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    Default Dew control for astro sketching



    Dew Control for Astro Sketching

    This has been one of the most confounded exercises for astro sketchers. There have been lots of ideas, but few effective solutions.

    For any solution, there should be a set of criteria that needs to be met:

    • Simple to make
    • Simple to use
    • Modest in power requirements
    • Be effective in keeping paper dry

    Not an onerous set of requirements, but these have proven difficult to achieve. Particularly as paper is so absorbent.

    This article will describe my experiences with dew and sketching, and the three solutions I’ve found that have proven most effective in controlling dew during my astro sketching sessions. As they say, “necessity is the Mother of invention”. Sometimes however, these solutions can have flippant origins!

    Avoid dew to begin with!
    The easiest solution requires no specialized equipment at all. Careful site selection can provide an environment that is dew free in the first place all night long. Dew and astronomy DO NOT need to be inseparable bed fellows. It is possible to actually find locations that are dew free. It does require knowing what to look for in the first place, and then to use this knowledge to finding the site. A big grassy field is actually the WORST possible situation for any astro activities. Grass means rich moist soil, and at night moisture saturation is quickly achieved with water vapour being released by the grass itself and from the soil, and dew settles very quickly as it is denser than air, and everything becomes very wet very quickly. There are even some popular astro sites that have had no appropriate site selection processes carried out to fully determine the location’s suitability for astro activities.

    However, it is possible to find dew free locations, if only being seasonal. The dark site locations my observing bubbies and I use have been painstakingly vetted for exactly this purpose, and these 19 times out of twenty are perfectly dew free during our dark sky sessions. On those rare occasions when dew does form, it also means that transparency is not as good as it can be, and usually we end up packing up early.

    That’s another thing that dew affects. The increase local water content in the air also reduces transparency. Finding a dew free location has many more added benefits than just no dew – it also brings with it improved transparency, and it can also bring improved seeing depending on the local geographic surrounds.

    You will find information on how to start looking for dew free locations in an article I wrote on the topic:

    Selecting a site for Astronomy Purposes - identifying the best possible location

    Dew shield solution
    However, sometimes dew is unavoidable, no matter what we do. I have been able to deal with modest amounts of dew with the very first sketching rig I made. My first solution followed the simple “dew shield” principle and I created an awning that wraps around the sketch rig. Very simple and effective as dew tends to fall around the paper largely without making the paper damp. This awning also provides a great location from which to perch the lights by which I sketch.

    Attachment 172279 , Attachment 172280

    The location of the light source is also extremely important. The worst location for the lighting is on top of one's head! With the light being square to the paper, the reflected glare that comes off the paper goes wholly into one's eyes! By having the paper being illuminated from the side, whatever glare there is is reflected straight off the side, and an absolute minimum of glare is reflected into one's eyes.

    Attachment 172281 , Attachment 172282

    Fans and localised evaporation
    Yet this initial simple solution has its limitations. When the sketch exceeds A3 in size or when dew is particularly heavy, the simple awning just doesn’t provide enough protection.

    I encountered this when I made my second sketching rig to accommodate a very large sheet of card to sketch the Large Magellanic Cloud. The first night I used this rig, dew was very, very light, but the absorbing properties of the paper meant that it became damp very quickly, despite the larger awning I made for the rig. I had to find another solution or this sketch would be impossible to do.

    Using fans to cool and control dew on telescopes is very common place, and when implemented correctly can be extremely effective. I had several 4” 12V fans at home that I had accumulated over the years as part of experiments with my scopes and from cannibalising them from old computers. Running out of time and options, and having nothing to lose, a somewhat flippant idea came to me to install a couple of these fans to the top of the sketch rig. To improve airflow efficiency and flow direction, I enclosed the fans with some thin black foam rubber. For this initial iteration, I connected the fans in series. Using a 12V power supply, this would mean a slower fan rpm rate, but I had to start somewhere. If the initial testing proved positive, I could always reconnect the fans in parallel to increase the fans rpm’s and increase airflow.

    Attachment 172283 , Attachment 172284

    Finally, the night came when dew was problematic, with my paper becoming damp very quickly. I had no alternative but to switch on the fans and see what happened.

    WOW! These fans were so very effective! Not only did they stop dew from dampening the paper, but they also dried out the damp that had already been absorbed by the paper! On this particular night, EVERYTHING became wet from dew. Our cars, telescopes, the ground, even my headlamp. Yet the modest rpm’s that these fans were working at proved totally effective in keeping my paper absolutely bone dry. It would have been impossible to work for three hours on the sketch that night if the paper wasn’t kept dry. Damp paper leaves the paper very fragile and impossible to work with. Just think of attempting to write on damp newspaper – an impossible task without causing damage to the paper, no matter how careful one is.

    One other solution
    Some people have suggested making a heated sketch pad. The biggest problems with this is it means a huge power requirement that only gets larger as the sketch pad becomes larger. It also means a more complicated fabrication, and it is not cost effective, nor simple to use. I quickly discounted this as a viable solution.

    ~x.X.x~

    This has been my journey so far in finding an effective dew control rig form my astro sketching. When a sketch at the eyepiece can take anywhere between half an hour to nine hours, I really need to be able to have an extremely effective way to prevent my paper from becoming damp. I now have three solutions – a dew free observing site, a simple awning reaching over my paper, and a fan powered sketching rig. I hope this has given you some solutions and inspired so ideas for you to explore in your own astro sketching journey.

    Alex.

  2. The Following 7 Users Say Thank You to mental4astro For This Useful Post:

    10538 (02-05-2018),bladekeeper (02-04-2018),dagadget (02-04-2018),Eagleheaf (02-04-2018),ic_1101 (02-05-2018),Mark Moyer (02-05-2018),SpyderwerX (02-04-2018)

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    Default Re: Dew control for astro sketching

    Outstanding, Alex! Thank you so much for taking the time to put this together for us!
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    Default Re: Dew control for astro sketching

    Excellent post and the information contained in it will help me set up here in Florida. I'll try to avoid areas near water and very grassy areas that have sprinkler systems. All in all an excellent well thought out article.

    Clears skies and good drawings.
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    Default Re: Dew control for astro sketching

    Very good Alex. Thank you.
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    Default Re: Dew control for astro sketching

    Nice Alex - Got me thinking, add fans to the laptop shield?
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    Moving on.......

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    Default Re: Dew control for astro sketching

    Thanks Alex! I tried using a floor fan last summer to help keep the mosquitoes away and found it also helped with dew issues. I never thought of an application using small computer fans on my sketch rig! What a great idea! Thanks for sharing this with us!
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    Default Re: Dew control for astro sketching

    Thanks for the kind words, folks

    I know this is a REALLY niche topic, even for astro sketching. Thing is there's SO LITTLE written on the topic, bugger all really, it needs somewhere for this sort of info to be readily at hand. So thank you to Bryan and the mods team for approving this thread to be made a sticky.

    I also hope that people will contribute their own experiences on this theme too. Dew is the mortal enemy of paper, and I'm sure that there have been some novel ways devised to deal with it.

    I'll be looking at modifying my original dew shield sketch rig to accommodate a fan or two. This is the rig I mostly use, so I need to pimp it up. So many nights I've thrown in the towel early because of dew. I now have a fighting chance to get some work done instead of driving home empty handed.

    Next topic I will cover is my experiences with using amber lights over red for fine stuff such as sketching, yes, but even more so for wider appeal such as map/chart reading and fine mechanical work that needs to be done on the fly out in the dark. A very challenging topic for the wider astro community as there has been so much time invested in the "red light" argument, but with actually little thought about how true the practicalities are for human eyes

    Alex.
    Last edited by mental4astro; 02-05-2018 at 05:30 AM.
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    Default Re: Dew control for astro sketching

    Well done article with useful information, thanks. I Florida where the highest peak in the state is 40 meters above sea level altitude is out of the question. I place a clear plastic sheet over my paper and flip it up to sketch with a red lamp on the side of the clipboard. No memory required as I can just take it to the EP with me. low humidity here averages about 50% so fans and dew heaters are a must.
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    Default Re: Dew control for astro sketching

    Back in Lijiang, dew was not a problem in winter. The humidity dropped below 50% so actually I had to put hand cream in my hands to keep then hydrated. That is a nice location to sketch.

    But here in BC....everything is just soaked during the nights in winter. I have not attemped to sketch here, but now that I have seen your anti-dew aids I may wanna try some of those.

    Thank you so much for such a very informative post Alex!
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    Default Re: Dew control for astro sketching

    I don't sketch but condensation in the winter is also a problem in my area and moving the scope shed isn't an option. We get both marine fog and radiation fog, sometimes both on the same night and at its worst the interior surfaces of the shed, the rolled off roof and everything in the shed gets wet, wet enough to drip from the counterweights, make the floor slick and before I took action ruin an IPS display.

    Passive dew shields didn't work. A fan to move air over the floor and wall surfaces didn't work. What is working is dew heaters and hoods on the scopes and a hood over the desk and a small 200W personal/desk heater. The personal heater is effective in drying the interior after a session and in preventing further condensation. I turn on the heater on nights when dew/frost/fog is likely but the scopes won't used. Since starting that I haven't had any further instances of opening the shed to find water dripping off the mount or the steel pier.

    I tried the fans because a.) I had them and b.) maybe I didn't quite comprehend the mechanics of dew formation. Moving air does not prevent condensation unless the moving air temperature is above the dew point. If the moving air is warmer than the air from which the water condensed it will speed the evaporation of existing condensation, but only if it is warmer. So moving an unlimited supply of saturated air over a surface that is below the dew point won't by itself prevent condensation. When it appears to prevent it the explanation is that the air moved by the fan is warmer than the air displaced.

    The hood over the desk area slows radiation heat loss from objects under the hood and from the desk top and the PC on the desk and sometimes the heater add sufficient heat to keep everything under the hood above the dew point.

    The dew shield slows radiation heat loss from the objectives and the heater the objective and the air just above it from cooling to or below the dew point.

    I might be wrong about this, but I think the reason for fans on Dob or Newt is to decrease or avoid the optical effect of the boundary layer and in an SCT to reduce convection plumes.

    I am not saying that the OP did not see condensation reduced or avoided when he used fans, but rather that when fans work the explanation is that the air has warmed above the dew point preventing dew or evaporation has cleared existing condensation. Heat is the preventative.

 

 
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