Tube currents and fans

[kalasinman]

After considering the issue of tube currents and vents/fans etc. I was wondering why it would be effective to install a fan in order to overcome tube currents. Is one type of air flow better than another? I can see, based upon the following video, that a fan my be effective in initial cool down, but during use is detrimental. I hope you find it interesting, as I did.

--Jack

P.S. The video does not address the possible effects of fan vibration.

[jetstream]

This is a great reference too http://www.fpi-protostar.com/bgreer/sep2000st.htm  The cooling fan should be isolated from the scope itself,and using the fan to pull air in from the font of the tube,exiting around the primary will help boundary later.Well worth a look and read anyway.

[kalasinman]

Very good. Seems like folks are pretty good at quantifying the problem, but short on solutions. I'm hopeful that as currents seem to be more of a concern when deltaT exceeds 5c, I may be ok. Our biggest high to low swing is 10c from early afternoon to dawn. Commonly a 5c swing is  all.

I'm hoping that with a modest size mirror (8") that a closed system (windowed newt) will work fine if the scope is kept near ambient for a few hours before use.--Jack

[Merlin]

Consider also this classic: http://media.skyandtelescope.com/documents/Alder-NewtThermal.pdf .

[kalasinman]

404, not found

[Merlin]

Agreed. Please try this: http://www.bing.com/search?q=thermal+management+in+a+newtonian+reflector&src=IE-Address

[Moonshane]

personally I only run fans for cool down. my understanding of the reason for fans is that they disturb the boundary layer as this creates a weak lens effect. as far as I can see the video shows this happening so the view at the eyepiece should be improved?

also as I understand it the size of the fan matters and also the output. there is no advantage to larger fans. for my 16" and 12" solid tube dobs, I use a single 4" fan blowing on the back of the mirror. once cooled I turn it off.

as with anything it's about compromise. there is always an optimal configuration but you usually create something in between nothing and optimal for a number of reasons. I am a visual observer only though and perhaps this matters more for very / ultra high magnification planetary imaging. I doubt it matters for low power DSO imaging.

I know that some of the best planetary observers in the world use large newts with tubes made of cardboard (sonotube) with fans constantly blowing on the mirrors at very high powers. e.g. http://www.cloudynights.com/topic/24123-refractor-vs-reflector/#entry305201

[kalasinman]

@Moonshane, I'm afraid the conclusions you draw from the video I posted to be opposite what I see happening. There is no statement in the video that there is a deltaT present. I believe the stated purpose of the video is to demonstrate the currents produced by the fans themselves. Having looked at yours and other postings, the common factor is a lack of agreement. I'm not advocating any approach. I was in favor of following the herd and adding a fan to the design I'm working on, and I still may. I was very surprised at the results shown in the video I posted as was the person making it. I found it interesting and thought I'd share it.--Jack

[paul mc c]

I would only use a fan to initally cool the scope,as when using it when observing deteriates the view greatly,especially using high power.

I have seen fans suspended using elastic bands attached to the ota and not the cell, which greatly reduces vibrations.

[ollypenrice]

One of the most, or perhaps the most, interesting approach to the problem was devised by Ralf Ottow, an optical engineer in the Netherlands. His 12.5 inch closed tube Newt has a watercooled primary. Chilled water from a flask is pumped round a spiral of silicone that separates the rear of the mirror from a glass blank, so forming a waterjacket. The cooling is only carried out for a few minutes at the start of a session until the mirror is just below ambient, then it is switched off. There are thermometers inside the mirror and within the tube. When these are all showing much the same value the scope is ready.

It is without doubt the best Newtonian I've ever used but there is more than the cooling at play. The optics are exquisite.

Olly

[kalasinman]

@Olly, thank you! That's an innovative approach indeed.--Jack

[Moonshane]

@Moonshane, I'm afraid the conclusions you draw from the video I posted to be opposite what I see happening. There is no statement in the video that there is a deltaT present. I believe the stated purpose of the video is to demonstrate the currents produced by the fans themselves. Having looked at yours and other postings, the common factor is a lack of agreement. I'm not advocating any approach. I was in favor of following the herd and adding a fan to the design I'm working on, and I still may. I was very surprised at the results shown in the video I posted as was the person making it. I found it interesting and thought I'd share it.--Jack

I am no expert on optics (clearly) and I do not know the impact on the image of the swirls etc shown in the video, nor the strength and output of the fans etc. people with a lot more knowledge than I will ever have report that the boundary layer of air needs to be 'messed up' to prevent the lensing effect. there's not much narrative in the video explaining what is going on which would have been helpful (assuming he knows).

it was indeed an interesting video and my interpretation of what I could see (other than the heat plumes from his hands) was that the boundary layer was being disrupted as I said.

What I can do is base my responses based on personal experience and what I see is the view improved more with time at y location than by the running of a fan unless there's a big initial difference between the temps of the mirror and ambient.

please don't take my responses any way other than constructive and occasionally helpful.  it's easy to come across on forums as pompous or combative sometimes when you mean nothing of the sort.

[Zakalwe]

There's also peltier cooling to consider. Paul Haese does this and his planetary images are pretty damn good.

http://paulhaese.net/peltierstoSCT.html

OK, it's a SCT, but the same principles would apply I would have thought.

I think that with a lot of these things, there are many other factors in play (seeing, collimation, location). When all these things are perfect, then in the hands of a top-class person, the mods will make a difference. In the hands of a very average numpty (such as me!), in a very poor location,  then I'd doubt if there's a huge amount to be gained.

[kalasinman]

@Moonshane, I agree about perceptions being difficult to control, especially without being in the same room, or hearing one's voice.

It is difficult to imagine the swirling of the air currents in the video from fans not working similarly to the swirling of the atmosphere. What I get from your statement about just taking your time implies that these currents may not be present if the mirror and air in the tube are close to ambient. I have read elsewhere that venting of the tube to facilitate cool down can themselves create distortions after the cool down period.--Jack

[ollypenrice]

There's always the opposite approach, too; keep the inside warm. This is much in vogue here in continental europe where people are insulating their SCTs very thoroughly. I guess what really matters is having a stable temperature within the whole tube.

Olly

[pietervdv]

I have recently upgrades the cooling fans on my imagingnewton. It used to have 3 rear mounted fans, thuis helped cooldown a lot nut my experience was the image only became crisp after 2 hours. I believen rear mounted fans leave a wake in the airflow that deels to stick tot the front of the mirror. My mod was tot add two cross flow fans spaced 120degrees to disrupt the airflow over the surface. The air that is blown tot the sides is then picked up by the rear blowing fans. The tube is 50 mm larger thans the primary. This had drastically improved image quality. I can start imaging in 30 minutes after the scope is set up. Excuses the horrible typing. Regards Pieter          

[John]

I have recently upgrades the cooling fans on my imagingnewton. It used to have 3 rear mounted fans, thuis helped cooldown a lot nut my experience was the image only became crisp after 2 hours. I believen rear mounted fans leave a wake in the airflow that deels to stick tot the front of the mirror. My mod was tot add two cross flow fans spaced 120degrees to disrupt the airflow over the surface. The air that is blown tot the sides is then picked up by the rear blowing fans. The tube is 50 mm larger thans the primary. This had drastically improved image quality. I can start imaging in 30 minutes after the scope is set up. Excuses the horrible typing. Regards Pieter          

Hi Pieter,

I've merged your two posts into a single one - I hope that was what you intended

[jetstream]

Disrubt the airflow over the surface. The air that is blown tot the sides is then picked up by the rear blowing fans. The tube is 50 mm larger thans the primary. This had drastically improved image quality. I can start imaging in 30 minutes after the scope is set up. Excuses the horrible typing. Regards Pieter

Pieter,I checked out your site-great images and equipment. I have an interest in controlling thermals and am wondering if you could show some pictures of how you configured the fans on your scope? The more I looked at the boundary layer issue,the more I realized how complicated it can be,never knew it "sticks" to the mirror or that vents after cooling caused issues. Thanks

[John]

On this topic, I take the passive approach to cooling and air currents in that, while there is a fan fitted to the rear of the mirror cell of my 12" dobsonian, I don't use it. I put the scope out 20-30 minutes before I intend to use it and that seems to do the trick. On really cold nights I will generally wait a little longer before using high magnifications.

The mirror cell in my scope is a very "open" design and allows air to circulate freely around the mirror. Taking the tube cap off when I put the scope out and pointing the scope at an angle of around 45 degrees seems to work well enough.

Incidently, on very cold nights, if I put an un-gloved hand on the tube, in the un-focused star image I can see the warm air currents coming through the tube wall and into the light path. One reason for wearing well insulated clothing on cold nights - keeps your body heat away from the tube.

[pietervdv]

Pieter,I checked out your site-great images and equipment. I have an interest in controlling thermals and am wondering if you could show some pictures of how you configured the fans on your scope? The more I looked at the boundary layer issue,the more I realized how complicated it can be,never knew it "sticks" to the mirror or that vents after cooling caused issues. Thanks

Hi,

Sure, here are some pictures that show how I installed the cooling fans on my 10" Newtonian.

The cross fans I only added a couple of months ago. I don't know if this is the best possible arrangement, but it works good enough for me.

At the rear there are 3 fans displacing 12.7 cfm together, and at the side 2 cross blowing fans spaced 120° displacing 4.7 cfm.

The cross blowing fans are arranged like this to minimize any light intrusion into the scope, also some thick filter

pads are installed inside to avoid light leaks.

Taking some pressure losses and recirculation into account the tube volume is flushed out about 5 times per minute.

This might seem excessive but although the carbonfibre tube is very rigid and temperature stable, it can

contain a lot of heat energy sucked up during the day which radiates away slowly at night. The fans also cool down the tube itself.

I tried several fans but found these to be very silent and vibration free. The only sound you hear is the air blowing.

The fanspeed is variable using a seperate control box, but I leave them on 100% of the time.

Side / rear View:

Bottom view:

Inside View:

You might want to check out how the Dream astrographs are cooled with the FAST system. They are the best astrographs money can buy (I you have this kind of money...).

These telescopes also use cellular mirrors to decrease cooldown times.

http://www.dreamscopes.com/pages/07/16inDAtube-02.htm

Best regards,

Pieter

[jetstream]

Pieter,thank you for this information and the excellent pictures.I will look up Dream astrographs to check out their arrangement too.Your images reflect an excellent set up and thermal management,I'm going to use your image of IC 1318 to help me pick out which areas may be the best for visual,if thats ok.A quick look already gave me a good idea where to start,what a great reference.

Thanks again