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Thread: Dew control for astro sketching

  1. #11
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    Default Re: Dew control for astro sketching



    HarmoniousSphere,

    Thanks for your input. But THIS article is geared firstly to sketching. If dew is so heavy, as you say you experience at your location, then sketching onto paper is never going to be an option for you. Paper will never remain dry.

    The problems that you encounter have little to do with sketching, so your discussion is out of context here. You have totally missed the purpose of the article which is noted in its title. But I will all the same address some of the problems encountered.

    It is a real shame that dew is such a problem for you. If dew is such a major problem, then unfortunately your passion for astro has hit a major impediment wholly because of your location. I sincerely hope that your shed is insulated as this will be the first line of defence against your dew problem. Heating may be the only solution available to YOU because of your location, but you need to be more proactive in reducing water collecting inside your scope shed. You may then argue that you want to have your scopes achieve thermal equilibrium by not having an insulated shed - so why are you not thinking about how your heating methods are affecting your instruments and optics, except in how it dries off dew?

    Professional observatories do not use heat. Ever. Yet we as amateurs fail to see this, and somehow believe that the location of these is arbitrary, with only distance away from light pollution as being the only requisite. Far from being the case.

    Heat is one solution, but it is the very worst solution, and depending on your niche in astro not an option at all. We try so hard to get our instruments to reach equilibrium, and then we go stick sources of heat all over the place, with nil consideration to the problems that introducing heat creates in the scopes!!!??? Of course heat will evaporate dew. Yet the consequences are massive power requirements for one, but the worst is the heat plumes generated inside OTAs and above optics, and the uneven expansion it provokes in optics meaning images are distorted because lenses and mirrors are not at thermal equilibrium.

    If the dew situation is so heavy as a result of your location, and heat is your only solution, then that is a real shame. But this is entirely the result of your location. Does your shed have a suitably strong ventilation system that operates continuously when it is closed? Along with insulation, ventilation is the only way you will avoid condensation inside your shed. If you are thinking that insulating your shed is only a thermal liability, think again. It can be an asset for you with your conditions. But you need to make use of a very efficient ventilation system with it in order to create an effective heat exchange before you begin your evening session. You are already using heat to prevent your optics from fogging up, without consideration to how this heat is affecting your gear. So, instead of dreading heat, use it to your advantage in redesigning your shed and how it works as part of larger heat exchange and ventilation system. You need to be thinking about dew mitigation methods not only way before you think about looking at the night sky, but it needs to be passively active all the time. The bonus of this is not only less dew issues during the night, but it will also keep your precious gear dry all the time.

    Moving of air does work. It is totally thermally passive too. If the local dew situation is so profound that a localised fan is not enough, you need look at more active ways of dealing with dew way before you begin your astro session. And letting the inside of your shed accumulate condensation is not the way to go. Insulating your shed will not only prevent condensation inside it, but it will protect your precious gear from excessive heat during the day, and an adequate ventilation system will create a powerful and effective heat exchange system along with keeping your gear dry while not in use. You may not get away with a wholly ventilation system to prevent dew, but there are some better, effective and efficient ways in which you can use heat.

    For those of you who reading this, appropriate site selection should will always be your first consideration for astro. It will not only give you better transparency and seeing conditions, but it will also mean your gear stays bone dry. This is the ultimate, and what can be learned by paying close attention to the geographic location of professional observatories. Remoteness from light pollution is only ONE condition of a long list of considerations that the people charged with finding the best location for these structures. And it is not professional astronomers who do this site selection process nor are they the one designing and building their scopes and buildings. The professional astronomers just get the keys to the joint only at the very end of the site selection, design and build process. They just tell the teams of engineers, technicians & geographers what they need. That's their only input into this process.

    But of course finding the optimal site for astro is sometimes impossible, or sometimes only a seasonal possibility. But that's the nature of the game and we then need to find the best location that we can.

    AND, of course, sometimes we need to make do with what we have, and then we need to be more creative in our approach to thermal and dew control, and even to how we participate in our own niche in astro.


    Local geography and climatic weather conditions need to be understood, no matter where you set up, and you then tailor your own set up to best deal with THOSE conditions, and this tailoring WILL NOT BE universal.

    Astronomical sketching has its own unique set of issues, none the less the very absorbent nature of the medium that paper is.

    Alex.

  2. #12
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    Default Re: Dew control for astro sketching

    I appreciate your thoughtful reply and I acknowledge that i strayed from the core idea of sketching. I apologize for the intrusion. I don't sketch although I tried only to discover that the small skill that I showed in a distant biology class had not improved in any way over the years. But for those with the talent and the perseverance I say hats off to you.

    On the general subject of condensation I expect that I wasn't clear however about how and when I use heat. I am not in a high humidity area where warm weather condensation is a problem and generally in cold weather condensation is a problem only after the roof is open. In cold damp weather I've experimented with running the little heater during a session just to see if I would be more comfortable. Thicker socks are the better choice. I do run the heater after a session where the interior surfaces show condensation. If I don't the water can persist for hours even though the shed is ventilated by the opening on all four sides above the top plates. There are covers and flaps to control wind driven rain but both light and air enter. On nights that I expect to use the heater after a session I close flaps on sides facing into and away from the prevailing wind so the heater is a bit more effective.

    When I converted the steel shed I added rigid foam insulation to the interior walls and the underside of the roof. It is effective in slowing heat gain on hot days, and having little mass it cools down rapidly.

    I am very much aware of things like tube currents and heat plumes, SCT users spot them very early. So unless the session is completely a spur of the moment thing, on warm days I crack the roof in advance to bring the scopes down to ambient. In winter though I think it is more a problem preventing the objectives from cooling below ambient or at least slowing that process.

    Your point about professionals and heat got me to wondering how they manage condensation if they manage it at all. How about the really large scopes? I found a book, First Light: The Search for the Edge of the Universe which had a section on the management of the Hale telescope at Palomar. Here are pertinent excerpts.

    Juan made up his mind about the fog. "I think we'll be alright," he said.

    The job of the night assistant is to operate the telescope for the astronomer. Given half a chance the astronomer will cleverly destroy a telescope. For that reason... the night assistants...have authority over the astronomers...especially when it comes to deciding whether to open or close the dome of the Hale. ...a professional astronomer famished for light, might open the Hale during cold humid weather. That could let dew settle on the mirror...which would turn into an acid mud that could etch the glass."


    Also as you said local conditions are critical. There is a university observatory a few miles from here. It opened in the late 60's with two optical scopes and a radio dish. The height of technology-- remote control using a command line interface. The 20" Schmidt or SCT (not sure) became so corroded that it was simply abandoned. Research work ended years ago. The 30" Ritchey-Chretien though dirty and corroded had automated shutters over the primary and it was refurbished for student use. The site has much less summer fog than other areas nearby including mine, but has similar winter conditions. Lacking the Hale night assistants the scopes must have been exposed to many damp nights. Everything under the domes and the domes themselves corroded to some extent. There were no cloud cameras, moisture sensors or rain sensors. A student at a remote site could and probably did fail to enter a correct instruction leaving the dome open. I do not recall seeing any heaters or dehumidifiers.

    So the pros, as you said don't use heat.

    Anyway I will slosh through the dew season which is odd given that we are apparently restarting a drought and I wish you good fun with sketching.
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  3. #13
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    Default

    Wider dew control considerations

    You know, Harmonious',

    You were right in bringing up your set of difficulties in this thread, even if you don't sketch!

    The difficulties you have to deal with are part of what many others need to deal with, including those who do sketch. AND, discussion of your situation, and what ever solutions may be developed for your situation may help others prepare for such conditions, including for those who sketch.

    So thank you for bring up your situation,

    I have spent a long time searching first for good dark sky sites. This included trips to big organised star parties. One star party in particular early on in my search was a BIG eye opener for the extremely poor conditions the site provided. This star party was held at a scout camp ground, which was in a very lush green valley in the middle of dairy country. There were two fields, one for "visual" the other for "imagers". The visual field was at the very floor of the valley right beside a slow moving creek, with very deep and moist soil all around. The imaging field was up higher on the valley slope - not too steep but far enough from the lush valley floor to make a significant difference. Just by chance I set up with the imaging lot because I felt this paddock had a better, less obstructed sky.

    That first night, the visual field was swamped with dew. The dew was so heavy that a few dob owners had to pour the water out from their rocker boxes! Yet up a little higher up on the more exposed slope, dew was much lighter. The visual observers ended up calling it a night around midnight because the heavy dew totally overwhelmed all their heavy duty dew mitigation measures. Up where I had set up, we were able to rock on with whatever dew control measures we had. This made me aware to two important aspects with dew:

    1, Geography and local land use dictating the formation of dew

    2, Formal campgrounds are not designed/located with astronomers in mind. These campgrounds are located with campers in mind for whom dew is less problematic than for astronomers.

    What we astronomers need to become aware of is the two above points, and then make the effort to find those sites that do provide better conditions for astro. Once these sites are identified and used, the benefits will be enormous, not just for less dew and improved conditions, but also for significantly less power requirements all round.

    ~x.X.x~

    But sometimes dew is just unavoidable, no matter what we do, can do, or where we find ourselves. It is then when we need to take what steps we can to best control dew and be productive. This includes my situation at home where I do my lunar and planetary sketching!

    So, hopefully this thread will develop a wider appeal.

    Alex.

 

 
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