Primary Mirror Fan

cjmarsh81 · #1 23-03-2006, 07:28 PM

Hello all.

I had the primary mirror out today and while it was out I noticed there is screw threads there ready for me to install a cooling fan. Seeing as it is ready for me to simply screw one on I thought why not. My question is, when you use it do you leave it on? or simply have it on long enough to cool down the primary?

I only ask because I don't know if leaving it on will be detrimental to viewing after the mirror has already cooled enough. Will it make the mirror too cold and then make the image worse?

Thanks

Starkler (Geoff) · #2 23-03-2006, 08:07 PM

Its there just to move some air over that big thermal mass of glass that we call a primary mirror.
By blowing a fan on the mirror it will shed the temperature difference between it and the surrounding air much quicker than without one. The fan will simply bring it closer to ambient temperature faster and it cannot make it too cold

rmcpb (Rob) · #3 23-03-2006, 08:08 PM

I leave mine on all the viewing session without any degradation in view that I can notice.

Give it a go and if it degrades your view turn it off when viewing after the mirror equilibrates.

davidpretorius · #4 23-03-2006, 09:25 PM

you will find that it still takes a long time to cool the mirror to the ambient temperature exactly, but and a big but is that installed with minimal vibration, then the movement of air up the tube stops the eddys and the seeing improves. hence leave it on!!

I believe starkler has this happen to him. I also have noticed it. Mind you, the bottom is sealed, so the air must move up the tube, not bounce of it and back towards the ground.

cjmarsh81 · #5 23-03-2006, 10:01 PM

How does the air move up the tube if the bottom is sealed?

I am pretty sure if I put a fan there the air will bounce off onto the ground.

davidpretorius · #6 23-03-2006, 10:17 PM

the guys will cut a disc the size of the bottom of the tube and attach, then cut a hole in the disc so that an attached fan can blow thru this hole. As the air is being blown into the area behind the mirror, the only escape is around the mirror and up the tube.

Starkler (Geoff) · #7 23-03-2006, 10:27 PM

Yep as Davo said, take a look at the thread titled "dob fans for the lazy".

I made a similar setup with a disk cut from plywood.

cjmarsh81 · #8 23-03-2006, 10:29 PM

Ok, I might just try the fan without the disc first. I am pretty sure I have a couple of old 80mm fans in the shed out of old computers.

BTW. The reason I had the primary out in the first place was to losen the mirror clips. I read in a astro book that the Chinese/Taiwanese tighten them too much so they are not damaged in transport. I losened mine a little, just enough to fit a business card between the glass and the clip. Put it back together and collimated it. I just looked at Saturn and it is the clearest I have ever seen it. I think my mirror was pinched before.

Astroman (Andrew Wall) · #9 23-03-2006, 10:55 PM

I have always had the air SUCKING, drawing the cooler air down onto the face of the mirror... learn something new every day.

davidpretorius · #10 24-03-2006, 08:03 AM

The other thing i have noticed is that if you do seal it, then the fan must be going. Having the seal effectively insulates, so the fan must be there and on!

Starkler (Geoff) · #11 24-03-2006, 08:16 AM

Quote:

Originally Posted by davidpretorius

The other thing i have noticed is that if you do seal it, then the fan must be going. Having the seal effectively insulates, so the fan must be there and on!

On a night of good seeing where the fan had been running for a few hours, I did some experimenting turning the fan on and off and couldnt detect a difference

So yes the fan baffle will inhibit cooling compared to an open cell if the mirror isnt at equilibrium, but once its there it doesnt make a difference.

Mine sits on velcro pads and is easily removable. I'll post a pic or two when I get around to it.

davidpretorius · #12 24-03-2006, 09:38 AM

for tassie and imaging, i must have a refridgerated version. I will have left the scope out from 9pm and then at 4am when i get up the mirror is at 9 degrees and ambient is at 5 degrees. Very frustrating!

It is the variations during the night that are the killer for me. Hence why i am copying bird.

iceman (Mike) · #13 24-03-2006, 11:29 AM

Quote:

BTW. The reason I had the primary out in the first place was to losen the mirror clips. I read in a astro book that the Chinese/Taiwanese tighten them too much so they are not damaged in transport. I losened mine a little, just enough to fit a business card between the glass and the clip. Put it back together and collimated it. I just looked at Saturn and it is the clearest I have ever seen it. I think my mirror was pinched before.

A common problem in most GSO dobs. Check Geoff's article here. It's worth checking your secondary as well.

cjmarsh81 · #14 24-03-2006, 07:57 PM

For anyone else considering taking out the primary mirror I suggest you follow Geoffs article on GSO dobs. Put the scope on the ground before unscrewing and removing the primary.

Unfortunately, I had not read the article. I was sitting on a chair with the scope horizontal and the primary facing me. When I unscrewed the primary and pulled it out I was suddenly wacked in the head with a scope that was now very top-heavy. If I had of realised how much a primary mirror weighs I would not have had this problem.

Pat Dotson · #15 25-03-2006, 07:37 AM

I you mount a bare fan on the back of the mirror cell, you can leave the fan running to help keep the mirror closer to ambient temperature as the temperatures falls. The bad thing about fans mounted to the mirror cell is that they *will* cause the scope to vibrate - especially when running full throttle! Once the mirror is cooled, it's best to run the fan at a lower speed.

Better performance can be gained if the fan is mounted on a baffle so that the back of the telescope is sealed. Air moving around the mirror and up the tube helps clear any turbulence from the tube. Even with a baffled fan, you need to turn the fan speed down once the mirror is cooled.

Pat Dotson
Newtonian Cooling Fans
www.scopefans.com

cjmarsh81 · #16 26-03-2006, 05:06 PM

I just mounted a fan on the bottom of the primary and will give it a whirl tonight if the weather holds out. If it does vibrate the primary I will either try to put some sort of foam washer to dampen the vibration or I will make a little Variable regulator so I can wind it down when it has cooled. See how it goes tonight.

davidpretorius · #17 26-03-2006, 09:11 PM

tried the baby bird cooling system out last night and was very happy to go out to an ambient of 7.6 and a mirror of 7.6.

had the pelitiers going for 4 hours.

cjmarsh81 · #18 27-03-2006, 09:32 AM

DP. What are you using to gauge the temperature of your mirror?

Well I tried it out last night and wow. I don't know if it was the fan cooling down the mirror fast or the fact that my mirror isn't pinched now or that I have just collimated the primary, but I was able to see Saturn at 480x clearly. No ripples or distortion. Had to turn the fan off while viewing as it was severely vibrating the image.

Oops.

For the first few minutes I kept wondering why the image kept going blurry. I thought maybe turbulent air pockets or something. I was looking through the EP and it would go blurry and then come clear again. Turns out I had some mozzie coils burning in front of the scope. Doh!

davidpretorius · #19 27-03-2006, 10:08 AM

Quote:

Originally Posted by cjmarsh81

DP. What are you using to gauge the temperature of your mirror?

a dual temp monitor that you can buy from d1ck smith for less than $30. i have attached the sensor at the end of the long wire to the side of the primary with heat paste, not at the top, but 1/2 way up.

matt · #20 27-03-2006, 03:12 PM

Those pre-made mrror cooling units on Pat's website look great.

Might have to get me one for the 8" Skywatcher.

BUT FIRST: I'd really like to try my hand at putting one together myself.

Now, I'm not the world's greatest DIY-er... so some help please, guys.

Fortunately, I didn't throw away the baffle from the mirror cell. Just put it aside. So I'm ready to craft mine into something resembling those on ScopeFans.com

Can anyone tell me what size fan I'll need to buy and where I might be able to pick up one of those variable fan speed units???

I'm pretty sure I can take care of cutting the holes for everything to fit and I can't imagine the soldering job would be too hard.

Is there any trick to securing the fan to the baffle to dampen vibration???

What do you reckon would be the best way to connect to power - off a cigarette lighter adaptor or straight plug-in power adaptor?

Thanks guys.