Mirror cooling stage II

[davidpretorius]

Just a quick update.

I have rec some thermal glue thanks to Wombat (Darren)

and have stripped away the existing sticky pad and then use the thermal glue to attach to the mirror.

I then cut a 2" piece of styrofoam about 10mm wide and 10mm thick with a slot for the actual sensor. This went over the sensor and glued into place.

I have measured the fan and will now look for a 56mm holesaw to start cutting a hole for sucking the air out. A few vent holes will help as well.

But first things first, another experiment tonight to see if the sensor is more accurately recording the mirror with hopefully the effects of the surrounding air temp in the tube having been reduced!!!

[matt]

Looking forward to those results Davo:)

It's an adventurous and full-on kinda bloke who starts cutting holes in his OTA.

I'm sure the end will justify the means:scared:

[davidpretorius]

actually it is so much fun, until i break something and someone gets hurt!!!

there are 5 or 6 screws at the bottom of the ota for removing the mirror cell. I was so scared when i first took these off!!

you get used to it i spose and more confident!

[ving]

looking forward to reading the results dave :)

i had a read of birds cooling write-up on his site. thats full of great info.... I plan on getting some heatsink action happening on my scope too now :)

[bird]

Good stuff DP, I'll watch out for your new results, they might be the same as the old results, but now I'd have more confidence in them :-) :-)

Bird

[Robert_T]

Onya DP :) I've got to admire your enthusiam and persistance to conquer the world of thermal equilibrium... :P


cheers,

[slice of heaven]

Good stuff Dave...another R&D project...have you thought about mounting in a way not to have vibes interfering?...any intensions of using variable speed on the fan?

[ballaratdragons]

Davo, you have done 4187 alterations to your 10" Dob, motorised, electrified and computerised everything, and you are still going. What are you going to do with this highly modified Robotic 10" when you buy the new 16" in March?
:lol2:

[davidpretorius]

vibes as in walking near it etc???

the dob buggy has pneumatic tyres whic would dampened any vibes i would think and yes some sort of computer controlled variable resistor can vary.

not sure in this one, birds system is on or off from memory. once the temp has stabilised, and mirror needs to quickly reach ambient and switch off the exhaust fan.

tonight is very overcast and threatening thunder. it seems to have reached a steady temp, so it will be intersting to see how long it takes mirror to catch up. no fan tonight. plan is to cut holes tomorrow and then redo experiment with exhaust fan.

[slice of heaven]

Nope, from the fan, I mounted my fan using 2 soft rubber grommets on each bolt to stop vibrations while the fans running, but I suppose you can turn the fans off while imaging if need be.

[bird]

Guys, I turn off all fans etc before I start imaging. Maybe it's just my dodgy-brothers workmanship but I can see vibration from these fans in the image, no matter how carefully I attach the fans.

Bird

[slice of heaven]

Thanks Bird ,That answered that :)
I dont image, but I've seen vibrations at the ep with the original solid mounted fan at high mags.

[ballaratdragons]

I don't get any Vibrations from my fan (even at 6.5mm) and some nights I leave it running all night whilst viewing to keep dew away from the mirrors.
Just lucky I guess. :confuse3:

[asimov]

I must say I never noticed visible vibration from my fan when I used to use it. Mounted it behind the mirror with rubber 'O' rings stretched between the fan & the 3 collimation bolts sticking out. The only thing noticable was the rotating mass of air in the OTA as it was drawn down the tube.......or up the tube, which ever way you hooked up the fan. Having it on while viewing was useless.

[asimov]

In your case Dave your sucking ACROSS the face of the mirror...a whole different ball game there...as far as 'rotating air mass' = I don't think so.

[slice of heaven]

And that's where having a variable speed comes in handy, finding a speed where you can still view but keeps the fan ticking over to kill the dew and keep the mirror cooling.

[davidpretorius]

yes i do agree with that!!!.

[asimov]

I have no doubt that would work too, yup!

[davidpretorius]

clouds are clearing and stars are out, so a temp drop might be in order

[ballaratdragons]

Might be a dumb question (typical from me) but when I build my Observatory, will it make much of a difference to viewing or imaging if I have a heater on in the observatory. In winter I want to keep my fingy's & toesy's warm.

[asimov]

Not advisable Ken.

[davidpretorius]

control of the air temp from the mirror up to the immediate area around the end of the tube and beyond.

like trying to view stuff over hot tin roofs!!!

[ballaratdragons]

I hate cold weather. All my bones ache. I'll just have to wear even more clothes and extra socks and get some gloves.

Might have to track down some of those electric undercothes!!

[davidpretorius]

we have reached ambient, so we can get an idea of how long it took to catch up from the graphat 8.45 pm, ambient was 17.5 and starting to flatten out. 1 hour and 15 minutes later it had stabilised at 17.1. the mirror in that time had dropped .8 of a degree.

so the old hour rule is pretty good!!!!

remember .5 of a degree difference or less is best!

[ballaratdragons]

Good info davo. 1 hour for good results.

What I can't get over is that it's 17 degrees at your place. It's freezing here!!!

[hogly52]

4am and its 28' here. I leave my air-conditioning off to ensure the scope is up to ambient.


Cheers,

Graeme :drink:

[davidpretorius]

ouch that is hot!!!

[davidpretorius]

Ok,

i am the brand new owner of a 57mm hole on the side of my tube!. The mirror is out of the cell so that sharp metal filings did not produce pretty patterns on the mirror and I may as well clean it whilst it is out. Off to the shop for de mineralized water!

Have cut all protusions off the computer fan off and the square base sits very nicely into the 57mm hole!

I will drill some small vents later into the side of the tube later, once i have seen how this suction action will go. I reckon for tube strength, i will drill two sets of holes (small to start) 1/3 of the way around to the left and right to keep it symmetric.

The bottom bit of the fan is placed flush with the top of the mirror, so the boundary layer should be affected by the suction of the fan

Nice hot day and a chance of no clouds, so another temp run tonight!!!

[ving]

hot in tassie?
never heard anything so bizare!

[asimov]

Hi Davo. Your gamer than I am mate! Well done. I'll be HIGHLY interested in this experiment!! I've been wanting to do that for 2 years now! Looking forward to the results...especially between sucking & blowing! Is it variable speed??

I'm going to make a prediction & say you will have better results blowing rather than sucking.

[asimov]

If this experiment is SPECIFICALLY designed to get rid of the boundary layer, if the mirror is at ambient there IS no boundary layer to get rid of.

[davidpretorius]

well, two fold:

1. i want to get the mirror quickly cooled to ambient in less than the hour that it roughly takes (from previous experiments) and in your case 4hrs!. ie if the temp is falling throughout the night, then the mirror will mean that i can start imaging sooner instead of waiting for the mirror to catch up.

2. now that i have a hole, it should also get rid of the dew that can now have easier access to the mirror!

[asimov]

Yep.

Another thing I thought of..& no doubt you've thought of this also!...The AREA of the holes you've made. The holes on the opposite side of the fan should be a bigger area if you know what I mean? If it can't suck (or blow) thru those holes at the same rate as the fan it could start drawing from the air in the OTA setting up tube currents. (but it may do this in any case) I suggest you try it out without any cool down. Defocus on a star & you will plainly see the heat haze from the mirror & then kick in the fan while your viewing & see what happens...

[davidpretorius]

i won't drill any extra holes yet, as at the end of the day, if there is enough air being sucked down the tube and from underneath (there a rather large air gaps in the mirror cell) and thus colder ambient air is being brought in from the outside to help cool the mirror through movement and convection, then i should be fine.

If i can get say a 10 minute or less lag to get the mirror within 1/2 degree then it is doing its job. If there is not enough air coming into the system, then more vent holes may be required.

To my mind, i will follow bird and not image whilst fans are on!

[asimov]

I was taking about visual....yeah, fans + imaging = not a great idea.

[ballaratdragons]

Davo,

With drilling all these holes and fitting fans etc, wouldn't it be easier to just build a truss OTA and place your optics in it?

[davidpretorius]

say a 16"????

[ving]

a 16"


do truss' have cooling problems? I'd say not much. :confuse3:

[ballaratdragons]

Eventually (March LOL!). Do you have the material to make a truss framed OTA for your 10"

[davidpretorius]

mine won't!!!!

[ballaratdragons]

Well to start with there are no tube currents!! :)

[asimov]

Very good point Ken...but I can't answer it! lol.

[ballaratdragons]

Lightweight! :)

Collapsable (more transortable) :)

[davidpretorius]

no, but i will look at some if it available.

I reckon i will be quite happy for another 6 months with the cooling and my dob motors.

I see no need to go a truss on the 10" at the moment. Like i say, I am happy to wait 10-20mins for the mirror to catch up!

[bird]

Hi Guys, always interesting reading what you're all up to...

Ken, there's been a lot of discussion about truss-style vs tube-style reflectors, it's not as simple as we might all hope...A truss doesn't have any ventilation problems, but on the other hand it lets air flow past the optics in any old way, and it's not unusual for air to be at different temps just a few feet off the gound vs close to the ground. Also if you're close by then you'll be heating the air a bit and it can be getting into the optical path.

Tubes keep all that out, and you get a column of air that sits above the mirror without moving very much - especially if you close off the mirror end of the tube...

My feeling is that for planetary stuff you want a closed tube + lots of spooky cooling, but for deep sky where you need a big mirror then a truss is more practical.

regards, Bird

[ballaratdragons]

That makes a lot of sense Bird! The slightest breezes would blow right across the front of the mirror at varied speed and temps. I have seen some with the curtaining around them but defeats the purpose Davo is after.

Davo, just continue on your present path :) :lol:

[ballaratdragons]

My fan cools my mirror in about 5 mins. Don't know any accurate measurements though.

[davidpretorius]

i am sucking, you are blowing. i am gettinig readings every 15mins, so yes it will be intersting to cross reference!

[davidpretorius]

well, i am a little confused

but looking at the data, i think it worked ok!!

When I first installed the fan inside the tube and right next to the mirror, the mirror cooled at an average rate 1.743 degrees per hour with the ambient falling at an average of 2.00 degrees an hour. This was very linear in nature and there was no levelling off in this time frame of 4 hours.

With the newest setup, the ambient again was dropping a very linear rate of 1.855 degrees per hour and the mirror was cooling at a very impressive 2.073 degrees per hour.

The actual fan was turned over half way thru the test and went from sucking to blowing cooler air into the tube. No noticeable change in the rate of cooling was observed.

So at this stage it looks to be a winner, but i would prefer to observe normal cooling now that a hole is in the side of the tube near the mirror.

Tonight, i will cool for an hour and then turn on the fan.

It is very hot at the moment, so hopefully there is a large temp difference to run the test under. And then a nice levelling off to see how the fan lags the ambient temp

[Wombat_In_Space]

Dave
Here is a a graph of the data that was captured through hyper terminal over a 6 hour period by capturing to a text file the run through excel.
At time of this post unit was still logging data once it finishes it's programed mission I will post the total data loged.:)

[davidpretorius]

that looks great.

the mirrror is warmer than 5 deg???

[Wombat_In_Space]

The data used was from the testing I am doing, the unit is yet to be fitted to the scope and wont be till I am sure it is all running correctly. All the lables are embeded in the microcontroller so that is why they come up as mirror temp, ambient temp and tube temp.
Testing was done using a "freezer block" as the "mirror" just so I could make sure the sensors all read a different temp.:)

[davidpretorius]

yes you had mentioned the freezer.

this stuff on cooling is great!

[ving]

a freezer block?
interesting, if the mirror temp is considerably lower than the ambient, how does this effect things?

[bird]

Ving, I've had it happen a few times where the mirror was too cold, and I've noticed that the mirror can warm up pretty quickly, but it's the cooling down part where it has trouble.

I found that to be a bit counter-intuitive, I'd have thought that a cold mirror would have trouble warming up, just as a hot mirror has trouble cooling down, but it seems that mirrors can be good at absorbing heat and bad at losing it.

Bird