Quote:
Originally Posted by 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.
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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/astc...is/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/t...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