Index of Air

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Re: Index of Air

Post by grimble_cornet » Wed Feb 11, 2015 10:30 am

Great presentation Bob - thank you.
This thread is great as it is also helping me to get my head around the tuning problems I am having with the Quark.
.

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Re: Index of Air

Post by Merlin66 » Wed Feb 11, 2015 11:02 am

Bob,
It seems the pressure tuner is "tuned" on band for 17.2psi absolute.

The absolute pressure in the chamber will be dependent of the local atmospheric pressure.
At Sea level (14.7psi)
<+2.5psi chamber pressure (<17.2psi absolute) will be off band more and more towards the red....
+2.5 psi chamber pressure (17.2psi absolute)will always be on-band (no matter what the "external" pressure is..)
>+2.5psi chamber pressure (>17.2 absolute) will be off band more and more towards the blue.

So, if we opened the chamber at 5000 ft altitude, the atmospheric pressure would be (14.7-2.5) = 12.2psi. Boyles Law (http://en.wikipedia.org/wiki/Boyle's_law) says that more volume must be compressed to bring the pressure up to the 17.2psi absolute (=5psi chamber) pressure. The volume compressed needs to be greater with altitude. It also infers that at higher altitude, with the tuning piston removed, that the etalon will be well off band into the Red.
Last edited by Merlin66 on Wed Feb 11, 2015 12:05 pm, edited 1 time in total.
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Re: Index of Air

Post by Bob Yoesle » Wed Feb 11, 2015 4:24 pm

Ooops - you're right Ken, it was late and my arithmetic skills must have been asleep! I will correct the entry.
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Re: Index of Air

Post by Merlin66 » Wed Feb 11, 2015 9:07 pm

Mike,
The quark uses a solid etalon, temperature dependent...
Not sure what impact (if any) pressure/ air refractive index would have.
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Re: Index of Air

Post by marktownley » Wed Feb 11, 2015 9:28 pm

The quark is a sealed unit (allegedly according to daystar) and so should be not affected by barometric pressure changes.
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Re: Index of Air

Post by Valery » Wed Feb 11, 2015 10:27 pm

marktownley wrote:The quark is a sealed unit (allegedly according to daystar) and so should be not affected by barometric pressure changes.
It is atmosphere pressure insensitive, cause it is not an air spaced, but mica spaced etalon. Sealed for the gel which is between the
etalon elements and blocking filter and cover - AFAIK.

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Re: Index of Air

Post by marktownley » Wed Feb 11, 2015 10:32 pm

good point there Valery ;)
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Re: Index of Air

Post by Merlin66 » Wed Feb 11, 2015 10:40 pm

Guys,
I just re-found the earlier spreadsheet by Gert which addresses the atmospheric and tilt issues...
I knew it was somewhere!!!
etalon_tilt_pressure_v4.xls
(293.5 KiB) Downloaded 33 times
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Re: Index of Air

Post by Valery » Thu Feb 12, 2015 6:25 am

Merlin66 wrote:Guys,
I just re-found the earlier spreadsheet by Gert which addresses the atmospheric and tilt issues...
I knew it was somewhere!!!
etalon_tilt_pressure_v4.xls

Hah! Good with a tilt. And does not corresponds to the reality in CWL vs pressure.

This last week the barometric pressure were changed between 759 and 767 mm Hg.
The temperature and relative humidity was nearly the same temp were 2-3C.

The day with 759mm Hg I have to add a very little tilt to come to the Ha CWL - i.e.
with slight tilt the picture becomes more contrasty and a bit darker.
The day with 767mm Hg the picture were already in Ha and any even slightest tilt makes
it much less contrasty and brighter.


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Re: Index of Air

Post by Merlin66 » Thu Feb 12, 2015 7:21 am

Valery,
I do believe it does a very good job.

I would need to see your images to evaluate.
I assume we are talking about a tilt tuned etalon at various atmospheric pressures...
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Re: Index of Air

Post by Valery » Thu Feb 12, 2015 4:49 pm

Merlin66 wrote:Valery,
I do believe it does a very good job.

I would need to see your images to evaluate.
I assume we are talking about a tilt tuned etalon at various atmospheric pressures...
Yes, tuned SM40. But the same with Lunt LS50F Ha.

I need to install this telescope on the equatorial mount and make a sequence of pictures at different
pressures and temp.


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Re: Index of Air

Post by mdwmark » Sun Feb 15, 2015 7:41 am

I found this interesting but if the temperature stays the same and you increase the pressure the bandpass moves red. You can check it on http://lightmachinery.com/optical-desig ... -designer/
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Re: Index of Air

Post by Merlin66 » Sun Feb 15, 2015 8:06 am

mark,
Yes, that's what Gert's spreadsheet shows....
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Re: Index of Air

Post by Bob Yoesle » Mon Feb 16, 2015 8:51 am

Well this is rather strange.

We know that Andy Lunt specifically states the pressure tuned etalons are made to be to the red side of the H alpha line, and that by adding pressure in the sealed etalon cell, the air density in the gap would increase, and therefore the refractive index should increase as well. This is shown in Mike’s graph as well.

If an etalon starts out on the red side of the Ha line, how would the Etalon Designer calculator’s results showing red-shifting get it on-band? Perhaps the calculator can not deal with these relatively small refractive index changes in a very accurate manner?

First, refractive index (n) with respect to wavelength is defined as λ = λ0 / n, where λ0 is the wavelength in a vacuum. It is therefore easy to see that for any transparent medium more dense than a vacuum, the resulting wavelength will become shorter, not longer.
The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values. Emphasis added.

http://en.wikipedia.org/wiki/Refractive_index
Refractive Index Diagram.jpg
Refractive Index Diagram.jpg (18.17 KiB) Viewed 1810 times
So if the pressure tuned etalon is designed to be on-band at a high refractive index (low altitude), and we go up in altitude and have a reduced refractive index (wavelengths become longer), we have to increase the refractive index to shorten them (blue shift) to come back on-band.

When we use the Etalon Designer with larger refractive index changes, this is exactly what we see. I have designed the etalon to be on-band for H alpha in a vacuum, where n = 1.00 :
N 1.00.jpg
N 1.00.jpg (93.8 KiB) Viewed 1810 times
Successively increasing the refractive index by 0.05 results in the expected shortening of the wavelengths:
N 1.05.jpg
N 1.05.jpg (93.83 KiB) Viewed 1810 times
N 1.10.jpg
N 1.10.jpg (93.65 KiB) Viewed 1810 times
N 1.15.jpg
N 1.15.jpg (93.7 KiB) Viewed 1810 times
Can’t say I’m an expert in any of this, but it does seem to me that if we move to a higher altitude with decreased air pressure, e.g. less air density and hence a lower refractive index, the wavelength will lengthen towards what it would approach in a vacuum – and shift to the red. So we need to either tilt the etalon to blue shift the CWL back onto the H alpha line, or increase the air pressure > density > refractive index to do the same.

Looking forward to more discussion and elucidation...
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Re: Index of Air

Post by Merlin66 » Mon Feb 16, 2015 10:09 am

Bob,
Could be the parameters you selected...
The air gap in a "normal" Coronado etalon is close to 0.21mm ( as per Gert's analysis)
In your sheet you have 0.4mm - double the standard.....also the reflectivity is more like 80% to 85%
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Re: Index of Air

Post by Bob Yoesle » Mon Feb 16, 2015 4:02 pm

Hi Ken,

I just tried using different values that would get me a line close to the central transmission peak in a vacuum. But I don't see how changing any of those parameters should cause an overall change in the etalon behavior due to refractive index. But the program seems to - for larger refractive index changes, the transmission peak(s) are shifted blue-ward, but for real-world very small changes, the peaks are shifted red-ward.

Using the values you gave, for very small changes in refractive index we again have red-ward shifts of the transmission peaks. For larger values we again have a blue-ward shift, but it is quite small even when going from a vacuum refractive index to that of say fused quartz. Again as above I used a secondary wavelength distance of 0.5 nm for reference. The adjacent peaks reveal the shortening of the wavelength:
2 N 1.00.jpg
2 N 1.00.jpg (79.87 KiB) Viewed 1795 times
2 N 1.5.jpg
2 N 1.5.jpg (80.06 KiB) Viewed 1795 times
And if we make a very small adjustment to the spacing parameter to center the H alpha line, we get essentially no CWL shift at all, but do again have the shortening of the wavelength of the adjacent peaks:
3 N 1.00.jpg
3 N 1.00.jpg (79.46 KiB) Viewed 1795 times
3 N 1.5.jpg
3 N 1.5.jpg (77.12 KiB) Viewed 1795 times
Again this may be an issue with the calculator and what type or degree of changes it is meant to be able to model. I have made an inquiry.
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Re: Index of Air

Post by swisswalter » Mon Feb 16, 2015 7:53 pm

Hi Ken, Bob and Mark

a very interesting discussion
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Re: Index of Air

Post by Merlin66 » Mon Feb 16, 2015 9:30 pm

http://solarchat.natca.net/viewtopic.php?f=10&t=13782
Bob,
The finesse for my SM60 was around 11A, which seems to match with a blocking filter of 10A.
Gert's work seems to confirm:
2.5psi change gives 0.4A shift, 1 degree tilt gives 0.4A shift (to the blue) at constant temp...
I had hoped that the design CWL point was a bit further into the red than the 0.2A being found...this barely gets the CWL into the red wing of Ha.
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Re: Index of Air

Post by Bob Yoesle » Tue Feb 17, 2015 4:24 am

Hi Ken,

Hmmm....

Christian's graphs show the same 0.4 A tilt shift as well, but only for a mica etalon. He notes that "If we assume Coronado filters are air-spaced, then a 1° tilt shifts the CWL by 1 A (versus 0.4 A for mica-spaced F-P etalon)."

Mica:
Daystar-tilt.JPG
Air:
Coro-60mm-profile-tillt.JPG
Finesse 11A? - Oh, I see - the FSR is 11A, and the FWHM is 1A. Finesse is FSR/FWHM, but is dimensionless. ;-)
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Re: Index of Air

Post by Merlin66 » Tue Feb 17, 2015 4:38 am

You are correct - FSR!

Re-checking Gert's spreadsheet, I was in error...it actually shows a 1A blue shift for a 1 degree tilt.
This agrees with your statement.
Sorry for the confusion.
Last edited by Merlin66 on Tue Feb 17, 2015 4:44 am, edited 1 time in total.
Reason: coorect an error
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Re: Index of Air

Post by mdwmark » Wed Feb 18, 2015 8:28 am

OK, from the calculator.
656.3nm,90% mirror,.15mm spacer,32mmCA,.1arcsec,1rms nm surface gives a .08nm HW, 1.43nm FSR. This is what your normal air spaced etalons are based on.
Using the index from the graph for air of 1.000280 you have a peck at 655.2058nm CWL.
Using the index from the graph at a higher pressure of 1.000300 you have the peck at 655.2230nm CWL.
As the pressure increases the peak shifted red.
I asked the etalon engineer from LightMachinery just to be sure.
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Re: Index of Air

Post by marktownley » Wed Feb 18, 2015 8:46 am

Thanks Mark. Always good to have some hard figures to work with.
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Re: Index of Air

Post by Merlin66 » Wed Feb 18, 2015 9:59 am

Mark,
Interesting but....
We find the spacing closer to 0.215mm, and the FSR at 11A.....the FWHM was approx. 0.8A
Based on measurements from a Coronado SM60 etalon.
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Re: Index of Air

Post by mdwmark » Wed Feb 18, 2015 6:27 pm

This is what I came up with as the design they are working for. I had checked an 60mm Coronado years ago. The inter-order spacing was about 15Ang. The main blocker bandpass would had been 6-8Ang HW to block the next peak.
Etalon Designer _ LightMachinery.pdf
(134.47 KiB) Downloaded 30 times
\
I don't know how you posted the image of the designer page. But if the attachment does not come out the spec are,
656.3nm CWL,95%T mirror,spacer .154+mm fuse Silica, 32CA,incident angle 1.3 deg, 50th wave, mirror error .02 arc sec,surface RMS 5nm, this give you 29%T,.77Ang HW,14 FRS. This matches what I measured before.
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Re: Index of Air

Post by Bob Yoesle » Thu Feb 19, 2015 12:01 am

Still looking into the complexities and waiting to hear back form some experts, but John Hunter from Light Machinery did get back to me and states:
Thanks for your inquiry. Etalon shifts are a little baffling. Increasing thickness and index and pressure definitely shift individual peaks to the red. It would appear that the original [Lunt] diagram you show is simply incorrect. Meanwhile, tilt tuning does shift things towards the blue and that is always counter intuitive to me but it is definitely the case.
Doing more research, I think I’ve found the answer to the question(s) of transmission peak wavelength shifts for tuning in FP etalons through the basic etalon theory formula:

λp = 2 n t cos θ

These are represented as:
fpplate.jpg
fpplate.jpg (22.64 KiB) Viewed 1726 times
See: http://www.cfht.hawaii.edu/Instruments/ ... icfp5.html

These parameters are defined as:

n gap refractive index
t gap thickness
θ angle of incidence (tilt)

Therefore we can see that:

1. As refractive index increases, the wavelength will increase (peak(s) shift red-ward);

2. As the gap thickness increases, the wavelength will increase (peak(s) shift red-ward);

3. As the tilt angle increases, the cosine decreases, and the wavelength decreases (peaks(s) shift blue-ward).

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.
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