How much pressure is in a pressure tuned etalon? And more

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Bruce G
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How much pressure is in a pressure tuned etalon? And more

Post by Bruce G » Wed Sep 09, 2020 5:54 pm

I could find only one other thread on the site that mentioned this and it seems to have ended shortly after the subject was brought up. I thought I would try to put some numbers to the question and what I found surprised me. I ask anyone who is interested in the subject to please help check my calculations. Specifically, this applies only to Lunt pressure tuned etalons, but I imagine that the same operating principle, and hence a similar pressure range, would be used by other PT systems

The rationale
Why do we care? If you have never operated a Lunt PT etalon, you should prepare yourself. You need forearms the size of Popeye's to do it. And the action of adjusting the etalon can be enough to throw the entire mount off alignment
Popeye3.jpg
Popeye3.jpg (46.28 KiB) Viewed 278 times
OK. Maybe I'm exaggerating a bit, but it takes some real force to adjust the etalon and it's nearly impossible to assess the changes as you are making the adjustment because of the motion. Wouldn't it be nice then, to simply carry a small compressed air source connected to the etalon with a couple of valves, one to increase the pressure in the etalon and one to bleed it off to the atmosphere? That's pretty much what's behind the subject question. How much air pressure do we need to deliver to an etalon to make it function within its normal range of operation? Lunt does offer such an accessory, but at a price point that immediately sends people looking for other solutions, almost as much as the etalon itself. So what pressure range would it take to operate a PT etalon?

Calculations
I made all of my measurements on a Lunt DS-II module. The units of measurement make no difference, as all units divide out in the end. I used millimeters. I also chose to use bar as the unit of pressure, as it makes things particularly easy compared to absolute pressure or gauge pressure. I do switch to kPa at one point when discussing actual pressure gauges because most likely you wouldn't choose to monitor pressure in bar and kPa makes the numbers easy.

Piston
The internal diameter of the piston tube is 45 mm and its length is 43 mm. But the o-ring of the piston sits 5 mm back from the face of the piston and therefore the piston travel can be no more than 43 - 5 = 38 mm. A 45 mm diameter with a 38 mm displacement results in a displacement volume of 60436 cubic mm.

The etalon pressure chamber
Things become more difficult here because it's not possible to obtain accurate measurements inside the sealed etalon and I don't really know much about its internal construction. But from what I can see, we have a chamber that appears to be 35 mm in diameter, based on the diameter of the glass end plates. Maybe a little less because the glass has to sit on some sort of support. The length of the chamber is much harder to determine, but appears to be not much more than 10 mm. Of course the glass and support structures inside the etalon also subtract from the volume. I decided to simply use a range of lengths to put bounds on what the pressures might be, calculating values for free air space of 3, 5, 7.5, 10 and 15 mm. There are small passages inside the etalon to allow air flow, but I'm neglecting those and saying that their volume is similar to the amount of volume that I failed to subtract earlier for support structures of the exterior glass.

Volume and pressure ratios
Our good friend, the Ideal Gas Law (PV = nRT), allows us to determine pressures from volume ratios. Assuming no leaks in the system, n is constant, R is already a constant and we can consider T to be constant if the temperature effects of compressing the etalon air are negligible or else we allow the system to restabilize to ambient temperature. We're left then, with PV = a constant. Pressure and volume are reciprocals. So if I know the ratio of the uncompressed to compressed air volumes, then I also know the associated pressures. The total uncompressed volume is the piston volume (Vp) plus the etalon chamber volume (Ve) and the compressed volume is simply the etalon chamber volume. So the volume ratio from uncompressed to compressed is (Vp + Ve)/Ve. Below is the relevant portion of my spreadsheet.
Etalon volumes2.jpg
Etalon volumes2.jpg (126.19 KiB) Viewed 278 times
The volume ratio can be thought of directly as pressure ratios. If I start at one bar, one atmosphere of pressure, and decrease the volume by a factor of 5, then the pressure must increase by a factor of 5, from 1 to 5 bar. The volume ratios in the spreadsheet represent the maximum pressure, in bar, that would be expected in the etalon chamber, given the the unobstructed air column lengths shown. Chances are that a length of 15 mm is too large and 3 mm is too small. I would lean toward the smaller numbers being more likely though, so the pressure range that I would expect to see inside the etalon chamber would vary from 1 bar up to 10 or maybe 15 bar, 1500 kPa or 225 psi. This was much larger than I had expected. Since density is mass divided by volume, this also tells us that the density of air inside the etalon, which is actually what changes the passband wavelength, varies by a factor of up to 10 or maybe 15.

Practical considerations
I was not able to provide precise values for the pressure range of a PT etalon, but I feel that I have put a pretty good window around it (and I invite comments and corrections). Given the values that I have calculated, it’s clear that this can’t be considered a low pressure system and leak control will be an issue. There are two ways to deal with leaks, prevent them or compensate for them. Most home-built things have trouble controlling pressures in the range that we’re talking about. O-ring seals are not for the average home machinist, and quality components can be expensive. I feel that it would be better to seal up what you can and compensate for the leaks as necessary.

The pressure source must also be considered. If I need to operate up to 15 bar, I’ll want to store air at a higher pressure than that, then I need a safety factor on top of that, so I would want a pressure tank rating of 25 or 30 bar. In imperial units, I would probably want to go for a 500 psi cylinder. Maybe a little overkill but I have worked with high pressure for a lot of years and you always want to stay on the conservative side (what if you drop the pressurized cylinder and it hits a sharp rock?). Never go cheap when it comes to high pressure. We don’t need very much volume at all. A 1 liter bottle would be far more than enough. A 500 psi pressure rating is nothing in a small diameter bottle, so we’re not talking much money.

The most critical part, and likely the most expensive part, is a good pressure gauge. It’s not at all necessary to know the true pressure of the etalon, but it’s essential that the pressure remains constant, once the etalon has been set. I don’t know how much the passband of an etalon changes from 1 bar to maximum pressure. If someone has a handle on that number, it would be helpful in knowing the pressure stability required to keep the etalon on band. But we can see that the pressure range is large. If I’m operating from 100 kPa (1 bar) to 1000 kPa (10 bar), then can we assume that we would need to know the pressure to at least 10 kPa (1% of full reading), or better, to 1 kPa (0.1% of full reading). The point at which pressure gauges start getting expensive is anything below 0.25% of full scale. I think that 0.25% would probably be good enough for this application. A pressure gauge of that accuracy is about $300.

I haven’t put much thought into the rest of the system, but at least a pressure source, a pressure control valve, a bleed valve, some tubing and connections will be needed. The connection to the etalon needs some thought. I think that leaks could be kept small enough to be a manually operated system (cheapest option). This is well within the capability of an Arduino to automate, but then you need to upgrade the pressure gauge to include an output and you need actuators to control the valves. Soon you begin to see why Lunt’s version costs as much as it does (does it contain an air compressor too?). But I think manual operation may be practical, making the major cost components the pressure gauge, an etalon adjustment cap (with plunger) from Lunt if you don’t want to sacrifice your own, and a couple of decent valves. I would love to get a two-way valve so that I could have a lever allowing me to press one way to increase pressure, the other way to decrease it.

If any of you can shed some more light on what the inside of the etalon looks like, it would help to put a much tighter window around the pressure range.

Thanks

Bruce G
Last edited by Bruce G on Thu Sep 10, 2020 1:07 am, edited 1 time in total.



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Re: How much pressure is in a pressure tuned etalon? And more

Post by DeepSolar64 » Wed Sep 09, 2020 6:20 pm

These may have some info too.

I have no personal experience with Lunt. I have two Coronado tilt-tuned scopes.





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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bruce G » Wed Sep 09, 2020 6:27 pm

Thanks. I've already disassembled to that level. What I need to know is what the inside of the sealed part looks like.

The videos do illustrate the pieces of hardware that I'm talking about very nicely though, so that is helpful



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Re: How much pressure is in a pressure tuned etalon? And more

Post by DeepSolar64 » Wed Sep 09, 2020 6:49 pm

They mention that the pressure is really low. Maybe 2-5 psi.


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Re: How much pressure is in a pressure tuned etalon? And more

Post by MapleRidge » Thu Sep 10, 2020 12:13 am

Hi Bruce...

I use the remote tuner for my pressure tuned Lunts, with a compressor that maintains pressure at the set point through a desktop app (or by push button on the main unit). It will run from 0-25PSI...the upper end is set to cut off to prevent damage to the PT.

On average, I find the 15PSI setting gives me the best tuning on my modded scope using a pressure tuned DSII module. I think the DSII module on the LS80T scope it was designed for needs a slightly different pressure but not off by much (the DSII module is on the modded scope more often than not).

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Re: How much pressure is in a pressure tuned etalon? And more

Post by MapleRidge » Thu Sep 10, 2020 12:20 am

Bruce...

My reason to go to the remote control is due to the cold winter weather making the PT knob very stiff to turn (I've imagined at -25C or so on occasion).

A second benefit is it allows the pressure to be re-set quite close to the same setting as I move form one AR to another (well when there are AR's on the disk) and adjust the pressure to image in the wings and be able to go back to the same pressure settings over the course of the imaging session. This can be done manually, but without the same precision.

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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bruce G » Thu Sep 10, 2020 1:06 am

Brian

Thank you. That helps a lot. There must be more internal volume in there than is apparent. A lower pressure makes everything a whole lot easier. I kind of wonder what would get overpressured above 25 psi. Internal components are solid, so a pressure differential appears only at the end seals and they're held in place by screw rings. Maybe the glass is thin and would pop.

You and I are at the same latitude and have similar temperatures. You understand what I mean about needing the forearms of Popeye to turn those things.

Being able to repeatably set the etalon based on observed pressure is certainly a benefit of having direct control of the pressure. I suppose it could be said that if I can afford a solar telescope, I can afford Lunt's little controller. But I don't know if it's because I'm cheap, because I'm a DIY kind of guy, because I'm an engineer or what, but something in me rebels against the microcontroller driven solution when it seems that a carpenter's portable air tank, two valves and a decent pressure gauge would put you in business pretty quickly.

One more question. Are you saying that you have a compressor that is controlled by the Lunt unit or that the Lunt unit contains a compressor?

I appreciate your input. I was definitely heading off in the wrong direction.

Bruce G



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Re: How much pressure is in a pressure tuned etalon? And more

Post by solarnoob » Thu Sep 10, 2020 1:26 am

MapleRidge wrote:
Thu Sep 10, 2020 12:13 am
On average, I find the 15PSI setting gives me the best tuning on my modded scope using a pressure tuned DSII module.
Brian
15PSI most of the time with my Lunt 100 SS.

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Re: How much pressure is in a pressure tuned etalon? And more

Post by DeepSolar64 » Thu Sep 10, 2020 1:31 am

Bruce, if I understand the principle right the pressure tuner traps air within it and one varies the pressure simply by screwing the cap in or out. There is a hole inside at the base of the unit that lets air inside to vary the pressure between the etalon plates. The thing is o ring sealed and you set it by removing the cap therefore setting the base pressure to your altitude. Then put the cap back on and then screw in to vary the pressure on the etalon. There is no powered compressor at all. My scope is tilt tuned and tilting the etalon is used to get on band instead of air pressure. Someone may correct me if I am a bit off on Lunt’s pressure tuners. I don’t own one.

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Re: How much pressure is in a pressure tuned etalon? And more

Post by DeepSolar64 » Thu Sep 10, 2020 1:45 am

The cap acts like a piston. Compressing the air as it is adjusted.

Brian, I never knew Lunt had a remote compressor available for the tuner.


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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bob Yoesle » Thu Sep 10, 2020 5:30 am

The LUNT PCUSB Controller:

https://luntsolarsystems.com/shop/acces ... ontroller/

This diagram from LUNT shows what at typical air-pressure tuned etalon does:

Etalon pressure tuning.jpg
Etalon pressure tuning.jpg (120.96 KiB) Viewed 213 times

Note that at sea level with no additional pressure the filter is off-band blue, and needs to be increased 17.237 kPa (2.50 PSI) to get on-band at 656.28 nm.


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Re: How much pressure is in a pressure tuned etalon? And more

Post by RodAstro » Thu Sep 10, 2020 6:51 pm

Hi Bruce
It seems to me the simplest and cheapest solution, would be to remove the existing pressure tuner and have it on a remote flexible pipe.
With the unit attached to a machined fitting that you could get hold of with one hand whilst turning the outer housing with the other hand it would be much easier to move and the flexible pipe would stop you moving the scope.

You could also make a second fine pressure tuner along the same principles but with a much smaller internal bore and a long throw (thread) to make it easy to adjust, use the original tuner to get up to pressure then the second one for getting perfectly on band.

Another option with the remote tuner would be to keep the tuner in a heated box when not tuning in cold weather so it moves easier when you come to adjust it. Once on band you may be able to make an electronic circuit using a pressure sensor and a PWM unit that adjusts the heat to keep it on band.
Then you could use heat to make fine adjustments.

Just a couple of ideas

Cheers Rod



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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bruce G » Thu Sep 10, 2020 7:23 pm

Bob,

Excellent! The plot tells me everything that I need to know. Where did you find that? I was looking for detailed information on the etalons, but never got much past marketing info or general descriptions.




Rod,

I like your suggestion of the two stage adjustment. Did you notice that the cap is quad-threaded? One turn of the cap advances the piston four thread pitches. That's part of why they're hard to turn. I can see a solution where the cap is cut off right at the back and then mounted on to a collar that could thread (with a fine thread) over where the back knurled area is. There's precious little material to work with there. And it's aluminum. Aluminum doesn't like fine threads. You could really do it right if you could make a new cap out of brass and with a little larger diameter to accept the threads.

It's an intriguing idea. Thanks

Bruce G
Last edited by Bruce G on Thu Sep 10, 2020 7:49 pm, edited 1 time in total.



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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bob Yoesle » Thu Sep 10, 2020 7:25 pm

You could also make a second fine pressure tuner along the same principles but with a much smaller internal bore and a long throw (thread) to make it easy to adjust, use the original tuner to get up to pressure then the second one for getting perfectly on band.
Ralston ( https://www.ralstoninst.com/calibration-test-pumps ) makes a precision hand pump that can do pressure, vacuum, or both, with both coarse and fine tuning adjustment. I use it on my vacuum-tuned internal etalon: viewtopic.php?f=10&t=15443&start=75#p196348


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Re: How much pressure is in a pressure tuned etalon? And more

Post by solarnoob » Thu Sep 10, 2020 7:38 pm

Amazing project! Thanks for sharing.

Frank


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Re: How much pressure is in a pressure tuned etalon? And more

Post by Bruce G » Thu Sep 10, 2020 7:51 pm

Bob Yoesle wrote:
Thu Sep 10, 2020 7:25 pm
Ralston ( https://www.ralstoninst.com/calibration-test-pumps ) makes a precision hand pump that can do pressure, vacuum, or both, with both coarse and fine tuning adjustment. I use it on my vacuum-tuned internal etalon: viewtopic.php?f=10&t=15443&start=75#p196348
Sweet! I'm going to have to look into that

Thanks

Bruce G



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Re: How much pressure is in a pressure tuned etalon? And more

Post by MapleRidge » Fri Sep 11, 2020 12:22 am

Bruce...

This is a link to their remote system...a tad expensive, but I wasn't into making up a system myself.

The control box has the electronics to interface with the PC via USB or you can adjust the pressure with the push buttons on the from to it. Inside is a small compressor, battery for stand alone operation, and some additional hardware to make it all work.

https://luntsolarsystems.com/shop/acces ... ontroller/

I'm not sure how widely they have been sold, though I have heard form a few people that have one.

Brian


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