Cooling newtonian

[Robindonne]

Im trying to solve as much issues i can face during the nightly hours.   And because im probably doing most of the imaging with a newton, im planning to install one big (or 3 small) fans, to cool the mirror.
i have some non-astro 3d parts being printed at a company and im planning to add one extra item.

 I have some margin-millimeters between the mirror and the tube.  Im wondering if a thin and inwards curved ring will help cooling the surface of the mirror. The idea is to let some cool air flow over the mirror surface instead of Only cooling from the back of the mirror.  
Added a small sketch to explain a bit the idea.  
if im inventing the wheel again i hope to hear where this is for sale and what its called.  Thx

[NGC 1502]

I’ve not seen that type of curved deflector before to direct air towards the mirror surface.  On the other hand someone has to be first and maybe that’s you 👍

The usual way to cool the mirror surface is a fan(s) in the side of the tube blowing across the optical surface, in conjunction with a rear fan.  Some favour fans that blow, others that draw air away from the mirror.

You’ve not mentioned the size of the mirror. The larger the mirror the more that active cooling is a good idea.  Other good factors are a mirror cell that’s not sealed but with plenty of ways for air to freely circulate.

There’s lots of online telescope making advice, just do a search.  Stellafane is a US based source that has loads of ideas.

HTH Ed.

 

 

[Robindonne]

3 hours ago, NGC 1502 said:

Some favour fans that blow, others that draw air away from the mirror.

Well this might be also e good solution to suck the air against the reflecting surface.  
 

3 hours ago, NGC 1502 said:

The usual way to cool the mirror surface is a fan(s) in the side of the tube blowing across the optical surface

That was probably the reason i wanted to try this.  I remember reading an old article about cooling mirrors. What i didn’t know was that the air just above the mirror gave the most problems.   So not just the mirror but the air from mirror to 20-30 inside the tube.   
 

2 weeks ago i had again a fogged mirror and correctorplate.   So these cloudy weeks are perfect to make time and solve some issues with my small collection 

[Robindonne]

3 hours ago, NGC 1502 said:

Stellafane

What an info-filled website.  Thx

Edited June 18, 2020 by Robindonne

[Ricochet]

5 hours ago, Robindonne said:

 I have some margin-millimeters between the mirror and the tube.  Im wondering if a thin and inwards curved ring will help cooling the surface of the mirror. The idea is to let some cool air flow over the mirror surface instead of Only cooling from the back of the mirror.  
Added a small sketch to explain a bit the idea.  
if im inventing the wheel again i hope to hear where this is for sale and what its called.  

As a concept it has been considered before but I doubt there is anyone making such a device commercially. If you do try this I think you want the air blowing across the mirror, not back down onto it. Disrupting the boundary layer of air is the goal rather than cooling the mirror from the front face. 

I have something similar on my dob, although mine is just a simple flat ring that sits on top of the mirror clips and is more to mask the edge of the mirror rather than redirect air flow from the fan behind. The outer diameter of the first ring I made was matched to the internal diameter of the tube to test if it made any appreciable difference. I could not see any improvement in the image when the fan was turned on, but the passive cooling was severely impeded and the telescope would only cool when the fan was running. As a result I changed to a thinner ring with a gap to allow air to more easily rise up the tube. 

[Robindonne]

No indeed, when i would change the direction of the fan, this ring is useless. But when ngc1503 mentioned about the suction instead of blowing, it also sounded like a way to definitely get the air in the tube moving against the surface. At least when the fan-tube connection is almost closed/sealed(less leaking)
 

But if its tried sometimes ago and not being used anymore then it probably means it wont work that good
 

i have to wait what Offer the 3d company Makes for the ring, i think not much, before i dive in the measuring etc.   
 

35 minutes ago, Ricochet said:

As a result I changed to a thinner ring with a gap to allow air to more easily rise up the tube. 

Maybe If you can find the time someday to make a picture from the inside of your tube Pls.?

Edited June 18, 2020 by Robindonne

[Peter Drew]

An interesting topic.  What has intrigued me over the years is the sheer variety of methods used by owners to address the issue.  You would think by now a consensus would have been arrived at to define the solution.  So many times I have read of an approach being advocated only for others to say that their experiment with it was a disaster.  There is also a definite school of thought that you shouldn't actively cool optics but rather to insulate them to maintain their ambient temperature, mainly for closed tube designs..  There are several small firms making a living supplying gadgets and materials intended to help matters.       🙂

[andrew s]

Interesting  indeed @Peter Drew, I was looking at this again recently where the boundary layer on spider vanes was being discussed.  

The issue in general is the formation  of a boundary layer with a different and changing refractive index to the  bulk air. To stop this it seems you have two options.

Insulate to keep the mirror and enclosed air at the same temperature or cooling the mirror to match the falling air temperature. 

The big boys air condition the whole telescope enclosure to match the expected night time temperature! 

Regards Andrew 

[Robindonne]

1 hour ago, Peter Drew said:

An interesting topic

Oke nice. Your reactions sounds like it will maybe help cool the mirror and nearby air in the tube. I will try something.  Not that im educated in whatever science is needed for this test, nor having any knowledge of whats possible with designing plastics(only wood), nor having a second quattro available to see the difference, ánd being one of the most inexperienced stargazers😅. But i will install the ring, and have it made the air flowing ánd towards ánd over the surface of the mirror.   
 

I will try to find the Article i read.   It mentioned not only a cool mirror but an (as important) cool area above the mirror.  Something thats effects the seeing more than the turbulent atmosphere.   
 

As i said, having really so less experience, i do know i have to cool the mirror one way or another. Just want to try to reach more than just the mirror if its proved to help.  
 

[andrew s]

Google "cooling telescope mirror" it gets a  number of interesting articles including some with video showing the air movement.

Regards Andrew 

[Robindonne]

37 minutes ago, andrew s said:

Google "cooling telescope mirror" it gets a  number of interesting articles including some with video showing the air movement.

Regards Andrew 

Oh wow.  Way more options and experiments than i thought.  When seeing all these different configurations, the most interesting is indeed why isn’t There one best way to follow. The single or multiple fans on the back of the mirror are likely not the best option.  But you can’t buy anything else!  At least Other options are not offered in probably all the astro specialized shops?

[Robindonne]

21 minutes ago, KevS said:

I always take a methodical approach to thermodynamics and heat transfer rather than "analogue bash". One first need to determine what is required as a physical quantity; subsequently.

If you know the volume of the glass in the main mirror it is be possible to determine the heat content of said mirror for a specific temperature. It would then be possible to calculate the volume of air required at a specific temperature (ambient) to reduce the temperature of the mirror to the same temperature as the  air that is now cooling it.; using this information fan sizes can be derived, as indeed could the cool down times.  For any cooling system to work at it's best there is a necessity to disrupt the boundary (stagnant) layer of the primary cooling fluid in this case air in relation to the object being cooled.  Subsequently the more random one can make the airflow the better.

Okay. I do have to put your explanation through a translator app.  But for the parts that i understand for now, you mean that its the best to let the volume of the mirror decide the diameter of the fan? Or go for the most random one and deal with the non corresponding mirrorvolume/fansize but make the airflow better?

I decided to try out a printed ring to see if the airflow will cool the mirror surface faster, and to see if a cooled area above the mirror makes big differences.  

ngc mentioned other option, to suck the air through the tube.  So a reversed fan.  Maybe the ring needs a reversed curve to suck the air over the mirror first.  
The detail that makes this option interesting is that, if the mirror needs to be the same temp as the ambient temp, i wont blow “fanheated” air against the mirror.  And therefore always Blow slightly warmer air into the tube.  Dont know if its really that big of a deal. 

[Robindonne]

2 hours ago, KevS said:

Give us a few days and I will thump a spreadsheet together so you can determine air flow requirements.

That of course would be very helpful.  Although to save you some work i can tell you its for a 8” Quattro.   If its a helpful (for us all) list you can make, then a big thank you in advance for all who benefits from that list.   To complete my planned setup, im considering 3x40 mm fans.   And if possible i will add a fan controller to it.  Dont know if its normal to shut it off after an hour orso, or let it blow on a lower speed for the whole night.  

[John]

Is the cooling about removing warmer air from within the tube and, if possible, disrupting the boundary layer or is it about actually cooling the glass temperature ?

I've always considered the former to be the key thing rather than the latter ?

 

 

[John]

Makes sense to me Kev

I don't find that I need to use the fan on the primary cell of my 12 inch dob. The tube seems to cool quite effectively in 30 minutes with the top cap off.

 

 

[Robindonne]

What I remember reading was due to the temperature lowering during the night, faster then the mirror can cool down without a fan, it would help to keep a constant flow over the mirrorsurface.  

Edited June 19, 2020 by Robindonne

[Robindonne]

And like mentioned before,The constant airflow helps to keep the Area right above the mirror cool. And prevent dew in whatever small form to build on the mirror

[Barry-W-Fenner]

52 minutes ago, John said:

Is the cooling about removing warmer air from within the tube and, if possible, disrupting the boundary layer or is it about actually cooling the glass temperature ?

I've always considered the former to be the key thing rather than the latter ?

 

 

Thats an interesting theory John, Ive not looked at it they way until now. I assumed that you needed to get the mirror temperature down. Not so much the temperature inside the tube.  However it makes sense to reduce the  temperature in the tube and make it less turbulent.

Baz

[Robindonne]

Those weird looking open or flex tubes aren’t that weird at all🙂

[John]

23 minutes ago, Robindonne said:

Those weird looking open or flex tubes aren’t that weird at all🙂

The truss design has advantages with cooling. In the winter months warm currents of air from the body can get into the light path though. I found that a light shroud helped with that and keeping stray light out of course. Also for keeping debris off the primary mirror.

I'm just not sure that, in the apertures that we amateurs generally use, that the cooling of the optics themselves, in terms of the optical figure, makes a lot of difference 

Keeping the light path clear of air currents certainly does make a lot of difference though.

 

 

[NGC 1502]

34 minutes ago, Robindonne said:

What I remember reading was due to the temperature lowering during the night, faster then the mirror can cool down without a fan, it would help to keep a constant flow over the mirrorsurface.  

I’m thinking that this applies to very large mirrors that can take hours to cool, and if the ambient temperature keeps on dropping struggles to get there.

I used to have a rear mounted fan on my 10” Dob, I fitted that more to prevent the primary dewing up, I had that happen twice at my local club’s dark site.   However to my horror on another occasion I turned the fan on when it got very damp and dewey - result was almost instant dewing of primary 😩

[Robindonne]

47 minutes ago, NGC 1502 said:

I’m thinking that this applies to very large mirrors that can take hours to cool, and if the ambient temperature keeps on dropping struggles to get there.

I used to have a rear mounted fan on my 10” Dob, I fitted that more to prevent the primary dewing up, I had that happen twice at my local club’s dark site.   However to my horror on another occasion I turned the fan on when it got very damp and dewey - result was almost instant dewing of primary 😩

Hm maybe that’s true indeed.  And maybe its a lot of extra work for small improvements.  But better safe then sorry and time or some extra couple of € aren’t a real headache.  I don’t know if besides the size of the mirror, the thickness filles a role in cooling down.  Its an f4 and im not really sure if that means the curve/radius is smaller. What would mean the thickness probably bigger.   When i bought this newton I remember comparing the weight against a carbon tube.  And that was not a big difference.  8,? vs 10 kg.  So the mirror must be heavy The heaviest part.   Comparing against a normal 8” newton, the Quattro8 is also 1,2 kg heavier while being 20% shorter than a normal 8”.  I cant find the thickness of these mirrors but based on these weightnumbers a conclusion is easy made that the f4 mirrors are thicker

Edited June 19, 2020 by Robindonne

[Peter Drew]

One good start to mitigate tube currents is to have a decent clearance between the tube and the lightpath to the mirror.  The OP mentions millimetres clearance, I would prefer centimetres.     🙂 

[NGC 1502]

8 hours ago, Peter Drew said:

One good start to mitigate tube currents is to have a decent clearance between the tube and the lightpath to the mirror.  The OP mentions millimetres clearance, I would prefer centimetres.     🙂 

Indeed, that’s one of several things I don’t like about OO UK Newtonians, the tube diameter is not much more than the primary diameter, and the upper tube trim has an opening virtually the same diameter as the primary.

Having said that, my OO UK 10” Dob is a great scope that I enjoy using,  but would be better without those issues.  I lessened those problems by increasing the primary cell ventilation (earlier OO mirror cells had a solid back with just a single 10mm diameter hole) and removing the lip from the upper tube trim.

Ed.

 

[alacant]

On 18/06/2020 at 15:50, Robindonne said:

trying to solve as much issues i can face

Hi

I've never come across this before.

How does a non-cooled mirror -without a fan- manifest itself? 

Cheers