Index of Air
Index of Air
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
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
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Re: Index of Air
Hi Colin
thank you very much for the formula. Makes it more easy to understand
thank you very much for the formula. Makes it more easy to understand
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Re: Index of Air
[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?
Robert
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Re: Index of Air
Hi Colin,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
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
You've lost me here Valery, why would we conclude that?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.
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Re: Index of Air
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.
Mark W.
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.
Mark W.
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Re: Index of Air
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.marktownley wrote:You've lost me here Valery, why would we conclude that?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.
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
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.
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
September - October season. And then also (when higher than 768mm Hg) December somewhere +2-3C.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.
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
I'm no good at Excel either Mark
But I've had a play anyway: 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 ( ) 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
But I've had a play anyway: 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 ( ) 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
.
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Re: Index of Air
"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.
Valery.
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.
Valery.
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Re: Index of Air
Yesterdays full disk - poor seeing, temperatures couple of degrees above freezing, 1040mB, most definitely off band...
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Re: Index of Air
Hi Mark,marktownley wrote:Yesterdays full disk - poor seeing, temperatures couple of degrees above freezing, 1040mB, most definitely off band...
Is it a DS or SS?
If it is SS, then I see no evidence of the off-band.
Valery.
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Re: Index of Air
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Re: Index of Air
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)...So, should we finally conclude that increasing n (pressure) is BLUE shifting?
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:
See: http://www.cloudynights.com/topic/21666 ... from-lunt/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)
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
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.
Valery.
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.
Valery.
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Re: Index of Air
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
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
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):
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):
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!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.
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Re: Index of Air
Hi folks
fantastic discussions, thank you so much
fantastic discussions, thank you so much
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Re: Index of Air
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
Yes please Mark, don't lock it at the moment, thanks in advance
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Re: Index of Air
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):
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.
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):
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
Great visual representation of this Bob!
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Re: Index of Air
Hi Mark, I just cut and pasted it together form the Lunt Blog ;-)
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Re: Index of Air
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.....
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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Re: Index of Air
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.
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
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.
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
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
Mike,
The quark uses a solid etalon, temperature dependent...
Not sure what impact (if any) pressure/ air refractive index would have.
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
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
It is atmosphere pressure insensitive, cause it is not an air spaced, but mica spaced etalon. Sealed for the gel which is between themarktownley wrote:The quark is a sealed unit (allegedly according to daystar) and so should be not affected by barometric pressure changes.
etalon elements and blocking filter and cover - AFAIK.
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Re: Index of Air
good point there Valery
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Re: Index of Air
Guys,
I just re-found the earlier spreadsheet by Gert which addresses the atmospheric and tilt issues...
I knew it was somewhere!!!
I just re-found the earlier spreadsheet by Gert which addresses the atmospheric and tilt issues...
I knew it was somewhere!!!
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Re: Index of Air
Merlin66 wrote:Guys,
I just re-found the earlier spreadsheet by Gert which addresses the atmospheric and tilt issues...
I knew it was somewhere!!!
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
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...
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
Yes, tuned SM40. But the same with Lunt LS50F Ha.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...
I need to install this telescope on the equatorial mount and make a sequence of pictures at different
pressures and temp.
Valery
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Re: Index of Air
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/
Mark W.
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Re: Index of Air
mark,
Yes, that's what Gert's spreadsheet shows....
Yes, that's what Gert's spreadsheet shows....
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Re: Index of Air
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.
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 :
Successively increasing the refractive index by 0.05 results in the expected shortening of the wavelengths:
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...
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.
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.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
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 :
Successively increasing the refractive index by 0.05 results in the expected shortening of the wavelengths:
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
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%
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
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:
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:
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.
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:
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:
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
Hi Ken, Bob and Mark
a very interesting discussion
a very interesting discussion
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Re: Index of Air
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.
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
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:
Air:
Finesse 11A? - Oh, I see - the FSR is 11A, and the FWHM is 1A. Finesse is FSR/FWHM, but is dimensionless. ;-)
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:
Air:
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
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.
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
Reason: coorect an error
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Re: Index of Air
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.
Mark W.
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.
Mark W.
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Re: Index of Air
Thanks Mark. Always good to have some hard figures to work with.
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Re: Index of Air
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.
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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"Imaging Sunlight - using a digital spectroheliograph" - Springer
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Re: Index of Air
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.
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.
MarkW.
\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.
MarkW.
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Re: Index of Air
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:
λp = 2 n t cos θ
These are represented as:
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.
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: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.
λp = 2 n t cos θ
These are represented as:
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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