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Edge925 - What to do about dew?

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#1 JP50515

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Posted 03 July 2019 - 09:18 AM

Hey guys...newb to the C&C forum here. I just got my first SCT an EdgeHD 925. 


So as you can guess, I set it up last night for first light, and of course it was like 90% humidity out there. I gave up after an hour for fear of my new scope suddenly filling with so much condensation that water would start pouring out the top lol.gif - jokes aside it was just an absurd amount of dew/condensation going on last night, so I brought the scope in and put it in front of a small fan before I went to sleep, which cleared it up pretty well minus the water marks on the glass now. 


Anyway I wanted to chat about dew prevention. I know the best option is a dew heater.

I keep seeing the astrozap heated dewshields though, and am wondering if anyone has any experience with those?

 

I plan to image with the scope once I get get a reducer and OAG, and I've heard hit or miss results with the non-rigid dew shields causing odd star shapes. 

Am I better to go with a plastic or aluminum dew shield and regular dew heater for this reason you think?  Is a dew shield necessary if you've got a heater?


Pardon my naivety here I am brand new to the world of glass...have always had newts previously. 

 

Thanks!



#2 ngc7319_20

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Posted 03 July 2019 - 09:44 AM

I've had good luck with AstroZap aluminum dewshields plus a heater band on the OTA just below the corrector.  You want to go easy on the heat if you are working at high resolution (planetary, etc.).

 

A small fan mounted inside the dewshield and aimed at the corrector will also help.

 

C9 fan.png


Edited by ngc7319_20, 03 July 2019 - 10:06 AM.

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#3 Jim Davis

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Posted 03 July 2019 - 09:46 AM

I have both a flexible dew shield and dew heaters. I have had the dew shield shift and screw up my imaging. A single dew strap seems to work just fine for me. Has keep it off on the heaviest of dewy nights.


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#4 Spikey131

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Posted 03 July 2019 - 09:53 AM

A dew shield is mandatory for all SCTs.  They should be included.

 

That is all I use, but I don’t do AP.  I use it every time I set up unless it is breezy.  I live in the northeast, so once in a while I will get dew on the corrector despite the shield, but most nights I am OK.

 

I never used heaters.  Once in a while I will employ a hair dryer.


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#5 barrett_flansburg

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Posted 03 July 2019 - 12:56 PM

 

A small fan mounted inside the dewshield and aimed at the corrector will also help.

 

I’m intrigued by the idea of the fan inside the dewshield. Can you recommend a fan? Something intended for computer cases or cpus? You don’t see turbulence the way that heated air can cause?



#6 ngc7319_20

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Posted 03 July 2019 - 01:30 PM

I’m intrigued by the idea of the fan inside the dewshield. Can you recommend a fan? Something intended for computer cases or cpus? You don’t see turbulence the way that heated air can cause?

The fan should be small and thin so you can tuck it in the dewshield just out of the light path.

 

https://www.jameco.c...an_2227655.html

https://www.jameco.c...ro_2224198.html

 

Turbulence is minimal since the air is well mixed.  The fan generates a tiny amount of heat that probably also helps keep the corrector clear.   And it will help break-up any boundary layer on the corrector plate (similar to seen with Newtonians).

 

You can put a speed control by wiring it up to a DC Universal Car Power Adapter with a selectable voltage.  Set it 12v for full speed, 5v for low speed, etc. 

 

https://www.midlandh...er=14396-139635

 

It works pretty well on my C14 while observing near the ocean.  I sometimes get a very slight haze on the corrector when the rest of the scope is dripping wet.


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#7 barrett_flansburg

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Posted 03 July 2019 - 03:00 PM

The fan should be small and thin so you can tuck it in the dewshield just out of the light path.

 

https://www.jameco.c...an_2227655.html

https://www.jameco.c...ro_2224198.html

 

Turbulence is minimal since the air is well mixed.  The fan generates a tiny amount of heat that probably also helps keep the corrector clear.   And it will help break-up any boundary layer on the corrector plate (similar to seen with Newtonians).

 

You can put a speed control by wiring it up to a DC Universal Car Power Adapter with a selectable voltage.  Set it 12v for full speed, 5v for low speed, etc. 

 

https://www.midlandh...er=14396-139635

 

It works pretty well on my C14 while observing near the ocean.  I sometimes get a very slight haze on the corrector when the rest of the scope is dripping wet.

Thanks! I’ll have to give it a try. 



#8 JP50515

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Posted 04 July 2019 - 10:39 AM

I put in for the dewheater yesterday. Gonna give that a shot first and wrangle in a dew shield if necessary down the road. Also picked up a pegasus pocket powerbox so I can finally get my cables sorted on my imaging rig. 



#9 jhayes_tucson

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Posted 04 July 2019 - 04:27 PM

...

The fan generates a tiny amount of heat that probably also helps keep the corrector clear.   And it will help break-up any boundary layer on the corrector plate (similar to seen with Newtonians).

...

 

Actually, unless it's really overheated, the fan isn't preventing much dew by the heat it produces.  The fan is forcing convection, which offsets the heat lost due to radiative heat loss to the sky with respect to the convective heat transfer from the surrounding air.  Remember that the if there isn't fog, the air temperature is above the dew point temperature.  Without the fan, radiative heat flow from the corrector surface will be larger than the natural convective heat flow from the air (warmer) to the corrector (colder,) which is why the front of the corrector can get pretty cold (approaching 6C below ambient under some conditions.)  When you force convection with a fan, the heat transfer from the air to the corrector goes way up.  That helps to keep the front surface of the glass at a temperature much closer to the surrounding air--and likely a bit above the dew point.  Fortunately, it doesn't take very much air flow to greatly increase the coefficient of convective heat transfer.  Unfortunately, a fan won't work as well as actively heating components when it's really humid.

 

The downside of the fan is potential vibration and image degradation due to turbulence--but that's usually better than getting totally shut down because of dew!

 

John


Edited by jhayes_tucson, 04 July 2019 - 04:29 PM.

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#10 ngc7319_20

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Posted 05 July 2019 - 07:20 AM

Actually, unless it's really overheated, the fan isn't preventing much dew by the heat it produces.  The fan is forcing convection, which offsets the heat lost due to radiative heat loss to the sky with respect to the convective heat transfer from the surrounding air.  Remember that the if there isn't fog, the air temperature is above the dew point temperature.  Without the fan, radiative heat flow from the corrector surface will be larger than the natural convective heat flow from the air (warmer) to the corrector (colder,) which is why the front of the corrector can get pretty cold (approaching 6C below ambient under some conditions.)  When you force convection with a fan, the heat transfer from the air to the corrector goes way up.  That helps to keep the front surface of the glass at a temperature much closer to the surrounding air--and likely a bit above the dew point.  Fortunately, it doesn't take very much air flow to greatly increase the coefficient of convective heat transfer.  Unfortunately, a fan won't work as well as actively heating components when it's really humid.

 

The downside of the fan is potential vibration and image degradation due to turbulence--but that's usually better than getting totally shut down because of dew!

 

John

Yes I think the main effect from the fan is that the motion of the air prevents water molecules from sticking and building up.  Heat from the fan is minimal, but I think it helps more than it hurts.

 

Fan vibration can be minimized by choosing a good fan, and mounting it on a somewhat flexible attachment (not too rigid).  Most of the recent high-end fans incorporate some sort of magnetic levitation of the fan rotor that minimizes friction and vibration (Adda HYPRO, Sunon MAGLEV, etc. bearings).  Using thin metal or flexible plastic for the mounting also helps. If you have room (and you probably wont in this particular dew shield application) -- the fan can be mounted on springs, or rubber bands, or rubber grommets. You can further reduce vibration by balancing the fan blades -- running tests and adding tiny bits of tape to the blades -- sort of trial and error.  A speed control can also be used to avoid any mechanical resonances.  If done with some care, there should be zero vibration.

 

In practice I've not seen much image degradation from turbulence.  My philosophy it that it is probably better to have the air at the corrector well-mixed, vs. some slow convection driven by temperature differences.  As a practical matter, you also have to weigh this against the air warm currents from a dew heater strap (less heat needed with fan running) or using one of these 100 Watt heat guns every 15 minutes (large temperature over-shoot), etc.  



#11 Cpk133

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Posted 05 July 2019 - 03:08 PM

Can't go wrong with an Astrozap stand alone heater strip, controller and flexible dew shield.  Been using that setup for years and dew free on even the most humid nights.  The only time I've had an issue is when I plugged into the wrong port on the controller.  I typically run from 33 to 50% capacity.  If dew is really heavy, I'll crank it up to 70%ish if I'm viewing near the zenith for any length of time.  I've had everything get soaking wet and remain dew free.  I didn't get the strip/ shield combo for good reasons.


Edited by Cpk133, 05 July 2019 - 03:08 PM.


#12 Jim-M

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Posted 06 July 2019 - 01:07 PM

First step is to use a flexible dew shield.  It will reduce the corrector heat loss by more than a factor of 10.  For me this is adequate for those nights that don't go too late.  It would have to be positioned obviously crooked (which I of course have done) to affect the viewing or image.  It also reduces the power required for the dew heater when you need to use one.  For me, in Southern Calif., this is 10 to 20% of the time.  



#13 Achernar

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Posted 14 July 2019 - 12:29 PM

It's a rare night on the Gulf Coast where the corrector won't dew up without a dew cap. Often I am required to use a heater strip and temperature probe together with the dewcap to keep my telescope clear. The heater strip goes around the tube just behind the corrector, and the probe will be placed according to the instructions that came with it. Mine clips to the strip  with the air temperature probe exposed to the air, while the other senor is in contact with the tube itself. When set up correctly, you won't have heat blooms spoiling the views. Just use a setting high enough to keep the corrector clear, and no more. Bring along a battery that can supply up to several amps all night long without discharging it too much, which will sooner or later ruins it. I use a 35 Amp-hour Absorbed Glass Mat or "gel cell" battery for this purpose, and it also powers the mount too. I quickly ruined one of those 17 amp-hour power tanks due to the current draw being too great for it. Size your battery accordingly to the power draw of your heater(s), mount and how long you tend to spend observing. With careful use and re-charging with the appropriate charger, they will dependably power your heaters and telescope for years before requiring replacement.

 

Taras


Edited by Achernar, 14 July 2019 - 12:45 PM.

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#14 JP50515

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Posted 15 July 2019 - 09:42 AM

It's a rare night on the Gulf Coast where the corrector won't dew up without a dew cap. Often I am required to use a heater strip and temperature probe together with the dewcap to keep my telescope clear. The heater strip goes around the tube just behind the corrector, and the probe will be placed according to the instructions that came with it. Mine clips to the strip  with the air temperature probe exposed to the air, while the other senor is in contact with the tube itself. When set up correctly, you won't have heat blooms spoiling the views. Just use a setting high enough to keep the corrector clear, and no more. Bring along a battery that can supply up to several amps all night long without discharging it too much, which will sooner or later ruins it. I use a 35 Amp-hour Absorbed Glass Mat or "gel cell" battery for this purpose, and it also powers the mount too. I quickly ruined one of those 17 amp-hour power tanks due to the current draw being too great for it. Size your battery accordingly to the power draw of your heater(s), mount and how long you tend to spend observing. With careful use and re-charging with the appropriate charger, they will dependably power your heaters and telescope for years before requiring replacement.

 

Taras

Thanks!

I went ahead and got got a shield and dew strap, along side a pegasus-astro pocket power box to regulate the power. 

Awesome little power distribution box, it has space for 4x - 12V and 2 Dew heaters with a Temp probe. As well as some other unrelated features like an 8V DSLR port.

I've only had one night out with it so far, (no do shield yet - still in the mail) but at 100% humidity it took a few hours to equalize and heat up that corrector plate, but it did manage to keep the dew at bay once up to temp. 

 

I run off a 10A AC-DC regulated power supply at home (90% of my imaging) and a 120AH deep cycle marine battery in the field so I think I should be good to go. My rig only draws about 3A at max power (while slewing hard) and about 1A-2A while imaging. 


Edited by JP50515, 15 July 2019 - 09:43 AM.


#15 Sarkikos

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Posted 15 July 2019 - 12:58 PM

I've had good luck with AstroZap aluminum dewshields plus a heater band on the OTA just below the corrector.  You want to go easy on the heat if you are working at high resolution (planetary, etc.).

 

A small fan mounted inside the dewshield and aimed at the corrector will also help.

 

attachicon.gif C9 fan.png

Is there any advantage to the aluminum dewshields vs the plastic ones?

 

Mike



#16 JP50515

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Posted 15 July 2019 - 05:59 PM

Is there any advantage to the aluminum dewshields vs the plastic ones?

 

Mike

I haven't seen plastic ones, but compared to the flexible ones the aluminum would prevent odd shaped stars due to misshapen flexible shields. 



#17 Sarkikos

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Posted 15 July 2019 - 08:16 PM

By "plastic," I mean the usual flexible dew shields offered by AstroZap, Meade and other companies.  IIRC, they are made of ABS plastic.  These dew shields are relatively flexible.  They can be rolled up somewhat to fit in an equipment bag or box.  

 

There are other dew shields that are softer and even more flexible.  They are made of something like foam sheeting.  

 

But I've never had any trouble with either kind of flexible dew shield deforming enough to disturb the image.  That just doesn't happen in my experience.  So if that would be the reason to prefer metal dew shields, I really don't think it's a valid reason. 

 

I don't see any reason to prefer the metal dew shields, except maybe as a matter of personal taste.  Some think they look better.  :shrug:

 

Mike


Edited by Sarkikos, 15 July 2019 - 08:18 PM.



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