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Dew


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Was it just your equipment or was it optics that dewed up?

cant see your signature, were you using a reflector or a frac? If a frac and the primary dewed up, best to get some dew strips.

A bit of dew on the surfaces of mount and scope won’t cause any damage.

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Yeah, if you are going to leave you equipment out for a couple of hours before use, you could get a scope cover. There are specific ones sold by astro retailers or less expensive ones sold as BBQ covers in well known DIY stores.

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I cover everything with a plastic sheet if setting up early for visual and or waiting for the scope to aclimate, just a loose covering nothing cinched or tied. I do the same for imaging and eaa. 

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I'm honestly a bit surprised it's not common to find telescopes with built-in dew heaters (aka plug-and-play anti-dew). It would add a few bob to the cost but it would certainly save the hassle of applying the heaters yourself.

It's impressive how effective even a light amount of heat can do. My camera's dew heater is barely warm yet kept the lens and front of the camera body bone dry while the rest of it was literally dripping in front of my eyes.

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15 hours ago, don4l said:

I'm not sure if I should admit this!

I use a travel hair dryer on the refractors and on the laptop screen.

It does make me a bit nervous, but it works.

I'm not recommending this to anyone.

It's probably not a bad short-term solution. It might cause some tube currents for reflectors but it'd probably prevent dew for a few minutes.

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15 hours ago, don4l said:

I'm not sure if I should admit this!

I use a travel hair dryer on the refractors and on the laptop screen.

It does make me a bit nervous, but it works.

 

I'm not recommending this to anyone.

 

 

I don't think you're alone.  I've seen a number of people use this method.

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Last night  had a brand new dew shield with built in heater on my 5" Newtonian and home brewed mat and tape dew shield on the finder guider (refractor). After about two hours, the finder was fine, the secondary on Newtonian was dewed up. I have neighbours so using a hair dryer isn't really an option, it's so frustrating sometimes.

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Dew is a fun challenge.

There are several solutions.   Firstly let's talk about the optics....

1. Dew shield - This helps to prevent the formation of dew by limiting the heat radiated from the optics.  Also, it helps to block down on stray light a little too.

2. Dew heater - This puts heat into the scope, targeted at a surface that dew normally forms on to prevent the heatloss that happens and allows dew to form

3. Dew Zapper - i.e. a hair dryer.  This has 2 things,  firstly the moving air can help to evaporate condensation and the heat put out can also help.   I use a 12v one and it works great when everything gets soggy. Used as a last resort but can save a night if needed.

4. A fan - moving air makes it harder for dew to form.

 

Away from the options, people have been known to forget about this bit....

1. toilet tent to put laptop, power supply and other bits in.     This is a great way of allowing my to sit near the scope and have the goodies out without having to worry about dew forming on everything.

2. plastic boxes.  Putting stuff in boxes helps to stop dew forming on anything electrical, the box doesn't need to be air tight or anything like that, just needs to keep the dew off.

 

So where does dew come from?

Answering this question will help to understand how to prevent problems with dew and make your life a whole lot easier.

Dew is formed on the cold surface when it temperature falls below the dew point.    The dew point is a temperature at which the air can no longer hold moisture, and it changes with the ambient temperature and humidity.  Wind also also effect the ability of moisture to condense - so can also have an effect on the dew point (hence why the hair dryer and fan above works)

When you put your object outside, it's surfaces will start to radiate heat out in all directions until it reaches a natural temperature.  The glass optics of your scope (mirrors, corrector lenses etc) are normally pointed upwards toward the night sky.   Space, obviously is very very cold, so your relatively hot telescope will start to radiate it's heat out to the entire visible part of the night sky, trying to balance it's temperature with everything else.      The dew shield cuts down the visible part of the sky by a huge amount, so it means this radiating of heat slows down exponentially, doesn't stop, but will slow the progress of something that could take say 30 mins, to take several hours.

 

The temperature of the optics will eventually fall below the dew point - this is because glass is good at radiating heat, once it falls below the dew point, moisture will start to form.    If that moisture gets cold enough, you get ice.  aka frost.

 

The dew heater works be providing extra heat energy to the scope.  This isn't to "heat up the scope", but to mitigate the heatloss.  Done correctly, the dew heater will keep the scope just about the dew point, meaning that everything stays stable, and therefore works efficiently.  This is why you can get expensive dew heaters that measure the temperature and humidity.  I personally use a simply PWM based heater and turn it up on a soggy night, and down on a no so soggy night, no science involved, just a guess.  Tends to work for the most part.

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1 hour ago, jjosefsen said:

I use dew heaters on refractor and guide scope, works well..

But how would you prevent dew in say a 150mm-200mm Newton reflector?

A specific secondary dew heater, such as a Kendrick. You could also add a small fan on to the bottom of the primary.

I find that I also need dew heaters on each end of my 50mm RACI as well as a cover over the Telrad on my Dob

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  • 1 month later...

Has anyone experience of dew controllers, such as the Kendrick, which have temperature and humid probes?

Generally your controller is just a manual control which doe snot allow for changes in environment as the night progresses.

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7 hours ago, iapa said:

Has anyone experience of dew controllers, such as the Kendrick, which have temperature and humid probes?

Generally your controller is just a manual control which doe snot allow for changes in environment as the night progresses.

Hi. I have the Pegasus Astro pocket power box with integrated dew controller. It has an automatic mode, where it uses it's climate sensor (temp and relative humidity) to set the power levels of the dew bands.

I certainly don't have problems with dew, but often it runs it on 100% But I also often have very high humidity..

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HI All and Merry Christmas,

I was out last night for 3 hours and suffered the worst dewing for a long time,hardly got any observing done with eyepieces and finderscopes dewing and freezing every few minutes . The temperature was  -3C but even so the freezing was inordinately bad,RH was also very high and living on a clay soil area doesn't help.

The frost melted overnight and my observatory floor was wet with the dripping from my equipment. Over 1mm registered on my raingauge with all the frost that thawed.

Of coarse telescope mirrors were still dewed today so had to force dry them with a hair dryer inside the tube so I could put the caps on.

Funny dew heaters are not something I've ever  got round to, and Santa didn't bring any last night.Maybe if I'd stay out to after midnight I could of called him over!

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On 02/11/2018 at 00:05, don4l said:

I'm not sure if I should admit this!

I use a travel hair dryer on the refractors and on the laptop screen.

It does make me a bit nervous, but it works.

I'm not recommending this to anyone.

Did this I once and heated the front objective of my refractor (lens) a tiny bit too much and dew formed on the inside of the lens.....so be careful :)

It completely dissipated but was quite worrying when I first saw it.

These days I just use some low wattage DIY dew heater strips which keeps dew and frost at bay.

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