Sunday 4 February 2018

Dew Control With Astronomical 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.

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:

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. 

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.

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.

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.

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.


No comments:

Post a Comment