fan operation without EFA

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Willem Jan Drijfhout:
John Hayes:
Yuxuan:
Wow, this makes me want to stay with my AGO iDK. Its thermal control unit is fairly simple: fans and heaters automatically work together to keep the mirror temperatures within a range above ambient set by the user.

It doesn’t monitor humidity, but I just set the target temperature 0.2C above ambient. Since dew point is always lower than the temperature, it keeps the mirrors dry.

I image in Florida with daily thunderstorms and have yet had dew on the mirrors. The scope, on the other hand, is drenched with dew in the morning.

Yuxuan,
That's exactly the way that the Planewave system works and it's no good. First, I don't want the heaters on at all when there's no danger of dew. Second, your approach of setting the target at 0.2C above ambient will indeed minimize heat radiating from the mirrors but you are forgetting about radiative cooling! The surface of your mirror can be as much as 2-3C below ambient simply due to heat lost to the sky. The actual temperature drop from radiative loss depends on a lot of factors. One of them is the kind of tube that you have. If it's an open truss tube, the drop will be larger than with a closed tube, which acts like a dew-shield by limiting the solid angle of sky exposure. There are other factors as well (such as the sky temperature) and that may be why it's working for you in FL. Either way, setting the target to be only 0.2C above ambient is really tight and I'm inclined to think that you may have just been lucky...so far.

John

Would a shroud help to reduce radiative cooling? Or would that be too thin to have any meaningful effect?

I don’t think a shroud has any effect on radiative heat loss of the mirrors, unless you treat your shroud with highly reflective coating like a hot water jug. But that obviously won’t work on a telescope…

5.01 Yuxuan #... · view views
John Hayes: Yuxuan, Yes, the PW system has the heating pad on the back of the mirrors. I’m not 100% sure where the sensor is located but I think that it’s also located on the back of the mirrors. Even if the sensor is on the edge of the mirrors, you still have to take into account the fact that you still aren’t measuring the temperature where it counts and the fact that unless you have VERY carefully calibrated your thermocouples, there are systematic errors in the measurement. Most “garden-variety” thermocouples are only accurate to +/- 1-2C, which is why I’d be very cautious about setting your temperature threshold at only 0.2C above ambient. The errors in what you are measuring are almost certainly larger than that! You also have to realize that there is some variability in the TDPS in cells of air that move past your scope. The chance of fog is quite high whenever the TDPS is less than 3C so that’s a good number to use for setting up your system as well. These are some of the factors that I’ve taken into consideration to set my TDPS threshold at 6C before I start the heaters. I then have the system set to hold the temperature of the mirrors at 2C above ambient when the heaters come on. John John, I’d say putting the sensor around the mirror is better than at the back, but it’s definitely not measuring the temperature where it counts. Yes, I did calibrate my temp sensors to eliminate systematic error. I’ll try setting the temp offset for secondary higher next time to see if it works better. Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. Like

5.01

#...

John Hayes:
Yuxuan,
Yes, the PW system has the heating pad on the back of the mirrors. I’m not 100% sure where the sensor is located but I think that it’s also located on the back of the mirrors. Even if the sensor is on the edge of the mirrors, you still have to take into account the fact that you still aren’t measuring the temperature where it counts and the fact that unless you have VERY carefully calibrated your thermocouples, there are systematic errors in the measurement. Most “garden-variety” thermocouples are only accurate to +/- 1-2C, which is why I’d be very cautious about setting your temperature threshold at only 0.2C above ambient. The errors in what you are measuring are almost certainly larger than that! You also have to realize that there is some variability in the TDPS in cells of air that move past your scope. The chance of fog is quite high whenever the TDPS is less than 3C so that’s a good number to use for setting up your system as well. These are some of the factors that I’ve taken into consideration to set my TDPS threshold at 6C before I start the heaters. I then have the system set to hold the temperature of the mirrors at 2C above ambient when the heaters come on.

John

John, I’d say putting the sensor around the mirror is better than at the back, but it’s definitely not measuring the temperature where it counts. Yes, I did calibrate my temp sensors to eliminate systematic error. I’ll try setting the temp offset for secondary higher next time to see if it works better.

Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

18.84 Ruediger #... · view views
The shroud has a devastating effect.It amplifies the the tube seeing dramatically. I can only use my CDK in summer only without the shroud. To reduce the tube seeing I keep the fans running while shooting - but they are so loud, that I have to switch them off not to late because the neighbors complain. Coming to the sensors: I have tried to find out where the sensors are together with PWI support. I became aware, they even do not have a real fixed position. The M1 sensors are placed by the technician based on his "daily mood". The do not have a strictly defined position. You only no that when opening. The back plate sensors are attached to aluminum. The M1 sensors we could not located for sure -not even asking the guys in the shop floor. M2 the same.You need to tear it down and check it. BTW: The heating is not attached to Mirror, but to metal of the back plate. That means, actually you are heating the back plate. To be honest, I was shocked and gave up in the end. Since using DHM I got it under control, but the heating / cooling concept needs serious rework. Also the cabling... It all looks like some wild tinkering. BTW: The shroud on the CDK produces additional diffraction spikes. You have to take counter measures. This also took some time to elaborate. Edited ... Like

18.84

Ruediger

#...

The shroud has a devastating effect.It amplifies the the tube seeing dramatically. I can only use my CDK in summer only without the shroud. To reduce the tube seeing I keep the fans running while shooting - but they are so loud, that I have to switch them off not to late because the neighbors complain.

Coming to the sensors: I have tried to find out where the sensors are together with PWI support. I became aware, they even do not have a real fixed position. The M1 sensors are placed by the technician based on his "daily mood". The do not have a strictly defined position. You only no that when opening. The back plate sensors are attached to aluminum. The M1 sensors we could not located for sure -not even asking the guys in the shop floor. M2 the same.You need to tear it down and check it. BTW: The heating is not attached to Mirror, but to metal of the back plate. That means, actually you are heating the back plate.

To be honest, I was shocked and gave up in the end. Since using DHM I got it under control, but the heating / cooling concept needs serious rework. Also the cabling... It all looks like some wild tinkering.

BTW: The shroud on the CDK produces additional diffraction spikes. You have to take counter measures. This also took some time to elaborate.

Edited ...

4.29 Willem Jan Drijfhout #... · view views · 1 like
Ruediger: Well, it depends a bit where you are located and climate conditions. I am facing very often huge temperature differences and jumps in humidity. Measuring the air temp and control the heating and fans based on that, will not work at all. E.g. I can measure an air temp of 10 degree and the dew point may be at 9 degree. BUT the M1 is at 18 degree from day time. So instead of cooling it you would fire up the heating. You will need the temperature reading of the glass of the M1 and M2. Measuring air temp somewhere will not work at all. I have been fighting for 1,5 year with overheating and dew because non of the boxes has a humidity sensor. I got very good support from PWI and I have received a second piece of software from PWI called "Dew-Heater-Manager" which can read an ASCOM humidity sensor and control the PWI software, which then switches heating and fans. Actually a second logic on top. Only with that tedious workaround it is possible to keep the mirror precisely 1 to 2 degree over dew point over the night. In my point of view, you can only work without EFA and Delta-T in areas, where dew is not an issue at all. CS Rüdiger Here an example of the DHM: Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important. Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC? One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down? Like

4.29

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· 1 like

Ruediger:
Well, it depends a bit where you are located and climate conditions.

I am facing very often huge temperature differences and jumps in humidity. Measuring the air temp and control the heating and fans based on that, will not work at all. E.g. I can measure an air temp of 10 degree and the dew point may be at 9 degree. BUT the M1 is at 18 degree from day time. So instead of cooling it you would fire up the heating. You will need the temperature reading of the glass of the M1 and M2. Measuring air temp somewhere will not work at all. I have been fighting for 1,5 year with overheating and dew because non of the boxes has a humidity sensor.

I got very good support from PWI and I have received a second piece of software from PWI called "Dew-Heater-Manager" which can read an ASCOM humidity sensor and control the PWI software, which then switches heating and fans. Actually a second logic on top. Only with that tedious workaround it is possible to keep the mirror precisely 1 to 2 degree over dew point over the night.

In my point of view, you can only work without EFA and Delta-T in areas, where dew is not an issue at all.

CS
Rüdiger

Here an example of the DHM:

Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important.

Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC?

One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down?

18.84 Ruediger #... · view views
Willem Jan Drijfhout: Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important. Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC? One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down? Yes, having DHM and a dew sensor read via ASCOM do work. It solved for me 98% of the issues. Please do not over interpret my screenshot. I have just grepped one to illustrate the two pieces of SW playing together. But you are right. M2 is very prone to radiative cooling. It changes hell fast. but here the cold comes from the back side, and not mirror surface. Also the heating is very strong for M2. It reacts within 15 to 20 seconds. DHM is currently implemented to use some kind of hysteresis and does a relative brute force approach. DHM was only a quick work around by PWI to get it somehow working. Before that they only offered some VBS scripts to trigger the web API from PWI SW. But I was not really happy with that, since I expect for that money a working system. So the tinkered DHM. But now it is OK for me. It is working. Not nice, but ... Like

18.84

Ruediger

#...

Willem Jan Drijfhout:
Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important.

Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC?

One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down?

Yes, having DHM and a dew sensor read via ASCOM do work. It solved for me 98% of the issues.

Please do not over interpret my screenshot. I have just grepped one to illustrate the two pieces of SW playing together. But you are right. M2 is very prone to radiative cooling. It changes hell fast. but here the cold comes from the back side, and not mirror surface. Also the heating is very strong for M2. It reacts within 15 to 20 seconds.

DHM is currently implemented to use some kind of hysteresis and does a relative brute force approach. DHM was only a quick work around by PWI to get it somehow working. Before that they only offered some VBS scripts to trigger the web API from PWI SW. But I was not really happy with that, since I expect for that money a working system. So the tinkered DHM. But now it is OK for me. It is working. Not nice, but ...

4.29 Willem Jan Drijfhout #... · view views · 1 like
Yuxuan: I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler. In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff! Like

4.29

#...

· 1 like

Yuxuan:
I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler.
In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff!

4.29 Willem Jan Drijfhout #... · view views · 1 like
Ruediger: Willem Jan Drijfhout: Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important. Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC? One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down? Yes, having DHM and a dew sensor read via ASCOM do work. It solved for me 98% of the issues. Please do not over interpret my screenshot. I have just grepped one to illustrate the two pieces of SW playing together. But you are right. M2 is very prone to radiative cooling. It changes hell fast. but here the cold comes from the back side, and not mirror surface. Also the heating is very strong for M2. It reacts within 15 to 20 seconds. DHM is currently implemented to use some kind of hysteresis and does a relative brute force approach. DHM was only a quick work around by PWI to get it somehow working. Before that they only offered some VBS scripts to trigger the web API from PWI SW. But I was not really happy with that, since I expect for that money a working system. So the tinkered DHM. But now it is OK for me. It is working. Not nice, but ... Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website. Like

4.29

#...

· 1 like

Ruediger:
Willem Jan Drijfhout:
Thank you both Rüdiger and John for sharing your solutions. John has built impressive hardware to achieve that. Probably a bit out of my league. For your solution to work, Rüdiger, is it fair to say that what is needed for this to work is to have this DHM from PWI and somewhere in your setup a humidity sensor, for example from the UPB? DHM would then collect the humidity info through an ASCOM connection from the UPB? I think also for my environment, being able to manage around dew point, rather than around ambient temperature will be important.

Just having another look at these graphs and keeping John's comments in mind around radiative cooling, is that what we see here for the secondary? Did you specify the required delta with dew point, in this case 3 ºC?

One other thing I noticed in the attached graphs is that secondary temp is fluctuating a lot, with heating going on and off. Is that on purpose? Would one not expect the heater to just stay on at a lower, but constant level, balancing out the inclination for the temperature to go down?

Yes, having DHM and a dew sensor read via ASCOM do work. It solved for me 98% of the issues.

Please do not over interpret my screenshot. I have just grepped one to illustrate the two pieces of SW playing together. But you are right. M2 is very prone to radiative cooling. It changes hell fast. but here the cold comes from the back side, and not mirror surface. Also the heating is very strong for M2. It reacts within 15 to 20 seconds.

DHM is currently implemented to use some kind of hysteresis and does a relative brute force approach. DHM was only a quick work around by PWI to get it somehow working. Before that they only offered some VBS scripts to trigger the web API from PWI SW. But I was not really happy with that, since I expect for that money a working system. So the tinkered DHM. But now it is OK for me. It is working. Not nice, but ...

Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website.

22.40 John Hayes #... · view views · 2 likes
=14.000000953674316pxWould a shroud help to reduce radiative cooling? Or would that be too thin to have any meaningful effect? Yes a shroud will help. You are just trying to block radiative transfer with the sky and I think that a shroud should work pretty well to help reduce heat exchange. John Like

18.84 Ruediger #... · view views · 1 like
Willem Jan Drijfhout: Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website. DHM was never officially released. You wont find it for download. But I was informed the try to incorporate it into PWI V4. But when... But I think when asking you will get it. Not a big deal. Like

4.29 Willem Jan Drijfhout #... · view views
Ruediger: Willem Jan Drijfhout: Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website. DHM was never officially released. You wont find it for download. But I was informed the try to incorporate it into PWI V4. But when... But I think when asking you will get it. Not a big deal. Its not in 4.0.99... Like

4.29

#...

Ruediger:
Willem Jan Drijfhout:
Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website.

DHM was never officially released. You wont find it for download. But I was informed the try to incorporate it into PWI V4. But when...
But I think when asking you will get it. Not a big deal.

Its not in 4.0.99...

18.84 Ruediger #... · view views · 1 like
Willem Jan Drijfhout: Yuxuan: I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler. In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff! This effect you can see in nature quite easily: You can find ripe on trees and plants, though the air temperature is above 0. The radiation into spaces make the temperature of the plants drop below zero and instead of dew you get ice on them. Here a link to a very good German article. https://de.wikipedia.org/wiki/Atmosph%C3%A4rische_Gegenstrahlung Unfortunately I have not found any English equivalent. There it is calculate an example how cold a plant leaf can get. Like

18.84

Ruediger

#...

· 1 like

Willem Jan Drijfhout:
Yuxuan:
I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler.
In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff!

This effect you can see in nature quite easily: You can find ripe on trees and plants, though the air temperature is above 0. The radiation into spaces make the temperature of the plants drop below zero and instead of dew you get ice on them.

Here a link to a very good German article. https://de.wikipedia.org/wiki/Atmosph%C3%A4rische_Gegenstrahlung
Unfortunately I have not found any English equivalent. There it is calculate an example how cold a plant leaf can get.

22.40 John Hayes #... · view views · 3 likes
Yuxuan: Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. Ah, yes...that's where a little optics comes into play. Since you are a physics guy, remember Planck's law, which describes how a body radiates according to its temperature (and emissivity.) Both the scope (and its components) are radiating energy (in the form of heat) and in order to conserve energy, they are also absorbing radiation from the surroundings. The sky typically radiates at about -25C to -30C so there's a net exchange of energy with the sky and that drops the temperature of surfaces exposed to the sky. Of course there is also heat exchange through conduction and convection with the surrounding air, which moderates the temperature drop that will occur. It's a bit complicated to compute the exact numbers but suffice it to say that the temperature of your mirrors will always stabilize *below* the ambient air temperature under a clear night sky. (BTW, radiative heat transfer is also the major component driving climate change but that a bit more complicated so we'll leave that subject for another time.) John Like

22.40

John Hayes

#...

· 3 likes

Yuxuan:
Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

Ah, yes...that's where a little optics comes into play. Since you are a physics guy, remember Planck's law, which describes how a body radiates according to its temperature (and emissivity.) Both the scope (and its components) are radiating energy (in the form of heat) and in order to conserve energy, they are also absorbing radiation from the surroundings. The sky typically radiates at about -25C to -30C so there's a net exchange of energy with the sky and that drops the temperature of surfaces exposed to the sky. Of course there is also heat exchange through conduction and convection with the surrounding air, which moderates the temperature drop that will occur. It's a bit complicated to compute the exact numbers but suffice it to say that the temperature of your mirrors will always stabilize below the ambient air temperature under a clear night sky. (BTW, radiative heat transfer is also the major component driving climate change but that a bit more complicated so we'll leave that subject for another time.)

John

5.01 Yuxuan #... · view views · 2 likes
Willem Jan Drijfhout: Yuxuan: I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler. In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff! Thanks, that’s interesting. I think in this radiation scenario the mirror is trying to reach thermal equilibrium with the universe which has a temperature of -270C! Like

5.01

#...

· 2 likes

Willem Jan Drijfhout:
Yuxuan:
I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler.
In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff!

Thanks, that’s interesting. I think in this radiation scenario the mirror is trying to reach thermal equilibrium with the universe which has a temperature of -270C!

18.84 Ruediger #... · view views
Willem Jan Drijfhout: Its not in 4.0.99... They work on it. They announced an Alpha in May, but .... The had more important things to fix first. The only problem is, if you have a Series 5 focuser / controller which requires PWI 4. Then you have pulled the short stick.... Like

5.01 Yuxuan #... · view views
John Hayes: Yuxuan: Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. Ah, yes...that's where a little optics comes into play. Since you are a physics guy, remember Planck's law, which describes how a body radiates according to its temperature (and emissivity.) Both the scope (and its components) are radiating energy (in the form of heat) and in order to conserve energy, they are also absorbing radiation from the surroundings. The sky typically radiates at about -25C to -30C so there's a net exchange of energy with the sky and that drops the temperature of surfaces exposed to the sky. Of course there is also heat exchange through conduction and convection with the surrounding air, which moderates the temperature drop that will occur. It's a bit complicated to compute the exact numbers but suffice it to say that the temperature of your mirrors will always stabilize *below* the ambient air temperature under a clear night sky. (BTW, radiative heat transfer is also the major component driving climate change but that a bit more complicated so we'll leave that subject for another time.) John Thanks, got it. So basically thermal equilibration with the sky via radiation “short circuits” that with ambient air via conduction and convection, the former being a faster process. Like

5.01

#...

John Hayes:
Yuxuan:
Being a physicist who have taught graduate level thermodynamics, I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

Ah, yes...that's where a little optics comes into play. Since you are a physics guy, remember Planck's law, which describes how a body radiates according to its temperature (and emissivity.) Both the scope (and its components) are radiating energy (in the form of heat) and in order to conserve energy, they are also absorbing radiation from the surroundings. The sky typically radiates at about -25C to -30C so there's a net exchange of energy with the sky and that drops the temperature of surfaces exposed to the sky. Of course there is also heat exchange through conduction and convection with the surrounding air, which moderates the temperature drop that will occur. It's a bit complicated to compute the exact numbers but suffice it to say that the temperature of your mirrors will always stabilize below the ambient air temperature under a clear night sky. (BTW, radiative heat transfer is also the major component driving climate change but that a bit more complicated so we'll leave that subject for another time.)

John

Thanks, got it. So basically thermal equilibration with the sky via radiation “short circuits” that with ambient air via conduction and convection, the former being a faster process.

7.56 andrea tasselli #... · view views
Yuxuan: Thanks, got it. So basically thermal equilibration with the sky via radiation “short circuits” that with ambient air via conduction and convection, the former being a faster process. There is no short-circuiting here, the telescope keeps loosing heat toward the sky (incidentally, as the highly reflective alluminium coating has a rather small emissivity compared to the usually blackened bodies of the telescope's outer shell and supports, much less than the tube hence the tube stars dewing far sooner) it will keep doing so at decreasing rate till is completely covered in dew at which point the rate drastically falls as humidity is opaque to most of the IR radiation. All of this assuming the ambient (air) is still above freezing. Conduction does not play a role in practical terms and convection ceases to be a significant factor once the temperature difference between mirro and air falls below 5 degrees C. Like

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andrea tasselli

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Yuxuan:
Thanks, got it. So basically thermal equilibration with the sky via radiation “short circuits” that with ambient air via conduction and convection, the former being a faster process.

There is no short-circuiting here, the telescope keeps loosing heat toward the sky (incidentally, as the highly reflective alluminium coating has a rather small emissivity compared to the usually blackened bodies of the telescope's outer shell and supports, much less than the tube hence the tube stars dewing far sooner) it will keep doing so at decreasing rate till is completely covered in dew at which point the rate drastically falls as humidity is opaque to most of the IR radiation. All of this assuming the ambient (air) is still above freezing. Conduction does not play a role in practical terms and convection ceases to be a significant factor once the temperature difference between mirro and air falls below 5 degrees C.

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A very interesting thread, I must have missed this. I run the CDK without the EFA, there is a little cable provided in most cases that allows you to power the fans. I use the Pegasus to keep them on all night. Tried 10x subs with the fan on and 10x with the fan off. At 0.3"/pixel the FWHM did not change so I doubt vibration is an issue. If that's the case leaving the fans on is better to both avoid dew and break up boundary layers I believe. I found the focus moves slightly with the fans on vs off. Tested it many times and its about 200 microns, it is very strange. I suspect the pressure behind the primary moves it somehow. As for the heaters, not having the EFA any more, I use the DeltaT to run the heaters, yes, it is blind to humidity and only sees the primary backplate NOT the primary mirror. John's solution is great, to add more heat as needed as humidity does up. I'm surprised PW don't use this. Even Celestron's new dew controller (few hundred $) accounts for humidity. NOTE There seems to be an issue with powering the DeltaT with the Pegasus UPBV2, my DeltaT dies a few weeks ago, this was a new one that replaced the previously burnt DeltaT. PW now say that they have such reports that seems there is an issue with the Pegasus boxes + DeltaT. Pegasus said its not the UPB and I need to follow up with PW. Also: I am now running the the CDK heaters with the Pegasus UPBV2 since I have nothing else for now. I am setting the power manually but the UBP does have an auto dew heater setting which I believe looks at the humidity. You can adjust the aggressiveness of the power. I might look more into this feature. Like

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A very interesting thread, I must have missed this.

I run the CDK without the EFA, there is a little cable provided in most cases that allows you to power the fans. I use the Pegasus to keep them on all night.
Tried 10x subs with the fan on and 10x with the fan off. At 0.3"/pixel the FWHM did not change so I doubt vibration is an issue. If that's the case leaving the fans on is better to both avoid dew and break up boundary layers I believe.

I found the focus moves slightly with the fans on vs off. Tested it many times and its about 200 microns, it is very strange. I suspect the pressure behind the primary moves it somehow.

As for the heaters, not having the EFA any more, I use the DeltaT to run the heaters, yes, it is blind to humidity and only sees the primary backplate NOT the primary mirror.

John's solution is great, to add more heat as needed as humidity does up. I'm surprised PW don't use this.
Even Celestron's new dew controller (few hundred $) accounts for humidity.

**NOTE**
There seems to be an issue with powering the DeltaT with the Pegasus UPBV2, my DeltaT dies a few weeks ago, this was a new one that replaced the previously burnt DeltaT.
PW now say that they have such reports that seems there is an issue with the Pegasus boxes + DeltaT.
Pegasus said its not the UPB and I need to follow up with PW.

Also:
I am now running the the CDK heaters with the Pegasus UPBV2 since I have nothing else for now. I am setting the power manually but the UBP does have an auto dew heater setting which I believe looks at the humidity. You can adjust the aggressiveness of the power. I might look more into this feature.

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Willem Jan Drijfhout: Hi Yuxuan, Surely, one buys a telescope for its optical performance, not for its fan-control box. But having to buy a €1,450.- controller to only use a €10 worth cable from it is just a big turn-off and a no-go out of principle. It makes one re-consider options again. Out of curiosity, what would be your reason to go from AG Optical to Planewave? *That little cable was provided with my CDKk14. The UPB can run the fans + heaters. In theory it can even run the stepper focus motor but Pegasus don't know the wiring, I contacted them and sent them pictures. No updates. Like

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Willem Jan Drijfhout:
Hi Yuxuan,

Surely, one buys a telescope for its optical performance, not for its fan-control box. But having to buy a €1,450.- controller to only use a €10 worth cable from it is just a big turn-off and a no-go out of principle. It makes one re-consider options again. Out of curiosity, what would be your reason to go from AG Optical to Planewave?

*That little cable was provided with my CDKk14.
The UPB can run the fans + heaters.
In theory it can even run the stepper focus motor but Pegasus don't know the wiring, I contacted them and sent them pictures. No updates.

8.40 Rouz Astro #... · view views · 1 like
Ruediger: Willem Jan Drijfhout: This system is long overdue an update. You are absolutely right! I was hoping they will clean up the mess with the Series 5 focuser, but they even made things worse: Not only not improving the EFA unit itself by adding a 5 Euro humidity sensor, they also switched the operating voltage to 24V! And question: which auxiliary gadgets (e.g. UPB) support 24V? When I became aware of it, I immediately cancelled my order and went straight for the Gemini. I have shared my anger with PWI and they discussed this point internally again, but it was too late to make any design changes. Also it should fit to the 24V of the Delta-Rho. I think, so good the optics are, electronic wise there is still some (actually more) homework to do. This is really not matching to the instruments quality. *DO you know if the series 5 controller needs the Delta T as well? I see it has a port called "Dew " I agree, the 24v makes it far less convenient. Also, Optec now have a 5inch Gemini as well. Its a bit more expensive but can work on scopes like the DR350. I'm also in talks with Optec and becoming a dealer some have asked me to set the Gemini up for them or the reducer setup. Edited ... Like

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Ruediger:
Willem Jan Drijfhout:
This system is long overdue an update.

You are absolutely right!
I was hoping they will clean up the mess with the Series 5 focuser, but they even made things worse: Not only not improving the EFA unit itself by adding a 5 Euro humidity sensor, they also switched the operating voltage to 24V! And question: which auxiliary gadgets (e.g. UPB) support 24V? When I became aware of it, I immediately cancelled my order and went straight for the Gemini.

I have shared my anger with PWI and they discussed this point internally again, but it was too late to make any design changes. Also it should fit to the 24V of the Delta-Rho.

I think, so good the optics are, electronic wise there is still some (actually more) homework to do. This is really not matching to the instruments quality.

*DO you know if the series 5 controller needs the Delta T as well?
I see it has a port called "Dew "

I agree, the 24v makes it far less convenient.

Also, Optec now have a 5inch Gemini as well. Its a bit more expensive but can work on scopes like the DR350.

I'm also in talks with Optec and becoming a dealer some have asked me to set the Gemini up for them or the reducer setup.

Edited ...

8.40 Rouz Astro #... · view views · 2 likes
Willem Jan Drijfhout: Ruediger: Willem Jan Drijfhout: Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website. DHM was never officially released. You wont find it for download. But I was informed the try to incorporate it into PWI V4. But when... But I think when asking you will get it. Not a big deal. Its not in 4.0.99... *Software here: http://planewave.com/files/tools/ Like

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Willem Jan Drijfhout:
Ruediger:
Willem Jan Drijfhout:
Well, it is yet another piece of software, but seems to be doing the job. Sounds like they should just make this standard available through their website.

DHM was never officially released. You wont find it for download. But I was informed the try to incorporate it into PWI V4. But when...
But I think when asking you will get it. Not a big deal.

Its not in 4.0.99...

*Software here:

http://planewave.com/files/tools/

8.40 Rouz Astro #... · view views
Yuxuan: Willem Jan Drijfhout: Yuxuan: I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked. To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler. In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff! Thanks, that’s interesting. I think in this radiation scenario the mirror is trying to reach thermal equilibrium with the universe which has a temperature of -270C! *The radiative heat loss is more the more transparency you have and the higher up the target is. Pointing at zenith, the shroud will have no effect on the radiative heat loss. I must say, in my case, I still haven't got dew on the mirrors from humid (80%) and cold Vancouver (200m for the Pacific ocean). My fans are always on though and my DeltaT setting are: M2 - 1.2 degrees over ambient Backplate - 0.9 degrees over ambient I got rid of the EFA so can't see the primary. Now the DeltaT is dead so manually running the heaters on low - not ideal. But the Pegasus auto function might be a good method, Will try to work out what aggressiveness works. It should use the RH reading from the sensor to regulate the power. I also tried to open the CDK rear plate to swap the wires from of the backplate and primary sensor so the DeltaT will read the mirror, its busy in there and didn't dare mess with it. Like

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Yuxuan:
Willem Jan Drijfhout:
Yuxuan:
I have to admit I still don’t understand how a mirror can cool itself to below ambient. It seems to violate the second law of thermodynamics in that it’s a refrigerator without needing power. Some non-equilibrium physics must be invoked.

To be honest, I had the same blank here and had to look it up. I guess it has to do with the three ways heat can be transported (convection, conduction or radiation). First two deal with kinetic energy, the last one is electromagnetic energy, emitted as IR. Under clear skies this IR radiation escapes earth, making the body that emits it cooler.
In fact, your example of a fridge without power, is a real thing, as weird as it sounds. See this article about making ice in the dessert... Fun stuff!

Thanks, that’s interesting. I think in this radiation scenario the mirror is trying to reach thermal equilibrium with the universe which has a temperature of -270C!

*The radiative heat loss is more the more transparency you have and the higher up the target is.
Pointing at zenith, the shroud will have no effect on the radiative heat loss.

I must say, in my case, I still haven't got dew on the mirrors from humid (80%) and cold Vancouver (200m for the Pacific ocean). My fans are always on though and my DeltaT setting are:

M2 - 1.2 degrees over ambient
Backplate - 0.9 degrees over ambient

I got rid of the EFA so can't see the primary. Now the DeltaT is dead so manually running the heaters on low - not ideal.

But the Pegasus auto function might be a good method, Will try to work out what aggressiveness works. It should use the RH reading from the sensor to regulate the power.

I also tried to open the CDK rear plate to swap the wires from of the backplate and primary sensor so the DeltaT will read the mirror, its busy in there and didn't dare mess with it.

8.40 Rouz Astro #... · view views · 1 like
Also, I tired PWM on the CDK fans, you can not modulate them at all. I didn't try lowering the voltage, that might damage them? @ Ruediger You might be able to swap the stock fans out with PWM PC fans, then you should be able to lower the speed which make it much more quiet to keep the neighbors happy. I see PW is adding side fans to all the OTAs, even the DR350. I even thought about drilling holes and adding those. Dean at PW said there is no need as the CDK14 mirror mask diverts the airflow over the front of the primary. The DR350 should be the same but they added fans there. Not sure why no on the CDK14. Like

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