Article: Cooling of Newtonian Optics

[iceman]

Alex Massey (mental4astro) has kindly written an article getting to the details and myths around Cooling of Newtonian Optics.

You can read the article on the IceInSpace Projects (http://www.iceinspace.com.au/projects.html) page, or directly by clicking the link below:

Cooling of Newtonian Optics (http://www.iceinspace.com.au/63-771-0-0-1-0.html)

Thanks very much to Alex for taking the time to write the article!

[iceman]

Article posted.

[Slawomir]

Thank you Alexander for clearly explaining the processes involved in cooling of Newtonian telescopes.

[Steffen]

I find dew heater straps a major source of grief for high-magnification viewing (which is what I mainly do), they just create so much turbulence at the top of the scope (Mak in my case). I don't have a sensor controlled strap, just a manual control, therefore my heater is either too cold or too hot most of the time.

This is why I rarely use the heater strap, at the first sign of haziness on the meniscus I use a 12V hairdryer to blow it away. This usually gives me another half hour of viewing before I need to check again. I still pack the heater, just in case, but try not to use it.

As for tube currents, I reckon that insulating the tube helps. I found this by accident after lining my Mak with FlockBoard. I have since found that others had good success with insulating their tubes on the outside. An insulated tube cools more slowly and evenly, therefore avoiding hot and cold spots and air currents.

Just my 2 cents. Thanks for the article, Alex!

Cheers
Steffen.

[mental4astro]

Mike, thank you for uploading the article.

Slawomir and Steffen, thank you for your kind words.

Steffen, thanks for mentioning how you deal with dew on your Mak. As I only use my C8 from home, I too use a hairdryer to clean off dew. I also use a dew shield that I made from an old yoga mat I found at a council chuck-out. This DIY shield is very long, and I pull it down over the OTA all the way to the Dec. bearings. I hadn't thought of it much, but the shield usually gives me 2, sometimes three hours dew protection before I need to pull out the hairdryer. After reading your post, I'm guessing that the oversized shield is giving more protection than I thought. As I'm typically pushing the C8 between 250X and 400X with the Moon, I can't say I've noticed the thermal interference that you mentioned after clearing off the dew. I just might be in a bit of a 'zone' and am able to ignore it or work with it until it calms down. The picture below shows my ol' C8 with its yoga-mat dew shield. The bright blue colour doesn't cause me any problems either.

[gregbradley]

I would have to disagree with several of the points here.

Most scopes that have fans have 3 or 4 not one. This then spreads the flow more evenly. I have not ever noticed distortion in my images from the fans on or off. Have you seen this effect with high powered eyepieces?

My Planewave CDK17 for example, will not come to tight focus if the temp difference between the primary and ambient is more than about .7C or 1C. You see it quite clearly.

The fans speed up the equalisation process and the images become noticeably sharper once the temp difference is around .3C or less.I leave the fans on all night. I almost never see dew on that scope.

This is an open truss system so the idea you don't need fans with an open truss system is not correct. It makes a huge improvement. It speeds the vital thermal equalisation process and I am not sure about the cool spot and distortion theory. Perhaps in a poorly designed fan system or one with only one fan (rare).

I have not trialled with and without fans on successive subexposures to see if there is a noticeable difference to sharpness but my educated guess it there would be none and if anything slightly better with the fans on as it keeps the temp difference in the case of my CDK to around .1 to .2C which is minor.

With my AP Riccardi Honders, it has 2 fans and 2 backplates that remove allowing circulation with the primary. The tube is wider by about 50mm than the primary so air can get around the primary. It also have thermal blankets which look like wet suit type material with velco strips to fasten them. They seem to work well. On 3 nights, one without the fan going, I did not get dew but there was a small amount first thing in the morning that must have formed at the end of the night. The 2nd time without fans (I thought they were on but they weren't) and no dew except again for first thing in the morning. The 3rd time no dewshield (it was windy) and fans going but rear plates on (I was worried about windborne dust).

Some very slight dew starting to form. So the fans, the backplates and the dewshield help a lot. The inside of the scope and dewshield is flocked.

Roland recommends to never use a hair drier. I know many use them on SCTs but thermal shock is a possible way to crack your corrector plate if done too hard. I also have a hair drier in the past and found it to be a very temporary solution and dew forms again relatively quickly.

Some Planewave CDK models now have 2 fans that blow across the primary. I think that is a great feature.

Greg.

[Moon]

Thanks for the article Alex.
Anyone interested in dealing with dew should google for "dew point" and do some further reading. Dew does not fall from the sky like rain. It forms on surfaces that are colder than the dew point. So in my view there is only 1 way to deal with dew: heat. You only need to be just above the dew point, and there are many ways you can achieve this.
James

[glend]

I was restraining myself on this one. While the article is informative , as someone who has built a few scopes myself , who has also researched this subject, it doesn't discuss the very good reasons that high end scope makers like Planewave still supply fans on their scopes - and it's not just because of market expectation.

The article does talk about the benefit of air flow across the mirror but doesn't point out the obvious, that simply tuning the rear fan around makes it into a puller instead of a pusher; that coupled with the standard fan shroud mod to enclose the back and create ducted flow, and the use of tube baffle just above the mirror, will draw air down across the mirror face and around it to exit out the rear.

Planewave CDKs use rear air extraction. The use of rear extractors can negate the need for mirror surface blowers, especially if a 'cuff' is used around the mirror to aid duct effect. In Planewave's case the fans are, I believe, computer controlled, but to what intent other than perhaps throttling the PWM I am not sure.

This maybe beyond the intent of the article in fairness, and it is impossible in a short article to cover all possble designs.

[gregbradley]

Thanks for that. What is PWM?

I am not sure what you mean by a "cuff". A circular tube sticking out past the end of something?

Yes the air is pulled out and it appears to be sealed as you say. I think the AP Riccardi Honders is doing the same thing.

I think air flow does indeed inhibit dew as well as heat. Having all of these various items - dew shield, flocking, air flow, heating if needed (as a last resort I imagine) thermal blankets add up to dew is not too much of an issue unless you are in Cairns or Florida.

Does regular dewing stain the mirrors?

Greg.

[glend]

PWM = Pulse Wave Modulation, commonly used for motor control, can also be used to control supply to things like thermoelectric cooler modules for camera cooling.

Re blower fans, I bought ten of these off ebay when I was building my imaging newt:

http://i.ebayimg.com/00/s/NTg1WDU4NQ==/z/u1MAAOxyEoFSVPd2/$T2eC16VHJGoFFvdy!+)TBSVPd2Qzc!~~60 _14.JPG

and they are great. Small, can be mounted just about anywhere with double sided tape or velco patches, no vibration and enough flow to be useful as a defogger. I am finding all sorts of use for them.

What I mean by a 'cuff' is a short tube rising above the mirror surface as we see with GSO RCs, Planewaves, etc. The short tube section provides the mirror with some obvious side light protection, but also ducts.

Re the question about 'dew staining,' I don't know. Most mirrors will require recoating over time. However, I imagine it can bind dust to the surface. The dreaded airborne eucalyptus sap is much more of an issue, in my opinion, if you use your scope anywhere near a stand of these trees (or downwind of them), as the sap is not water soluble due to the oil content.

[mental4astro]

Glen, I completely agree that a closed rear cell with the fan pulling air down out of the scope instead of the blowing onto the rear of the primary is an excellent solution. It was an oversight of mine not to have included that solution in the article. Thank you for bringing it up! If you notice, the article deals with open rear cells. I had used a pulling system on one scope many years ago, and had forgotten about it. Ta for jogging my memory.

Fans pulling air will have a distinctly different affect. While there will be an initial temperature differential in the mirror when the fans are first turned on, I do have to agree that this is a much gentler way of cooling and the mirror will respond differently. A closed rear cell allows for air to be pulled around the mirror from the front of the scope. The air flow is distributed over a large area rather than concentrated on one spot as blowing onto the mirror, particularly if the cell is an open one. You will not get any disagreement from me here. The differential does also occur, but over time it is eliminated. If this is how Planewave uses its fans, then that is a completely different way of doing so from what the article deals with. Faster? Maybe. And if it does, I have no problem with it. I am grateful that the article has started some debate on the matter. If from the debate everyone, including me, learn something new from our peers, then the article has played its role. From some of the posts already made I've found ways to improve the article too.

The added bonus of a closed cell pulling system is that you also get the air flow benefit to controlling dew.

The 'cuff' is mentioned I guess you can call a dewshield, same way Glen describes the 'cuff' he mentioned.. Come to think of it, it really is a dewshield. This was not meant to be fancy but a simple confusion on my part thinking a dewshield was put over an objective lens or corrector plate. James, I agree that dew does not fall from the sky like rain. The cuff, or dewshield, that I put around my mirror boxes can end up soaked, outside an in. But the primary mirror is dry. Temperature difference between outside the mirror box to the outside? I doubt it after an 8 hour session. Water is attracted to water due to its molecular shape, so if the dewshield is wet, dew will gather more readily on where there is water already. This shelters the primary from dew forming.

Using a hairdryer is offered as one solution. If you have caused a corrector plate to crack after using one, then it dryer was set too hot, placed too close, or for too long a period, or the ambient temperature is very cold. I also don't experience frost where I use my C8, so the scope is never exceptionally cold. Common sense needs to play a part in this. I will ask for an amendment to be made to the article about this. Thanks for suggestion.

[mental4astro]

Please understand that the article is an insight into cooling, and looks mainly at poor implementation of some systems. It is not an absolute article. Some of the points already made by some people have brought up items I had not mentioned, like pulling air around rather than blowing onto a mirror, and i welcome this. I am also not denying the good work that others have put into systems that work, and I also welcome people adding this to the discussion. These systems also respect the thermal properties at play. This supported by the phrase "controlled cooling" as mentioned in the article. To work in terms of absolutes is like insisting that the Earth is flat.

We all welcome helpful comments that build our information pool. So please keep in mind the intention of the article.

[Shiraz]

Hi Alexander.

In the interests of filling in the information pool on some more aspects of what “controlled cooling” might mean when imaging, the general advice in the literature is that you should definitely use a fan when imaging with closed tube Newtonians (planetary and DSO). For example, Martin Mobberley in his book on webcam planetary imaging goes as far as suggesting that fitting a cooling fan to a planetary Newtonian may perhaps be the “most important decision you would make in your observing career”.

A fan with a baffle plate speeds up the initial cooldown (from maybe 2 hours to ¾ hour with my 10 inch for example) and keeps the mirror closer to ambient through an imaging session, so that boundary layer and tube current effects do not stuff up the image (http://www.fpi-protostar.com/bgreer/fanselect.htm and http://www.fpi-protostar.com/bgreer/images/hwaves.jpg plus other Brian Greer sites). Ambient air blowing on the back of the mirror can initially cause a cold spot, but that goes away when the rest of the mirror reaches ambient. Mirror figure does change with temperature, but my experience is that boundary layer effects over the mirror are much more deleterious when it comes to image quality http://www.eso.org/gen-fac/pubs/astclim/papers/lz-thesis/node61.html (note an extra ~0.3 arcseconds of seeing per degree C temperature differential !!)

In terms of thermal stability and heat transfer, I think that borosilicate glass is actually better than ordinary glass, although both cool down fairly slowly without fan assistance http://www.cruxis.com/scope/mirrorcooling.htm .

Some of us who have done planetary imaging even go to the extreme of using Peltier cooling to speed up mirror cooldown and tracking http://acquerra.com.au/astro/cooling/ – this produces intense thermal gradients, but these can be managed and the cooling is still worth it overall to get rid of the boundary layer and help the mirror track a dropping temperature. My understanding is that large professional scopes also employ surface cross-flow fans, primarily to cool the mirror and break up the convection structure http://adsabs.harvard.edu/full/1979MNRAS.188..249L, - they also probably help with dew control. Some semi-professional scopes may use both back fans and boundary layer fans (eg the latest CDK17 has 4 fans blowing across the mirror front surface and 3 evacuating the region behind the mirror) to clear away the boundary layer and cool the mirror http://planewave.com/products-page/telescopes/17-inch-cdk-optical-tube-assembly/ .

I am sure that you can get good results without fan cooling if you use a thin mirror in an open Dobsonian structure, but I think that users of imaging Newtonians with solid tubes and moderately thick mirrors will definitely be better off with back fans - I certainly would not consider imaging without a fan running. The literature also suggests that we may be even better off by installing front fans to scrub off the boundary layer and cool the mirror from both sides, but I haven’t tried that yet. I also shudder ever so slightly when people suggest heaters for dealing with dew - even though it might do the primary task, one has to wonder what damage it does to image resolution. I have found that covering all sky-exposed OTA external surfaces with IR reflecting tape and keeping up a steady and gentle airflow through the OTA keeps dew at bay under all but the most extreme conditions.

Regards Ray

[Steffen]

That would be an issue with real hair dryers, but the 12V foldable camping model I'm using barely puts out lukewarm air in a cold night. I certainly cannot imagine drying my hair with one (and I have very little). That makes these 12V hair dryers ideal and quite safe to use on optical surfaces. Restraint is the key, as with anything. You don't want the corrector or meniscus to actually become warm.

Cheers
Steffen.

[gregbradley]

Or worse crack from thermal shock. But as you say moderation is the key. I am not sure if its a lasting handling though as I have done it myself in the past and as I recall the dewfreeness did not last long.

The Honders does 3 things, fan air extraction, open mirror air at the back with an opening around the mirror of around 25mm all the way round, thermal blankets, flocking of the main tube and dewshield and quite a long dewshield.

I used to use a simple wrap around insulation blanket on my refractors around the objective area (like those windscreen sun protectors). As long as you put it on before sunset it worked great. It did not work putting it on during the night as the whole point was to slow down the heat loss of the lens to slow down the dewing process. Simple but effective.

Greg.

[mental4astro]

Ok, following the customary gentle and subtle way of IIS members (like a sledge hammer! :rolleyes: :lol: ), I have revised the article and made a new submission to the Iceman to exchange the article for the new one.

I am actually very grateful for those who did take the time to post in this thread to bring to my attention my unintended bias towards the visual aspects of optics cooling. Yes, I was aware of the methods of cooling mentioned by Greg, Glen and Ray. But as I have not dealt with closed tube instruments for so long, the best practice methods for these instruments and for photo applications did slip my mind. I apologize for this. I am not so arrogant to not acknowledge this.

While Mike looks into the change of the article versions, you can find the revised article in my blog:

http://alexanderastrosketching.blogspot.co m.au/p/telescope-bit-and-bobs.html

Corrr, visual upsetting photo? :scared2: What's the world coming to... :lol:

Cheers

Alex.

[Moon]

Alex, no need to apologize - you made the initial effort and got a really interesting discussion going.
James

[mental4astro]

Thank you James. Most gracious of you.

Yes, it has sparked discussion. That was an intention of the article, and help fill some gaps in my own knowledge as I don't profess to know it all. I don't mind putting my head up to admit this, and relish the chance to grow. Another was to create an article where different considerations to do with cooling could be centralized as a convenient single starting spot. Information is very scattered, and many different ways are described. But there are distinct visual and photo requirements and many, many different instruments. Things can get very confusing on best practice for your individual needs and instruments. The incorrect implementation can result in poor outcomes or a waste of time, money and effort.

But the initial discussion of the article did flush out some excellent suggestions. This gave me the fuel to revise the article, and so give the best outcome for our astro community.

Alex.

[glend]

Alex, I was OK with what you put forward initially, as a primer for newbies it was informative and correct for most aspects of the topic. So thanks for that effort. As we see, the IIS community colaboration has a some benefit for all. Sort of like 'peer review' in research.

[dannat]

Alex article was fine -only thing some may have wanted is a title revision
Cooling of Newtonian Optics - For Beginners/Visual instrument

[gregbradley]

Its been a good.discussion
Methods of.active cooling would.make a good thread
Greg

[mental4astro]

Why not this thread, Greg?

The article is intended as a starting point on the best practice principles . It would be and is beyond the scope of an article in IIS to be a definitive article on the subject. It would be too long, and eventually out of date.

But this thread now gives the topic a home for discussion. Links have already been posted, ideas bandied about.

I don't own this thread. As discussion has evolved, it has become bigger than just myself too.

This revised article I've posted on my blog. I've added a link to this discussion thread exactly for this purpose:

http://alexanderastrosketching.blogspot.co m.au/p/telescope-bit-and-bobs.html

From what I can see, all the ingredients are now here for the next stage in the evolution of the topic to develop.

[The_bluester]

Regards dew control on an SCT. I have found that with the 9.25 and a dew shield (Which is a couple of hundred mm long) the only really viable solution is a heater belt for the corrector plate. I used the hairdryer solution for a couple of years until I finally went out and bought a heater and controller for it, but it is only so-so effectiveness wise.

My viewing has been more or less purely visual observation and without a heater it does tend to slowly spoil your night. You fiddle around and look at the sky to see if transparency has fallen off drastically as the contrast and detail has gone, then eventually have a look at the corrector plate and realize that half an inch of potential rainfall has condensed there. A quick blow with a hairdryer and you are OK for a while before it happens again.

I can not comment on if the heat from the belt is enough to change the figure of the corrector enough to see a difference (You would assume it must have some effect as the heat will be in a gradient from edge to center, with a nice big secondary mirror there as a heatsink to boot) but the lack of condensation on the corrector meaning you can actually still use the scope IMO outweighs any potential lack of crispness the heat may lead to.

[tlgerdes]

My 2c, I've enjoyed it so far. One thing that seems to been "missed" in the discussion is the title, we were talking about newtonian optics, not CDKs, SCTs, closed back scopes etc. Each of these others have very different requirements and different problems.

[The_bluester]

True enough, I even did read the title (And the original article that started it) but responded to comments on SCT designs.

Though the discussion seems to have moved on from simply cooling of Newtonians, even for just Newtonians themselves to include dew control.

[Akwestland]

I would like to add my opinion on this from a complete newbie perspective.

First off, Alex, you wrote a great article on something that I am just starting to look at with my 2 scopes, a 4" SCT and a 8" Newt. Congrats on a great article that has provided an insight to this topic.

Secondly, to everyone that has contributed. I have followed this thread intently. It has been a great learning process to hear from so many users and all of their opinions/techniques.

Only a couple of weeks ago I started to explore the concept of needing more equipment to overcome this problem. This thread has really given me a lot of advice on what I do or do not need. It has possibly stopped me from a purchase that I do not really need. This is the great thing about this forum, everyone gets to have a say, and for a newbie, this is invaluable.

Just my thoughts.
Andrew.

[mental4astro]

I wrote the article expressly to deal with cooling. As the article developed, I noticed that dew control shared some common techniques, and possible problems, so I added it at the end. It would have made for an unfinished article if I didn't mention this added bonus from the efforts of cooling a mirror that benefited our productivity, and if also incorrectly applied would lead to unwelcome consequences that we worked so hard to eliminate in the first place!

I guess I could have mentioned dew control in the title. Oh, well, another modification for the Iceman to do...

[Paul Haese]

The tests that Anthony and I did many years ago show that active cooling is the only way to get optics to within 0.5 degrees of ambient.

You need the optics to be at this temperature to reduce thermal distortion of the object being imaged. Boundary layer controls are a poor attempt at managing thermal distortion and do have an effect but no where near that of cooled optics.

Mirrors cooled naturally will take many hours to come to within 2 degrees of ambient. My C14 takes until near dawn for natural cooling to take effect and even then there will be some thermal distortion.

Mirrors can be rapidly cooled and will performed flawlessly once cooled. They may need further cooling and then after each cooling there will be a period of settling usually 5-10 minutes and then you will be able to undertake imaging or high resolution viewing for an extended period.

I have used a hair dryer on my C14 corrector plate for 8 years and never had a single crack. So I am dubious about causing a fracture. That said, I do wave the dryer around vigorously to prevent me heating one area more than another. In dewy conditions I will get around 15 minutes of imaging done before I need to hit the corrector with more heat.

Anyway, these are just my views and having spent a lot of time discussing this with Anthony over the years I know his views too.

[gregbradley]

Paul,

Have you considered taking this active cooling approach to your RC12? I am surprised some of the scope makers haven't adopted this approach as it would be a good marketing advantage and not that difficult to implement.

I would think potentially there would be a considerable gain to doing this.

It would be great if there were some commercially available bolt on unit to cool the backs of mirrored scopes.

Bird told me the goal was to have the mirror a few degrees below ambient. Is that what you go for with your C14 or just equalised?

Greg.

[Paul Haese]

Because a Newtonian is an open tube it is inherently different to a SCT. I found that if I went just below ambient on the cooling at around 0.7 of a degree this would rebound and equalise at about 0.5 degrees above ambient.

At this stage I had not put my mind to doing this with the RC12. In truth, I have been managing all the other problems, however it might actually be worth doing. The rear plate is made of aluminium which would be the perfect surface to mount peltiers. Sealing it up would be relatively easy too. Something to think about. :thumbsup:

Also worth considering is that it really depends on whether I was aiming for very high resolution imaging. Planetary imaging is very high resolution and so the effects of mirror cooling are very much more evident over native DSO imaging. F8 imaging is really rather sedate compared to f40.

Not to dismiss the idea and just to further show there might be something in it, is that Tony Hallas has applied my SCT cooling system to one of his own and is doing DSO imaging with it. Celestron is now considering the design (the more well known Tony made a difference) and applying it as an imaging option to its scopes.

[gregbradley]

I have a portable AC unit at my dark site weekender house. I think I will drag it into the observatory and see if it makes an appreciable difference or if it simply accelerates dewing!

My CDK17 gets to within .1 C of ambient after about 2 hours even on a warm day and within .5C within an hour routinely. The difference in focusing is substantial. The difference between .1 and .5C is marginal but 1C is noticeable.

Greg.

[iceman]

Version 2 of Alex's article is now live.
Same URL: http://www.iceinspace.com.au/63-771-0-0-1-0.html