Newtonian primary Dew Heater - First iteration

[philjdowns]

Hi All,

After some re-thinking, finally built the first (well second really - won't mention the first one ?) operational version of my heater for my 8" Newtonian primary mirror. As you may have seen from earlier posts, I have serious dew problems! 

Soak testing here at 12.2v (0.912A - 11.4W) been running for a couple of hours and it's about as hot as I can stand on my fingers. Doubt very much if it'll ever be running this high for real, but I need to know it will not have any issues if accidently left on max output (though I doubt this much heat will do the mirror any favours!) 

Based on a standard ladder resistor configuration, using 52 x 680Ω resistors (rated at 0.5W but running at 0.22W max). Initially tried for a radial pattern but couldn't get a uniform distribution so decided to go for a grid layout.
The overall thickness will be 4mm, 3mm for the resistors and 1mm for a insulating sheet back, the resistors will be touching the back of the mirror.

Anybody else tried this?

Phil.

[philjdowns]

Hi All,

Well, no-one commented that I shouldn't so, I've mounted the lattice to a 1mm PVC sheet, and mounted the controller as well. 

Then fitted the unit into my primary mirror housing. (you can see where the original fan and connection points were) There will be a small plastic cover over the controller when it's all back together and tested. The added weight of this is only just over the weight of the removed fan so that's good.

Now to fit the mirror, and re-install the housing into the OTA, re-collimate and find my new focus point as the mirror is 4.5mm closer to the front of the tube.

I've got an elasticated dust cover that I'll try over the back end of the OTA when I use this, thinking it'll reduce any air currents up the tube from the heater and cut down on any moist air being drawn into the base of the tube, this should (in theory) reduce the dew on my secondary by using the heat radiated from the primary to keep the secondary just a tiny bit warmer. Might need a secondary heater though, but I still have loads of components left.

All I need now are some clear skies - Please, it's been soooo long....

 Clear Skies All!

Phil.    

[Jkulin]

@phil I Phil, only just seen this.

As far as I know applying that much heat to the mirror can cause badly shaped stars, far better would have been to fir 3 fans to the back of the cell to get air movement, that's all that my RC has apart from a dew heater on the secondary. You really don't want lots of heat being applied.

I'd be interested to see how it turns out.

Good Luck

[philjdowns]

@Jkulin Hi, yes I agree that my heater is capable of outputting way more heat than I'll ever need, I just wanted to make sure that if it accidentally got connected at full voltage (12v) it wouldn't go up in flames.

I'm going to start at about 0.8v (if I ever see the sky again ? ) so that's calculated at 0.05W total (0.001W/in²), I'm not sure how to work out the rate of heat loss from the mirror but it's likely higher than 0.001W/in², so I need to find the voltage that matches the heat loss and keeps the mirror above the dew-point.

It's going to take a while to calibrate things I know, but I'm planning to restrict the controller to 8v (for now) which would give a maximum of 0.6W or 0.098W/in². Hoping to run at less than 5v, so when I have some usage stats I can restrict the controller further.

I did have two fans attached but it made almost no difference, my location is on a hill by the south coast so I get warm sea air coming in and rising up the hill, cooling as it does. I get dew on everything in the garden almost every night. That's why I'm looking at heat and air-flow restrictions in my OTA. Very gentle heating and minimising the moist air entering the bottom of the OTA works in my head but time will tell.

Hoping to get it all re-assembled this weekend, though no clear skies forecast for at least a week - the joy of astro in the UK!

Thanks for your comments.

Phil.    

[KevS]

Phil, the attached may be of use. The cooling time calculated is I emphasise only an approximation as I have not allowed for air moisture. (That's the next addition)

K.

Mirror Cooling.xlsx

 

[philjdowns]

@KevS Thanks Kev, very interesting. though I'm not using fans to cool my mirror (it's kept in an unheated outhouse so it's always at ambient or very close) it's given me an idea of the radiant heat loss from my mirror. 

I'll be actually heating my mirror to about 1° above ambient then hopefully maintaining that temperature, my lowest setting for my heater grid (200mV) in theory equated to 0.00015W sop that's looking promising.

My plan is to set-up and then set the heater to 3v? for 20 mins (I'll be checking the temperature all the way at first) I'm looking for a guide voltage that will give me a 1° increase in about 15-20 mins, then turn is down to 0.8v?? and see if it maintains the temperature over ambient. I'll have to see how things go as the temperature drops during the night.
As it's cloudy for the foreseeable future, I'll be doing some of this early calibration in the outhouse area, though the mirror won't be seeing 'clear sky' so that'll skew any numbers I get.

I'll keep you all posted how things get on.

It's almost like I don't think this Astro Photography lark isn't complicated enough ?  

Phil.      

[philjdowns]

Quick update, I have run a test with the mirror assembly removed from the OTA, and in my workshop.
Ran the heater at 3v for 30 mins, then switched off for 5 mins, then on at 250mV for 150 mins, logging the mirror temperature every 5 mins.

Started at 16.1°, after 30 mins it was at 17.6°. A larger temperature increase than I wanted, but still safe. 
Setting the heater at almost the lowest setting (250mV) the temperature gradually fell over the next 2.5 hours to 16.5°

So I now know that 3V is too high for the initial heat for my mirror, so the next test will be at 2.5V, and I'll up the the steady state voltage to 500mV and see how that goes. 

I see now that I've made this way over rated, but I guess it'll last so much longer! 

This test was obviously not done under dark skies or outside (and a good 12° warmer that the outside temperature) but I'm hoping that it'll all behave in a relative fashion, though I think I'll lose more heat when the mirror is under a dark sky. 

Looking forward(ish) to the next 3 hours watching my telescope mirror - I know how to have fun ? and with luck, the time I spend can benefit others in the same position!

Phil.

[philjdowns]

Finally!

Got an evening where it's worth setting up (apparently ?) so here's my completed heater installed, connected and running at 750mV (0.04W) to keep the primary a little above ambient as the temperature drops - fingers crossed.
Not expecting to get much imaging done, for a start my NB filters still haven't arrived - two months overdue ? and as I've re-located my primary going to have to spend a while finding the correct focus point again.
I've got an elasticated weather seal bag for the bottom of the OTA but again that still hasn't arrived so I'll be getting some air flow up the tube at the moment.

As you can see, the back of the mirror is not exposed any more. The fly lead is the input 12v supply, and the power socket is across the restive load so I use this to set the output voltage. If I need to, this will be where a secondary mirror heater can be connected.  

Been so long since I've seen the sky, we'll see how it goes!

Phil.