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Insulation (airspace) layer for under Reflectix

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#26 Sarkikos

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Posted 14 December 2018 - 10:36 AM

Maybe because the corrector plate and secondary are exposed to the outside cold air.  The heat loss is then conducting through the glass and secondary.  Inside your tube you have the room temp air next to the cold surfaces ( your corrector plate glass and the mirrored secondary).   Because of the temperature difference, convection is taking place and right next to those surfaces you have air movement and a temperature gradient.......hence air density difference.......bending your light all over the place, until you reach equilibrium.   I suppose the temperature gradient is much smaller at the primary, so light is bent less through the lower air density difference.   The greater the temperature difference, the longer the mass takes to reach equilibrium.   

The corrector plate and secondary will always be exposed to the outside cold air.  You can't observe through Reflectix.  :grin:  But you can minimize the exposure by attaching a long dew shield to the front of the scope.

 

Mike



#27 Sarkikos

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Posted 14 December 2018 - 10:46 AM

One thing to consider is, the scope will come to equilibrium with ambient temps faster without the insulation. The insulation material slows down that process. So, for large temp differences like you have, you will probably be better off leaving the insulation off until you're ready to observe.

But I thought the rationale for using the Reflectix was to have the OTA retain its heat and not come to equilibrium with the ambient air at the observing site.  If we are going back to acclimating the scope, don't use the Reflectix at all.

 

Or is the idea to only partially acclimate the scope, to lessen the temperature difference between OTA and ambient air, then wrap with Reflectix to insulate the scope at that point to prevent further heat loss?

 

Mike


Edited by Sarkikos, 14 December 2018 - 10:50 AM.


#28 NMBob

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Posted 14 December 2018 - 11:02 AM

My preference is Velcro.

 

grin.gif

Mike

As long as you attach it with duct tape we won't kick you out of the club.

 

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Bob

 

Oh boy! Another insulation thread! :) I have fans. Never have a problem.


Edited by NMBob, 14 December 2018 - 11:04 AM.

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#29 Sarkikos

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Posted 14 December 2018 - 11:05 AM

You also live in New Mexico.  Could that make a difference?  Not about the duct tape.  About the insulation.  But then Precaud also lives there.

 

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Mike


Edited by Sarkikos, 14 December 2018 - 11:07 AM.


#30 choward94002

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Posted 14 December 2018 - 11:39 AM

Ah, another cooling/ thermals/ Reflectix thread ... yay!

 

Let's see if we can summarize things before this party gets started ... there are two different issues that people seem to fixate on: dew/ frost prevention and tube current thermals prevention.  Two totally different, apples and oranges kinds of things.  There are three kinds of heat transfer mechanisms that affect cooling: radiative cooling, convective cooling and conductive cooling.  Radiative cooling isn't getting heat FROM something, it's losing heat TO something (and that's a function of the emissivity of an object).  Conductive heating is losing/ gaining heat TO/FROM something via an intermediate substance (could be air, wool, foam, aluminum, glass, whatever).  Convective heating is heat being redistributed by an intermediate substance from an area of high heat to low heat (could be air, could be magma, could be water in your tea, whatever) ...

 

Radiative cooling is why your black car is colder in the morning than your white car sitting right next to it.  Conductive heating is why your hand gets cold picking up a cold can of soda.  Convective heating is why your pot of water is able to reach the same boiling temperature even though the heat is at the bottom ...

 

If you want to minimize tube current thermals then you need to get your entire OTA (tube, glass, interior air, rear cell, mirror, everything) at the exact same temperature as the outside ambient air ... if there is no difference then there's no conductive heating to cause internal convective heating to get started (tube currents).  If you kept your OTA inside of a meat locker then you'd be in tube current nirvana, never see a single one.  But it's hard to image inside a meat locker so you need to take it outside where there are natural forces trying to get your OTA at a different temperature than the ambient air, which will cause tube currents ... changes in the ambient air temperature over time as the night gets cooler and radiative cooling from the OTA and from the interior of the OTA through the corrector plate.  To offset these two cooling forces we do two things: coat the exterior of the OTA with a substance with a very low emissivity (like mylar, or aluminum foil, or something tougher like Reflectix) to cut down on the radiative cooling and run fans (tempest fans seem popular) to exchange the air inside the OTA with the changing outside air.  There is a third thing that's done very successfully by some fellows, and that's cooling the rear mirror cell itself (which is a large heat sink) down below the anticipated lowest temperature of the night so that the thermal mass/ mirror doesn't heat up the interior OTA air ... again, keep the interior air of the OTA at the exact same temperature as the ambient air no matter what forces are conspiring to change that ...

 

If you want to minimize dew/ frost formation you need to keep dew/ frost from forming in the first place so you need to keep the interior of the OTA air and the corrector plate glass at a temperature above the dew point of the ambient air (which is a function of relative humidity and temperature).  You can't control the humidity of the outside air (you can control that of the air inside the OTA) but you can control the temperature using dew heaters and dew shields ... keep the temperature high enough and you won't hit the dewpoint and no dew/ frost.  To keep the temperature high enough to avoid the dewpoint we can do two things: coat the exterior of the OTA with a substance with a very low emissivity (like mylar, or aluminum foil, or something tougher like Reflectix) to cut down on the radiative cooling, further insulate the OTA with something with a higher R value than Reflectix (like bubble wrap, or another coat of Reflectix) and use a heater on either/ both the corrector plate itself and on the tube ... again, keep the interior OTA temperature above the dewpoint temperature inside the OTA and keep the corrector plate temperature above the dewpoint of the ambient air ...

 

Note that these two (other than using aluminum foil/ mylar/ Reflectix on the outside of the OTA) are diametrically opposed to each other: if you are doing air exchanges to lower the tube currents then you're at the mercy of changes in dewpoint and dewing/ frosting over your scope/ internals [search for "frosty scope" and my current favorite, "why is my OTA filled with water" threads here].  If you are keeping your OTA warmer than ambient so you don't dew/ frost over your bits then you're going to get tube currents ... maybe not too much, but they *will* be there ...

 

So, years of threads neatly summarized, let the festivities begin! :)  Remember, play nice and don't get the thread locked!


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#31 NMBob

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Posted 14 December 2018 - 11:45 AM

You also live in New Mexico.  Could that make a difference?  Not about the duct tape.  About the insulation.  But then Precaud also lives there.

 

grin.gif

Mike

I saw that. It looks like he's in "upper" New Mexico. It gets a little wilder up there, but I'd agree with the team that says get the temps stabilized and keep them there. We'll go from the 90's/80's down into the 70's/60's here in Socorro in the summer. 50's/40's to below freezing in the fall/winter. We're at 4700', so it cools off kinda quickly once the sun goes down. 20 miles west it goes from cool to freezing AS the sun is going down. It's at 7000'. I just fire up the fans as I'm getting things put together and leave them on until the noise starts to bug me. If I see bad air I turn them on again for a little while, but that is rare. They seem to do a good job. The 800 and 1100 are in those semi-rigid Celestron cases, so kinda insulated. They usually go from my place or from my 'U stor it' storage place for a short truck ride in the bed to where ever I'm setting up. The ceiling in that storage place is really well insulated, so it doesn't get quite as hot or cold as outside. I had one night of really bad dew, once, so I bought dew heaters for everything. I can count on no hands how many times I've used any of them since. That could factor in to everything that you would want to do, but I think it would be simpler to just get the scope temp stabilized and then fight the dew in the usual manner.

 

Is duct tape the same across the Universe like we assume things like Planck's constant to be? Never thought about that.

 

Bob


Edited by NMBob, 14 December 2018 - 11:46 AM.

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#32 precaud

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Posted 14 December 2018 - 12:05 PM

But I thought the rationale for using the Reflectix was to have the OTA retain its heat and not come to equilibrium with the ambient air at the observing site.  If we are going back to acclimating the scope, don't use the Reflectix at all.

 

1. Your question suggests that it is possible for a scope to achieve temp stability on its own while immersed in a colder air mass, but its not.

2. Using phrases like "retains heat" is confusing, IMO. Insulation is not 100% capable of retaining heat, it only slows down the transfer between two bodies/masses.

 

Or is the idea to only partially acclimate the scope, to lessen the temperature difference between OTA and ambient air, then wrap with Reflectix to insulate the scope at that point to prevent further heat loss?

I think that is one way to look at it. But again, it's not "preventing heat loss",  that is impossible. Insulation is only capable of slowing it down. Bottom line, it really comes down to the Tdelta, the temperature differential, and the time within which the adjustment happens. If the scope is taken from 70F into 26F, that's a huge Tdelta. It will take time for the scope to "lose its heat" and come closer to ambient. The metal parts will do so more quickly than the glass ones. And insulation will only slow down the process. No reasonable amount of insulation is going to "insulate" the scope from the ambient temps. But for smaller Tdeltas, such as when temps drop while observing, insulation can help all parts of the scope cool down simultaneously, which will lessen thermals and (maybe) keep the glass figures closer to optimum.

 

It's just a matter of degrees (pun intended...)   wink.gif


Edited by precaud, 14 December 2018 - 12:11 PM.

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#33 ChrisGTS

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Posted 14 December 2018 - 12:12 PM

Just adding my experience here, I recently wrapped both my C5 and C8 in Reflectix.  I have had two observing sessions with those telescopes since then.  Both of them had similar weather -- the temperature was about 40 degrees F outside, and I took the telescopes straight out from my ~70 degree house to observe both times.  The first night (with my C5), everything seemed fine -- I didn't notice any tube currents and I had a good session.  The second night, I started off with the C8, and the tube currents were massive and totally obvious as such in out-of-focus star images.  So I took that scope in and replaced it with the C5, and had the same experience -- massive tube currents.  I am not sure why the experience was so different on the two nights, but I feel like it may not be viable to take the scopes out from my house in a temperature delta that large, and I may start putting them outside 1-2 hours before observing.  Perhaps in other seasons, when the outside temperature is not so different from indoor temperature, it will be viable to just take the scopes out and observe right away.  I'll do some more testing to see what I can observe.

 

Chris


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#34 jhayes_tucson

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Posted 14 December 2018 - 12:25 PM

I was under the impression that slowing the cooling was what I wanted to do. Slow and steady wins the race, as they say! But if someone who has experience with insulating their OTA can chime in and confirm that this is indeed the goal of insulation, that would help. And if 8m wrong, let me know. I also thought that insulation helped with frost issues. Again, correct me if I am wrong.

There has been a lot written on this stuff—and there is a lot of bad advice floating around.  Simply wrapping a scope in Reflectix will not eliminate thermal issues and it may make things worse.  I don’t know if you read the last (very long) thread on Reflectix but I suggest that you read (or re-read) post #71 here:

 

https://www.cloudyni...eflectix/page-3

 

The bottom line is that you want a thermally stable system—not an insulated system.  Reflectix can help but without fans, it’s not a sufficient solution.  As you’ve found, simply wrapping the system only helps to prolong a state of instability, which isn’t any good.

 

John


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#35 precaud

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Posted 14 December 2018 - 12:28 PM

Hi Bob,

 

I saw that. It looks like he's in "upper" New Mexico. It gets a little wilder up there, but I'd agree with the team that says get the temps stabilized and keep them there. We'll go from the 90's/80's down into the 70's/60's here in Socorro in the summer. 50's/40's to below freezing in the fall/winter. We're at 4700', so it cools off kinda quickly once the sun goes down.

 

I'm at 7000 ft, and day/night temp swings are probably comparable to what you see, mostly due to the low humidity. 30ºF swings are pretty normal; last night's low was 17, today's high is supposed to be 48. Most of the loss occurs from 1/2 hour before sundown to about 1am, at a rate of 4º to 6º per hour. Seeing seems to stabilize somewhere around 10pm, but temps keep dropping. The absolute low is typically just before sunrise, but after midnight the dropping really slows.

 

Regardless of which scope I'll use, If possible, I put it out as soon as the sun no longer can shine on it directly.


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#36 Paradoxdb3

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Posted 14 December 2018 - 12:41 PM

---

"It's simple, really: think about it. Insulation slows down what we call "heat transfer", which moves from hot to cold. Let's say your scope is stored in a 70ºF environment and taken out into a 26ºF space. And for now let's assume that outside temps don't drop further. The scope will take longer to reach 26ºF with the insulation on. So for large Tdelta, the insulation prolongs a large, necessary adjustment.



Now lets add in the fact that temps are still dropping while you observe, we'll say at 4ºF per hour. And we'll assume the scope starts out somewhere close to outside temp. The scope body and other metal parts will cool faster than the glass. In this case, it helps to insulate the body so that the entire mass of the scope can cool down in sync.



So insulation helps for the slower, more gradual adjustments, but not the big one..."

---

And this, right here, may be the strongest argument against insulating my OTA. I don't think redoing my work with a layer of felt between the Reflectix and the tube will do me any favours. In fact, insulation of any kind at all may not be the best course of action for me. It may be best for me to remove the insulation entirely.

If anyone else living in sub zero temperatures can confirm that this insulation has helped them, I'd be very curious to hear from you. Otherwise, it seems the idea is still not an open/shut case. Looking forward to hearing from others is colder climates (-20°C and colder)

#37 precaud

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Posted 14 December 2018 - 12:51 PM

I think you're looking for one absolute answer and there isn't one. In the simplest terms, there are two.

 

: Going from living space to winter outdoors, no insulation for the initial cooldown.

: Once its at or close to ambient, then insulation will help with the slower temperature drops.

 

So the insulation "jacket" needs to be removable.


Edited by precaud, 14 December 2018 - 12:52 PM.


#38 choward94002

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Posted 14 December 2018 - 01:07 PM

---

"It's simple, really: think about it. Insulation slows down what we call "heat transfer", which moves from hot to cold. Let's say your scope is stored in a 70ºF environment and taken out into a 26ºF space. And for now let's assume that outside temps don't drop further. The scope will take longer to reach 26ºF with the insulation on. So for large Tdelta, the insulation prolongs a large, necessary adjustment.



Now lets add in the fact that temps are still dropping while you observe, we'll say at 4ºF per hour. And we'll assume the scope starts out somewhere close to outside temp. The scope body and other metal parts will cool faster than the glass. In this case, it helps to insulate the body so that the entire mass of the scope can cool down in sync.



So insulation helps for the slower, more gradual adjustments, but not the big one..."

---

And this, right here, may be the strongest argument against insulating my OTA. I don't think redoing my work with a layer of felt between the Reflectix and the tube will do me any favours. In fact, insulation of any kind at all may not be the best course of action for me. It may be best for me to remove the insulation entirely.

If anyone else living in sub zero temperatures can confirm that this insulation has helped them, I'd be very curious to hear from you. Otherwise, it seems the idea is still not an open/shut case. Looking forward to hearing from others is colder climates (-20°C and colder)

You're neglecting that your OTA is going to continue to get colder than ambient due to radiative cooling ... a LOT colder, since it's essentially radiating to the black body temperature of the upper atmosphere (which I read was around -60C) ...

 

Reflectix (or my current choice of aluminum foil) doesn't have much insulation value ... that's not it's function.  It's function is to keep the OTA from getting COLDER due to radiative cooling than ambient air (and thus setting up convective currents inside the OTA) ... 



#39 Conaxian

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Posted 14 December 2018 - 02:07 PM

I peeled the original two wraps and re-insulated my small mak with just one layer of reflectix over the tube only, leaving the rear cell and the corrector cell uncovered so the AstroZ dew cap fits right. The temp difference here is from 70F to 20-25F many times.

I put the scope out in the un-attached garage in the afternoon.  This way it will slowly cool to within 10 degrees of outdoors. Once outside it will continue to cool, but slowly.  With a bare dew cap and an uncovered rear cell both ends are losing heat (not equally, but close) and the scope doesn't get shocked. Images were fairly sharp with no apparent tube currents, although I think it is less than perfect with a bit of SA. I've seen no dew or frost after two hours outside even though it's quite humid here. The insulated tube is also a lot more comfortable to handle when it's time to bring it in.

At that point I remove the diagonal, put on the lens cap and focuser cap, put it in its scope bag, and bring it inside.  Leaving it untouched for several hours it will have warmed up very gradually, I haven't seen any moisture.  The eyepieces and diagonal are also sealed in their now cold case and left to warm up slowly.  I opened the accessory case briefly (too soon) the first time and saw some haze forming so I quickly closed it. Lesson learned.

People with small scopes without a garage might consider clearing a space for it in the bottom of the refrigerator.  That should be almost as good as an unheated garage to get it ready.

So far so good, but I have only had two chances to test.



#40 Paradoxdb3

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Posted 14 December 2018 - 02:21 PM

My plan is to NOT redo my insulation job by adding a layer of felt between the tube and the Reflectix. In fact, my plan is to do nothing at all, and just leave everything as is for now. I'll still take my OTA out to the trunk of the car >1 hour before I plan to use it.

As for taking the Reflectix off to let the tube cool first, and then put it on after some cooling has taken place, this is not very conducive to my environment. Skin freezes in under a minute some days, so working with bulky mitts and gloves is the only other option, which is to say it's not an option! Haha.

I've also heard some people putting ice packs on their tube before taking them outside to help bring everything to equilibrium. The results were positive, from what I read.

I can't wait until we're out of winter and heading into spring. I got some of my best images starting in April and May.

#41 NMBob

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Posted 14 December 2018 - 02:28 PM

Hi Bob,

 

 

I'm at 7000 ft, and day/night temp swings are probably comparable to what you see, mostly due to the low humidity. 30ºF swings are pretty normal; last night's low was 17, today's high is supposed to be 48. Most of the loss occurs from 1/2 hour before sundown to about 1am, at a rate of 4º to 6º per hour. Seeing seems to stabilize somewhere around 10pm, but temps keep dropping. The absolute low is typically just before sunrise, but after midnight the dropping really slows.

 

Regardless of which scope I'll use, If possible, I put it out as soon as the sun no longer can shine on it directly.

Yup, pretty much the same, usually just 3-5 degrees warmer "down" here.


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#42 Peter Besenbruch

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Posted 14 December 2018 - 02:45 PM

I was under the impression that slowing the cooling was what I wanted to do. Slow and steady wins the race, as they say! But if someone who has experience with insulating their OTA can chime in and confirm that this is indeed the goal of insulation, that would help. And if 8m wrong, let me know. I also thought that insulation helped with frost issues. Again, correct me if I am wrong.

Yes, that's precisely what you want to do: Slow the cooling until you stop getting internal thermals. Racing rivers and flowing streams do not look like tube thermals. They sound like seeing. As someone else mentioned, check a star higher in the sky, and make sure it's a star.


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#43 WadeH237

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Posted 14 December 2018 - 03:02 PM

At the risk of looking foolish, I'm going to jump in here...

 

I think that part of the reason that these discussions keep going round and round is that the telescope is a complex system.  There is another recent thread where folks are trying to model it accurately, and it's not a trivial task.

 

That said, I think that there are two extremes that are easy to understand:

 

1) If all parts of the optical tube are at the exact same temperature as the ambient air, then there is no air movement inside the telescope (which is the direct thing that harms the image).  This is a stable situation and will result in the best views that the telescope can deliver.

 

2) If there are large differences in the temperatures of internal parts of the telescope (ie. if the mirror is significantly warmer than other parts inside the tube - or even the inside walls of the tube itself), then there can be significant convection inside the tube, which harms the image.  This is bad.

 

But I think that there is some middle ground here.  First off, I think that convection currents inside the baffle tube are (much) worse than a convection that's along the inside wall of the tube.  A convection between the corrector plate and the primary mirror outside of the baffle tube is probably in between the first two convection scenarios that I described.

 

And this is where I think that insulation may help - or harm - the use of the telescope.  If the insulation is sufficient to slow the cooling of the tube, such that it minimizes convection in the baffle tube, then it can help the views early in the evening - even if it does nothing to prevent convection between the corrector plate and other parts inside.  Of course, this delays the opportunity for the scope to reach a stable temperature equilibrium with the outside air.  As such, it may prevent the telescope from ever reaching its full potential.  And if the insulation is insufficient to prevent convection from happening inside the baffle tube, then it may keep the telescope in its worst state for a very long time.  This would obviously be bad.

 

I think that this may explain some of the different results that people are getting.  As for me, I've had good luck with insulating my C14 for early evening viewing, where it's obviously better than no insulation.  And I readily admit that it may prevent the scope from reaching peak performance later in the night.  But I've not see huge temperature extremes.  In the case we're talking about here, the temperature difference may be enough that bad convections form even with insulation.  If that's the case, there is no upside - and considerable downside - to insulating the scope.

 

Anyway, that's just my musings on this...


Edited by WadeH237, 14 December 2018 - 03:03 PM.


#44 Paradoxdb3

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Posted 14 December 2018 - 06:20 PM

I wonder if there is still an upside, even in the extreme conditions. Do you think that an insulated tube, along with an insulated dew shield, will help keep the meniscus frost free for longer? Or am I just farting out nonsense here?

#45 jhayes_tucson

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Posted 14 December 2018 - 07:17 PM

I wonder if there is still an upside, even in the extreme conditions. Do you think that an insulated tube, along with an insulated dew shield, will help keep the meniscus frost free for longer? Or am I just farting out nonsense here?

 

 

The single most effective thing you can do to prevent dew or frost is to use a dew shield.  Without a dew shield, the danger of dew or frost starts when the relative humidity goes above about 65%.  A dew shield makes dew or frost unlikely up through about 85%-90% (depending on the temperature and the size of the shield.)  Even a dew shield won't help when the air is still and the RH goes above ~95% (which qualifies as "extreme conditions".)  Adding heat to the dew shield can keep you safe all the way up to RH=100% but the amount of heat that you have to add depends on the size of the shield.  If your shield length is 1.5x the diameter of the scope, raising the temp of the shield by 5C-9C above ambient will prevent dew and frost.  When I say "heat the shield", I'm talking about a metal dew shield wrapped in heater strips (2 or 3).  Floppy plastic shields with the heater over the front corrector don't count.  Frost forms at a slightly higher temperature than dew by less than 1C for most common conditions so add a little extra protection when it's below freezing.  It is very important to keep the temperature of an unheated dew shield close to (or above) the ambient air temperature.  Wrapping the dew shield in Reflectix or painting it bright white is an effective way to help reduce radiative cooling.  Using heater strips is even better.  

 

Insulating the OTA tube with something reflective can help to prevent internal dewing and/or frost but it should be combined with fans to A) circulate air from the outside and B) to break up convective currents that will interfere with image quality.  It does not take very much air circulation to be effective.  Simply wrapping the OTA in Reflectix will do nothing to prevent dew or frost on the front optical surface.

 

John


Edited by jhayes_tucson, 14 December 2018 - 07:17 PM.

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#46 Sarkikos

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Posted 14 December 2018 - 07:46 PM

 When I say "heat the shield", I'm talking about a metal dew shield wrapped in heater strips (2 or 3).  Floppy plastic shields with the heater over the front corrector don't count.  Frost forms at a slightly higher temperature than dew by less than 1C for most common conditions so add a little extra protection when it's below freezing.  It is very important to keep the temperature of an unheated dew shield close to (or above) the ambient air temperature.  Wrapping the dew shield in Reflectix or painting it bright white is an effective way to help reduce radiative cooling.  Using heater strips is even better.  

In my experience, a dew shield of plastic* with one warming strip wrapped around the base (or built-in) will prevent dew on the corrector of a Cat or the objective of a refractor.  I observe at very dewy sites.  On a really dewy/frosty evening, the corrector or objective would dew over within an hour without the heated dew shield.  With the heated dew shield, I can observe all night.  These are black plastic* dew shields, not white, and not covered with Reflectix.

 

So in my experience, black plastic* dew shields do count.  I've been going to the same very dewy/frosty dark site for the past 11 years.

 

*  "High quality ABS-grade polymers" according to the AstroZap website.

 

Mike


Edited by Sarkikos, 14 December 2018 - 07:50 PM.


#47 jhayes_tucson

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Posted 14 December 2018 - 08:21 PM

In my experience, a dew shield of plastic* with one warming strip wrapped around the base (or built-in) will prevent dew on the corrector of a Cat or the objective of a refractor.  I observe at very dewy sites.  On a really dewy/frosty evening, the corrector or objective would dew over within an hour without the heated dew shield.  With the heated dew shield, I can observe all night.  These are black plastic* dew shields, not white, and not covered with Reflectix.

 

So in my experience, black plastic* dew shields do count.  I've been going to the same very dewy/frosty dark site for the past 11 years.

 

*  "High quality ABS-grade polymers" according to the AstroZap website.

 

Mike

Yes, black, plastic dew shields work; but, they don't work as well as white ones and they don't count for what I was talking about.  The folks at AstroZap have sold a lot of those things (I even have one.)  Unfortunately, applying heat on top of the corrector plate is absolutely the worst place to apply heat.  It's a good place to generate air currents and it feels good; but, it doesn't put heat where it's most needed.  Heating the dew shield itself is the best and heating the OTA behind the corrector (only for a SCT -- not a Mak or worse, a refractor) is second best.  Heat conduction at the edge of the corrector is very inefficient and won't provide much protection near the center.  When I used the AstroZap shield, it was common to find the center of my corrector covered in dew.  Most of the protection with that strategy comes from the shield itself.

 

John



#48 Cfreerksen

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Posted 14 December 2018 - 08:22 PM

In my experience, a dew shield of plastic* with one warming strip wrapped around the base (or built-in) will prevent dew on the corrector of a Cat or the objective of a refractor.  I observe at very dewy sites.  On a really dewy/frosty evening, the corrector or objective would dew over within an hour without the heated dew shield.  With the heated dew shield, I can observe all night.  These are black plastic* dew shields, not white, and not covered with Reflectix.

 

So in my experience, black plastic* dew shields do count.  I've been going to the same very dewy/frosty dark site for the past 11 years.

 

*  "High quality ABS-grade polymers" according to the AstroZap website.

 

Mike

This has been my experience too. I use the cheap black plastic shield that I got with the scope. The one from Celestron that fits 6 and 8. I have imaged well below freezing when everything around me has a good frost. On the outside of the dew shield frost. On the inside a few crystals, but nothing on the corrector plate. I was imagining near zenith and had no dewheaters. So in my case cheap plastic dew shields do work. Maybe not as well as others but better than nothing.

 

Chris


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#49 Kokatha man

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Posted 14 December 2018 - 08:25 PM

<"I think that part of the reason that these discussions keep going round and round is that the telescope is a complex system.  There is another recent thread where folks are trying to model it accurately, and it's not a trivial task.">

 

In 11,000+ posts I'd have to say that 95%+ of them are in the S.S. Imaging forum...I've dropped in here a few times because we use a C14 in our imaging - & before that a C11...so I guess I can post occasionally in "Cats & Casses."

 

And I have to confess that these types of threads, particularly this & a couple of other current similar ones have piqued my amazement with some of the claims - & likely doubled my post count on this forum in the last week! lol.gif

 

What amazes me is the degree to which anecdotal evidence rules over empirical evidence on topics like this! bigshock.gif

 

I'm sorry if this is akin to dropping a bomb in these topics & it is definitely NOT suggesting that anyone is fibbing, deluded or otherwise encumbered when they avow the benefits of insulation: anecdotal evidence has its place, but should always be tested rigorously, subjected to vigorous examination &/or scientific rebuttal just as "anecdotal" is in the Law in democratic societies that I'm familiar with...

 

With all due respect SCT telescopes aren't uber complex systems & are considerably more robust than many might imagine! (especially when it comes to the shock-resistance of the glass components!) 

 

Anyone who has set a hair-dryer to "Max" & played it from a few inches onto a corrector - which might be at -5°C...consistently gathers some idea of that particular property...

 

You cannot "model" something without monitoring a series of temperatures throughout the system under varying conditions/parameters, nor can you "predict" or "determine" the most effective means of operating said telescope optimally by simply stating (or speculating, I've noticed) upon various modes of heat transfer or retention - no matter how many formulae you might reference as possible factors.

 

And of course the "hard evidence" or is that "smoking gun"..?!? lol.gif

 

I won't rehash what I've said on this subject recently, but I will include another quote from Ron in one of his posts in that long "Reflectix" thread:

 

<"...unbelievable 'myth' I've seen posted in this SCT forum is that some think those great planetary images you and others get, are essentially faked by photoshop type processing adding details that are not really there in the raw images!"> 

 

I didn't respond to those comments at the time, which Ron himself described as "unbelievable" but I did muse upon the following, which I think should really dispense with such ridiculous thoughts & further reinforce the fact that it is strong "evidential" outcomes that confirm our approach, simultaneously savaging the alleged benefits of insulative methods...dewing situations notwithstanding, which I have comprehensively addressed earlier. (Mike's reply as I type about a "couple of hours or less" being much greater timespans than that which we deal with without any real concerns...)

 

Back in March 2013 we captured a single consistent r-g-b sequence of Saturn & I processed said capture, pleased with the amount of disk banding & overall detail in an image so early in that apparition.

 

It was only when a fellow CN'er observed that the North Polar region appeared to be hexagonal in outline that the discussion really picked up...incredulous comments with ayes & nays followed: this aspect of Saturn's N.P. had indeed been imaged (very poorly) by the Pic du Midi 1M. scope back in the 1980's but not from Earth since...our subsequent use of WinJupos to create a NP map finally confirming the fact & dispelling the doubts of those that thought such a feat was impossible - & now considered a relatively "easy" feat! lol.gif

 

Similarly in June last year when we imaged an Equatorial storm feature on Neptune several weeks before the Keck team picked it up, a fact acknowledged by them in the recently released Icarus paper by them...we did not appreciate the amazing aspect of its existence in such savage (1500km+) winds at the time - but we did confirm it was not an anomaly by capturing it several times with different filters on that night.

 

We're not prescient...nor do we "fabricate" our planetary images on the bizarre possibility that every now & then we might strike it lucky & "invent" details that might turn out to be real...these & many, many other images are examples by us & others of the clear, unambiguous evidence that making sure your instrument's primary temperature in particular is operating at, or as close to, the current outside air/ambient temperature is the best way to ensure optimum optical performance...collimation, seeing etc pertinent naturally! smile.gif 


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#50 Jaimo!

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Posted 14 December 2018 - 09:13 PM

popcorn.gif


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