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Advice needed for 9.25 Edge dew protection

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9 replies to this topic

#1 scrufy

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Posted 24 August 2019 - 08:25 PM

Hi all,

I’ve just finally decided to upgrade after 20 years with a 1960s Selsi 60mm refractor and a (recently acquired) Monolux 4380.

I went with Celestron CGX / Edgehd 9.25 with astrophotography as an end result goal along with visual.(still need to get a guidescope but that’s a whole different topic)

I’ve borrowed friends 5 and 6” SCTs before and never had an issue with dew in my short viewing sessions over the years, but have been advised that now I need to worry about it.

 

The question is what to get, and do I really need it but I think the advice was pretty sound.

There is strip heaters, heated dew shields, just dew shields, heated eyepiece strips, etc...

Enough stuff out there to go crazy if you wanted to.

 

For detail specific to my use:
I’m in Southern California at 1400’ elevation in the foothills north of Los Angeles.

No snow, lowest temp is every couple of years it may get high 30s F. then go up to 60 daytime for a couple days. Summer can get about 95 to rare 105 day and drop to 80 that evening.

Still in the city, but just a couple blocks from where the mountains really begin.
We do suffer from orthographic lift if that is a consideration for the type of system I get.

 

Power drain isn't an issue. I have several huge 18650 battery banks I’ve made from scraped laptop battery packs and set up for 12 volts and can regulate any of them from .25 to 5 amps. Any one of these could power the mount alone for a couple days and I’d just make one for dedicated use for the dew application.

 

I just want to “Dew” this right the first time - sorry couldn’t help it, with getting good stuff, but also don’t want to waste money on things I’ll never need.

 

Any advice from the experts here would be greatly appreciated.


Edited by scrufy, 24 August 2019 - 08:26 PM.


#2 DaveSD

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Posted 24 August 2019 - 08:45 PM

Hi,

 

I also have the CGX 9.25 EdgeHD and live by the coast in So Cal.  I can't go for more than an hour in the summer without dew buildup, if I don't use a dew shield.  I use the Astrozap dew shield/heater combo and it works great/no dew on the corrector.  You need the 1 notch version (AZ809N-1).  I made my own pulse width modulated controller and cable.  But if you have enough battery power, you could just use the simple adapter and let it run full blast.

 

If you are going to guide with this OTA, you probably want to get an off-axis guider, so no dew control needed for that.



#3 scrufy

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Posted 25 August 2019 - 12:09 PM

Thanks Dave.

I looked at the Astrozap. That looks like a nice unit. I’ll give it a try.



#4 DaveSD

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Posted 25 August 2019 - 02:34 PM

You're most welcome.  It is odd that Celestron doesn't make a dew shield for the 9.25...



#5 Napp

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Posted 25 August 2019 - 02:51 PM

Whether to get a heated dew shield or separate dew shield and heater - the main consideration.  Do you want or need to be able to modify the dew shield?  Some shields come with a notch.  If you need more notches you have to cut them yourself.  You can’t do that with a heated shield because you would have to cut the embedded wire.


Edited by Napp, 25 August 2019 - 02:51 PM.


#6 macdonjh

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Posted 25 August 2019 - 05:49 PM

My experience is soggy SE Texas: I always used a passive dew shield (Kendrick flexible) and a 20W dew strip with my C11.  Never had dew problems after that.  I was not on batteries, so I allowed the dew strip to run full power.

 

Prior to having the dew strip I used a hair dryer and often used it several times per hour.



#7 scrufy

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Posted 26 August 2019 - 07:53 PM

Well I ended up getting the heated 1 notch but figured if I ever have to I can hopefully just slip anything extra under the edge.

thanks for all the great suggestions.



#8 Rustler46

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Posted 08 September 2019 - 11:44 PM

Well I ended up getting the heated 1 notch but figured if I ever have to I can hopefully just slip anything extra under the edge.

thanks for all the great suggestions.

I'm late to this discussion. With my metal tube C-11, dew prevention has been on ongoing issue living in the moist Pacific Northwest. If you are into double stars or lunar/planetary imaging/viewing, you'll want to use just enough applied heat to prevent dewing. Otherwise you risk distorting the images via heat plumes, tube currents, etc. One can apply maximum heat for completely dew free observing or imaging. But you'll be using a hot telescope. So here's what I currently use:

  1. One heater strip mid-way on the tube
  2. A single layer of Reflectix insulation on the tube over the heater strip
  3. A home-made metal heated dew shield with the heater mid-way on the shield
  4. A single layer of Reflectix over the dewshield
  5. DewBuster dew controller - website

There is a wealth of good information on that website. The advantage of the DewBuster system is there are temperature sensors for ambient air and air inside the OTA. The controller sets how much warmer you want internal versus external air - just enough to prevent dew is best. This is also an advantage for those set up at a remote dark site. Less battery power will be needed.

 

The insulation system in items 1-4 is what came from an extensive discussion on the following thread:

I entered the discussion on page 10. But you might want to read the entire thread. But according to the DewBuster website, applying heat to the metal corrector cell is not the best method. Most of that heat goes up the "chimney", distorting the air just above the telescope. The corrector is quite well insulated by spacers between the lens and its cell. Better to heat the air inside the OTA, behind the corrector. In the past I had removed the heater from my AstroZap heated dew shield, placing it just below the corrector cell on the metal tube. In fact I placed a second heater strip just below that - two in total. This allowed for notching the dewshield (sans heater strip) for upper and lower dovetails.

 

The arrangement just described worked fine to keep dew at bay if the DewBuster was set for 7°F above ambient temperature. When I applied the advice on the Reflectix thread above (items 1-4 in the list), I found that a 3°F setting was all that was necessary to prevent dew. And this was achieved with only a single heater strip on the OTA. I believe this has improved planetary images as a consequence of the reduced heat input. Also at the start of an observing session, I can leave the dew shield off to allow for access to the collimation screws. When that is optimized, the dew shield goes on for the rest of the evening.

 

C-11 Dew Prevention-1.jpg

This shows the insulated OTA and heated metal dew shield attached.

 

C-11 Dew Prevention-2.jpg

Here the Reflectix insulation is in place over the dewshield.

 

So that is my take on the subject of dew prevention. You'll find there are widely diverging opinions as to what is best technique. So try take in as much knowledge on the subject as you're able, then decide on how to apply what you've learned in your observing environment. The variety of that environment is the source of the differing options expressed.

 

Best Regards,

Russ


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#9 scrufy

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Posted 15 September 2019 - 11:42 PM

Thanks Viking.

I’ve had pretty good luck so far with just a home made regulator but I can add an ambient sensor and make it stay just a little above. Control circuits are my job so it shouldn’t be that big of a deal.

With the tips here I’ve got a great start.



#10 whizbang

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Posted 21 September 2019 - 07:12 AM

I always attach a non heated dew shield, and, a heat strip.

 

When you notice the shield getting moist, plug in the heat strip.  I always run full power.




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