Topics

Cooling Experiment with TEC Mak

vahe352@...
 

“Wouldn’t it be better to use a non-metal tube to start with? Carbon-Fiber or Krupax-50 for example”

 

 

The idea of wrapping insulation is an attempt to help out EXISTING instruments that were designed with aluminum tubes and had to rely on passive cooling to reach thermal equilibrium for optimum performance.

 

CF is typically used for imaging scopes, the material does not expand as much with dropping temperatures as metal so refocusing is typically not required.

 

Will it make all that much of a thermal difference if the aluminum tube is replaced with a non metal CF? I do not know, but even with CF tubes I am willing to bet that additional layer of insulation would definitely help out the thermals.

 

That said, for my next project, modifying MC250/20, I am using an aluminum tube, but I have figured out a way to maintain a total separation between the metal tube and the volume of air inside it. The aluminum tube offers advantages in producing cooler air that if captured, stored and re-circulated within the tube in a controlled manner can be used to eliminate tube currents and cool the mirror very efficiently without having to resort to filter/fan units which are only effective in cooling the mirror, with this design the less insulation the more effective the cooling will be.

 

So, it all depends on the specifics of the design.


Vahe

"Pedro Santos" <pbsastro@...>
 

OK Mike, you have a “slight” cost advantage there…  J

 

Pedro

 

From: tec-scopes@... [mailto:tec-scopes@...] On Behalf Of mike@...
Sent: sexta-feira, 4 de Abril de 2014 15:37
To: tec-scopes@...
Subject: RE: [tec-scopes] Re: Cooling Experiment with TEC Mak

 




I thought of this also Pedro. However, a carbon fiber tube for a C14 is $600 and the matching dew shield is another $300. The Reflectix material cost $16 and was enough to do a C14 tube and dew shield and a TEC 300 MAK tube and dew shield with enough material to do another scope :)

 

Mike




mike@...
 

I thought of this also Pedro. However, a carbon fiber tube for a C14 is $600 and the matching dew shield is another $300. The Reflectix material cost $16 and was enough to do a C14 tube and dew shield and a TEC 300 MAK tube and dew shield with enough material to do another scope :)


Mike

"Pedro Santos" <pbsastro@...>
 

Wouldn´t it be better to use a non-metal tube to start with? Carbon-fiber or Krupax-50 for example.

Not to mention the added bonus it is much more comfortable in the cold.

 

From: tec-scopes@... [mailto:tec-scopes@...] On Behalf Of vahe352@...
Sent: quinta-feira, 3 de Abril de 2014 02:15
To: tec-scopes@...
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak

 




Keep on experimenting and let’s hear the results. As for insulating the mirror end of the tube I know that I am not going to worry too much about that, that end is just too busy even in my Maks with no fans and relying on slow passive cooling.

I am hoping to try the insulation on 250/20 to see how that one performs, unfortunately the Houston spring offers never ending overcast skies this time of the year and I am not sure if I will be lucky with a night of clear sky anytime soon.

I am not convinced that the mirror is the main source of Mak cooling problems, to me tube currents in an uninsulated metal tube can and will mess up the image big time, even if the mirror is perfectly acclimated, insulating the tube helps the situation.

 

Vahe 




vahe352@...
 

Keep on experimenting and let’s hear the results. As for insulating the mirror end of the tube I know that I am not going to worry too much about that, that end is just too busy even in my Maks with no fans and relying on slow passive cooling.

I am hoping to try the insulation on 250/20 to see how that one performs, unfortunately the Houston spring offers never ending overcast skies this time of the year and I am not sure if I will be lucky with a night of clear sky anytime soon.

I am not convinced that the mirror is the main source of Mak cooling problems, to me tube currents in an uninsulated metal tube can and will mess up the image big time, even if the mirror is perfectly acclimated, insulating the tube helps the situation.


Vahe 

mike@...
 

Vahe, Todd and I decided to try your "Reflectix" solution. Although you suggested removing the insulation during the cool down period we both decided not to and see if it was extended. We both are actively cooling our scopes with fans etc. I saw no delay in the scope mirror reaching ambient with the insulation attached. As evidenced by the temperature gauge, the C14 mirror reached ambient approximately one (1) hour after sunset. In the picture the "OUT" reading is actually the mirror temp as the external gauge probe is mounted directly on the mirror and the "IN" reading is ambient. I will need to do further testing such as attaching a temp probe against the scope tube to see if it remains at ambient. You also suggested attaching insulating material to the scope back but this may be impractical with gauges, handles, focus knob etc. If the ambient temperature starts dropping through the course of the night, will the insulation impede the ability of the mirror to follow? Possibly. I will be looking for the answer in the coming imaging sessions.


http://mikeharden.net/scopes/C14_Side_Lowres.jpg

http://www.mikeharden.net/scopes/C14_Rear_Lowres.jpg

http://www.mikeharden.net/scopes/Ambient_Mirror_Temp_Gauge.jpg 

http://www.mikeharden.net/scopes/TEC300_sideview_lowres.jpg


Mike



 


Tim Khan <timkhan@...>
 

see below:


From: Bruce MacEvoy
To: "tec-scopes@..."
Sent: Wednesday, April 2, 2014 1:13 AM
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak



like alan i need a little clarification ...
Meanwhile, we found experimentally, with different glass samples that the meniscus would cool many times faster without the need of forced cooling. In our first experiments, while cooling the scope, it was completely useless and required some redesign of hardware components. This was ironed out and we were able to use the scope while it was cooling down being no different to a Mak or SCT without a cooling system.
i am guessing this means: the meniscus would cool radiatively many times faster than the mirror, and therefore was not the primary reason to use forced air cooling.

no, Combination of Radiation, Convection
Conduction.


and forced air cooling (of the meniscus?) was useless and besides being useless required redesign of hardware, which made it doubly impractical. 

in the end, you could use the 12" TEC scope while it was cooling down (with the forced air system running behind the mirror but not around the meniscus?) and this gave you image quality comparable to using an SCT or Mak while it was cooling down without a forced air system at all. (but isn't the 12" TEC a mak?)

Yes, two spereated volumns and yes its a mak that could keep up with falling temperatures.

The end result was that we were able to effectively pump out 90,000 joules out of the primary. The meniscus and secondary heat was removed simply from IR (to blackbody) and exterior convection for its stored heat of about 52,000 joules (these numbers are for a dt=-15C).
this means: all things equal, at the start of cool down the meniscus holds approximately half as much heat as the mirror?

you cannot make that assumption, depends on the specific heat of the glass materials used.


bruce




Bruce MacEvoy <bmacevoy@...>
 

like alan i need a little clarification ...
Meanwhile, we found experimentally, with different glass samples that the meniscus would cool many times faster without the need of forced cooling. In our first experiments, while cooling the scope, it was completely useless and required some redesign of hardware components. This was ironed out and we were able to use the scope while it was cooling down being no different to a Mak or SCT without a cooling system.
i am guessing this means: the meniscus would cool radiatively many times faster than the mirror, and therefore was not the primary reason to use forced air cooling. 

and forced air cooling (of the meniscus?) was useless and besides being useless required redesign of hardware, which made it doubly impractical. 

in the end, you could use the 12" TEC scope while it was cooling down (with the forced air system running behind the mirror but not around the meniscus?) and this gave you image quality comparable to using an SCT or Mak while it was cooling down without a forced air system at all. (but isn't the 12" TEC a mak?)
The end result was that we were able to effectively pump out 90,000 joules out of the primary. The meniscus and secondary heat was removed simply from IR (to blackbody) and exterior convection for its stored heat of about 52,000 joules (these numbers are for a dt=-15C).
this means: all things equal, at the start of cool down the meniscus holds approximately half as much heat as the mirror?

bruce

Tim Khan <timkhan@...>
 

We only cooled the air....and its a tight fit back there.



From: Alan French
To: tec-scopes@...
Sent: Tuesday, April 1, 2014 9:11 AM
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak



Tim,

Interesting, and thanks for sharing.

Do the coolers directly cool the mirror? (I'm trying to picture how they are set up inside the scope.)

Clear skies, Alan

On 4/1/2014 9:02 AM, Tim Khan wrote:
In our experiments with closed force cooled system with the 12" TEC it always cooled in just under an hour (about 50-55 minutes no matter the external ambient temp and starting mirror temp). We employed two 60 watt thermoelectric coolers behind the mirror. The IR radiation exchange between the back plate and mirror was only a small percentage (2-3%) the net loss is a yet a lower number.

Meanwhile, we found experimentally, with different glass samples that the meniscus would cool many times faster without the need of forced cooling. In our first experiments, while cooling the scope, it was completely useless and required some redesign of hardware components. This was ironed out and we were able to use the scope while it was cooling down being no different to a Mak or SCT without a cooling system.

The end result was that we were able to effectively pump out 90,000 joules out of the primary. The meniscus and secondary heat was removed simply from IR (to blackbody) and exterior convection for its stored heat of about 52,000 joules (these numbers are for a dt=-15C).

To date this scope gave me the best views of Jupiter I have seen.


From: Alan French
To: tec-scopes@...
Sent: Monday, March 31, 2014 2:08 PM
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak



Ram,

I couldn't find a number from aluminum coated mirrors, but shiny aluminum has an emissivity of about 0.05, so it does not radiate well at all. Perhaps, if the heat is being removed from the rear, this is a good thing. Convective cooling, especially forces convective cooling (a fan) removes heat from Newtonian primaries quite quickly.

Clear skies, Alan



On 3/31/2014 1:55 PM, Ram Viswanathan wrote:
Rolando, that's a very helpful explanation. I guess the aluminum on the mirror is analogous to mylar emergency blankets that do such a good job of insulating despite being so thin. Except that in the former case, we want the heat to radiate away rather than be stored!

--Ram




On Mon, Mar 31, 2014 at 9:51 AM, <chris1011@...> wrote:
 
>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>
 
Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.
 
Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.
 
Rolando 









j.blazey@...
 

Well my Intes scopes , MN of course included, do not have spots but have secondary assemblies that give off heat and it shows up as little tornadoes or, well, plumes in the out of focus star images. You're right though, turning the fan on does disrupt the plume. The exhaust fan flow in the M815 is rather small IMO and it still takes quite a while to cool the primary. However, the MN76's fan has a large flow and is quite effective at cooling the primary. What I do with that scope is point the back end up, remove the eyepiece and turn on the fan. That impinges air on the seconday assembly and forces the mirror air out the top. Very effective for initial cool down.

My MW/Intes MN86 is exactly as you describe, thin mirror with open back. That leaves me with the secondary plume which I manage in the same way as the MN76, except passively as the MW scope has no fan.

Jeff
Quoting chris1011@...:

Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>

Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.

Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.

Rolando

tec@...
 

Here are few pictures related the subject:
https://groups.yahoo.com/neo/groups/tec-scopes/files/MC300/

Alan French <adfrench@...>
 

Tim,

Interesting, and thanks for sharing.

Do the coolers directly cool the mirror? (I'm trying to picture how they are set up inside the scope.)

Clear skies, Alan

On 4/1/2014 9:02 AM, Tim Khan wrote:
In our experiments with closed force cooled system with the 12" TEC it always cooled in just under an hour (about 50-55 minutes no matter the external ambient temp and starting mirror temp). We employed two 60 watt thermoelectric coolers behind the mirror. The IR radiation exchange between the back plate and mirror was only a small percentage (2-3%) the net loss is a yet a lower number.

Meanwhile, we found experimentally, with different glass samples that the meniscus would cool many times faster without the need of forced cooling. In our first experiments, while cooling the scope, it was completely useless and required some redesign of hardware components. This was ironed out and we were able to use the scope while it was cooling down being no different to a Mak or SCT without a cooling system.

The end result was that we were able to effectively pump out 90,000 joules out of the primary. The meniscus and secondary heat was removed simply from IR (to blackbody) and exterior convection for its stored heat of about 52,000 joules (these numbers are for a dt=-15C).

To date this scope gave me the best views of Jupiter I have seen.


From: Alan French
To: tec-scopes@...
Sent: Monday, March 31, 2014 2:08 PM
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak



Ram,

I couldn't find a number from aluminum coated mirrors, but shiny aluminum has an emissivity of about 0.05, so it does not radiate well at all. Perhaps, if the heat is being removed from the rear, this is a good thing. Convective cooling, especially forces convective cooling (a fan) removes heat from Newtonian primaries quite quickly.

Clear skies, Alan



On 3/31/2014 1:55 PM, Ram Viswanathan wrote:
Rolando, that's a very helpful explanation. I guess the aluminum on the mirror is analogous to mylar emergency blankets that do such a good job of insulating despite being so thin. Except that in the former case, we want the heat to radiate away rather than be stored!

--Ram




On Mon, Mar 31, 2014 at 9:51 AM, <chris1011@...> wrote:
 
>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>
 
Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.
 
Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.
 
Rolando 





Tim Khan <timkhan@...>
 

In our experiments with closed force cooled system with the 12" TEC it always cooled in just under an hour (about 50-55 minutes no matter the external ambient temp and starting mirror temp). We employed two 60 watt thermoelectric coolers behind the mirror. The IR radiation exchange between the back plate and mirror was only a small percentage (2-3%) the net loss is a yet a lower number.

Meanwhile, we found experimentally, with different glass samples that the meniscus would cool many times faster without the need of forced cooling. In our first experiments, while cooling the scope, it was completely useless and required some redesign of hardware components. This was ironed out and we were able to use the scope while it was cooling down being no different to a Mak or SCT without a cooling system.

The end result was that we were able to effectively pump out 90,000 joules out of the primary. The meniscus and secondary heat was removed simply from IR (to blackbody) and exterior convection for its stored heat of about 52,000 joules (these numbers are for a dt=-15C).

To date this scope gave me the best views of Jupiter I have seen.



From: Alan French
To: tec-scopes@...
Sent: Monday, March 31, 2014 2:08 PM
Subject: Re: [tec-scopes] Re: Cooling Experiment with TEC Mak



Ram,

I couldn't find a number from aluminum coated mirrors, but shiny aluminum has an emissivity of about 0.05, so it does not radiate well at all. Perhaps, if the heat is being removed from the rear, this is a good thing. Convective cooling, especially forces convective cooling (a fan) removes heat from Newtonian primaries quite quickly.

Clear skies, Alan



On 3/31/2014 1:55 PM, Ram Viswanathan wrote:
Rolando, that's a very helpful explanation. I guess the aluminum on the mirror is analogous to mylar emergency blankets that do such a good job of insulating despite being so thin. Except that in the former case, we want the heat to radiate away rather than be stored!

--Ram




On Mon, Mar 31, 2014 at 9:51 AM, <chris1011@...> wrote:
 
>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>
 
Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.
 
Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.
 
Rolando 





Alan French <adfrench@...>
 

Ram,

I couldn't find a number from aluminum coated mirrors, but shiny aluminum has an emissivity of about 0.05, so it does not radiate well at all. Perhaps, if the heat is being removed from the rear, this is a good thing. Convective cooling, especially forces convective cooling (a fan) removes heat from Newtonian primaries quite quickly.

Clear skies, Alan



On 3/31/2014 1:55 PM, Ram Viswanathan wrote:
Rolando, that's a very helpful explanation. I guess the aluminum on the mirror is analogous to mylar emergency blankets that do such a good job of insulating despite being so thin. Except that in the former case, we want the heat to radiate away rather than be stored!

--Ram




On Mon, Mar 31, 2014 at 9:51 AM, <chris1011@...> wrote:
 

>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>

 

Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.

 

Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.

 

Rolando 


Ram Viswanathan <ramviswanathan@...>
 

Rolando, that's a very helpful explanation. I guess the aluminum on the mirror is analogous to mylar emergency blankets that do such a good job of insulating despite being so thin. Except that in the former case, we want the heat to radiate away rather than be stored!

--Ram




On Mon, Mar 31, 2014 at 9:51 AM, <chris1011@...> wrote:
 

>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>

 

Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.

 

Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.

 

Rolando 


chris1011@...
 

>>Actually, my biggest problem with my Maks, including Mak Newts are
thermal plumes off of the secondary assembly.>>

 

Thermal plumes actually don't come off the secondary assembly. There are Maks that have just a spot on the corrector, no seperate secondary, and guess what? they still have the same thermal plumes. To find out where the plumes come from I did an experiment by removing the secondary baffle. The plumes disappeared, but of course now the contrast went to h.ll because light from the sky goes right to the focal plane. Cannot live without a secondary baffle.

 

Your Intes draws air down the tube thru the secondary baffle and breaks up the plume inside the secondary baffle. However that does not fix the seeing problem inside the tube. The moving air is turbulent and does not produce a perfectly clear image - stars still dance around like crazy all the while that the fan is on. As soon as you turn off the fan, the plume is back like a bad penny. The real culprit is the primary mirror, which stores a lot of heat that must be eliminated somehow. Thin mirror, open back to radiate the mirror heat is the best solution.

 

Rolando 

j.blazey@...
 

Actually, my biggest problem with my Maks, including Mak Newts are thermal plumes off of the secondary assembly. The Intes cooling system works quite well and when I had my TEC 8/15.5 and Intes M815 together, the Intes instrument was more usable more of the time. It draws filtered air gently in from around the meniscus and out the back. This significantly controlled the tube and mirror boundary layers while also surrounding the secondary assembly with cooler air.

A friend of mine modified his C8 by drilling 6 holes around the perimeter of the front of the tube and installing clip in filter disks and an exhaust fan on the back. The filtered holes directs the ambient air towards the secondary assembly where it impinges, mixes and flows out the back. The system is VERY effective and one I'm going to copy for my C14. But I could not bring myself to cutting up my TEC 8. My TEC 6 was thermally quite well behaved but it, of course, had significantly smaller glass and air volumes.

Jeff
Quoting Bruce MacEvoy <bmacevoy@...>:

RC wrote:

"I have done quite a bit of experimentation over the years on closed tube catadioptrics. There are 3 ways to mitigate tube currents. The first is to try to get rid of the heat in the primary mirror, either by cooling fans or by opening the back and aiming the back of the mirror up to the clear sky to take advantage of radiative cooling. This usually takes about 1 hour for medium sized instruments (8 to 12"). It helps to have as thin a mirror as possible - thick mirrors store a lot of heat."

the method roland describes is the one that was recommended to me by the manufacturer of my commercial SCT -- tilt the front toward the ground and open the visual back to let air escape. 

however it seemed to me that the mirror radiated into the airspace behind it, which was trapped in the half of the tube higher than the opening, like warm air trapped along a room ceiling; warm air could not ventilate up the baffle, because the end of the baffle was lower than the mirror; and the metal back is quite thick, at least 1/3", so conduction through it would be slower than radiation (the entire back cannot be removed to expose the mirror). the method i developed instead was to point the SCT straight up. this lets warm air rise to the corrector, radiates heat directly through the glass, makes best use of the temperature "sink" at zenith, and distributes the heat flow evenly around the mirror circumference. although this seemed to produce more rapid and satisfactory results, given daily variations in temperature changes and lacking a control telescope for comparison, i can't say for sure. 

Vahe wrote:

"I became interested in this issue a long time ago and started reading everything on the topic that I could get my hands on, for sure there are lots of theories out there, but I can not think of a single paper that offered an all out practical solution, so I decided to resort to the only option available to me which was “Trial & Error”, and I started experimenting, believe me this is not a rocket science, if you apply a little common sense and approach it with an open mind you will get there."

my perception is also that there is a variety of sources, diversity of opinions, and lack of an "all out practical solution". the remarkable thing, in my view, is that many amateur astronomers have either a physics, mechanical or engineering background, and therefore the community should converge rather efficiently on a consensus view of the solutions -- the problems seem to be easy to identify. i've admired an aggressive meade SCT rebuild done by tom krajci, so perhaps the extent of mechanical work involved is the main obstacle.

as for opinion: i purchased a DK scope touted with a "conical mirror" that supposedly would radiate heat more efficiently from the thin mirror edges, but what i have found instead is that the fat mirror core is a tenacious heat sink and it takes the mirror a very long time to stabilize (in an open tube, with a ventilator fan pulling air across it). in fact, the 10" ventilated DK takes much longer to cool down than the 12" enclosed SCT mounted next to it in the same observatory enclosure!

i had to smile to see "trial and error" as the last resort for vahe as well. but here's the thing. even trial and error, repeated enough times by enough people, should converge on the same solution for the same problem in the same situation. but that hasn't happened: for example roland insulates the inside of the tube, vahe doesn't. (and does removing the exterior insulation really help the cooling, if the inside insulation on the tube is not also removed?) 

that's where i come back to the fundamental test/control approach, using analysis of the image formation -- not the schlieren analysis of unmounted mirrors, not the measured temperature difference of tube and air, not the gut check intuition, but the visual effect on the image at the focal plane -- of different methods applied to one of two identical scopes mounted at the same location with identical cooling histories pointed at the same object in the sky. comparison of the images in those two scopes, across all seasons, will tell you *exactly* how much benefit comes from any remedial customization. 

i realize optical manufacturing companies are overworked and under resourced. but i put the main responsibility on them. the average end user can't be expected to do physical research and mechanical work, and if the mechanical adjustment is really beneficial then it should be built in at the start. the requirement for two telescopes in the test/control procedure is better met by a manufacturer, who often has multiple telescopes available before shipment to buyers. telescopes are all manufactured differently, so source testing each specific design is better than generic remedies that are recommended for many different designs. and finally, manufacturers do not even post specific, tested and proven cool down procedures for their instruments at their web sites. all this implies that the manufacturers really don't see it as their problem. which leaves ... individual "trial and error."

apologies for the rant, and thanks to those who have helped my education on this topic. 

bruce




On Thursday, March 27, 2014 6:47 PM, "vahe352@..." <vahe352@...> wrote:

 
Tom
Writes;
“Why
can’t we assume the meniscus releases heat into the optical path, especially
200mm or larger instruments, as described in this article?”
 
Every
component in a telescope, optical or mechanical, contributes to the cooling
problems, that includes the massive meniscus and the separate secondary with
its baffle attached to it, but their contributions are minimal and shortlived
compared to the big offenders which are the primary mirror, the metal tube and
let us also not forget the center baffle in this mix.
 
Also,
consider this, the meniscus is a refractive element, meaning light passes
through it just once, the mirror on the other hand by virtue of reflecting the
incoming light, doubles up whatever problems that are created by the metal tube
in front of it, so everything being equal, size and mass, the mirror is in
distinct disadvantage when it comes to amplifying the image distortions, and if
that is not bad enough in TEC Maks the mirror is totally enclosed with no
contact with the outdoor air for cooling.
 
Bruce
writes;
“I do not
understand the actual physics of the problem, so I am left with trial and
error”
 
I became
interested in this issue a long time ago and started reading everything on the
topic that I could get my hands on, for sure there are lots of theories out
there, but I can not think of a single paper that offered an all out practical
solution, so I decided to resort to the only option available to me which was
“Trial & Error”, and I started experimenting, believe me this is not a rocket
science, if you apply a little common sense and approach it with an open mind
you will get there.
 
The
insulation wrap can only help minimize the problem in TEC Maks, insulation can
not entirely prevent the problem in these totally enclosed Maks but they are
very helpful,  without the insulation under rapid cooling Maks that
relying on passive cooling just can not keep up with the changing temperatures.
 
Is it
possible to totally prevent the problem? Yes, it is, but it is a massive
undertaking if the goal is to redo an existing TEC Mak, I am currently
modifying my 10” Mak, the existing tube is set aside and I am starting with a
completely new tube design, the cooling principals employed are based on
continuous re-circulation of the entire air volume in a closed loop system,
there is more to it but as I said before this one is a story for another day.


Vahe


Bruce MacEvoy <bmacevoy@...>
 

RC wrote:

"I have done quite a bit of experimentation over the years on closed tube catadioptrics. There are 3 ways to mitigate tube currents. The first is to try to get rid of the heat in the primary mirror, either by cooling fans or by opening the back and aiming the back of the mirror up to the clear sky to take advantage of radiative cooling. This usually takes about 1 hour for medium sized instruments (8 to 12"). It helps to have as thin a mirror as possible - thick mirrors store a lot of heat."

the method roland describes is the one that was recommended to me by the manufacturer of my commercial SCT -- tilt the front toward the ground and open the visual back to let air escape. 

however it seemed to me that the mirror radiated into the airspace behind it, which was trapped in the half of the tube higher than the opening, like warm air trapped along a room ceiling; warm air could not ventilate up the baffle, because the end of the baffle was lower than the mirror; and the metal back is quite thick, at least 1/3", so conduction through it would be slower than radiation (the entire back cannot be removed to expose the mirror). the method i developed instead was to point the SCT straight up. this lets warm air rise to the corrector, radiates heat directly through the glass, makes best use of the temperature "sink" at zenith, and distributes the heat flow evenly around the mirror circumference. although this seemed to produce more rapid and satisfactory results, given daily variations in temperature changes and lacking a control telescope for comparison, i can't say for sure. 

Vahe wrote:

"I became interested in this issue a long time ago and started reading everything on the topic that I could get my hands on, for sure there are lots of theories out there, but I can not think of a single paper that offered an all out practical solution, so I decided to resort to the only option available to me which was “Trial & Error”, and I started experimenting, believe me this is not a rocket science, if you apply a little common sense and approach it with an open mind you will get there."

my perception is also that there is a variety of sources, diversity of opinions, and lack of an "all out practical solution". the remarkable thing, in my view, is that many amateur astronomers have either a physics, mechanical or engineering background, and therefore the community should converge rather efficiently on a consensus view of the solutions -- the problems seem to be easy to identify. i've admired an aggressive meade SCT rebuild done by tom krajci, so perhaps the extent of mechanical work involved is the main obstacle.

as for opinion: i purchased a DK scope touted with a "conical mirror" that supposedly would radiate heat more efficiently from the thin mirror edges, but what i have found instead is that the fat mirror core is a tenacious heat sink and it takes the mirror a very long time to stabilize (in an open tube, with a ventilator fan pulling air across it). in fact, the 10" ventilated DK takes much longer to cool down than the 12" enclosed SCT mounted next to it in the same observatory enclosure!

i had to smile to see "trial and error" as the last resort for vahe as well. but here's the thing. even trial and error, repeated enough times by enough people, should converge on the same solution for the same problem in the same situation. but that hasn't happened: for example roland insulates the inside of the tube, vahe doesn't. (and does removing the exterior insulation really help the cooling, if the inside insulation on the tube is not also removed?) 

that's where i come back to the fundamental test/control approach, using analysis of the image formation -- not the schlieren analysis of unmounted mirrors, not the measured temperature difference of tube and air, not the gut check intuition, but the visual effect on the image at the focal plane -- of different methods applied to one of two identical scopes mounted at the same location with identical cooling histories pointed at the same object in the sky. comparison of the images in those two scopes, across all seasons, will tell you *exactly* how much benefit comes from any remedial customization. 

i realize optical manufacturing companies are overworked and under resourced. but i put the main responsibility on them. the average end user can't be expected to do physical research and mechanical work, and if the mechanical adjustment is really beneficial then it should be built in at the start. the requirement for two telescopes in the test/control procedure is better met by a manufacturer, who often has multiple telescopes available before shipment to buyers. telescopes are all manufactured differently, so source testing each specific design is better than generic remedies that are recommended for many different designs. and finally, manufacturers do not even post specific, tested and proven cool down procedures for their instruments at their web sites. all this implies that the manufacturers really don't see it as their problem. which leaves ... individual "trial and error."

apologies for the rant, and thanks to those who have helped my education on this topic. 

bruce



On Thursday, March 27, 2014 6:47 PM, "vahe352@..." wrote:
 
Tom Writes;
“Why can’t we assume the meniscus releases heat into the optical path, especially 200mm or larger instruments, as described in this article?”
 
Every component in a telescope, optical or mechanical, contributes to the cooling problems, that includes the massive meniscus and the separate secondary with its baffle attached to it, but their contributions are minimal and shortlived compared to the big offenders which are the primary mirror, the metal tube and let us also not forget the center baffle in this mix.
 
Also, consider this, the meniscus is a refractive element, meaning light passes through it just once, the mirror on the other hand by virtue of reflecting the incoming light, doubles up whatever problems that are created by the metal tube in front of it, so everything being equal, size and mass, the mirror is in distinct disadvantage when it comes to amplifying the image distortions, and if that is not bad enough in TEC Maks the mirror is totally enclosed with no contact with the outdoor air for cooling.
 
Bruce writes;
“I do not understand the actual physics of the problem, so I am left with trial and error”
 
I became interested in this issue a long time ago and started reading everything on the topic that I could get my hands on, for sure there are lots of theories out there, but I can not think of a single paper that offered an all out practical solution, so I decided to resort to the only option available to me which was “Trial & Error”, and I started experimenting, believe me this is not a rocket science, if you apply a little common sense and approach it with an open mind you will get there.
 
The insulation wrap can only help minimize the problem in TEC Maks, insulation can not entirely prevent the problem in these totally enclosed Maks but they are very helpful,  without the insulation under rapid cooling Maks that relying on passive cooling just can not keep up with the changing temperatures.
 
Is it possible to totally prevent the problem? Yes, it is, but it is a massive undertaking if the goal is to redo an existing TEC Mak, I am currently modifying my 10” Mak, the existing tube is set aside and I am starting with a completely new tube design, the cooling principals employed are based on continuous re-circulation of the entire air volume in a closed loop system, there is more to it but as I said before this one is a story for another day.

Vahe


mike@...
 

Vahe, I would be interested in following your progress on the Mak mods. Maybe you could periodically post pictures in the photo section.


Mike

vahe352@...
 

Tom Writes;

“Why can’t we assume the meniscus releases heat into the optical path, especially 200mm or larger instruments, as described in this article?”

 

Every component in a telescope, optical or mechanical, contributes to the cooling problems, that includes the massive meniscus and the separate secondary with its baffle attached to it, but their contributions are minimal and shortlived compared to the big offenders which are the primary mirror, the metal tube and let us also not forget the center baffle in this mix.

 

Also, consider this, the meniscus is a refractive element, meaning light passes through it just once, the mirror on the other hand by virtue of reflecting the incoming light, doubles up whatever problems that are created by the metal tube in front of it, so everything being equal, size and mass, the mirror is in distinct disadvantage when it comes to amplifying the image distortions, and if that is not bad enough in TEC Maks the mirror is totally enclosed with no contact with the outdoor air for cooling.

 

Bruce writes;

“I do not understand the actual physics of the problem, so I am left with trial and error”

 

I became interested in this issue a long time ago and started reading everything on the topic that I could get my hands on, for sure there are lots of theories out there, but I can not think of a single paper that offered an all out practical solution, so I decided to resort to the only option available to me which was “Trial & Error”, and I started experimenting, believe me this is not a rocket science, if you apply a little common sense and approach it with an open mind you will get there.

 

The insulation wrap can only help minimize the problem in TEC Maks, insulation can not entirely prevent the problem in these totally enclosed Maks but they are very helpful,  without the insulation under rapid cooling Maks that relying on passive cooling just can not keep up with the changing temperatures.

 

Is it possible to totally prevent the problem? Yes, it is, but it is a massive undertaking if the goal is to redo an existing TEC Mak, I am currently modifying my 10” Mak, the existing tube is set aside and I am starting with a completely new tube design, the cooling principals employed are based on continuous re-circulation of the entire air volume in a closed loop system, there is more to it but as I said before this one is a story for another day.


Vahe