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Questions About Insulating SCT's with Reflectix

catadioptric DIY equipment SCT
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#226 Adun

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Posted 20 January 2019 - 07:30 AM

So I've made an insulator + dew/light shield for my 6" SCT, using a thermal material that is meant for campers, consisting of some ~5mm black matte foam, with mylar on one side:

 
It's a little long, but I understand that will help delay the appearance of condensation. I'm not sure about the effect of it also being a full body cover (the "insulator" part) but I'm hoping that will help the tube retain heat longer and afford me more observing time before dew shows up. The place I'm going  to has 85% humidity, and dew point at 18°C, which is reached at just 8pm, with temperatures dropping further to 14°C at 1am. I only intend to observe for a couple hours (from 7:30 to 9:30pm maybe) so there's hope this might be enough despite the lack of a heater.
 
If I find something interesting I'll post it here. 

 

Quick update:

 

I'm at the final location of my trip, where it's cold and humid at night. Last night I was able to observe without dew problems.

 

The 6" SCT had been stored in the cabin through the day. Around 8pm when I went to take my tripod out of the car, the car was already covered in dew.

 

I set up the tripod/mount, went inside, installed the DIY dew/light shield/insulator, finder, eyepiece, and then went out.

 

I had no problems observing, although when I tried to use my hoodie to block some stray light, I ended up fogging my Baader zoom, so I had to switch to my 82° eyepieces and not use the hoodie.

 

By 10:30pm dew still had not formed on the SCT, but I was done and tired, so I picked everything up and went inside to sleep.

 

So the insulator was a success, and gave me what I needed: ability to observe for a couple hours without dew. 


Edited by Adun, 20 January 2019 - 07:31 AM.

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#227 Rustler46

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Posted 06 July 2019 - 01:11 AM

Well, here I am late to the "discussion", which I've found both interesting and at times confusing. So I'm reviving this thread, not to rekindle any controversy, but to ask for brief opinions on what I propose to do with my Celestron-11 (2008 version, metal tube, non-edge, without provision for fans).

 

My circumstances:

  • Moderate coastal climate, high humidity at times, but minimal temperature change from day to night
  • Presently using the DewBuster system with two dew heaters just below the corrector cell
  • Unheated black plastic AstroZap dew shield
  • Infrequent blasts with a hair dryer, if I haven't adjusted the temperature setting correctly on the DewBuster
  • Mostly visual, but at rare times have done planetary imaging. Here's the best I've done with the C-11 on Mars, when it was less than 14 arc-seconds:

Mars0021 10-02-07 21-32-19B 2.jpg

 

 

This is what I'm proposing to do for improving dew control and provide for decent image quality:

  1. Continue using my DewBuster system
  2. Continue using my black plastic AstroZap dew shield with a dew heater located 1/2 way up above the corrector. See possible variation below.
  3. Paint the outside of the dew shield white
  4. Position one dew heater strip on the OTA around 1/3 the distance from corrector to primary
  5. Wrap the OTA with Reflectix, using Velcro

One possible variation on #2 is to use a Reflectix dew shield fashioned like recommended on the DeBuster website. This has already been constructed with the interior coated with flat black paint. With this one the dew heater would also be located 1/2 way up above the corrector. One advantage to using this dew shield is that it is easily removed for use of Bahtinov mask or for reaching collimation screws.

 

One observation I have about moving one of the dew heaters from the metal tube to the dew shield. It would seem that heat would be less damaging in this location compared to inside the tube. Light only has traverse this area once on the way to focus. Inside the tube it traverses 3 times.

 

Your comments or suggestions would be welcome.

 

Best Regards,

Russ



#228 luxo II

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Posted 06 July 2019 - 05:30 AM

Putting the dew heater in the dew shield makes it basically useless. You could pour 100w into the heater and it will go straight up to space, meanwhile your corrector will still dew over.

Have tried before. Way back in the 1980s dewcaps dewcaps for celestrons often had heaters which were essentially useless.

Edited by luxo II, 06 July 2019 - 05:32 AM.


#229 Sarkikos

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Posted 06 July 2019 - 09:20 AM

I observe at sites which are often very dewy.  In my experience, a dew shield with built-in warming strip does prevent dew better than a dew shield without a warming strip.  I have no doubt about this.  But is it the optimum position for stabilizing thermals in the telescope?  No.  Is it even the best position for preventing dew?  No. 

 

But I wouldn't say built-in dew strips are basically useless for preventing dew.  That would go against what I've seen in the field.  They are better than just a dew shield alone in preventing dew.

 

Mike


Edited by Sarkikos, 06 July 2019 - 09:28 AM.

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#230 Rustler46

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Posted 06 July 2019 - 10:08 AM

I removed the dew heater from my dew shield years ago. What I intend to do is move one of the two dew heaters I have on the metal tube just behind the corrector cell to 1/2 way up in the dew shield. The other dew heater is moved from just behind the corrector to down the metal tube a bit further.

 

What John Hayes is saying makes sense to me. Some heat radiating from the dew shield down toward the surface of the corrector keeps the corrector clear. Some of that heat will chimney up out of the way. Better here than where the light has to pass 3 times (inside the tube). Painting the outside of the dew shield white will reduce heat loss via radiation on that surface and help provide a dew shield that appears warm as seen from the corrector. Thus from an infrared heat exchange point of view the corrector will radiate heat away slower than if the dew shield was cold.

 

Also the fact that glass is a poor conductor of heat applied to the inside means heat inside the tube is less effective in keeping the corrector clear. Reflectix on the tube reduces heat loss from the metal tube. This reduces the amount of heat to be applied internally to keep the OTA just above ambient temperature. Otherwise the black metal tube radiates heat away to the night sky, needing constant input of heat inside.

 

I know not all will agree with what I do. What works for dew prevention depends upon the widely varying climate/weather circumstances we each observe under. But the two heater strips on OTA (and unheated dew shield) has worked for me for years up to now. I'll report back on how the changes work for my purposes.


Edited by Rustler46, 06 July 2019 - 10:28 AM.

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#231 jhayes_tucson

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Posted 06 July 2019 - 11:12 AM

Russ,

My scope is at DSW where the dew can be really bad--particularly during the winter.  (Yep, that may be a surprise for those of you who haven't actually visited the Rowe plateau in New Mexico where there can be dense fog, high humidity, and extreme dew conditions.)  I use two heater strips on the dew shield and one about 1/3 of the way down the OTA (below the corrector,) which uses a carbon fiber tube.  All of the heaters are covered by Reflectix.  The heaters are run through a Dew Buster, which is set to hold them about 6-7 degrees above the ambient air temperature (as I recall.)  The Dew Buster power is controlled by an Arduino controller that only turns it on when the temperature/dew-point spread at the corrector plate is computed to within 6 degrees using a black probe at the corrector along with an ambient air temperature probe and a RH sensor.  This whole thing works really well and I haven't been shut down by dew ever since I got it going (early last October.)  It might be possible to turn some of the temperature limits down on my system but it's not fully controllable remotely and I'm not out there enough to fine tune it.  I think that I might be able see some minor effects on image quality with the heaters running but that's never been a show stopper.  Since the OTA tube is made of carbon fiber, I suspect that most of the benefit comes from radiant heating from the dew shield heaters.  Using a metal dew shield is important if you choose to heat the shield itself.  Just don't get anything too hot.  Using "just enough" heat is the best approach.

 

John

 

 

PS  I've done a fair amount of thermodynamic modeling that confirms that heating the dew shield is a very effective way to limit dew.  It would be pretty straightforward to make some interferometric measurements to demonstrate the optical effects of air turbulence induced by  these different schemes; but unfortunately, I don't have access to the equipment any more.


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#232 Sarkikos

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Posted 06 July 2019 - 11:24 AM

In Maryland, we call dew in winter frost.

 

:grin:

Mike


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#233 Rustler46

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Posted 06 July 2019 - 04:35 PM

Russ,

My scope is at DSW where the dew can be really bad--particularly during the winter.  (Yep, that may be a surprise for those of you who haven't actually visited the Rowe plateau in New Mexico where there can be dense fog, high humidity, and extreme dew conditions.)  I use two heater strips on the dew shield and one about 1/3 of the way down the OTA (below the corrector,) which uses a carbon fiber tube.  All of the heaters are covered by Reflectix.  The heaters are run through a Dew Buster, which is set to hold them about 6-7 degrees above the ambient air temperature (as I recall.)  The Dew Buster power is controlled by an Arduino controller that only turns it on when the temperature/dew-point spread at the corrector plate is computed to within 6 degrees using a black probe at the corrector along with an ambient air temperature probe and a RH sensor.  This whole thing works really well and I haven't been shut down by dew ever since I got it going (early last October.)  It might be possible to turn some of the temperature limits down on my system but it's not fully controllable remotely and I'm not out there enough to fine tune it.  I think that I might be able see some minor effects on image quality with the heaters running but that's never been a show stopper.  Since the OTA tube is made of carbon fiber, I suspect that most of the benefit comes from radiant heating from the dew shield heaters.  Using a metal dew shield is important if you choose to heat the shield itself.  Just don't get anything too hot.  Using "just enough" heat is the best approach.

 

John

 

 

PS  I've done a fair amount of thermodynamic modeling that confirms that heating the dew shield is a very effective way to limit dew.  It would be pretty straightforward to make some interferometric measurements to demonstrate the optical effects of air turbulence induced by  these different schemes; but unfortunately, I don't have access to the equipment any more.

Hi John,

I was hoping you would see my post and could add some more insight. I don't have a metal dew shield - mine is plastic. But I could add a wide strip of metal on the inside with the dew heater attached. Would a layer of aluminum foil on the inside work if painted black?

 

You say to cover the dew shield with Reflectix. With the plastic shield lined with metal, would the Reflectix go on the outside with the inside colored black? Would painting the outside white be of benefit? Or does the Reflectix insulation play a critical role in a warm dew shield's effectiveness?

 

The only temperature sensors I have for the DewBuster system are - (1) ambient air temperature and (2) on the metal tube for measuring temperature inside the OTA. Lacking anything better, that's what will be used. I'll just experiment to see what temperature differential works to keep dew at bay.

 

Any advice you can offer would be much appreciated.

 

Best Regards,

Russ


Edited by Rustler46, 06 July 2019 - 04:47 PM.


#234 jhayes_tucson

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Posted 07 July 2019 - 01:32 AM

Russ,

The main reasons that I suggest a metal dew shield is that it's easy to mount the heater strip on the outside of the shield and the metal will conduct the heat over a wider area on the shield to heat the inside surface where it radiates.  Ideally, you want the whole shield to be at a slightly elevated temperature, which is why I use two heater strips.  It should still work if you just mount the heater on the inside of the flexible shield; though maybe not quite as well.  Reflectix on the outside of the shield over the heater strips reduces radiative cooling of the shield and helps to provide a little insulation on top of the heater strips so that most of the heat goes into the shield itself (rather than lost by conduction and radiative losses to the outside of the shield.)

 

Although it seems to work pretty well, my configuration may not be perfect so I suggest experimenting a bit to see what works best on your system. The main thing is to make sure that there is just enough radiated heat from the inside of the dew shield to keep the front corrector surface from falling below the dew point.  I never had as much success with a floppy, plastic dew shield using this scheme but maybe you can get it to work.  If not, a metal shield might be the answer.

 

John


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#235 Rustler46

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Posted 07 July 2019 - 01:59 AM

Russ,

The main reasons that I suggest a metal dew shield is that it's easy to mount the heater strip on the outside of the shield and the metal will conduct the heat over a wider area on the shield to heat the inside surface where it radiates.  Ideally, you want the whole shield to be at a slightly elevated temperature, which is why I use two heater strips.  It should still work if you just mount the heater on the inside of the flexible shield; though maybe not quite as well.  Reflectix on the outside of the shield over the heater strips reduces radiative cooling of the shield and helps to provide a little insulation on top of the heater strips so that most of the heat goes into the shield itself (rather than lost by conduction and radiative losses to the outside of the shield.)

 

Although it seems to work pretty well, my configuration may not be perfect so I suggest experimenting a bit to see what works best on your system. The main thing is to make sure that there is just enough radiated heat from the inside of the dew shield to keep the front corrector surface from falling below the dew point.  I never had as much success with a floppy, plastic dew shield using this scheme but maybe you can get it to work.  If not, a metal shield might be the answer.

 

John

Thanks for that information, John. I can see the wisdom here. Now there is a reduced fraction of what the corrector sees at outer space temperature. The rest of what it sees is the inside of the dew shield which can be warmed. I may try my hand at making my own metal shield since commercial units are $175.


Edited by Rustler46, 07 July 2019 - 01:04 PM.


#236 Rustler46

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Posted 07 July 2019 - 01:28 PM

Thanks for that information, John. I can see the wisdom here. Now there is a reduced fraction of what the corrector sees at outer space temperature. The rest of what it sees is the inside of the dew shield which can be warmed. I may try my hand at making my own metal shield since commercial units are $175.

I looked into materials to make a metal dew shield:

This could be cut, shaped into a tube and secured with pop rivets. But knowing my skills, it would likely be somewhat crude. Then there's the problem of attaching it to the OTA. The commercial units have welded seams and nylon-tipped screws for attachment to the telescope.

 

Looking at other options I came across this product:

This is just heavy aluminum foil, black on both sides. I'm wondering if I could attach a heater strip to one side of this using something like aluminum duct tape. Then this metal jacket could be attached to the inside of my plastic dew shield, perhaps using gaffer's tape. The outside of the dew shield could then be covered with Reflectix. I may well try this out to implement some of John's suggestions. Having a slightly warmed metal surface inside the dew shield should radiate some heat to the corrector's front surface.

 

Another less expensive option is to make my own blackened aluminum foil. Using heavy duty kitchen foil I can paint one side of the foil with black paint using the directions on the DewBuster website. Then the dew heater strip can be attached to the other side as described above.

 

But first I will wrap the main metal body of the Celestron-11 with one heater strip inside, a short distance down from the corrector cell. Since the temperatures day to night don't entail a tremendous differential, just this partial wrap should greatly reduce the amount of heat introduced inside the OTA.

 

I'll report back on my results.

 

Russ


Edited by Rustler46, 07 July 2019 - 01:56 PM.


#237 jhayes_tucson

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Posted 07 July 2019 - 02:12 PM

Russ,

Yeah, I cringed at the price for the AstroZap aluminum dew shield too and I still have the rolled sheet metal sitting in the corner from when I decided to make my own.  I gave up on making it simply because I ran out of time to get it done before I had to visit the observatory.  I bought a piece of aluminum from On Line Metals and then took it to a local steel cabinet maker where they rolled it for me at no charge.  (I tipped the guys in the shop with some beer money and they seemed happy.)   You can mount the shield on the outside of the end cap or on the inside of the end cap.  If you get the size right, I've found that Velcro felt works pretty well to create a nice snugg fit.  At the observatory, I use outdoor masking tape (or aluminum tape) to secure the shield so that it can't move in the wind.  One note:  The adhesive on aluminum tape pulls off on everything so consider that stuff to be permanent.  You can remove the adhesive but it's a hassle to clean up the mess.

 

One other thought:  If you use a single piece of Reflectix to cover both the scope and the dew shield, it's pretty stiff once it's rolled up.  That can go a long way to support and secure your dew shield (depending on the dimensions.  If you use big pieces of Velcro on the Reflectix, it's easy to remove the whole thing after each session.

 

John


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#238 Sarkikos

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Posted 07 July 2019 - 02:36 PM

As long as the Cat had a plastic dew shield with either built-in warming strip or add-on warming strip, I've never had condensation form on a corrector up to a C6 or EdgeHD 8", the largest the Cat I've ever owned.    And I observe under very dewy conditions. 

 

For my Dob I use a Dew-Not controller.  For my Cats, I just Velcro one or more small 12v portable power supplies to the top of the mount.  As long as the power supply keeps cranking out the power, there is no condensation on the corrector.  These will last for five hours or so.  Lately I've upgraded to a somewhat larger power supply that gives out substantial heat all night.  No controller.  Even for the Dob, I usually end up setting the controls to high in order to keep the dew off eyepieces and the finders.  So why bother with a controller?

 

At least for keeping off the dew, I don't see any reason to upgrade to metal dew shields.  Why deal with the extra expense and weight if it's not necessary?  Personally, I never felt a need or desire for metal dew shields.

 

Mike


Edited by Sarkikos, 07 July 2019 - 02:44 PM.


#239 Rustler46

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Posted 07 July 2019 - 05:53 PM

As long as the Cat had a plastic dew shield with either built-in warming strip or add-on warming strip, I've never had condensation form on a corrector up to a C6 or EdgeHD 8", the largest the Cat I've ever owned.    And I observe under very dewy conditions. 

 

For my Dob I use a Dew-Not controller.  For my Cats, I just Velcro one or more small 12v portable power supplies to the top of the mount.  As long as the power supply keeps cranking out the power, there is no condensation on the corrector.  These will last for five hours or so.  Lately I've upgraded to a somewhat larger power supply that gives out substantial heat all night.  No controller.  Even for the Dob, I usually end up setting the controls to high in order to keep the dew off eyepieces and the finders.  So why bother with a controller?

 

At least for keeping off the dew, I don't see any reason to upgrade to metal dew shields.  Why deal with the extra expense and weight if it's not necessary?  Personally, I never felt a need or desire for metal dew shields.

Mike, the advantage of a controller like the DewBuster or Kendrick models is that excessive heat isn't introduced into the telescope's optical tube. Only the amount of heat needed (to keep the tube at the temperature differential you've dialed in) is being supplied - nothing more. Thus inside tube currents could be minimized. If these currents aren't a a problem, then any kind of controller will work, or none at all. A power supply will happily provide all the heat (and more) that will keep the dew away. For those who observe remotely, being frugal with power usage in dew prevention can make portable batteries last longer.

 

Yeah, the metal tube is an expense that may not be necessary in most circumstances. Our needs will vary, not only by how much moist air we are under, but also by how fast the temperatures are dropping and how great the day to night temperature different we experience.

 

So as they say, YMMV - our mileage may vary and it usually does. That's why we have such a diversity of opinions on threads like this.


Edited by Rustler46, 07 July 2019 - 07:32 PM.


#240 Rustler46

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Posted 08 July 2019 - 12:53 AM

But first I will wrap the main metal body of the Celestron-11 with one heater strip inside, a short distance down from the corrector cell. Since the temperatures day to night don't entail a tremendous differential, just this partial wrap should greatly reduce the amount of heat introduced inside the OTA.

 

I'll report back on my results.

I finished wrapping the optical tube with Reflectix. My dear wife was a great help with her crafting skills - sewing, quilting, etc. Without that help I would have made a mess of things. There were four holes that had to be cut in the covering - one each for top and bottom dovetail extrusion, one for Telrad and one for the DewBuster controller. Each of these needs to be of the correct size and spaced properly from the other cutouts. Here are some photos:

 

C-11 & Reflectix Wrap-01466.jpg

This view shows the cutout for the upper dovetail on left and on right the DewBuster already within its cutout. The mount for the Telrad is barely visible on the left. The twin-lead wire for the OTA heater strip is shown coming out of the cutout for the dovetail. The heater is wrapped midway on the metal tube between front and rear castings.

 

C-11 & Reflectix Wrap-01470.jpg

This one shows upper and lower dovetails along with the Telrad in its cutout. Notice the Telrad has its own dew heater being fed by the twin-lead wire. I found the commercial Telrad dew shields to be worthless. But this home made heater keeps dew away by application of continuous medium heat from the DewBuster controller.


Edited by Rustler46, 08 July 2019 - 06:25 PM.

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#241 Rustler46

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Posted 08 July 2019 - 01:21 AM

I finished wrapping the optical tube with Reflectix. 

Here are some more photos:

C-11 & Reflectix Wrap-01467.jpg

 

Here can be seen the Telrad, upper dovetail (with tube rings for my AT115EDT refractor) and the DewBuster controller on right. Next to the tube ring on the bottom can be seen the wire leading to the sensor for measuring the OTA's internal air temperature. This is plugged into the DewBuster controller. The sensor for measuring ambient air temperature can be seen silhouetted against the front casting.

 

C-11 & Reflectix Wrap-01463.jpg

This shows the Reflectix before any holes were cut in it. Around 1/3 of the circumference has a double layer of Reflectix. The wrap is held in place with some heavy duty Velcro on the Reflectix. The DewBuster controller is held on the tube with some more Velcro. I may eventually tape the seams and exposed ends of the Reflectix to seal things against air leakage.

 

I may eventually wrap the rear casting around the optical port and focus knob. But just covering the telescope's metal tube will do a lot for thermal management. Next in this project is to equip my plastic dew shield with an internal aluminum layer, warmed by a heater strip. As a trial I will just use some heavy duty aluminum foil, painted black to enhance thermal emission. The intent is to have it supply heat to the front of the corrector lens. The outside of the dew shield will need a layer of Reflectix or white paint. The heater on the metal telescope tube will supply some energy to warm the corrector from behind.

 

When more has been finished, I'll report on the results.

 

Clear Skies,

Russ


Edited by Rustler46, 08 July 2019 - 01:31 AM.

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#242 Rustler46

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Posted 09 July 2019 - 02:27 AM

Russ,

Yeah, I cringed at the price for the AstroZap aluminum dew shield too and I still have the rolled sheet metal sitting in the corner from when I decided to make my own.  I gave up on making it simply because I ran out of time to get it done before I had to visit the observatory.  I bought a piece of aluminum from On Line Metals and then took it to a local steel cabinet maker where they rolled it for me at no charge.  (I tipped the guys in the shop with some beer money and they seemed happy.)   You can mount the shield on the outside of the end cap or on the inside of the end cap.  If you get the size right, I've found that Velcro felt works pretty well to create a nice snugg fit.  At the observatory, I use outdoor masking tape (or aluminum tape) to secure the shield so that it can't move in the wind.  One note:  The adhesive on aluminum tape pulls off on everything so consider that stuff to be permanent.  You can remove the adhesive but it's a hassle to clean up the mess.

 

One other thought:  If you use a single piece of Reflectix to cover both the scope and the dew shield, it's pretty stiff once it's rolled up.  That can go a long way to support and secure your dew shield (depending on the dimensions.  If you use big pieces of Velcro on the Reflectix, it's easy to remove the whole thing after each session.

 

John

Thanks, John for the insight on dew shields. I'll be looking at getting some thin aluminum sheet to make a metal dew shield. Placing blackened foil on the inside of my plastic dew shield sounded easy, but in practice would be difficult. I suspect something that thin wouldn't be very efficient in conducting heat from a dew heater to locations distant from the heater. So some thin sheet metal would be interesting to see how it could be formed into a tube. I like the thought of it setting inside the end casting. Then some Velcro fastenings could hold it down in place. If a suitable sized sheet of aluminum can be sourced locally at a good price I'll give it a try. Otherwise On-Line-Metals is not too expensive. 

 

I have an extra dew heater that was one of two on the C-11 metal tube. But now that it is covered with Reflectix, only one heater was needed. Here's the spare I'll be attaching to the outside of the metal  dew shield:

 

C-11 Dew Heater.jpg

 

This could be attached to the outside of the metal with aluminum tape and covered with Reflectix. The inside of the dew shield will be blackened by following the instructions on the DewBuster website. First the metal is primed with 3M adhesive. When dry this will be spray painted flat black. 

 

Thanks for all the information. Just understanding the thermodynamics going on at the front side of the corrector was fascinating. The conditions at home where I do most of my observing are not hard to deal with. If all else fails I have an electrical outlet at the base of my pier, with a hair dryer at the ready.

 

All the Best,
Russ



#243 earlyriser

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Posted 09 July 2019 - 02:58 AM

I've heard that the reflective nature of the Reflectix is a major factor in why it works to retain heat.  It works by reflecting heat.  I can understand how reflecting heat coming up from the OTA back into the OTA would make sense. 

 

But why would reflecting away heat coming onto the OTA from the environment - the sky, the grass, the trees, whatever -  help the OTA retain heat?  That doesn't make sense to me.  So why does the outer surface of the Reflectix need to be shiny?  Why can't we just blacken it with paint?  Why would that affect the Reflectix' ability to help the OTA retain heat?  It's the inner surfaces of the Reflectix that face the OTA which are important as heat retaining surfaces.  The OTA never "sees" the outer surface of the Reflectix.

 

Mike

Somebody probably answered this already, but good reflectors are bad emitters. A reflective outside surface radiates less heat away from the scope.


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#244 Rustler46

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Posted 09 July 2019 - 10:31 PM

Thanks, John for the insight on dew shields. I'll be looking at getting some thin aluminum sheet to make a metal dew shield. Placing blackened foil on the inside of my plastic dew shield sounded easy, but in practice would be difficult. I suspect something that thin wouldn't be very efficient in conducting heat from a dew heater to locations distant from the heater. So some thin sheet metal would be interesting to see how it could be formed into a tube. I like the thought of it setting inside the end casting. Then some Velcro fastenings could hold it down in place. If a suitable sized sheet of aluminum can be sourced locally at a good price I'll give it a try. Otherwise On-Line-Metals is not too expensive. 

 

I have an extra dew heater that was one of two on the C-11 metal tube. But now that it is covered with Reflectix, only one heater was needed. Here's the spare I'll be attaching to the outside of the metal  dew shield:

 

 

This could be attached to the outside of the metal with aluminum tape and covered with Reflectix. The inside of the dew shield will be blackened by following the instructions on the DewBuster website. First the metal is primed with 3M adhesive. When dry this will be spray painted flat black. 

 

Thanks for all the information. Just understanding the thermodynamics going on at the front side of the corrector was fascinating. The conditions at home where I do most of my observing are not hard to deal with. If all else fails I have an electrical outlet at the base of my pier, with a hair dryer at the ready.

My local hardware store had a suitable material for the metal dew shield. This was 30 gauge (0.010 inch, 0.25mm) aluminum flashing. In a 20-inch width it ran $2.29/foot.

 

The inside diameter of the C-11 sky-end casting is around 12.1 inches. This gives a circumference of  38 inches (12.1 x π = 12.1 x 3.14 = 38). So I had them cut me a piece of flashing 40 inches long. This will give a 2-inch overlap. The good guys at the hardware store charged me for 3 feet. So it cost only $6.87. Such a deal. This material is stiff enough to hold its shape quite well. It is even lighter than the plastic dew shield it will replace - 10 oz vs. 20 oz. 

 

Next is to blacken the inside and rivet the cylinder to shape. Some 1/8-inch pop-rivets will do the job. Then tape the heater strip to the outside and cover with a strip of Reflectix - viola! It sure beats the price of a commercial metal dew shield (less than $10 vs $175).

 

Edit:

 

I got the metal trimmed to 1.5X the lens diameter (1.5 x 11 = 16.5 inches). After cleaning up the sharp edges with a file, I sprayed the 3M 77 Super Spray Adhesive as a primer for the flat black paint to be applied inside. The Krylon Ultra-Flat black paint shouldn't be applied in high humidity.  I do read all labels thoroughly for safety and proper application of product. Since it was drizzling, the humidity was near 100%. Yeah, it rains at times here in Oregon.

 

So I'll let the adhesive dry thoroughly overnight, hoping that tomorrow will be much less humid. It is supposed to be clear in the afternoon. So then I'll paint the inside black and work on forming the cylinder with some pop-rivets. Then some strips of HD Velcro will secure the dew shield to the OTA. Then I can attach the heater, secured with aluminum tape and covered with Reflectix.

 

I'll get a photo of the finished dew shield.

 

Clear Skies,

Russ


Edited by Rustler46, 10 July 2019 - 02:41 PM.


#245 luxo II

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Posted 10 July 2019 - 04:13 AM

 I sprayed the 3M 77 Super Spray Adhesive as a primer for the flat black paint to be applied inside.

Now there's an interesting idea. Must try that with Black 3.0.


Edited by luxo II, 10 July 2019 - 04:14 AM.


#246 Sarkikos

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Posted 10 July 2019 - 05:52 AM

For those who already have a plastic dew shield, why not just cover it with Reflectix?

 

Mike



#247 Rustler46

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Posted 10 July 2019 - 09:52 AM

For those who already have a plastic dew shield, why not just cover it with Reflectix?

Mike, that would be an improvement. The corrector would likely "see" a dew shield that was not so cold. John's insight on the matter was to provide a dew shield that is warmed so that it would radiate heat toward the corrector, helping to keep it dew free. Now the corrector sees a piece of dark, cold sky toward which the corrector radiates heat. But now it is also receiving heat radiated from the warm dew shield. The net effect is to keep it warmer.

 

The metal shield will receive heat from the dew heater and is able to conduct that heat over the entire surface of the metal. This provides the corrector with a much warmer sky to see - that sky being space plus the inside of the warm dew shield. Covering the metal dew shield with Reflectix reduces the amount of heat it would otherwise lose by its outside surface radiating to the cold sky it sees. A metal dew shield is preferred over plastic, since it better conducts heat (provided by the heater) throughout its mass. Being painted black inside contributes to better thermal radiation, compared to a light colored surface.

 

The corrector also receives some heat from the warmed air inside the tube. But glass doesn't conduct heat very well. So it does better if the heat is given it from the front, which is the surface more likely to dew up. In that case less heat is needed on the inside of the OTA, which helps reduce tube currents. Covering the metal OTA with Reflectix further reduces the amount of heat needed in that area.

 

Another advantage for front heat is that the light on its way to focus only passes through that region once. So heat has less deleterious effect compared to inside the tube, which light traverses 3 times.


Edited by Rustler46, 10 July 2019 - 10:03 AM.

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#248 freestar8n

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Posted 10 July 2019 - 10:24 AM

Heating the dew shield will certainly help reduce dew formation but it requires the dew shield to be significantly warmer than ambient - and that will introduce a chimney effect directly in front of the corrector.

In contrast if you apply heat below the corrector all you need to do is maintain the corrector near ambient and no tube currents are generated. That is why almost everyone applies dew heaters this way with scts and refractors.

The view of the sky from the corrector has a central extremely cold region directly facing it (the cold night sky) and serving as a strong heat sink. In order for the dew shield to compensate it must be well above ambient to balance the loss - especially since it is radiating from the side as opposed to face on.

The turbulence may not have strong impact on deep sky imaging but for visual or planetary work it is likely detrimental.

In my case with edgehd11 I use a short dew shield to reduce wind effects in combination with a dew strap below the corrector. I regularly post deep sky images with low fwhm along with diffraction limited views of the airy pattern. I don’t use insulation or reflectix on anything. The scope is outdoors under a telegizmos cover and I just uncover it and turn on the dew heater.

One night I had left the dew heater unconnected and in the morning the front of the scope was soaking wet. So dew is a problem for me but everything is fine with gentle heating to keep the corrector near ambient and nothing in the optical path is hot.

Frank
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#249 jhayes_tucson

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Posted 10 July 2019 - 11:51 AM

Heating the dew shield will certainly help reduce dew formation but it requires the dew shield to be significantly warmer than ambient - and that will introduce a chimney effect directly in front of the corrector.

In contrast if you apply heat below the corrector all you need to do is maintain the corrector near ambient and no tube currents are generated. That is why almost everyone applies dew heaters this way with scts and refractors.

The view of the sky from the corrector has a central extremely cold region directly facing it (the cold night sky) and serving as a strong heat sink. In order for the dew shield to compensate it must be well above ambient to balance the loss - especially since it is radiating from the side as opposed to face on.

The turbulence may not have strong impact on deep sky imaging but for visual or planetary work it is likely detrimental.

In my case with edgehd11 I use a short dew shield to reduce wind effects in combination with a dew strap below the corrector. I regularly post deep sky images with low fwhm along with diffraction limited views of the airy pattern. I don’t use insulation or reflectix on anything. The scope is outdoors under a telegizmos cover and I just uncover it and turn on the dew heater.

One night I had left the dew heater unconnected and in the morning the front of the scope was soaking wet. So dew is a problem for me but everything is fine with gentle heating to keep the corrector near ambient and nothing in the optical path is hot.

Frank

 

Actually, it doesn’t have to be heated “significantly warmer than ambient.”  The amount of heat required depends on the dimensions of the dew shield—and the local temp/RH conditions.  A passive, unheated dew shield all on it’s own does a really good job of reducing heat exchange with the sky so that in a lot of cases, heat isn’t needed at all.  Adding heat to the dew shield does two things:  1) It allows for a the use of a short dew shield (L~= telescope aperture.)  For larger scopes, that helps to reduce the effects of wind on system stability so that the system can be used with a bit more wind before getting shut down.  2) Adding heat to the dew shield allows the system to remain dew free up to within a temperature-dewpoint spread of 1-3 degrees.  That’s basically at the point where the probability of fog formation is pretty high.  The amount of heat that’s required depends on a lot of factors but it typically doesn’t have to be more than about 4C above ambient—and 1-2C will work quite well under all but the very worst conditions (>~95% RH.)  Any heat in front of the telescope is undesireable but at a delta of 1-2C, the effects on image quality are minor.  Remember, the goal is NOT to heat the corrector.  The goal is to compensate for radiative heat loss to the sky so that the corrector remains at the ambient air temperature.  In that case, convective heat transfer from the corrector to the air will be zero.

 

The amount of heat required from the dew shield can be further reduced by splitting the heat exchange between radiative heat from the dew shield with heat from the OTA.  Placing a heater strip about a third of the way down the tube behind the corrector generally works well.  I prefer to keep heat applied to the OTA at a very low setting to avoid tube currents and internal dewing.  I wrap the OTA in Reflectix so it takes almost no power to the keep dew out of the OTA and to provide a little bit of heat flowing out through the corrector.  Under ideal conditions, the temperature inside the OTA should be at the same temperature as the radiatively heated front surface of the corrector, which should be at the outside ambient air temperature—so there will be no heat flow.  It is especially important to be careful to avoid tube currents in a SCT because the light path may traverse the same volume of air twice.  Heat induced turbulence inside the OTA is just as undesirable as it is in front of the telescope, but again, keeping the heat to a minimum will not seriously degrade image quality.  Simply wrapping the OTA in Reflectix goes a long way to preventing the inside of the OTA from cooling in the first place so the amount of heating, should be VERY small.

 

John


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#250 Sarkikos

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Posted 10 July 2019 - 11:53 AM

Mike, that would be an improvement. The corrector would likely "see" a dew shield that was not so cold. John's insight on the matter was to provide a dew shield that is warmed so that it would radiate heat toward the corrector, helping to keep it dew free. Now the corrector sees a piece of dark, cold sky toward which the corrector radiates heat. But now it is also receiving heat radiated from the warm dew shield. The net effect is to keep it warmer.

 

The metal shield will receive heat from the dew heater and is able to conduct that heat over the entire surface of the metal. This provides the corrector with a much warmer sky to see - that sky being space plus the inside of the warm dew shield. Covering the metal dew shield with Reflectix reduces the amount of heat it would otherwise lose by its outside surface radiating to the cold sky it sees. A metal dew shield is preferred over plastic, since it better conducts heat (provided by the heater) throughout its mass. Being painted black inside contributes to better thermal radiation, compared to a light colored surface.

 

The corrector also receives some heat from the warmed air inside the tube. But glass doesn't conduct heat very well. So it does better if the heat is given it from the front, which is the surface more likely to dew up. In that case less heat is needed on the inside of the OTA, which helps reduce tube currents. Covering the metal OTA with Reflectix further reduces the amount of heat needed in that area.

 

Another advantage for front heat is that the light on its way to focus only passes through that region once. So heat has less deleterious effect compared to inside the tube, which light traverses 3 times.

Do the commercially-available metal dew shields have interior flocking, or are they at least painted flat black on the interior?  Leaving the interior a shiny or even light-colored surface might be better to stabilize thermals, but it is much worse for light baffling.  I wouldn't think of having a dew/light shield with a shiny interior.   And in my experience and opinion, good flocking is better than flat-black paint.

 

It's not a good thing to concentrate on one way to improve the image (only visual for me) while ignoring another.  Light baffling, dew prevention and thermal stabilization are all important.  In my book, they are at least equally important.  Actually, I would say that dew prevention is most important - depending of course on where you observe.  Then I would say that light baffling is more important then thermal stabilization.  Poor light baffling will degrade contrast at all levels of observing, while poor thermal control will mostly affect moderate to high magnification.

 

Mike


Edited by Sarkikos, 10 July 2019 - 12:06 PM.

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