Article: Cooling of Newtonian Optics

[gregbradley]

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

[mental4astro (Alexander)]

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 It would be too long, and eventually out of date.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.blogs...-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 (Paul)]

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 (Trevor)]

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 (Paul)]

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 (Andrew and Kim)]

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 (Alexander)]

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]

Originally Posted by 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.

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.

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 (Mike)]

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