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Show me your insulation projects!

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#1 rustynpp

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Posted 27 June 2019 - 07:47 AM

Hi all,

 

I have a new-to-me EdgeHD 8 OTA on the way, and one of the first things I plan to do is make an insulation jacket to mitigate the effects of thermal currents. I have a plan to do so, and I wanted to run it by you fine folks to get your feedback. I'd also like to see how others have approached this project before I start buying, cutting, and gluing.

 

I plan to wrap just the aluminum portion of the OTA in two layers of Reflectix (with glue in between), then add a layer of adhesive felt to the outside (not a fan of the look of bare Reflectix on a scope). I'll need to cut out a portion of the jacket so that I can access the dew heater (the dew heater should go under the jacket, correct?). I'll spray some hydrophobic solution on the felt to repel moisture and all the bad things that come with it. I'll then run some black all-weather duct tape around the perimeter to stop it from fraying/peeling before finally wrapping it around the tube, closing the seam with duct tape and hiding it under the dovetail.

 

The dew shield will be similarly constructed, except it'll have just 1 layer of Reflectix (is a double layer necessary?) and two layers of felt - one inside and one outside. I don't plan on using the hydrophobic spray on the inside of the dew shield, since I'd rather any dew that forms get absorbed into the felt rather than run down into the corrector. I'll add a 2" strip of velcro to close it up.

 

Two questions:

  1. Right now I do not plan to insulate the mirror cell, just the aluminum OTA and corrector plate. Is this a bad idea? I'll admit that my reasoning is mostly superficial, but I think I'd have a hard time wrapping the cell in such a way that it'll be effective, won't look like a mess, and won't be in the way when using the scope. It also seems to me that insulating the mirror cell is less important than insulating the aluminum OTA, but I could be wrong.
  2. Being an Edge, this scope will have vents that will actively work against my goal of insulating the tube. How should I address this? I could just cut a couple chunks of Reflectix to stick over the vents, but that's ugly and probably not super effective. My thought, having not yet handled the OTA in person, is to remove the vent cover and fit a small piece (or pieces) of Reflectix into the opening such that I can put the cover back on. Is this feasible? Hopefully it's possible to do it in such a way that minimizes the chances of the Reflectix falling into the tube.

Btw, the felt I plan to use is linked here. It's quite inexpensive and not made for outdoor use, but I couldn't find a better product. Hopefully the hydrophobic spray will help protect it. https://smile.amazon...lv_ov_lig_dp_it

 

Let me know what you think, and show off your projects so I can get a sense of how others have solved this problem!

 

Thanks all!

Nick


Edited by rustynpp, 27 June 2019 - 08:32 AM.


#2 jhayes_tucson

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Posted 27 June 2019 - 10:00 AM

The best optical performance happens when the telescope is in thermal equilibrium with the environment.  Without fans, all that insulation is just going reduce heat exchange through the tube so it will take a long time to stabilize.  While that happens, most of the heat flow will be by convection through the corrector plate, which will cause thermals.  In my view, the only valid reason to use Reflectix on the OTA is to reduce thermal gradients due to radiative cooling.  It is essential to use fans with the Reflectix to A) prevent tube currents and B) allow forced convective cooling of the OTA components to speed thermal stability.  It doesn’t take much air flow and Tempest fans work quite well for this purpose.  Using multiple layers of Reflectix does not provide any additional benefit.  The only other advantage of using Reflectix on the tube is to better insulate an anti-dew heater strap so that you can use a lower power settting.  Heating the dew shield is generally a better way to prevent dew than heating the tube, but both will work...and this is where violent disagreement kicks in so we’ll leave it at that.  In either case, the goal is to minimize heat flow.  All you have to do is to maintain the temperature of the front surface of the corrector at the ambient air temperature to prevent dew formation.

 

Bottom line:  My advice is to:  A) Wrap the tube in a single layer of Reflectix, B) Install tempest fans, and C) give the system plenty of time to stabilize before use.  Remember, the whole idea is to get the telescope and all of it’s components at the same temperature as its surrounding.

 

John


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#3 yellobeard

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Posted 27 June 2019 - 12:53 PM

However, with proper insulation, the cooldown of the inside of the scope is slowed down drasticly.
And where a lot of people find this against nature, some have found the advantages!

Very slow temperature change inside your scope means at least three important things:

-You immediately can start your observations! No cooling time needed anymore!

-The optics inside can now 'follow' the slow cooldown, without any deformations caused by temperature differences.

-The air inside quickly finds its equilibrium, so no aircurrents.

This only applies to closed optical systems, like SCT, refractor and Mak (and schmidt-newton, wright-newton etc)

Think of the scope as your home: When your home is not properly insulated, you feel drafts., and drafts is turbulence!
But in a properly insulated house there are no drafts.
Being way smaller than your house (i hope..) a C8 is much easier to insulate properly.
If you want to use fans, only use them to circulate the inside air, NOT take colder air inside that scope, its the same as open your window in a well insulated house, and let in the cold air!

Many people here already tried, and had their own eye openers, look for the threads about it.
As it is an quite new insight, there still are many non-believers, most of them reluctant to even try, or did not insulate in a proper way (everything needs to be covered by insulation, exept eye piece and corrector plate!)
Insulation material is cheap, so definitely worth a try don't you think??

Still a dew shield is prefrable, but the corrector plate will stay clean for significantly longer, because of the warmer inside.

It was Texereau(?) I believe, who already very early advised to close an RC with a parallel window, which already gave way better results because of the air inside being less turbulent..


.

Edited by yellobeard, 27 June 2019 - 01:03 PM.


#4 rustynpp

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Posted 27 June 2019 - 01:47 PM

John and yellobeard, thanks for your responses.

 

John - My understanding is that the conventional wisdom of the last several decades has been supplanted by the realization described by yellobeard. It was on this basis (and the advances of the Edge scopes) that I decided to dip my toes back into the SCT world, after selling my Nexstar 8i from about 15 years ago. It seems to have taken CN by storm, with many users reporting excellent results by insulating their OTAs instead of attempting to acclimate them.

 

yellobeard - Do you have any thoughts about the extent to which leaving the mirror cell un-insulated will be detrimental to avoiding thermal currents? As you state, ideally everything would be covered except the EP and the corrector, but I wonder if the heat loss from behind the mirror is really significant. Any opinions?

 

Thanks both!



#5 Astrojedi

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Posted 27 June 2019 - 03:30 PM

Here is my EdgeHD 8 with a single layer of reflectix only on the tube (not the back edge). Setup for an outreach event in my backyard.

 

I recently tired this experiment. What I found was that the OTA is usable almost immediately in Spring/Summer temp (haven't tried Winter yet). It was quite amazing to take out the scope and be able to observe Jupiter immediately.

 

But... the image quality was not quite as good (very close though) as when the scope is fully thermally stable but this only matters for Jupiter and Mars where some low contrast detail was being hurt. This could also be because I did not insulate the back. For Lunar and deep sky it was a stunning improvement. No more bloated, shimmering stars

 

IMG_7715.jpg


Edited by Astrojedi, 27 June 2019 - 03:32 PM.

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#6 Mike Mc

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Posted 27 June 2019 - 03:51 PM

I have a reflectix blanket for my Evo 8HD Edge that looks just like Astrojedi's. I did glue some black felt onto the scope side of my blanket for added insulation, but I'm not sure how much it helps. My experience using it is similar to that described by him as well. I find that on cool nights it reduces internal currents, but observing planets is still problematic for the first hour or so at least. But it eliminates dewing for me at my observing locations in North Texas, even when the car and eyepiece case are dripping wet. 

 

My  thoughts on what's going on: I agree completely with both Yellobeard and John Hayes. If a scope could be well-insulated, then heat flux / thermal currents could be significantly reduced. However, reflectix is not an insulator against convective heat transfer ( note it's low resistance rating). But it is an effective radiant barrier. So it doesn't stop my scope from cooling down due to convection, although it slows cooling somewhat. However, it keeps the scope from cooling below ambient temperature by reducing radiant heat loss, thus avoiding dew formation. In a more aggressive dew environment I could see needing dew heaters under the reflectix to add heat back in to keep the scope temp above the dew point. But so far I haven't needed dew strips.

 

My primary objective was to eliminate dew formation, and it seems I have succeeded for my location. However along the lines of John's thinking, I am thinking seriously of adding TEMPEST fans to speed up cool down to support quicker access to high magnification views in cool weather, but I haven't yet done so.

 

Hope this helps. 

 

Clear skies. 


Edited by Mike Mc, 27 June 2019 - 04:19 PM.

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#7 rustynpp

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Posted 27 June 2019 - 04:15 PM

Astrojedi and Mike - what do you guys do about the vents in the OTA? Do you plug them up somehow, or just leave them alone?



#8 Astrojedi

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Posted 27 June 2019 - 04:22 PM

Currently I leave the vents open just by virtue of how I have applied the reflectix. Unclear if it is hurting or helping. My initial goal was to minimize surface area of heat exchange to control thermals but still allow the tube to slowly cool down.



#9 Mike Mc

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Posted 27 June 2019 - 04:23 PM

I'm basically with John on this: let it cool down, assist with fans even. But cover with reflectix to avoid further cooling below ambient temp that is caused by radiant heat loss. Following that line of thinking, I've left mine open, but I might put Tempest fans on at some point.


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#10 rustynpp

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Posted 27 June 2019 - 04:59 PM

Interesting. I was under the impression that if you go the insulation route, it's best to go all the way and block as much heat exchange as possible. I figured you either try to acclimate it or you insulate it, but anything in between doesn't really make sense to me. The point of insulating is to reduce the heat transfer to a trickle that you can barely see, as opposed to a waterfall that stirs up the air a whole bunch.

 

I'm basically with John on this: let it cool down, assist with fans even. But cover with reflectix to avoid further cooling below ambient temp that is caused by radiant heat loss. Following that line of thinking, I've left mine open, but I might put Tempest fans on at some point.

Is this to prevent dew formation or to prevent thermals? I can see how this would help with dew control, but I wouldn't imagine that cooling below ambient temp would induce thermal currents that spoil the view.



#11 yellobeard

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Posted 27 June 2019 - 05:33 PM

Interesting. I was under the impression that if you go the insulation route, it's best to go all the way and block as much heat exchange as possible. I figured you either try to acclimate it or you insulate it, but anything in between doesn't really make sense to me. The point of insulating is to reduce the heat transfer to a trickle that you can barely see, as opposed to a waterfall that stirs up the air a whole bunch.
.

That is correct!

Insulate properly, or not at all! Parts that are not insulated, still lose heat too fast, which causes turbulence inside the tube.
That is why some of you indeed see a much improved image quality, but just not as sharp as it can be with the scope fully acclimated.

I use a combination of reflectix on the outside, as a radiation buffer, with Armaflex inside the tube.
Armaflex is a professional, closed cell structure , very light weight black insulation sheet, which is avaible in thickness from 9 to at least 30mm.
My 16" SCT has 15mm thickness Armaflex inside the tube, with sometimes reflectix on the outside
Works great!

But the challenge is in the fact that, apart from not much room for insulation inside the commercial tube's, many don't want to open their telescopes to apply the insulation inside.


.

Edited by yellobeard, 27 June 2019 - 05:38 PM.


#12 Bean614

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Posted 27 June 2019 - 06:16 PM

A few shots....

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#13 Bean614

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Posted 27 June 2019 - 06:33 PM

One more...

 

A note about Dew and Reflectix, since I'm in Western Mass., about 120 miles, as the crow flies, from the OP.  I was out this past Saturday night, as it was clear, but with rapidly falling temps. (84*F at 6pm down to 42*F the following morning).  I attached my Talent Cell Battery, with the Dew Controller attached to it, it the arm of my Evo-8 Mount.  At 9pm I took the Scope Out, WITH the Dew Shield already attached, with a plastic bag over the opening to prevent dust and whatever from getting in.  Attached the OTA to the mount, turned on the heaters, removed the plastic bag, and aligned the scope---- all in 5 minutes from walking out the door from an inside temp of 68*F.  Did the 'usual' for the date, with a Moon about to rise in a few hours---Jupiter, some nice PN's, Saturn, etc.  Dew was Heavy, and by 11pm the Reflectix, Mount, Tripod, EP Cases, etc., were soaking wet.  By 1am, I had to get some sleep, and before re-covering the OTA/Heated Dew Shield Combo, I looked at the Corrector, first with a red light, then an LED light (I was going to tear down anyway!).  ZERO Dew!  

    It was then, while disconnecting and shutting off the Dew Controller and Talent Cell, that I realized that I FORGOT to plug the Cable from the Dew Shield INTO the controller!  4 Hours in a dew filled night, with flawless views from the get-go, without anything but Reflectix and an unheated Dew Shield!

 

  Now, you'll read arguments hear from 'doubters', and read a lot of 'scientific' explanations for why this Does, or doesn't work.  Well, as Jon Issacs has stated, obviously it DOES work, so more time should be spent, by those who are capable of such calculations, figuring out WHY it works, instead of bashing it.

Good luck to you!

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#14 yellobeard

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Posted 27 June 2019 - 11:00 PM

When people say "it is best to make the optics and air inside have the same temperature as outside your scope" They are absolutely right! It gives the best image!

But, acheving it, especially with bigger scopes, is another story, it needs time and effort, and even then, when you are plagued with continous dropping ambient temperatures, your scope is always too slow to follow.

And, as Bean already wrote, insulation does have some very important advantages!

#15 Mike Mc

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Posted 28 June 2019 - 12:11 PM

Interesting. I was under the impression that if you go the insulation route, it's best to go all the way and block as much heat exchange as possible. I figured you either try to acclimate it or you insulate it, but anything in between doesn't really make sense to me. The point of insulating is to reduce the heat transfer to a trickle that you can barely see, as opposed to a waterfall that stirs up the air a whole bunch.

 

Is this to prevent dew formation or to prevent thermals? I can see how this would help with dew control, but I wouldn't imagine that cooling below ambient temp would induce thermal currents that spoil the view.

My take is that reflectix is primarily to avoid dew formation caused by the scope surface temp being driven below the ambient temp by radiation. I agree with you that to go insulation route it's best to "go all the way" along the lines of Yellobeard's approach. I think that reflectix insulates to some limited degree, but I consider it as primarily a radiant barrier to suppress dew formation. For controlling thermals, I won't go his route because my scope is only 8" and I don't have the nerve to disassemble my scope to internally insulate it. So I'm left with living with the thermals, or installing fans to speed up internal cooling.



#16 Astrojedi

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Posted 28 June 2019 - 12:32 PM

When people say "it is best to make the optics and air inside have the same temperature as outside your scope" They are absolutely right! It gives the best image!

But, acheving it, especially with bigger scopes, is another story, it needs time and effort, and even then, when you are plagued with continous dropping ambient temperatures, your scope is always too slow to follow.

And, as Bean already wrote, insulation does have some very important advantages!

This is how I see it as well.

 

Nothing is perfect. But reflectix is just so simple and cheap to try (and risk free - no OTA modifications required) that if you have a SCT you should give it a try and find out first hand whether you like it rather than debating it out here.

 

The cost benefit ratio is really high. Worst case you will be out $10. Best case it will completely change the way you use your SCT and deliver views you never thought possible. 


Edited by Astrojedi, 28 June 2019 - 12:32 PM.

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#17 jhayes_tucson

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Posted 28 June 2019 - 12:42 PM

However, with proper insulation, the cooldown of the inside of the scope is slowed down drasticly.
And where a lot of people find this against nature, some have found the advantages!

Very slow temperature change inside your scope means at least three important things:

-You immediately can start your observations! No cooling time needed anymore!

-The optics inside can now 'follow' the slow cooldown, without any deformations caused by temperature differences.

-The air inside quickly finds its equilibrium, so no aircurrents.

This only applies to closed optical systems, like SCT, refractor and Mak (and schmidt-newton, wright-newton etc)

Think of the scope as your home: When your home is not properly insulated, you feel drafts., and drafts is turbulence!
But in a properly insulated house there are no drafts.
Being way smaller than your house (i hope..) a C8 is much easier to insulate properly.
If you want to use fans, only use them to circulate the inside air, NOT take colder air inside that scope, its the same as open your window in a well insulated house, and let in the cold air!

Many people here already tried, and had their own eye openers, look for the threads about it.
As it is an quite new insight, there still are many non-believers, most of them reluctant to even try, or did not insulate in a proper way (everything needs to be covered by insulation, exept eye piece and corrector plate!)
Insulation material is cheap, so definitely worth a try don't you think??

Still a dew shield is prefrable, but the corrector plate will stay clean for significantly longer, because of the warmer inside.

It was Texereau(?) I believe, who already very early advised to close an RC with a parallel window, which already gave way better results because of the air inside being less turbulent..


.

 

We are very close to being in violent agreement but I want to clarify a couple of things.  First, your comparison of a scope with an insulated house is pretty good except that you haven't mentioned that single pane windows are a huge source of heat flow in any home--and the same goes for a telescope.  It is easy to show that the thin corrector plate on a C14 conducts 14.8 Watts of heat for only a one degree temperature difference between the inside and the outside surfaces of the corrector.  That is a significant amount of heat lost through a single pane of glass--and it's why high quality windows are made to be double or triple pane.  What that means for a telescope is that if the tube is highly insulated, most of the heat transfer that occurs while the scope cools will be through the front element.  That may not necessarily be a bad thing as long as you don't care how long it takes to get a scope cooled down and you aren't actively heating the inside of the scope.  Once you start adding heat to the inside of the scope, a couple of bad things can happen.  First, without active mixing, you may generate thermal gradients, which can degrade the imaging performance of the telescope.  Second, with a highly insulated, actively heated scope, you have to be very careful not to create a "hot tube" with the inside of the tube significantly warmer than the surroundings.  A hot tube will be really dew resistant but it will also shed a lot of heat through the corrector, which will create turbulent air flow that will seriously degrade image quality.

 

A single layer of Reflectix is very effective at reducing differential radiative cooling over the OTA tube, which can cause internal currents and thermal gradients.  In addition, having the top of the tube at a colder temperature than the bottom of the tube helps with the formation of laminar baffle plumes.  The goal should not be to reduce convective heat flow from the tube to zero.  The goal is to reduce differential cooling.  My telescope sits at a remote observatory so I never have "thermal shock" problems."  It is wrapped in Reflectix simply to reduce differential radiative cooling.

 

I've done a lot of experimenting with Tempest fans and convinced myself that they are extremely effective at eliminating laminar convective flow such as baffle plumes.  I can turn off the fans, defocus a star image and watch baffle plumes develop and then turn off the fans and watch them instantly disappear.  I've also seen them instantly eliminate double imaging due to a thermal tube gradient.  Of course a big concern is their effect on imaging performance, but I've never been able to measure any problems.  When the atmosphere cooperates, I've been able to achieve 1" FWHM stars using 20 minute exposures with the fans running.  That's not to say that I'm 100% convinced that their effect on imaging performance is zero, but it's close--at least for long exposure imaging.  Their single biggest advantage is that they gently circulate exterior air to more quickly equalize the temperatures of all of the internal OTA components with the outside air temperature.  In my view wrapping a SCT in Reflectix without using fans is only half of a solution--particularly for a larger scope.

 

 

John


Edited by jhayes_tucson, 28 June 2019 - 12:45 PM.


#18 jhayes_tucson

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Posted 28 June 2019 - 12:49 PM

My take is that reflectix is primarily to avoid dew formation caused by the scope surface temp being driven below the ambient temp by radiation. I agree with you that to go insulation route it's best to "go all the way" along the lines of Yellobeard's approach. I think that reflectix insulates to some limited degree, but I consider it as primarily a radiant barrier to suppress dew formation. For controlling thermals, I won't go his route because my scope is only 8" and I don't have the nerve to disassemble my scope to internally insulate it. So I'm left with living with the thermals, or installing fans to speed up internal cooling.

 

Mike,

If you are using Reflectix to prevent dew formation, how do you put Reflectix on the corrector plate where you really don't want dew to form?  It seems like that might limit the amount of light you can get through the system.  tongue2.gif

 

John



#19 Mike Mc

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Posted 28 June 2019 - 06:05 PM

Mike,

If you are using Reflectix to prevent dew formation, how do you put Reflectix on the corrector plate where you really don't want dew to form?  It seems like that might limit the amount of light you can get through the system.  tongue2.gif

 

John

Yup.



#20 yellobeard

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Posted 29 June 2019 - 12:39 AM

your comparison of a scope with an insulated house is pretty good except that you haven't mentioned that single pane windows are a huge source of heat flow in any home

John

Hi John,
Of course, you are right.. The strange part is, that somehow, in all my scopes, no matter what the temperature difference with the outside air, the still somewhat insulating capacity of the glass of the schmidt corrector, seems to be enough to not give any problems.
And there are more people here that have the same experience..
Even with the best seeing I ever experienced, I never saw any turbulence in front of the corrector plate of my 16" SCT, when doing a star test, or a ronchi test in the field.

And you also forgot to mention something:
The single pane windows you mentioned, still have a free heat exchange with the sky, and with an insulated scope, I still use an ergonomically as big as possible dew shield to as much as possible limit that exchange..

Where you see a problem when you calculate the heat transfer through the glass, I put it to the test in the field, and still immediately, somehow got razor sharp images, when seeing permitted of course.

In my country, there are a few amateur astronomers, who are very determined to make Maks and SCT's perform at their best in any environment, and they also found that, with closed optical systems, proper insulation is the best way, not because they calculated it, but because they tested it in the field for years.

They even completely grabbed out all optics from scopes that gave top test bench performance, and put them in an ugly looking, wooden tube, with astonishing field results!

Calculating things is one, but a few people don't just want to thrust on it, and will test it in the field.

Big example: In my country, there is a new insight about very thin curved (meniscus) mirrors, using the curvature to make them stiffer. They are even made out of that very high expansion type, greenish window float glass, something we would not ever consider some 20 years ago!
And with a plop calculated support, an F/3 60 cm (24") mirror, only 17 mm (0.67") thin, was thought to be able of giving quite good results.
Then came the people with their calculators and computers:"No way you ever can make it work that way, AS WEAK AS A TOWEL!!" Was their comment.

But guyzz like Rik ter Horst are not very suseptible to those remarks, which is a good thing, because now, Rik has that great 24" dobsonian, with a very light weight, only 0.67" thin mirror, working splendidly!
And, where he thought he would need a 54 points support, he found out that an 18 point even was sufficient!

Of course, in some cases, you need to accept the caculations as true, but if you feel you can, then put it at test in the field as well, you might still encounter a few surprizes..

Is insulation the most perfect solution? Perhaps I must admit that it might be 'just not', and, as I always say, you must do it in a proper way, but then indeed you still cannot insulate the corrector plate, but I for sure found out that insulation works, and I enjoy it every time I am outside with my scopes.


.

Edited by yellobeard, 29 June 2019 - 01:11 AM.

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#21 yellobeard

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Posted 29 June 2019 - 01:33 AM

Here is a kind of field knife edge test on Vega, with my 16" SCT, under quite good seeing conditions.

There are no visible turbulences from schmidtplate or obstruction.

The only thing I saw needing to be addressed, is a minor problem at the fucussing tube: at the right image, its that small colder airflow that comes down into view at the top.

In the dobsonian like mount, in my case, the focussing mechanism always is on top of the tube, and not insulated!

So the cooling of that focusser, makes a colder airflow, fall down in the main beam in the scope tube.

Warming up the focusser with a hairdryer solved my problem

However, that airflow was small enough to not do any harm to the image quality of the 16", I only noticed it during this test..

I probably don't need to tell you, at the left image, that plume is at the left.

Any other differeces you see in the pictures, only are seeing effects.

 

Ohh, forgot: This test was done two weeks ago, some 15 minutes after the scope was brought to 50F temperature, from 70F room temperature

 

 

Screenshot_20190629-081824_Gallery.jpg


Edited by yellobeard, 29 June 2019 - 02:01 AM.

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