DIY Newtonian Cooling Fan - which fan?

[AbsolutelyN]

I'm looking to add a DIY cooling fan to the back of my 250pds but I'm unsure how powerful the fan needs to be.  

The idea of the fan being USB appeals for simplicity as I'm no electrician. Does anyone know if this would be suitable? 

https://www.amazon.co.uk/gp/product/B06XCTHQG3/ref=ox_sc_act_title_2?smid=A3EP4QEYMLH6KY&psc=1

Many thanks

[Dr_Ju_ju]

Personally I always go for Noctua fans, for long life, reliability, being quiet, efficient etc, e.g. https://www.amazon.co.uk/Noctua-NF-P12-redux-900-Ultra-Silent/dp/B07C5KZX85   

[CraigT82]

I think Maglev fans are good for their low vibration, something like this...

https://www.corsair.com/uk/en/Categories/Products/Fans/Magnetic-Levitation-Fans/ml-pro-config/p/CO-9050040-WW#tab-tech-specs

I'ts 12v not USB, but wiring it to a 12v supply with a speed controller (https://www.amazon.co.uk/GELID-GAMER-system-speed-controller/dp/B002ZO9MVC/ref=sr_1_11?dchild=1&keywords=PWM+fan+speed+controller+12v&qid=1602080220&sr=8-11) is not very difficult, just a pair of wire cutters/strippers and some wire connectors and you're good to go.

You don't actually need a fan that big but my preference is for a large fan run slowly. I keep mine mechanically decoupled from the tube by 2 layers of velcro  (fan is velcroed to the plywood baffle, then baffle is velcroed to the OTA)

[martinl]

Another vote for Noctua fans, they are usually very quiet (=smooth), which is what you want for a telescope. I use a 92mm Noctua on my 220mm Newton and it is plenty even at reduced speed. I bet a slower revving 12V Noctua powered directly by a 5V USB supply would work fine. Just attach the red and black cables from the (non-PWM) fan to the plus and ground of the USB cable (USB pin-outs can be found on line). I’ve thought about doing this on my scope and simply power the fan using a small USB power bank attached to the mirror cell. One less cable to trip over.

A little air movement goes a long way:

https://www.fpi-protostar.com/bgreer/fanselect.htm

[AbsolutelyN]

Thanks for the input here, really appreciated. I like the USB adaptor idea. 

If I understand all this correctly the items below should do the job. Does these appear to have all the cables and parts required to build one? 

https://www.amazon.co.uk/Noctua-NF-P14s-redux-900-Ultra-Silent/dp/B00KF7NGUE/ref=sr_1_2_sspa
https://www.amazon.co.uk/gp/product/B002ZO9MVC/
https://www.amazon.co.uk/dp/B07C4RZ4RF

I was planning on mounting it on some Perspex I have and attaching with velcro as suggested. 

[martinl]

It should work. No guarantees, of course. Personally I’d skip the speed controller, a 900 RPM fan at 5V should spin slow enough on its own, but it won’t hurt anything. 

[martinl]

The main issue with undervolting fans is that they may not have enough power to start spinning. Usually you can get them going by giving them a flick with a finger in that case, kind of like starting an old airplane. I’m not sure if larger fans, like this 140mm one, is more likely to suffer this than smaller ones. I would expect so and would probably look at a slightly smaller one.  

[AbsolutelyN]

Thanks all, very much appreciated. Will see if I can get it all working over weekend. 

[AbsolutelyN]

21 hours ago, martinl said:

The main issue with undervolting fans is that they may not have enough power to start spinning. Usually you can get them going by giving them a flick with a finger in that case, kind of like starting an old airplane. I’m not sure if larger fans, like this 140mm one, is more likely to suffer this than smaller ones. I would expect so and would probably look at a slightly smaller one.  

You were absolutely right on the USB not having enough power to spin the 140mm. Unfortunately I'd already ordered it. However I found a better solution than USB - just a simple battery. Obviously need to sort the wiring better and find a case for battery but I like the idea of it being self contained so no wire trailing from back of scope. 

Edited October 8, 2020 by AbsolutelyN

[AbsolutelyN]

One of the links above mentions airflow direction - I've pasted the text here. From this I assume the idea is to blow air at the mirror? I had assumed it would pull air through the tube. Any consensus on which direction is better? 

https://www.fpi-protostar.com/bgreer/fanselect.htm

"When using a rear-mounted fan, you have a choice of airflow direction; blowing onto the back of the mirror, or sucking air out the bottom of the tube. Unless the bottom of your tube is closed and carefully ducted, blowing the air stream at the mirror will be more efficient (i.e., a smaller fan can be used to get an equivalent cooling effect). In practice, this slight advantage is almost meaningless, and other more pragmatic considerations take precedent. If you regularly observe from a dry, dusty, location, consider pulling air out the bottom of the tube to reduce the accumulation of dirt on your optics."

[CraigT82]

I use the blowing up the tube method, I thought the air in the tube would be mixed together more effectively this way and I support that with absolutely no testing or evidence whatsoever!

[AbsolutelyN]

4 minutes ago, CraigT82 said:

I use the blowing up the tube method, I thought the air in the tube would be mixed together more effectively this way and I support that with absolutely no testing or evidence whatsoever!

Brill thanks . Turned out to be really easy to make of these, all ready to test tonight assuming forecast stays clear. Thanks to everyone for help, much appreciated. 

[martinl]

Blowing is generally considered more effective for cooling. It also keeps hot air from the observer to be pulled in from the top and interfere with high power views. That said, I’m a contrarian and opted to suck. That works as well, and keeps moist air close to the ground from being pushed into the tube.

If you attach the fan or baffle with Velcro you can easily swap it around to see if it makes a difference to your views. 

[AbsolutelyN]

7 minutes ago, martinl said:

Blowing is generally considered more effective for cooling. It also keeps hot air from the observer to be pulled in from the top and interfere with high power views. That said, I’m a contrarian and opted to suck. That works as well, and keeps moist air close to the ground from being pushed into the tube.

If you attach the fan or baffle with Velcro you can easily swap it around to see if it makes a difference to your views. 

Yes I've used velcro for fan, battery and to attach to bottom of scope so easy to change direction. It's a bit rough and ready but will hopefully do the job. I'll clean it up and make it a bit more presentable and secure over time. 

 

Edited October 8, 2020 by AbsolutelyN

[Space Hopper]

The way i've always thought, is that if you draw in air from below the tube and up through it, 

you are only cooling the rear of the mirror, rather than the business side of it.

If the mirror is warm, there will be a boundary layer of warm air directly above it, which will stay largely undisturbed.

Eventually it will dissipate, as the entire mirror reaches equilibrium, but this will take time.

Theres also more dust and moisture at ground level. Is this what you need blowing up the tube ??

I know Orion Optics tubes work the opposite way : they draw air in down the tube from above. I've had 3 of them.

 

This topic of course has been debated over and over at length before. But its certain that either way is better than no fan at all.

But i myself consider sucking to be better than blowing.....🤭😆

 

Edited October 8, 2020 by Space Hopper

[martinl]

My reasoning for opting to pull rather than push air was similar to @Space Hopper’s. Pyrex, and especially quartz, mirrors don’t change their figures massively due to thermal expansion. The main goal of mirror cooling is to reduce the boundary layer in front of the mirror and to reduce tube currents. Best is probably to have a fan blow across the mirror face to scrub the boundary layer, but that often involves cutting holes in the tube (been there, done that, didn’t like it). Since my mirror has a slight turned edge I opted to put a baffle a few cm in front of the mirror with a hole a few mm smaller than the mirror to mask the turned edge and to work as a Mauro da Lio baffle. Air gets sucked down the telescope tube and turns 90 degrees after the baffle in front of the mirror, hopefully sucking some of the boundary layer with it. I know some people use pushing fans to do something similar. The baffle then forces the air to pass in front of the mirror, mixing the (hot) boundary layer with cold air. 

[Wyvern]

Just throw my hat into the ring on this thread.

A while back, I invested in a replacement focusser for my 8" f5 SkyWatcher and discovered as a result of shorter focus travel, that I'd need to move the main mirror up the tube by some 10-15 mm. Whilst having the mirror backplate off the scope, I reasoned it a good time to fit a "cooling fan" to the setup. 

In time honoured tradition I followed the NUMPTI approach - (Never Use More Product Than Intellect).

Located "redundant" 4" dia, 6V computer cooling fan and connected wiring through on/off switch to USB connector. This then plugs into my USB hub for power. 5v output from hub is enough to "get things turning." I'd wired it up to blow air from the mirror back, up and out the top of the tube. Unlike Space Hopper, I'm clearly more of a "blower" than "sucker". 😉

Chunked out 4"ish dia hole in mirror backplate and attached fan through rubber "O" rings to outside of backplate.

Did the whole move mirror/re collimate thing and remounted backplate to scope body.

Switched on power and on removing front dust cover, felt a "reasonable" flow of air being pushed out therefore assuming that everything was working as "planned".

In practice, on entering the obsy I power up and remove front dust cap from scope. By the time I've rolled the roof back, fired up the laptop, initiated Stellarium and FireCap and then sent the scope to a decent 1st mag star to do a focus test on, it's time to switch off the fan and get down to "serious business". It certainly does help get the scope down to working temp much quicker with a minimum of fuss.

Would be delighted to show some piccys of the setup, but fractured my ankle so am out of action for the next couple of weeks. Please make the most of the exceptionally clear and transparent skies that you'll now be getting as a result of my "sacrifice". 💩

Regards to all,

Les  👨‍🦽 

[philhilo]

i have come across this thread and am interested in the outcomes from adding a fan to the rear of a Newt for imaging

My position is not about getting the OTA to ambient temperature as it lives outside, however I do have a problem with gradients in images, and I know some of these are caused by dew on the secondary and primary (despite being told its very rare for the primary to dew up, tell me that when I am stood with a hairdryer, again!). I have looked into secondary heating, but if I can do it with one mod then even better. 

I have read many articles extolling the virtues of a fan, cool down times, dew prevention, improved resolution due a disruption of the boundary layer on the primary (better when blowing rather than sucking).

My concerns with the fan are three fold:

How does it impact guiding - you are applying a force to the tube with the fan blowing, and with an elastic mount  momentum during corrections?

Can the vibration be eliminated sufficiently to not be seen in the images?

I have spent time trying to stop light getting in through the rear of the OTA, can a fan be installed without compromising the light proofing?

Cheers,

Phil

 

 

 

[markse68]

Can’t comment on imaging but I did have a situation recently where I started observing at one location (my workplace) then decided to go closer to home with the scope. It was a cold damp evening and of course when i arrived at the second location and set up the scope again, the primary and secondary were completely fogged from the warm car journey. Disaster I thought 🤦‍♂️ I switched the fan on and within 5 mins or so they were both clear and I was observing again 😊 

The fans get my scope from my bedroom warmth to tight double splitting ability in less than 30mins I’d say- compared with some evenings before I fitted them when even after hours the mirror wasn't functioning to it’s full capability. They blow and it’s a pyrex mirror. The baffle plate they’re mounted to does quite a good job of sealing the rear of the OTA and they shift quite a lot of air so clear the tube of warm air pretty quick. Yes you can see spiral currents in a star test but it’s a lot better than thermal eddies and they can always be switched off for critical observing though i rarely bother. They’re great for when bringing in frozen dew dripping scope back indoors- leave them running for half an hour and scope is back to warm ambient temp and condensation free. One of the best upgrades I’ve made I think.

Mark