fan operation without EFA

Hi Yuxuan,

I am using an Optec Gemini on my CDK 14, which replaced the Hedrick (analog Rouz’ setup). I am still using the EFA box for the fans and to get sensor readings for the primary mirror and backplate. Without I have no sufficient control to avoid dew on the mirror. The problem are the special plugs/cables. These special cables make it very hard to substitute the EFA box.
If no precise control about the heaters is required you can use for example a PA UPB or something equivalent. I still use the Delta-T box, though I have a PA Box. I have to use it, since the PWI Dew-Heater-Manger requires both. Without I cannot avoid dew, without overheating the mirrors. 
The heaters have Chinch connectors. But for the fans you have to tinker.

Actually the complete EFA and Delta-T setup is somehow a very weird concept.

CS
Rüdiger

I would also prefer none of the boxes - also for the reason they are totally overpriced. But I found no way to replace them without serious functional losses.

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:

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.

Yuxuan:
My understanding is that, based in California, they didn't initially realize the need for dew control, and Delta-T came as an afterthought. 

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?

How about run two UPB in series

Yeah, please more boxes! Let's play Jenga with UPBs.

Yuxuan:
Also, Dave Tandy told me he will soon get out of the amateur telescope business, and will no longer support second-hand owners like me. This is unacceptable to me.

Ah, I see, that is good to know.

Ruediger:
I was hoping they will clean up the mess with the Series 5 focuser,

Just looked at the Series 5 controller, and it says 'controls fans of CDK12.5-24'. No mention of the heaters. So looks like they're not changing their strategy.

Willem Jan Drijfhout:

Ruediger:
I was hoping they will clean up the mess with the Series 5 focuser,

Just looked at the Series 5 controller, and it says 'controls fans of CDK12.5-24'. No mention of the heaters. So looks like they're not changing their strategy.

Looking at the back of the new controller, there is a mysterious “dew” port. Their website is nice in terms of graphical design, but boy it’s confusing.

I'm coming to this discussion a little late but I'll add my two cents anyway.  Like Rouz, I use a Gemini focuser on my PW scope and that reveals a VERY strange design choice that PW made.  As has been pointed out, to get full control of the functionality of the Delta-T controller and the PW3 software, you need the PW focuser!  Furthermore the dew control software (PW3) is totally brain dead!  It has options to turn the heat on at certain times and below certain temperatures but it has no capability to measure the temperature-dew point spread (TDPS), which is all that counts to prevent dew.  I have spoken to the folks at PW about this many times and even offered to help them fix the problem; but to no avail.  They have admitted the problem, but they've been unresponsive to my comments or offers of help.

So, I gave up on them and solved the problem myself.  A long time ago I built an Arduino controller to control the anti-dew power supply/controller on my C14.  That system has been perfectly reliable and I've never had dew form on the optics in New Mexico so it works REALLY well.  Given that history,  I simply adapted that design for the PW system.  Since I didn't want to control power to the Deta-T controller, I set up this controller to interrupt power to the heaters themselves.  My controller measures both ambient temperature and dew-point, it computes the TDPS, and controls relays that open or close to allow current to flow to the mirror heaters.  There are two temp/humidity sensors on each side for signal averaging and to handle a failure should that occur.  The software is latched to prevent instability around the threshold set point and I can control it and set parameters remotely.  The anti-dew control box with the sensors is mounted on top of the scope for accurate ambient measurements and I've calibrated the temperature sensors agains various other thermocouples on the scope.  As I recall, I have the TDPS threshold set at 6C to allow for time to raise the temperature of the mirrors above ambient as set in the Delta-T controller.  The system is completely automatic and comes online when I power up the scope so I don't have to do anything other than turn on the Delta-T controller.  There's not a lot of dew in Chile but it can (and does) happen and so far, the scope has stayed dew free--at least as far as I can tell!

To control the fan, I simply power it through a small power brick on the IP power switch.  I usually just run the fan for about 30 minutes after bringing the scope online and then leave it off.  The fans on the PW20 are big enough that I'm worried about vibration so I prefer to normally keep them off.   With manual control, I can watch my guide data to determine if they should stay on or can be switched off based on whether I see evidence of baffle plumes.

Here are some photos of my controller temporarily strapped to the top of the Delta-T controller (before I labeled everything and mounted the Dewpoint controller on it's own bracket away from the Delta-T controller..)

Ruediger:

John Hayes:
I'm coming to this discussion a little late but I'll add my two cents anyway.  Like Rouz, I use a Gemini focuser on my PW scope and that reveals a VERY strange design choice that PW made.  As has been pointed out, to get full control of the functionality of the Delta-T controller and the PW3 software, you need the PW focuser!  Furthermore the dew control software (PW3) is totally brain dead!  It has options to turn the heat on at certain times and below certain temperatures but it has no capability to measure the temperature-dew point spread (TDPS), which is all that counts to prevent dew.  I have spoken to the folks at PW about this many times and even offered to help them fix the problem; but to no avail.  They have admitted the problem, but they've been unresponsive to my comments or offers of help.

So, I gave up on them and solved the problem myself.

Hi John,

same here. Talked multiple times to them and offered help. But actually no mind change.

You have tinkered a nice solution, but it has a huge draw back, which actually caused me the trouble and forced me to stay with the EFA box: You measure the air temp. That’s fine as long as the mirror temp is close to that. Here I have fluctuations about 30 degree and more. In the evening the mirror is far above air temp, even with fans continuously running, in the morning the air is much warmer than the mirror. 

I had either tube seeing, which starts when M1 is warmer than 2 degree ambient and the stars are distorted, or in the morning water was pouring out of the scope. I had so much water in the backplate, that I had water mark inside everywhere and it needed service. 

In the end PWI confessed that they never had tested, nor designed, their anti dew resp. cooling concept for such climate conditions. The only tested in dry, dessert like areas, areas or AC controlled domes.

To sum it up: the optics is great, the anti dew resp. fan control is simply for the bin.

Remember that what counts is the temperature of the front surface of the mirror; not the temperature of the rear of the mirror.  In my C14, I have a thermal couple mounted right at the front corrector plate to handle this issue (due to radiative cooling.)  On the 20", I've carefully monitored the temperature difference of my optics and I never see a spread of more than 3C between the reading for the secondary (the coldest mirror in my scope) and the ambient air temperature measured behind the backplane of the scope.  That's another reason that I have the TDPS threshold set to be fairly high--radiative cooling is a big deal!  Ideally, that value should be determined both by the maximum rate of change possible in the TDPS and the rate at which you can heat the mirrors.  In principle this approach will work anywhere but I agree that things get really challenging for anyone operating where it's hot and where it's extremely humid at the same time.  Those are the conditions when the air can hold the most water and when you might wind up needing a bilge pump if you can't get the mirrors warm enough.

As I've pointed out in my many posts about my 20", Planewave makes a good scope but it's not without problems and I just can't understand why they are unresponsive about fixing this stuff.  I've concluded that they must be too busy building and shipping scopes to be able to stop to fix things.  That will eventually bite them.  I love those guys but when I eventually step up to buy a 24" or 28" it isn't likely to be a PW.

John

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.

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

On PW CDKs, are the temp sensor and heating pad glued to the back of the secondary mirror? That would be problematic if there’s radiative cooling. On AG iDKs they are wrapped around the secondary, closer to the surface. They can at least partly detect and account for radiative loss of heat.