herrointment
Carpal Tunnel
Reged: 03/12/11
Loc: North of Hwy. 64
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: WarmWeatherGuy]
#5370434 - 08/15/12 05:25 PM
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Whew!
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MikeRatcliff
Carpal Tunnel
Reged: 06/12/04
Loc: Redlands, CA
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: RodgerHouTex]
#5370459 - 08/15/12 05:37 PM
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I didn't realize how many heat transfer experts we had here at CN.
Anyway, heat transfer ONLY occurs in one direction, from hot to cold. Not vice versa.
Right but in the case of radiation, the night sky high above the earth is effectively colder than the ambient air temperature close to the telescope. Radiation is shining up from the telescope without as much coming back from the sky.
In fact, if it weren't for the radiative losses to the sky, the night time air temperatures would not cool down. The ground surface cools off from radiation to space which gradually cools the air.
Mike
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MikeRatcliff
Carpal Tunnel
Reged: 06/12/04
Loc: Redlands, CA
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: MikeRatcliff]
#5370484 - 08/15/12 05:54 PM
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And I will add that air itself is mainly transparent to radiation, except for the small amounts of water vapor, dust, and carbon dioxide. (Otherwise a few hundred ppm of CO2 would not be a global warming issue).
So the radiation from telescopes will jump past the warm ambient air next to the telescope and go to space, with only some returning from space due to the deep column of water vapor and CO2 above us.
Mike
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Ed Holland
Pooh-Bah
Reged: 06/16/10
Loc: San Jose, CA and Oxford, UK
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: MikeRatcliff]
#5370565 - 08/15/12 07:02 PM
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I've never seen dew on a cat. Maybe it's the fur - something adaptable to our telescopes perhaps?
Leopardskin scope-cosies anyone? 
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EFT
Vendor - Deep Space Products
Reged: 05/07/07
Loc: Phoenix, AZ
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Tori]
#5371445 - 08/16/12 11:27 AM
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I still don't fully understand why it's a problem to cut a few holes and stick in a few fans. I'd rather have 10 years of not having to fight with thermal currents, knowing that I'm seeing as well as my collimation and seeing conditions allow. If I have to send the telescope or mirror away in 10, 15, or 30 years so be it. Wouldn't a few holes destroy the big arguments against cats? I'm sure I must be missing something because otherwise it would be being done, aftermarket if not by the manufacturers. Surely cleaning/recoating mirrors mustn't be the only reason, is it?
I've had a half dozen smallish refractors over the last 35 years, but I'm not a physicist and I don't build telescopes. I don't know the science behind it all. My current C14 EdgeHD is the largest scope I've owned, by far, and I want to make the most out of it...
Thank you!
I suspect there are several things behind this. First is the fear of taking apart and cutting up a new, or even used, SCT. This is not a job for everyone. Next is that the fans that most people are familiar with are in the 40mm or larger range. Adapting fans that large to an SCT is difficult because it requires accurately cutting very large holes, in a surface that is often not flat or even, not an easy task. Large fans also take up a lot of space, so many people who have tried that end up mounting them on the outside of the OTA which gets kind of ugly. Filtering has also been an issue for many people since the standard computer fan filter is not really suited for this purpose. What most people do not realize is that large fans are not only unnecessary, but they are more likely to introduce vibration into the system. Smaller fans can be mounted with drilled holes rather than cut holes and can be easily filtered with metal filter mesh and take up very little space. But such fans are comparatively expensive and not usually just laying around. Vibration is definitely a concern but is easily addressed by using the right fans (which are generally not going to be your average computer fan).
Overall, it is interesting that it has taken so long to start venting SCTs when MAKs have been passively and actively vented for quite a while.
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Loc: NE Ohio
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: EFT]
#5371468 - 08/16/12 11:41 AM
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Overall, it is interesting that it has taken so long to start venting SCTs
It's way too late to "start" incorporating fans in SCTs. Both Meade and Takahashi were doing it in the 90's - and Meade never stopped.
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EFT
Vendor - Deep Space Products
Reged: 05/07/07
Loc: Phoenix, AZ
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: jrcrilly]
#5371502 - 08/16/12 12:04 PM
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Overall, it is interesting that it has taken so long to start venting SCTs
It's way too late to "start" incorporating fans in SCTs. Both Meade and Takahashi were doing it in the 90's - and Meade never stopped.
I don't know of any Meade SCTs or MAKs that currently have fan systems. Some of the MAKs may have in the past but I haven't seen any Meade SCTs with fans or even vents. I am not familiar enough with all of the Tak scopes but some of the larger Mewlons are the only ones I am familiar with that have recently added fans. The only scopes that I am familiar with that have routinely had fans or the ability to easily add them have been the Intes Micro MAK and Mak/Newt scopes.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: EFT]
#5371976 - 08/16/12 06:17 PM
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By coincidence I own an early 7" lx50 mak and just a few weeks ago got around to taking it apart to remove the weight that is inside. I had learned about the weight but I didn't fully appreciate it until I took it apart. Yes, the OTA has a fan and vent in the back - but it blows directly onto a 1" thick 8 pound hunk of steel. Not only does this provide a huge thermal mass that dwarfs the mirror and corrector - but it completely blocks the flow of air from the fan.
Separately - possibly to confuse the whole issue of ventilation in scopes - I think that some high end maks have fans inside - but no vents at all. All they do is stir the air and disrupt any semi-persistent turbulent layers that would hurt the image. I can believe this would be very effective but I haven't tried it.
Finally - several people have mentioned how bad carbon fiber is for tubes - but I don't know what evidence there is to support it. It is so thin I don't know how much it prevents cooling of the system. I had a thread in this forum a couple years ago on my sct cooling study and there was some discussion - but in my measurements I didn't see any cause for concern. I think it does help with focus stability, so it may be too bad that they tend to be aluminum now.
Frank
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Alph
Pooh-Bah
Reged: 11/23/06
Loc: Melmac
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: freestar8n]
#5373427 - 08/17/12 03:58 PM
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it blows directly onto a 1" thick 8 pound hunk of steel. Not only does this provide a huge thermal mass that dwarfs the mirror and corrector
That hunk of steel (aluminum ?) actually acts as a heat sink. The less air trapped behind the primary, the better. If all space behind the primary mirror were filled with aluminum, then the fans could just blow at the rear casting.
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I think that some high end maks have fans inside - but no vents at all. All they do is stir the air and disrupt any semi-persistent turbulent layers that would hurt the image. I can believe this would be very effective but I haven't tried it.
Yes it is indeed effective. This was reported on this forum few months ago.
Atmospheric seeing is mainly effected by uneven distribution of temperature gradient across time and space. Turbulent air flow in itself or pressure gradients don't have much effect on atmospheric seeing. Fans inside the tube mix air and make temperature gradient disappear.
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Loc: NE Ohio
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: EFT]
#5373893 - 08/17/12 09:56 PM
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The only scopes that I am familiar with that have routinely had fans or the ability to easily add them have been the Intes Micro MAK and Mak/Newt scopes.
That's why I pointed out two specific SCT examples (90's Tak, 90's to present Meade). If we leave the SCT world they become positively common - look at the many RC and DK variants from a wide variety of manufacturers that incorporate them.
I realize that as an SCT fan vendor you have an incentive to make them appear rare and new but folks have the right to know the facts.
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EFT
Vendor - Deep Space Products
Reged: 05/07/07
Loc: Phoenix, AZ
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: jrcrilly]
#5374144 - 08/18/12 02:30 AM
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The only scopes that I am familiar with that have routinely had fans or the ability to easily add them have been the Intes Micro MAK and Mak/Newt scopes.
That's why I pointed out two specific SCT examples (90's Tak, 90's to present Meade). If we leave the SCT world they become positively common - look at the many RC and DK variants from a wide variety of manufacturers that incorporate them.
I realize that as an SCT fan vendor you have an incentive to make them appear rare and new but folks have the right to know the facts.
Come on John. It is not my intention to make them appear rare at all. I am truly unaware of very many examples of SCTs that had venting built in, passive or active. If I was aware of such things I would simply say so. I have no need to decieve people nor any incentive that I consider sufficient to do so in this regard. Being in business is not what I personally consider incentive to lie or decieve. Nor do I need to toot my own horn like that in this thread since I can promote products in the vendor forum all I want.
My only point has been that the current crop of SCTs, including most if not all Celestrons back to the start and Meades back through at least the LX200s have been neither passively or actively vented. I am unsure to what Meade SCTs or MAKs that Meade currently produces that have passive or active venting as you imply. If I am wrong on that, then I am happy to see the examples. I think that the Meade LX400 had fans but it is one of the only SCTs of either brand that I can think of before the Edge HD. MAKs have been fairly inconsistent on this but far more likely to have fans than SCTs for some reason. There are plenty of smaller MAKs like the Orion 127 and 150 that have no vents or fans. There are however plenty of examples of open tube scopes with fans and there are good and similar reasons for that.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Alph]
#5374200 - 08/18/12 04:50 AM Attachment (9 downloads)
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That hunk of steel (aluminum ?) actually acts as a heat sink. The less air trapped behind the primary, the better. If all space behind the primary mirror were filled with aluminum, then the fans could just blow at the rear casting.
Apparently you regard a large hunk of metal behind the mirror as a good thing in big SCT's or maks. Such an attitude is quite unique on earth, but may be common on Melmac, where the ground is cold and the sky is hot.
The purpose of the metal is to act as a counterweight so the long tube can fit in a fork. In normal use the 7" lx50 has the ota connected to the fork so I was never aware of the extra mass in the ota. Even when I took the ota off the fork I didn't realize it contained an extra 8 pounds. But if you take the fan off you would be in for a surprise because the fan is blowing directly onto a hunk of metal.
Of course it isn't aluminum - because it's a counterweight. It's presumably either iron or steel.
Of course it isn't a heat sink - because in normal use the telescope is cooling when you take it outside. It is a massive heat source with high thermal capacity - and right where you don't want it - behind the mirror keeping it hot while the front is facing the cold sky.
Attached is an image showing views of the weight during overhaul. Note that there is no gap around the weight in the tube - it is in fact held in with sealant all the way around, connecting it to the tube. It is attached to the back of the scope with thick dollops of sealant giving it about 5mm gap for air flow. There is a hole visible - but it is not for the fan - it is for the focuser. The fan blows directly onto a thick metal plate and a trickle of air would then flow over the plate and into the ota through the coma shaped opening in the middle of the weight. In the upper right image the hole on the left, with the square imprint around it, is for the fan and you can see the metal inside. The hole on the right is for the focuser and it is clear.
This is definitely how *not* to cool a cat/mak.
Frank
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Jon Isaacs
Postmaster
Reged: 06/16/04
Loc: San Diego and Boulevard, CA
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: EFT]
#5374234 - 08/18/12 06:40 AM
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I am unsure to what Meade SCTs or MAKs that Meade currently produces that have passive or active venting as you imply. If I am wrong on that, then I am happy to see the examples.
I would like to know more myself. I have seen more than a few Meade and Celestron SCTs of a variety of vintages, I have either missed something or the standard Meade and Celestron SCTs, the LX-200's SCTs, the LX-90s, the LX-3s the Celestron Ultima 2000s, the Nexstar SE series etc, were not vented nor cooled with a fan...
Jon
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Loc: NE Ohio
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Jon Isaacs]
#5374407 - 08/18/12 09:48 AM
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I am unsure to what Meade SCTs or MAKs that Meade currently produces that have passive or active venting as you imply. If I am wrong on that, then I am happy to see the examples.
I would like to know more myself.
Hi, John. The 16" LX200 kas always incorporated fans.
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EFT
Vendor - Deep Space Products
Reged: 05/07/07
Loc: Phoenix, AZ
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: jrcrilly]
#5374534 - 08/18/12 11:34 AM
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I am unsure to what Meade SCTs or MAKs that Meade currently produces that have passive or active venting as you imply. If I am wrong on that, then I am happy to see the examples.
I would like to know more myself.
Hi, John. The 16" LX200 kas always incorporated fans.
I was thinking that the 16" might, but since I don't have one of those sitting around I couldn't remember.  It's not surprising that an observatory class OTA would have fans. I will have to take a look at my club's 16" the next time I get a chance.
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Alph
Pooh-Bah
Reged: 11/23/06
Loc: Melmac
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: freestar8n]
#5374535 - 08/18/12 11:34 AM
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This is definitely how *not* to cool a cat/mak.
That's how they cool the CCD in a camera. They put a heat conductor as close to the CCD as physically possible. I am not saying that that piece of metal was designed as a heat sink. However it will not make cooling the mirror any harder.
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Jon Isaacs
Postmaster
Reged: 06/16/04
Loc: San Diego and Boulevard, CA
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Alph]
#5374559 - 08/18/12 11:52 AM
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That's how they cool the CCD in a camera. They put a heat conductor as close to the CCD as physically possible. I am not saying that that piece of metal was designed as a heat sink. However it will not make cooling the mirror any harder.
The issue is that the chuck of steel itself needs to cool down. Thermal equilibrium is just that, the scope needs to be in thermal equilibrium, otherwise there are tube currents and thermal gradients in the optical path. It's like putting a warm rock in a kettle of water, there will be convection currents until the rock has equilibrated.
Cameras are quite a different situation, you are using heat condition through an aluminum block to cool the CCD, it's conduction rather than convection and aluminum is one of the best materials for heat conduction.
Jon
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Alph
Pooh-Bah
Reged: 11/23/06
Loc: Melmac
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Jon Isaacs]
#5374580 - 08/18/12 12:04 PM
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The issue is that the chuck of steel itself needs to cool down.
The chunk of metal will cool off much faster than a mirror made of glass. Ah! Did you see a kettle with a bottom made of copper. The principle is the same. Cameras are no different.
It is like putting a cold rock in a kettle of water. 
Edited by Alph (08/18/12 12:07 PM)
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Steve Cobb
member
Reged: 04/26/10
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: jrcrilly]
#5374680 - 08/18/12 01:06 PM
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Perhaps its a simple question but I recently bought a C14 non-HD which however came with 2 small vents. What do people think of the effectiveness of these Celestron vents? Do they speed of cooling effectively?
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elwaine
Pooh-Bah
Reged: 06/18/06
Loc: S.W. Florida
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Re: Oh Big Cat thou art as wise as thou art beautiful
[Re: Alph]
#5374698 - 08/18/12 01:21 PM
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This is definitely how *not* to cool a cat/mak.
That's how they cool the CCD in a camera. They put a heat conductor as close to the CCD as physically possible. I am not saying that that piece of metal was designed as a heat sink. However it will not make cooling the mirror any harder.
Apples and oranges comparison, Alph. The mass of iron in the Meade Mak stores heat and does not conduct heat from the mirror nor does it do much of a job radiating heat out of the OTA. The heat sinks in CCDs and on some computer chips are light weight aluminum or copper and have large surface areas via fins. Unlike iron, they do not efficiently store heat. Unlike iron, they do radiate heat efficiently. Furthermore, the heat sinks on chips contact the chip and thus remove heat via conduction, and then radiate the heat via fins. (If you cook, you know the difference between a heavy iron skillet and a thin, lite weight aluminum pan.)
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