Some thought on cooling my C14

pmrid (Peter) · #1 09-08-2010, 07:24 AM

I have recently acuquired an old "orange" C14 that has been de-forked and is due to be mounted on a Titan (when it finally arrives). The process of deforking exposed the holes arounf the rim of the base of the OTA where the fork was previously attached. There are 4 of them and there a few other smaller holes where various weight bars were attached). I am thinking about a way of using these holes (suitably masked for filtering effect) to assist in creating an airflow through the tube to aid in cooling the mirror down to ambient. The idea I have been toying with is to mount a reasonably-sized computer case-type fan on a 2" adapter and fitted into the focuser tube at the back so that it sucks air out of the tube and therefore through those exposed holes.

The air flow through those holes would be across the base of the mirror and then up into the OTA and down the baffle tube. To increase the cooling effect I was also thinking about attaching 3 peltiers to the outside of the base of the OTA with a heat sink and fan on top in the hope that this would cool the baseplate of the scope across which air is being drawn from those exposed holes and therfore adding to the cooling effect.

I am trying to avoid having to do unduly ugly things to the OTA like removing the primary and suchlike. That's why I am hoping to make use of what is already there. Since I haven't used Peltiers before I am unsure whether they would do much good if, for example, I attach the cold side directly to the outside of the baseplate (with some thermal grease of course). Anyone have any ideas on all this?

Peter.

DavidTrap (David) · #2 10-08-2010, 06:59 AM

I think this link to Paul Haese's site might be the go.

DT

pmrid (Peter) · #3 10-08-2010, 12:50 PM

Thanks David. I was looking for that article on IIS but couldn't find it.
Looks like the awful option of opening up the tube and removing the primary is going to be unavoidable. Gulp!
Peter.

Paul Haese · #4 13-08-2010, 03:14 PM

Thanks for the recommendation David.

Peter if you follow my tutorial closely you should not have any trouble. Read and re read this tutorial several times so you know what to do when. If you need help or want to ask any questions then just ask them here and I will post a reply as quickly as possible.

pmrid (Peter) · #5 13-08-2010, 03:29 PM

Quote:

Originally Posted by Paul Haese

Thanks for the recommendation David.

Peter if you follow my tutorial closely you should not have any trouble. Read and re read this tutorial several times so you know what to do when. If you need help or want to ask any questions then just ask them here and I will post a reply as quickly as possible.

Thanks Paul,
Just as a preliminary question: I see that you have installed 2 Peltiers with corresponding case-fans on the inside. Is that sufficient or should I target a third - since I have the opportunity to do so from scratch. Have 2 been sufficient in your experience? Would 3 be likely induce frosting on the optics and related problems?
Peter.

Paul Haese · #6 13-08-2010, 03:55 PM

I think two is fine, I have never found the need to have more than that operating. Two can drop the temperature about 30 degrees C in 2 hours. So that is 15 degrees an hour.

pmrid (Peter) · #7 14-08-2010, 02:42 AM

Quote:

Originally Posted by Paul Haese

...Two can drop the temperature about 30 degrees C in 2 hours. So that is 15 degrees an hour.

Paul, I had assumed (in my blissful SCT ignorance) that the object was to achieve thermal equilibrium - by which I have understood bringing the mirror down to ambient temperature. On a 5 degree night, you want the primary (and the rest of the OTA as well) down to the same level - but not lower. Or have I missed something quite fundmental? Do you cool the OTA below ambient?
Peter

AlexN · #8 14-08-2010, 03:55 AM

The ota does not need to be cool itself, but the air sealed within the OTA **NEEEDS*** to be at the same or very close to the temperature of the primary mirror. Otherwise you suffer the same thermal distortions we saw in your C14 the other night, at a lesser scale.

Essentially, you're aiming to get all the air inside the tube and primary to within 3 or 4 °c of ambient temps... The closer you can get to matching the optics to ambient, the less internal tube currents you have to deal with.. Then you are only at the mercy of local seeing conditions, and not the conditions set by the thermal currents happining inside your OTA as a result of the mirror being cooler that the OTA internal temperature, or vice versa, the mirror being warmer than the air inside the OTA and ambient temp causing a warm boundary layer on the mirror....

This warm boundary layer of air on the primary is what we saw the other night when observing Jupiter earlier in the evening.. Obviously we were battling the seeing when Jupiter was at a low altitude, but once it was up at say 60° altitude and the OTA tube had cooled to a reasonable level, the seeing was still a problem but the views of Jupiter was markedly improved..

Pauls cooling setup addresses every problem that there is essentially. If I were to change anything, I would have added a low power internal dew heater directly behind the corrector plate [READ: touching the glass) as no matter what you do with those external heater bands of yours, you will still suffer dew problems on a humid night... We are after all talking about a piece of glass less that 0.5mm thick... Lots of surface area, but not much thermal mass... It will dew up...

Paul informed me back when I had my C11 that regardless of anything else he'd tried (dew shields, heater bands, keeping everything at equal temps etc.) the best, without out fail, way to keep dew off a 14" corrector plate is to hit it every 3 or 4 minutes with a hair drier.

Lets think about it logically though. 90% of the time, you will be using the C14 @ F/2 in hyper star config. At this focal length, the thermal currents you will introduce by having an internal dew heater DIRECTLY touching the interior side of the corrector is going to make SFA difference to the overall image quality.. For planetary imaging, heater bands kill the image quality because you're looking at imaging at 14m focal lengths... with hyperstar, much like with standard wide field imaging... Thermal currents and seeing conditions (atmospheric or dew heater induced seeing issues) mean very little.... and you have the 12.5" F/20 DK for planetary imaging and observing... I would go to every length to make sure the C14 hyperstar setup is as fool proof as possible at F/2. This includes cooling the mirror to ambient temp to avoid internal currents, and doing whatever (within reason) to avoid the ingress of dew onto the corrector.....

Its been a big day for me and I've rambled here, but I think I've conveyed a few worth while things to consider...

Decide first what the primary use of the scope is... full on pelt cooling may not be necessary if the primary use of the scope is going to be wide field imaging using the hyperstar.... Keep planetary imaging to the DK, visual observation with the DK... and the occasional F/7 imaging through the C14 that is going to require some pretty serious attention. The person to ask about that is 'Spearo' Frank produces amazing images with his C14 setup... and I don't believe he has done much if any modification to the OTA to permit this...

Pauls setup is exceptional, but he demands exceptional things from the optics... Imaging at 14m FL as he does for planetary work is a far cry from 2.5m @ F/7 and a whole different ball game than 710mm @ F/2

To be honest with you mate.. I'd say lets leave it as is for the time being until we can assess what you want to do, and what major issues your plans are going po present before we go any further......

Thats enough out of me for now.. its late and technically.. im supposed to be working... but yeah... Hopefully somewhere in all this rambling I've been somewhat helpful..

In anycase, you know I'm with you every step of the way, no matter what you decide to do, I'll be there to give you a hand and an extra half a brain to think through options.

Regards,
Alex.

Paul Haese · #9 14-08-2010, 10:10 AM

Peter,

you missed my point a little. Yes the point is to get it to ambient, but if the mirror is say at 39 degrees after a hot day in the sun and the night time temperature drops to 9 degrees it will take 2 hours with 2 peltiers to remove that heat. You must always think of the differential temperature points. You can go below ambient but I usually stop the cooling about 0.3 above ambient and let the scope settle. So two peltiers will work fine in even the most extreme cases. Three I think is overkill.

pmrid (Peter) · #10 14-08-2010, 11:22 AM

Quote:

Originally Posted by Paul Haese

Peter,

you missed my point a little.

G'day Paul. Yes. That's always a strong possibility. I'm comfortable with it now and not too concerned about the process.

Alex, thanks for those observations too. I'm heading offshore for work now and will be away for about 10 days - plenty of time to mull over the issues/options. But I think the answer will be HyperStar at F2 first and foremost. For other FL's I've got the 8" Newt to play with and the Dall-Kirkham for lunar/planetary - pretty darn good combination I think.

Peter.

Paul Haese · #11 14-08-2010, 09:11 PM

Well done Peter, please ask me anything if you get stuck during the modification.