269 replies to this topic

[graffias79]

Rather than add on to a very long thread taking it further off topic, I wanted to ask this in a new thread.  Soon I'll be getting my first cat - a 150mm Mak Cass.  I was reading in another thread that insulating the tube is a good way to cut down on thermal issues.  I know that these scopes can have some trouble in certain conditions.

 

What I gathered is that I'm supposed to let the scope get to ambient temperature first, then cover the optical tube with insulation to help keep it from radiating more heat to the sky.

 

If that is the case, what material would be ideal?  I saw aluminum foil mentioned but I don't want to risk anything getting scratched.  Perhaps I could buffer the foil with some felt?  Perhaps an aluminum felt sandwich?  It would be cute to make a little telescope jacket for when it gets too cold. lol

[DLuders]

I have a Celestron 127mm Maksutov-Cassegrain Telescope (MCT), and know that in hot conditions and cold, the telescope must reach Thermal Equilibrium with the atmosphere to provide steady images.  As the night wears down and the air cools even further, it's important that the telescope's optics keep up with the changes and not be insulated.  If you defocus a star and the "donut" is swimming with wormlike movement, the telescope has not achieved equilibrium with its surroundings.

 

A lightweight, foam-type Dew Shield (like I have) is handy to keep the corrector lens from getting obscured with moisture, but it is not meant to insulate anything.

Edited by DLuders, 14 January 2018 - 04:57 PM.

[yellobeard]

Hi graffias,
Off topic... Yes, I think that I can agree on that, being also the cause of it..

What you 'gathered' is not written anywhere if i recall correctly, but if I didn't have my glasses on, and overlooked it, then still it certainly didn't come from me!

Proper insulation can stay around your scope tube permanently! The purpose is the drasticly slowing down of the temperature drop inside your scope, causing the total inside be able to 'follow' that slow drop, including the most massive part, the primary mirror.
Ok, the way you thought you 'gathered' also works, but needs more time, because you still need to cool down that scope first, and then also apply the insulation.

No, my scopes are insulated permanently, and when I put them in the much colder outside air, they almost instandly give a very sharp image, as the internal's simply are prevented from cooling down fast.

Ohhh, and the aluminum foil that was mentioned is not real aluminum foil! Its a bubbely structured insulating foil with an ultra thin aluminum deposition layer on one side, totally harmless to your scope!

Edited by yellobeard, 14 January 2018 - 05:20 PM.

[Jon Isaacs]

I have a Celestron 127mm Maksutov-Cassegrain Telescope (MCT), and know that in hot conditions and cold, the telescope must reach Thermal Equilibrium with the atmosphere to provide steady images.  As the night wears down and the air cools even further, it's important that the telescope's optics keep up with the changes and not be insulated.  If you defocus a star and the "donut" is swimming with wormlike movement, the telescope has not achieved equilibrium with its surroundings.

 

A lightweight, foam-type Dew Shield (like I have) is handy to keep the corrector lens from getting obscured with moisture, but it is not meant to insulate anything.

 

 

Yelowbeard is from the Netherlands and builds his own SCTs as well as modifying commercial SCTs.  Look at his equipment list.. 

 

In another thread, he discusses the reasons why insulating an SCT or MAK is the correct approach to thermal management of these closed scopes.  Another Dutch ATM, Rik ter Horst uses the same approach.  Not many are up to building SCTs from scratch, these guys are and from what I know both are not just amazing ATMs, both are professionals in the fields related to telescope optics.

 

I'd pay close attention to this this thread and what Yelowbeard has to say.  It may disagree with the conventional wisdom but it has a wisdom all it's own based on both experience and theory.  

 

Jon

[DLuders]

It may matter that, in the dry parts of the western US, the temperatures can rapidly drop 30 degrees in short order after the sun sets.  I don't think that The Netherlands has those wide temperature swings.  Also, the time it takes to "cool down" a 5"-6" Cat is a lot less than it would be for a large, custom SCT.  I can set my telescope outside my house for an hour to achieve equilibrium, and keep it out there all night long to maintain equilibrium.  For my situation, the last thing I want to do is slow down the heat transfer -- the quicker the better.       

[yellobeard]

Of course it theoretically works well when you cool down your scope to ambient temperature first.

But then, practically, most people don't have the facilities to have their scopes at the expected night temperature all time, like the professionals do with for example the VLT.

And then, without insulation, you are forced to wait till your scope is in balance with the environment, and the bigger that scope, the more hours it takes. Furthermore, during a good night, the temperature continues to drop, only small scopes can hope to keep up with that drop, insulated scopes don't need to keep up, as they are not suseptible to slow or rapid changes in temperature.

Edit: In a dry environment you probably don't have issues with damp on your front corrector, and indeed the cooling down of small scopes is reasonably acceptable, but in a more moist area, the big advantage of insulation is that it at least will take much longer before mist is forming on your corrector.

Edited by yellobeard, 14 January 2018 - 05:55 PM.

[Jon Isaacs]

It may matter that, in the dry parts of the western US, the temperatures can rapidly drop 30 degrees in short order after the sun sets.  I don't think that The Netherlands has those wide temperature swings.  Also, the time it takes to "cool down" a 5"-6" Cat is a lot less than it would be for a large, custom SCT.  I can set my telescope outside my house for an hour to achieve equilibrium, and keep it out there all night long to maintain equilibrium.  For my situation, the last thing I want to do is slow down the heat transfer -- the quicker the better.       

 

I am not going to argue with you. 

 

But I will say that the Netherlands is a cold climate and taking a scope outside from a warm environment represents a serious change in temperature. And look at Yelowbeards equipment list,  modified C-5s. 

 

This idea that insulation is the right approach is new to us in the US but it's worth thinking about. The cool quickly concept is from the Newtonian world, we have no choice.  This a new Paradigm.. 

 

For what it's worth,  even here in San Diego where the temperatures are very mild,  I never felt my 127 Orion Mak cooled in an hour. If I still had it,  I'd be giving insulation a try. 

 

Jon

[yellobeard]

Insulation also is very simple and cheap, so why not try...

[sonny.barile]

Insulation also is very simple and cheap, so why not try...

 

Hi Yellowbeard.   I started the thread you guys went “off topic” in and I did not mind it. It is now chock full of great technical information and will become a resource for the future. 

 

I have a C8 and at this particular time my climate is very cold. It is 18 degrees f at this moment. I keep the scope in my house at about 70 degrees. Are you stating that if I insulate before bringing it out it will be immediately usable?

 

There are many clear nights that I do not have enough time to do cool down so I don’t get to observe. If this were to work I would get many more opportunities. 

[Richard O'Neill]

 Permanently installed Reflectix insulation (technically a radiant barrier) works well on my C8. This scope was a testbed to evaluate the idea. I also have a 7" Mak. that is completely surrounded with this material. As a consequence the radiation barrier allows observations to begin immediately and dew is no longer a problem. What's not to like?

 

 

Edited by Richard O'Neill, 14 January 2018 - 09:06 PM.

[Crow Haven]

Reflectix is great stuff!  I've got a big roll of it already and will be using it for my 7" Mak and C11.

[azure1961p]

Rather than add on to a very long thread taking it further off topic, I wanted to ask this in a new thread.  Soon I'll be getting my first cat - a 150mm Mak Cass.  I was reading in another thread that insulating the tube is a good way to cut down on thermal issues.  I know that these scopes can have some trouble in certain conditions.

 

What I gathered is that I'm supposed to let the scope get to ambient temperature first, then cover the optical tube with insulation to help keep it from radiating more heat to the sky.

 

If that is the case, what material would be ideal?  I saw aluminum foil mentioned but I don't want to risk anything getting scratched.  Perhaps I could buffer the foil with some felt?  Perhaps an aluminum felt sandwich?  It would be cute to make a little telescope jacket for when it gets too cold. lol

I haven't read all the replies here, I'll just add my own...

 

 

The only reason to insulate is so the corrector stays slightly warm fending off few. Fair enough.

 

Here's how you really want to do.it, if I were a bigger mak owner...

 

 

Pipe in outside air behind the primary or at least halfway up the tube. That'll cut your mirror cool down to thirty minutes. Ok now the corrector... Get a little heating ring for it just enough heat to fend off dew. Yes you are cooling the scope while heating the corrector, get over it.  The corrector neerds just a liitttttlllleee heat to be effevtive.

 

In warmer climes this us not needed as much if your scope is at outside temps all day. Where inside storage and outside air differ greatly you may want to consider the dual front approach lined.

 

Pete

Edited by azure1961p, 14 January 2018 - 09:02 PM.

[cbwerner]

Furthermore, during a good night, the temperature continues to drop, only small scopes can hope to keep up with that drop, insulated scopes don't need to keep up, as they are not suseptible to slow or rapid changes in temperature.

This is a new concept to me, so let me restate what I think you are saying to make sure I understand.

 

The conventional approach is to set the scope outside in advance and/or install fans to make the telescope change its temperature to the ambient as quickly as possible.

 

But with insulating the idea is to slow thermal changes as much as possible so that the telescope's temperature change is so slow that the ambient temperature is not a factor and you have the same result that you would have if the scope were at ambient temperature and stable.

 

Is that the crux of it?

[James Ball]

 

Furthermore, during a good night, the temperature continues to drop, only small scopes can hope to keep up with that drop, insulated scopes don't need to keep up, as they are not suseptible to slow or rapid changes in temperature.

This is a new concept to me, so let me restate what I think you are saying to make sure I understand.

 

The conventional approach is to set the scope outside in advance and/or install fans to make the telescope change its temperature to the ambient as quickly as possible.

 

But with insulating the idea is to slow thermal changes as much as possible so that the telescope's temperature change is so slow that the ambient temperature is not a factor and you have the same result that you would have if the scope were at ambient temperature and stable.

 

Is that the crux of it?

 

This is what I am understanding here too.  If you think about it, what causes unsteady views is air inside the tube that is a mixture of different temperature air that is moving around.  Since air at different temperatures has different refractive index, when you have that swirling around inside the tube you have unsteady views, if the air inside the scope is a nice even temperature, then even if it is different from the surrounding air it should still give a steady view.

 

Wouldn't this essentially be the same reasoning why a refractor doesn't have the problem that a SCT/MCT does? 

 

If the temperature inside the insulated tube is constant and you are not having air movement due to convection currents, then the only difference in refractive index will be at the boundary of the air at the surface of the corrector plate which shouldn't be much of a problem.

 

Tube currents are caused mostly by the cold tube causing turbulence as it cools air inside the tube, and the warm air circulates over from the center to the side of the tube to cool further, causing a convection current and until the air inside the tube all becomes the same temperature as the tube wall, then the currents will continue to flow.  Insulated tube would not cause the cooling and not currents will form.

[vahe]

Here is my 8" Mak wrapped with 1/4" Reflectix, not a total solution but it helps.

.

Vahe

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[yellobeard]

Is that the crux of it?

That indeed is one way to look at it yes..

The point is, no matter what the ambient temperature does, the insulation drasticly slows down the negative effects of it inside your scope, giving the inside and all optics the oppertunity to gradually and 'easily follow' the temperature changes, without temperature differences that are compromizing image quality..

More so: As an optical specialist, I of course want perfect performing telescopes, and with that, there comes that feeling that you cannot make a good mirror from that commonly aviable green type of float glass, having a high termal expansion..
And if, some 10 years ago, you would have told me that I eventually would use that green glass in even my biggest scopes, I would have walked away laughing!!
Right now, my 16"SCT includes all my experience that is build up in some 40 years, including the latest insight: A set of mirrors, made from that cheapest green glass! The disatvantages of that green glass are totally eliminated by the proper insulation.

Of course, I'm a visual observer, so when you are fully into photography, then stable focus over hours of time will be your greatest wish, and then I'm not sure that cheap glass will be your friend..

Edited by yellobeard, 15 January 2018 - 01:08 AM.

[yellobeard]

I haven't read all the replies here, I'll just add my own...

Pete

Sorry Pete, but If this thread would have some 100+ posts, I would understand.. but here...? And I have the feeling that you didn't read the most important posts.. Well, it's a choice..

Edited by yellobeard, 15 January 2018 - 01:41 AM.

[yellobeard]

Permanently installed Reflectix insulation (technically a radiant barrier) works well on my C8. This scope was a testbed to evaluate the idea. I also have a 7" Mak. that is completely surrounded with this material. As a consequence the radiation barrier allows observations to begin immediately and dew is no longer a problem. What's not to like?

Exellent example Richard!

Some in my country go a bit further, and also insulate the total back (mirror cell) of their scopes, which can help if temperature changes are severe.

But mostly, insulating the back is not needed, as the amount of material in that mirror cell, together with the smaller area that is exposed to the outside temperature, already cause a slow down in temperature drop, as proven in this particular case..

Both my C11's (bino scope) also are insulated the same way as shown here..

Edited by yellobeard, 15 January 2018 - 02:03 AM.

[davidc135]

Another reason to use insulation. Apologies if the same point has been made before. It's not just that the tube's temperature is following that of the ambient air down. It's as much the other way around. All surfaces, tube, ground etc will cool by radiation and the surrounding air is then cooled by contact with the surface. I think the aluminium coating radiates less effectively than, say, white paint at the relevant wavelengths.

 

David

[Richard O'Neill]

"Some in my country go a bit further, and also insulate the total back (mirror cell) of their scopes, which can help if temperature changes are severe."

 

 

 I agree, I have yet to cover the back end of my C8 but I did cover all exposed surfaces of my 7" TEC Mak, including the rear port extension tube and diagonal mirror housing. With both scopes the Reflectix extends beyond the front of the telescope to form a very effective dew shield.

 

 Another feature of the TEC Mak that aids immediate use is the integral fan which continuously mixes internal air to eliminate any residual stratification that could result from the sky radiating meniscus. After an hour or so I usually discontinue use of the fan unless temps continue a rapid decline.

Edited by Richard O'Neill, 15 January 2018 - 10:31 AM.

[vahe]

 "Another feature of the TEC Mak that aids immediate use is the integral fan which continuously mixes internal air to eliminate any residual stratification that could result from the sky radiating meniscus. After an hour or so I usually discontinue use of the fan unless temps continue a rapid decline."

 

 

The concept of mixing internal air is a good one, but in the case of TEC all their Maksutovs use the same tiny fans without regard the tube size and volume, for 6” and 7” these fans are somewhat effective but once you go above size these fans are just too small to make any difference, additionally mixing fans are situated right up against the metal tube which restricts air flow on the back of the fan.

.

My 8” Mak has two fans one at the mirror and one at the meniscus, it helps a little, but the 10” with one fan is totally hopeless. it makes absolutely no difference with fans on or off.

.

Vahe

[graffias79]

Thanks for the clarification.  I'm definitely going to give this a shot when I get the Mak Cass!

[Richard Whalen]

Vahe, I wonder if Yuri changed the fans or improved this in later models? I believe you have early ones? In my 8" TEC the fan works really well (for my climate) think its a single unit? Mine was built in 1999/2000 if I remember right. I also think the quartz mirrors helps give good views in falling temps. Here in Florida most of the year cool-down is not an issue, though in the winter it can be, so I keep my OTA in the wine cellar at 48 degrees until the sun no longer shines on the mount if its going to be cold.

 

When I take it to NM I will have to try the insulation idea as temperature swings can be 40 degrees or more up in the Gila.

It is not bad up until around midnight, after which it drops like a rock in the early spring, late fall.

 

Do you wrap the backside of the tube also?

[Richard O'Neill]

 Thanks, Vahe. That's helpful information. I wasn't aware of fan placement in the TEC but their use combined with the Reflectix has worked well in my east coast Maryland environment.

 

 For those who own a 7" Meade Mak my internal air mixing experiments may be of interest. To avoid introducing particle contamination and humid air I routed exhaust port air through a flexible tube to the input port. Although this strategy didn't cool internal components as quickly as routing cool external air throughout the interior would have it did help to ameliorate internal stratification. Unfortunately I have yet to cover the Meade Mak with reflectix so I don't know what the combination would yield but assume the overall performance would be at least similar to the TEC Mak. which, of the three telescopes, now gets the most use. So many scopes, so little time.

[Cpk133]

Who's going to be the first to put there scope in a bucket and shoot the periphery with expanding foam?