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Dew shield/Heater question

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8 replies to this topic

#1 W7VL

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Posted 17 January 2020 - 02:19 PM

Just purchased Celestron EdgeHD 11". Just getting back into hobby with limited experience.  Need some advice about whether or not I need to install dew shield/heater. I  had a Nexstar 6se for years and never had an issue with dew or frost at my location in SW Colorado(elevation 7500'). Relative humidity is very low here and rarely do we experience frost in the winter or dew in summer. 

Do I need to install?


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#2 Phil Sherman

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Posted 17 January 2020 - 02:37 PM

Get a large piece of black construction paper from an art supply store, roll it into a cylinder that just fits over the tube and you'll have a dew shield for pennies. If you have dew issues with this in place, consider adding heat strips.


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#3 MikiSJ

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Posted 17 January 2020 - 03:27 PM

I also have a new C11 EdgeHD. First night that I had it out the dew was almost like rain, $35 for dew shield. Phil's suggestion for construction paper is good but I fear it will only last a couple of nights.

 

I have two Astrozap type heater strips and controller left over from my APM 152 so I won't have to go for that.

 

I don't want to spend your money (yes, I do!) but you paid $3,500 for your OTA, $50-75 for dew removal is cheap.


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#4 Prasad

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Posted 17 January 2020 - 04:02 PM

Just purchased Celestron EdgeHD 11". Just getting back into hobby with limited experience.  Need some advice about whether or not I need to install dew shield/heater. I  had a Nexstar 6se for years and never had an issue with dew or frost at my location in SW Colorado(elevation 7500'). Relative humidity is very low here and rarely do we experience frost in the winter or dew in summer. 

Do I need to install?

Bruce, I go to Cherry Springs State Park in PA where it can get quite humid on some nights and yet, I never faced dew problem. I do keep dew heaters as well as a 12 V dc jot air blower (hair dryer) handy. 



#5 Phil Sherman

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Posted 18 January 2020 - 03:06 PM

......

Phil's suggestion for construction paper is good but I fear it will only last a couple of nights.

..........

Construction paper works well for a quick verification that a dew shield is all that's necessary. Oaktag, a lot heavier than paper should last a lot longer. I've also used flexible cardboard which is even stronger. Commercial packing places use flexible cardboard to wrap irregular shaped objects.



#6 Blackbelt76

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Posted 18 January 2020 - 05:41 PM

I' use sleeping bag egg crate foam. It comes in a roll.

Easy to cut, easy to install, AND, the added benefit of the "egg" shaped foam acts like a light trap.

Sadly, even smooth black cardboard reflects light.

I find a dew shield more useful as a lens shade.

 

 

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#7 Eddgie

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Posted 18 January 2020 - 05:51 PM

I to think that having spent $3000+ for a scope and not getting a dew heater is kind of, well, stopping short of a complete kit.

 

These big corrector can start dewing in mintues and once it starts, if you don't know it has started, by the time you realize it, you have enough dew on your corrector that you are faced with the prospect wiping it off, running a hair dryer to blow it off, or calling it a night. 

 

If you live in the north east, coating etching is a real possibility. People do not believe this happens, but I had an Astro-Physics Apo that would make you weep to see what dew did to the coatings.

While dew shields can indeed slow down dew, once again, once it starts, you are in a pickle.  The entire value proposition of the dew heater is to prevent the dew so that you never have to make one of these other choices.

 

I used to simply leave my dew strap on my C14 all the time.  I had the power cable run down under the dovetail to the balance point on the dovetail.  Is all I had to do was plug it in.  No extra steps to take, nothing to loose or store.  

 

Few things are more aggravating than dew starting to form on your corrector. Once it starts, you are stuck with the ramifications.  You can wipe it off, but it comes back.  You can hair dry it off, but it comes back.  If the dew shield is on it and it does dew up, you can pull the dew shield, wipe it down, and put the dew shield back on it, but it is going to come back.

 

Why would anyone not want to have a 100% solution???

 

I have had a lot of problems in astronomy that I could not solve, but dew on my SCT corrector?  That was easy.


Edited by Eddgie, 18 January 2020 - 05:53 PM.


#8 whizbang

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Posted 18 January 2020 - 09:39 PM

If you don't have a dew problem, count your blessings.

 

You should probably get a unheated dew shield anyway.  It will keep dust and stray reflections off the corrector plate.


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#9 MalVeauX

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Posted 19 January 2020 - 08:54 AM

Get a heat strip. They're inexpensive. Run from a lithium battery. You don't need a controller and all that. You can literally get a heat strip with a built in regulating temperature switch that goes from warm to hot with a button, no controller beyond that, comes with the strip, and plug it into a TalentCell lithium battery. You don't have to warm your optics to 100F, just a few degrees warmer than ambient and it'll be dew free. SCT correctors are magnets for dew, that surface cools down rapidly before the primary mirror does, and if the humidity is there, it will soak it up and steal your light. Dew shielding isn't enough with SCT in my experience. Maybe it's fine in bone dry deserts for some people. I'm in Florida, its 99% humidity at all observing times at night, 365 nights a year, and a dew shield is literally useless for dew, it's just a lens shade for stray light and prevents falling dew/water from hitting the corrector plate immediately. Heat is the only way to stop it. And it's easy and inexpensive to do it.

 

Very best,




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