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Is Astrozap Aluminum Dew Shield for Celestron 8" EdgeHD worth it?

Astrophotography
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#1 D.T.

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Posted 10 April 2021 - 09:14 PM

I have a Celestron EdgeHD 8.  I live in the Portland, Oregon area.  I have an AP location in my backyard where I use the EdgeHD. I have significant problems with dew.  Currently I have, and use, the Flexi-Heat Dew Shield.  It works, but it has a tendency to sag, and cause some vignetting problems. I am considering the Astrozap aluminum dew shield, but it is very expensive, and I wonder if it is worth it?  And I wonder why it is not heated?  I would think that once the shield cools off it would not be of any benefit.  I could put some heating elements on it, and it would seem that the aluminum would spread the heat all around the front of the scope.  But since the product is not shipped with heating elements, there must be some reason why this is not done.  Can anybody explain this to me?

 



#2 mlord

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Posted 10 April 2021 - 09:33 PM

Dew condenses on the front corrector plate of the OTA because that plate is radiating heat off to cold deep space faster than the surroundings are radiating heat back to it.  Add in just the wrong humidity/wind conditions, and dew condenses on it.

 

A dew shield limits the amount of sky seen by the OTA to just the amount that it needs for viewing.  Thereby reducing the amount of cold deep space that can absorb radiated heat from the OTA.  So it doesn't lose heat as quickly, and has time to absorb some back from the rest of the body which may be slightly warmer.  Thus preventing or delaying dew condensation.

 

Even so, with the right conditions, on a cold enough night, dew may still condense.  That's where heaters become necessary.

 

The downside of heaters, is they often result in more tube currents, which distorts the view.


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#3 D.T.

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Posted 10 April 2021 - 10:13 PM

Dew condenses on the front corrector plate of the OTA because that plate is radiating heat off to cold deep space faster than the surroundings are radiating heat back to it.  Add in just the wrong humidity/wind conditions, and dew condenses on it.

 

A dew shield limits the amount of sky seen by the OTA to just the amount that it needs for viewing.  Thereby reducing the amount of cold deep space that can absorb radiated heat from the OTA.  So it doesn't lose heat as quickly, and has time to absorb some back from the rest of the body which may be slightly warmer.  Thus preventing or delaying dew condensation.

 

Even so, with the right conditions, on a cold enough night, dew may still condense.  That's where heaters become necessary.

 

The downside of heaters, is they often result in more tube currents, which distorts the view.

Helpful, but questions remain:
1) Would heaters be more likely to cause heat currents on the aluminum tube than on the flexible tube?

2) Is the aluminum tube worth it vs. the flexible tube?



#4 wrnchhead

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Posted 10 April 2021 - 10:30 PM

Helpful, but questions remain:
1) Would heaters be more likely to cause heat currents on the aluminum tube than on the flexible tube?

2) Is the aluminum tube worth it vs. the flexible tube?

I think with things like dew, your own location may make a big difference, so another person may not be able to give you set-in stone advice. As they say, you just have to try it and see. The three most common places I image have three different reactions to changing weather. Aluminum (and probably most metals as thin as were are talking about) radiate heat pretty fast. And dew control vs tube currents kind of come down to this... Dew = no imaging at all. The idea is you keep your optics just above the dew point, not so much super heat them so that they radiate a ton. 

 

Re: that floppy dew shield, I got one of those with a scope purchase, and I can't stand the thing. I agree the aluminum ones are kinda pricey but that comes down to shipping and the fact that the market is tiny for such an item so no real economy of scale in production. 



#5 Richard O'Neill

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Posted 10 April 2021 - 10:37 PM

"Is Astrozap Aluminum Dew Shield for Celestron 8" EdgeHD worth it?"

 

Not at that price! Have you considered making your own?  https://www.randombi.../dewshield.html

 

Here's a simpler one. http://www.hamiltona..._Dew_Shield.pdf

 

 I've covered my C-8 optical tube with Reflectix for better thermal stabilization and found that it works very well to quell internal tube currents. I have yet to encounter a problem with dew on the corrector, without the need of a heater. The Reflectix shield extends 12" beyond the end of the telescope to provide a very effective dew shield.

 

100 5146
Album: Insulated C-8
1 images
0 comments

 
 Added 24" Reflectix Daylight Shield for Questar. Very useful when observing Venus when it's near the sun.
Also have a 14" shield for nighttime use.
 
Image 1
Image 2

 


Edited by Richard O'Neill, 11 April 2021 - 02:40 PM.

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#6 wrnchhead

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Posted 10 April 2021 - 10:48 PM

image0.jpeg

 

Mr O'Neill, I did the same thing, WHY DIDN'T I THINK OF MAKING IT INTO A DEW SHIELD ALSO?!? 

 

Fortunately the smallest amount of Reflectix you can buy is enough to insulate Hubble, so I need to re-do mine. Great tip! 


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#7 Richard O'Neill

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Posted 10 April 2021 - 11:19 PM

You could redo, or simply add another longer layer for increased insulation. Northern Kansas can get mighty cold in Winter. flowerred.gif



#8 wrnchhead

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Posted 11 April 2021 - 01:31 AM

You could redo, or simply add another longer layer for increased insulation. Northern Kansas can get mighty cold in Winter. flowerred.gif

The cold is definitely bad, but it was cloudy here from December 21st until maybe two weeks ago when I got a clear night. It's clear tonight but muddy everywhere and seeing terrible. 



#9 speedster

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Posted 11 April 2021 - 04:54 AM

There is a trick to get rid of the sag.  Velcro it together before you put it on the scope and velcro it so it's a tight fit - then wiggle it on the scope and it fits much tighter than velcro-ing it around the scope.


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#10 Richard O'Neill

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Posted 11 April 2021 - 02:31 PM

  BTW, adhesive aluminum duct tape is a perfect match for securing Reflectix insulation seams. Much cleaner looking than ordinary types.


Edited by Richard O'Neill, 11 April 2021 - 02:33 PM.


#11 D.T.

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Posted 11 April 2021 - 07:19 PM

 

"Is Astrozap Aluminum Dew Shield for Celestron 8" EdgeHD worth it?"

 

Not at that price! Have you considered making your own?  https://www.randombi.../dewshield.html

 

Here's a simpler one. http://www.hamiltona..._Dew_Shield.pdf

 

 I've covered my C-8 optical tube with Reflectix for better thermal stabilization and found that it works very well to quell internal tube currents. I have yet to encounter a problem with dew on the corrector, without the need of a heater. The Reflectix shield extends 12" beyond the end of the telescope to provide a very effective dew shield.

 

 
 
 Added 24" Reflectix Daylight Shield for Questar. Very useful when observing Venus when it's near the sun.
Also have a 14" shield for nighttime use.
 
 
 

 

 

Make my own?  I have done that kind of thing in the past.  My typical experience is, building my own drags on forever, and costs me more that if I had just bought the expensive off the shelf item.  These days, the only way I will try building my own is if what I need just isn't available anywhere.

 

I think I can see why Astrozap doesn't add heating elements to the aluminum tube.  Maybe the heat would be conducted on the tube and then radiate outward.

 

I fail to see why wrapping reflectix around the tube would help with dew protection?  Would it not keep the optics from achieving temperature equilibrium?  I thought that was bad thing to do?



#12 whizbang

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Posted 11 April 2021 - 07:43 PM

First off, forget the swanky aluminum dew shield.  That baby will draw dew to your corrector like flies to do-do.

 

The flexiheat is a top of the line dew shield.  Mine never sagged.  Do you have the velcro tight enough?

 

Lastly, I am just north of you in Seattle.  Conditions here are darn damp.  Just after the first of the year, I finally caved in and wrapped my SCT OTA's and dew shields in Reflectix.  I haven't had to run a dew heat strip ever since.  And, I can go to higher power immediately out the door.   You should give it a go.  Cheap.  $17 at a big box home improvement store.


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#13 Richard O'Neill

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Posted 11 April 2021 - 08:32 PM

"I fail to see why wrapping reflectix around the tube would help with dew protection? Would it not keep the optics from achieving temperature equilibrium? I thought that was bad thing to do?

 

 Not at all, provided the rate of temperature change can be significantly reduced to an acceptable level. An insulated dew shield effectively limits the objective corrector's angular field of view to the sky which reduces the rate of heat loss. Adding tube insulation greatly reduces or eliminates tube currents. Many people on this forum have written about it. You might find the threads in this link interesting as well as educational. Good luck. flowerred.gif

 

https://www.cloudyni...h&fromMainBar=1


Edited by Richard O'Neill, 11 April 2021 - 09:53 PM.

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#14 D.T.

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Posted 12 April 2021 - 12:15 AM

So, the Reflectix works because it is an insulator?  What is the point of the reflecting surface on the outside?  It would seem you could wrap the SCT with many different kinds of insulating material that don't have to have a reflecting surface?



#15 Richard O'Neill

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Posted 12 April 2021 - 11:04 AM

D.T., I don't have the time to rewrite a college/ graduate level paper on the physics of thermal radiation shields or the emissivity of materials such as aluminum, but perhaps these links will aid your understanding of the 'why' of things. flowerred.gif

 

https://ntrs.nasa.go...19680008577.pdf

 

https://www.norbord....ics-of-Foil.pdf


Edited by Richard O'Neill, 13 April 2021 - 09:44 AM.

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