb 20, 2015 11:40 am

Great thread fella's, i'm enjoying reading it :)
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Re: Index of Air

Post by swisswalter » Sat Feb 21, 2015 7:47 am

Hi solar Freaks

most interesting, keep on with the good explanations
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Re: Index of Air

Post by Bob Yoesle » Tue Feb 24, 2015 3:43 pm

Bob, now we have clarified that, we need to address the sweet spot in tilt systems vs pressure tuned...
In a previous thread (http://solarchat.natca.net/viewtopic.php?f=9&t=14573), we established:

Therefore the Jacquinot spot peak wavelength variation limit can be mathematically expressed as:

Equation 1: Δλ = √2 x FWHM

The tilt (field) angle verses wavelength change can be found with Christians formula above for the CWL shift:

(Equation 2): Δλ = ½ (CWL / n^2) θ^2

We can now solve for θ:

√2 x FWHM = ½ (CWL / n^2) θ^2

θ^2 = √2 x FWHM ÷ ½ (CWL / n^2)
This fits perfectly with the curves derived by Christian (http://www.astrosurf.com/viladrich/astr ... P-Coro.htm):
Jacquinot-Fig.JPG
Jacquinot-Fig.JPG (85.58 KiB) Viewed 1493 times
So we can see that for the relatively small changes in refractive index n that occur with pressure tuning, there will be a very small (likely imperceptible) change in the size of the Jacquinot ("sweet") spot. It appears for tilting, the size of the spot will not change, but will also shift to follow the CWL shift:
Coro-60mm-profile-tillt.JPG
As can be seen, the change in CWL varies quadratically with tilt, so the greater the tilt the the more the edge of the sweet spot will shift off-band as well.

And getting back to Robert's original question:
Does that mean an etalon designed for tilting will be wrongly setup to be pressure tuned with positive pressure increase?
If my assumptions from the above post are correct, the answer appears to be quite definitely "yes." However, it would therefore appear possible to apply a negative pressure or "vacuum tuning" to a tilt tuned etalon enclosed in an identical pressure-tuned embodiment in order to shift the CWL blue-ward...

Hand vaccuum pumps are readily available, and it would appear to be an excellent DIY project:
hvpo_04010.jpg
hvpo_04010.jpg (112.12 KiB) Viewed 1493 times
Of course, one could get quite more elaborate ;-)
Last edited by Bob Yoesle on Tue Feb 24, 2015 6:07 pm, edited 1 time in total.
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Re: Index of Air

Post by swisswalter » Tue Feb 24, 2015 5:23 pm

Hi Bob

most interesting with the vacuum. We could do that easely with a valve at the black PT knob of the LUNTS ;) (pat. pending :lol: )
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Re: Index of Air

Post by Bob Yoesle » Tue Feb 24, 2015 5:58 pm

Hi Walter,

I would make an intermediate coupler (between the etalon housing and the attached pressure tuner) with a vacuum line connection and stop valve, and then use the pressure tuner for fine adjustment.

(pat. pending :lol: )

Yeah, I should have patented the enclosed/vented "safe" Herschel wedge as well... http://www.cloudynights.com/topic/33027 ... wedge-mod/
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Re: Index of Air

Post by swisswalter » Wed Feb 25, 2015 5:13 am

Hi Bob

a cool design of that vented herschel wedge
Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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