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Importance of cool down

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#1 GOLGO13

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Posted 25 October 2021 - 09:24 PM

One of the reasons I like refractors is quick cool down. Also, the scope is still pretty usable during cool down.

 

However, we shouldn't forget that refractors need cool down also. And the larger the refractor the more cool down.

 

Certain designs also require more cool down than others (say triplet vs doublet). Sometimes the glass used can make a difference.

 

My new TSA-120 appears to have a good 20-30 minute cooldown in the times I've used it so far (going from 73 inside to 55 outside). This isn't too different from the Vixen 130mm refractor I had in the past...it was usually about 30 minutes. My Vixen 103s I've always felt was 15-20 even with extreme temperature changes. My 60mm doublet is pretty darn minimal in cool down. Probably 5-10 in a worst case scenario.

 

Cooldown with Mirrored scopes is incredibly important and also a reason I tend to use refractors 95% of the time in the winter. I think cool down is not given enough discussion sometimes in these forums. It's extremely important with any telescope (including refractors), but especially important with mirrored scopes. 


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#2 vtornado

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Posted 25 October 2021 - 09:51 PM

My 6 inch cat and 12 inch dob easily trump my 4 inch refractor.

But sometimes they aren't ready to go when I am.


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#3 GOLGO13

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Posted 25 October 2021 - 10:00 PM

My 6 inch cat and 12 inch dob easily trump my 4 inch refractor.

But sometimes they aren't ready to go when I am.

No doubt there. Usually I bring both my 10 inch dob and 4 inch refractor to a dark site. Then, as the big dob is cooling I use the refractor. Once the dob is cooled I switch to it. 

 

A 10 inch dob with a fan is usually 45 minutes to an hour. No fan 2 hours. 

 

In the past I would get so frustrated with cool down. I'd put the scope out and the conditions were good. 1-2 hours later the clouds had moved in and I'm bringing the scope back in with no observing! 

 

I find a 6 inch Cat is not too far better than a 4 inch refractor, but yes has a bit more resolution and brightness. Of course totally different FOV. However, I do think a 6 inch cat is a wonderful scope. I really like the 6SE as a starting scope. One of my first scopes was a Celestron 5i (predecessor to the SE line)...really enjoyed using that scope. But now I'm more of a refractor guy, but I do have a 5 inch and 8 inch SCT.


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#4 bobzeq25

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Posted 25 October 2021 - 10:11 PM

Another advantage of having an observatory.  A well designed one is well on the way to cool down before the roof is even opened.


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#5 Redbetter

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Posted 25 October 2021 - 11:02 PM

In general in the forums I see cool down emphasized too much, rather than too little. Ironically, for the given type it is perhaps mentioned most in the Cat and Reflectors forums, and least in Refractors.  The cool down time for the Z10 with fan is pretty short, enough that it takes only 15 minutes or so more than the 110ED to cool--not much of a factor at all.  That is what I have seen in side-by-side in the backyard looking at Jupiter.  The Z10 rapidly overtakes the frac and then runs away from it.  

 

The SCT and Mak can take longer, although the typical way I handle them is to set them out early to get a head start.  For quick looks I can cheat a little with the 127 Mak and put its thermal blanket on, and just take it right out to observe.  It is several times more portable than the the big frac.  It isn't quite the same as being fully equilibrated, but sufficient for quick looks.  The frac requires some cool down by comparison.

 

The 20" takes considerably longer, but most often it needs to drop by 30+ F for the observing site, vs. what it was in the garage...somewhat of an apples to oranges comparison to back yard.   For the big scope the first 20 minutes of cooling are during set up, the next 20 minutes are early observation/evaluating conditions.   Another 20 minutes rids the most objectionable thermal effects, and often brings it close to what the sky/seeing will allow, unless the seeing is particularly good.   In good seeing it will be another half hour or so before the scope is showing signs of topping out.   The thing of it is, if the seeing is even mediocre, the 20" will be running away from others much sooner than that.      

 

Yes, cool down matters most for quick short looks, and favors smaller apertures.  But if seeing is suitable for more than quick looks, a larger frac, cat, or reflector will be worth it before long.  

 

One of the advantages to the smaller aperture refractors is that they better handle the lousy seeing suffered by objects low in the sky.  And being mounted higher (and often more portable) these scopes can better avoid observing obstacles.   


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#6 GOLGO13

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Posted 25 October 2021 - 11:09 PM

Probably should have mentioned that cool down issues can vary greatly by the observing location. Certain locations have rapid drop in temps during the night compared to other locations. And certain times of the year also.


Edited by GOLGO13, 25 October 2021 - 11:17 PM.

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#7 havasman

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Posted 26 October 2021 - 12:30 AM

In general in the forums I see cool down emphasized too much, rather than too little.

waytogo.gif waytogo.gif

 

Coming to thermal equilibrium throughout our optical systems is an achievable goal but there is observing that can be done well while the thermals optimize. 


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#8 Muffin Research

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Posted 26 October 2021 - 01:47 AM

I usually prefer the winter with reflectors as here temperatures are more steady, temps drop fast to a certain temperature and stay there for hours over the bulk of the night.


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#9 ziggeman

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Posted 26 October 2021 - 02:57 AM

Including dew heaters? ;)



#10 Muffin Research

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Posted 26 October 2021 - 03:14 AM

Including dew heaters? wink.gif

Ah yes, cooldown isn't as much as a worry, you're starting to think about heating things up again :D although the dew shield helps, by the time they dew over it's kinda time to pack up and get some sleep anyway.

 

But I love dry and icecold conditions for viewing, everything is just so crystal clear, no twinkling stars, and heatplumes are easily spotted and avoided.



#11 Jon Isaacs

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Posted 26 October 2021 - 03:51 AM

In general in the forums I see cool down emphasized too much, rather than too little. Ironically, for the given type it is perhaps mentioned most in the Cat and Reflectors forums, and least in Refractors.  The cool down time for the Z10 with fan is pretty short, enough that it takes only 15 minutes or so more than the 110ED to cool--not much of a factor at all.  That is what I have seen in side-by-side in the backyard looking at Jupiter.  The Z10 rapidly overtakes the frac and then runs away from it.

 

 

My 10 inch F/5 GSO Dob has a fully baffled fan and I normally set it out at least an hour before sunset.  I do not compare it's cool down to other scopes, I compare it to itself, i.e., when has it fully cooled to the point where it gives it's best possible views...  

 

IMG_21092021_184554_(1024_x_700_pixel).jpg
 
That is how I judge the importance of thermal equilibrium, How long must it cool until the scope is ready to give it's best possible view?  How close to it's best possible views are it's initial views when first set outside?
 
These are the areas where refractors excel.  Ultimately, the 10 inch Dob provides the better views of the planets and close double stars than the 120mm F/7.5 ED Orion Eon but the Eon's views are initially very good and soon enough, less than 20 minutes in San DIego's mild climate, it will be providing it's best possible views.  
 
The Dob, even with the fan running, takes more than an hour to start providing clean splits on those sub-arcsecond doubles.  
 
Coming to thermal equilibrium throughout our optical systems is an achievable goal but there is observing that can be done well while the thermals optimize.

 

 

:waytogo:

 

My situation is quite different now than it was a dozen years ago.  I no longer travel to a dark site for a one nighter, arriving around sunset and setting up. Most of my dark sky stuff is done from our little place in the high desert, the scopes are permanently setup and they live in a well ventilated garage. When I do travel, it's in our motor home and the scope is setup well before sunset and will remain assembled for several days.

 

But in any event, when viewing DSOs from dark skies, I have never felt it necessary to wait for the scope to come to thermal equilibrium, I think of it like I do the seeing, I just choose my targets based on both the seeing and the thermal state of the scope.  Most DSOs are not particularly sensitive to seeing/thermal equilibrium so it's not a big deal.  

 

If I were taking a scope that had been at 72 degrees in a heated garage or house outside where it was 35 degrees, I might think a little differently.  But the way it works is that if it going to be 35 degrees outside, it's probably about 45 degees or even less in the garage.  

 

When I used to do one nighters, I just choose my objects in accordance with the scope and the seeing.. the same thing I do now.

 

Jon


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#12 Ihtegla Sar

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Posted 26 October 2021 - 08:29 AM

Probably should have mentioned that cool down issues can vary greatly by the observing location. Certain locations have rapid drop in temps during the night compared to other locations. And certain times of the year also.

Exactly. People who think cool down issues are overemphasized tend to live in mild climates where they don't have to contend with extreme day/night temperature swings and temps that drop continuously all night.

The old saw about just setting the scope out for an hour at sunset may work in a mild climate but not so well when it's 95 degrees at 10:00 pm just as the sun sets, 70 degrees an hour after sunset, 60 degrees an hour later at midnight and 50 degrees an hour before dawn at 3:00 am. That's a typical summer day where I'm from.

Cool down isn't much of an issue in winter when the nights are long and temperature swing is only 20 degrees. Of course clouds prevent observing 95% of the time in winter.

But in the summer when the "night" is only four hours long and the temperature can drop an average of 10 degrees an hour all night, cool down is a major issue.

My 20" never reaches equibrilium in the summer despite a bunch of fans and storing it at ambient in a well ventilated shed. By the time fall rolls around, when the nights are longer and temperature swings are closer to 20 degrees than 40, cool down is no longer a problem.

Edited by Ihtegla Sar, 26 October 2021 - 08:31 AM.

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#13 unimatrix0

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Posted 26 October 2021 - 08:39 AM

I usually prefer the winter with reflectors as here temperatures are more steady, temps drop fast to a certain temperature and stay there for hours over the bulk of the night.

Also, winter is a great time to use a DSLR with full frame or ASP-C if someone doesn't have those in cooled Cmos versions.   When it drops below freezing at night, the DSLR sensor will be cooled for sure and with the crystal clear air they pull in a lot of nice details on DSOs without much noise. 



#14 vtornado

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Posted 26 October 2021 - 10:14 PM

some of the cool-down over emphasis is for new-bee's who keep their scope in a 70 degree house,

and bring it outside to 30 degrees in the winter, immediately start viewing and complain about the views.

 

When I started I did not know this and wondered why my 8 inch cat was not very sharp.

Although to be fair to myself there was no cloudynights to get valuable info from.

 

Many of the new-bees are not die hard observers.  They are casual and don't plan to observe.

They take their scope out immediately and want to observe.

 

Many new-bee's don't have a second scope to occupy them when the big one is cooling down.


Edited by vtornado, 26 October 2021 - 10:22 PM.


#15 Redbetter

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Posted 27 October 2021 - 12:09 AM

Exactly. People who think cool down issues are overemphasized tend to live in mild climates where they don't have to contend with extreme day/night temperature swings and temps that drop continuously all night.

The old saw about just setting the scope out for an hour at sunset may work in a mild climate but not so well when it's 95 degrees at 10:00 pm just as the sun sets, 70 degrees an hour after sunset, 60 degrees an hour later at midnight and 50 degrees an hour before dawn at 3:00 am. That's a typical summer day where I'm from.

Cool down isn't much of an issue in winter when the nights are long and temperature swing is only 20 degrees. Of course clouds prevent observing 95% of the time in winter.

But in the summer when the "night" is only four hours long and the temperature can drop an average of 10 degrees an hour all night, cool down is a major issue.

My 20" never reaches equibrilium in the summer despite a bunch of fans and storing it at ambient in a well ventilated shed. By the time fall rolls around, when the nights are longer and temperature swings are closer to 20 degrees than 40, cool down is no longer a problem.

With respect to the assertion in the first paragraph:  not really.  "Mild climate" is misleading since the temperature drop here is typically much greater than the more temperate climates I have lived in.  In fact, thermals are a bigger issue here than other places I have lived, but this area has what I consider a very mild climate.  It is hot and clear during the summer, but the low humidity makes even 105+ comfortable enough if one is not sitting in direct Sun...milder than mid 90's in summer in most places I have lived/worked.   "Winter" is laughably mild, at least from the perspective of my back yard.  

 

A 20 degree swing would be nice here, usually coincides with cloud cover and more humid conditions.  30+ F delta is a given most nights, and it can be considerably more.  The difference is that the more arid climate means larger late afternoon to late evening temperature drops, even for backyard viewing.   And if I go up a mountain the swing is typically 40 to 50 F.  That is what my 20" often sees.  Mid-summer the temp in the garage will be 90 F with the temp on the mountain in the 50's (sometimes 40's in the AM hours.)  The scope has it easy by comparison, I am typically running in 100-105 F conditions, then observing at 50 F in summer.   Colder parts of winter here will be 60 in the garage with the temp on the mountain in the 20's at warmest, and down below 10 at times.   

 

So what I see with the 20" is that most of the limitation from thermals is in the early phases of cooling.  I can see that while collimating and sighting in (then touching up collimation once the initially cooling has taken effect.)  Things settle in after that, even with the temp still declining.  Bulk seeing problems generally swamp residual mirror thermal issues after the initial cooling.  These atmospheric seeing problems can be observed as the directional "flowing river" effect, or fluctuating breezes/ambient temp, random drift in and out, etc.   It isn't a problem with the scope's cooling that is holding it back; and I know this from nights when the ambient temperature does stabilize for hours, yet there is no net change in the performance of the scope during the period.   There are other times, when the seeing sharpens noticeably for an hour or so, before returning to what it had been.  And that sharpening and worsening tend to coincide with any breeze.  They don't seem to have much correlation to further cooling the scope.  Similarly, when I have a refractor set up beside the 20", I can observe the same seeing effects, proving that it isn't specific to the Dob. 

 

That is also why I have not added additional cooling to the 20"--I realized after a time that it would be only the early cooling that was significantly impacted.  And honestly, that period doesn't matter much to me.  Why?  Because early evening is novice observing time anyway for DSO's.  I often find myself giving tours or helping someone during that time.  It is the time when there always seem to be a lot of interruptions, traffic (general public and other observers), and people who set up early and then pack up and leave just before things get good.  It is the most frustrating/least productive time to actually try to observe.  My motto is:  the real observing doesn't start until midnight.  That is when I can settle in looking for threshold objects.  



#16 Ihtegla Sar

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Posted 30 October 2021 - 02:46 PM

With respect to the assertion in the first paragraph: not really. "Mild climate" is misleading since the temperature drop here is typically much greater than the more temperate climates I have lived in. In fact, thermals are a bigger issue here than other places I have lived, but this area has what I consider a very mild climate. It is hot and clear during the summer, but the low humidity makes even 105+ comfortable enough if one is not sitting in direct Sun...milder than mid 90's in summer in most places I have lived/worked. "Winter" is laughably mild, at least from the perspective of my back yard.

A 20 degree swing would be nice here, usually coincides with cloud cover and more humid conditions. 30+ F delta is a given most nights, and it can be considerably more. The difference is that the more arid climate means larger late afternoon to late evening temperature drops, even for backyard viewing. And if I go up a mountain the swing is typically 40 to 50 F. That is what my 20" often sees. Mid-summer the temp in the garage will be 90 F with the temp on the mountain in the 50's (sometimes 40's in the AM hours.) The scope has it easy by comparison, I am typically running in 100-105 F conditions, then observing at 50 F in summer. Colder parts of winter here will be 60 in the garage with the temp on the mountain in the 20's at warmest, and down below 10 at times.

So what I see with the 20" is that most of the limitation from thermals is in the early phases of cooling. I can see that while collimating and sighting in (then touching up collimation once the initially cooling has taken effect.) Things settle in after that, even with the temp still declining. Bulk seeing problems generally swamp residual mirror thermal issues after the initial cooling. These atmospheric seeing problems can be observed as the directional "flowing river" effect, or fluctuating breezes/ambient temp, random drift in and out, etc. It isn't a problem with the scope's cooling that is holding it back; and I know this from nights when the ambient temperature does stabilize for hours, yet there is no net change in the performance of the scope during the period. There are other times, when the seeing sharpens noticeably for an hour or so, before returning to what it had been. And that sharpening and worsening tend to coincide with any breeze. They don't seem to have much correlation to further cooling the scope. Similarly, when I have a refractor set up beside the 20", I can observe the same seeing effects, proving that it isn't specific to the Dob.

That is also why I have not added additional cooling to the 20"--I realized after a time that it would be only the early cooling that was significantly impacted. And honestly, that period doesn't matter much to me. Why? Because early evening is novice observing time anyway for DSO's. I often find myself giving tours or helping someone during that time. It is the time when there always seem to be a lot of interruptions, traffic (general public and other observers), and people who set up early and then pack up and leave just before things get good. It is the most frustrating/least productive time to actually try to observe. My motto is: the real observing doesn't start until midnight. That is when I can settle in looking for threshold objects.


Okay, so you don't live in a "mild climate" but you think cool down issues are overemphasized because seeing issues often overwhelm cooling issues where you live due to thermals and your scope has plenty of time to cool because you don't start observing for several hours after sunset because of all the novices and traffic at your observing site?

How thick is your mirror? Mine is 2" (Obsession).

There is no one ever at my observing site after dark, so I don't have to worry about that. And there aren't any thermals nearby as I'm out on a small steppe and it's quite a few miles to the nearest mountains. But I do have to contend with high altitude seeing issues as I'm right under the jet stream.

I too often set up a four inch refractor (FC100DF) next to my 20" dob. Many (maybe most) nights the refractor does better than the 20" on planets and double stars due to cooling issues with the big mirror that renders planetary views complete mush and makes tight double stars impossible. I installed corner boundary fans but that didn't seem to help. The only time I've gotten good planetary views out of the 20" during the summer planetary season is early morning right before the sun comes up when temperatures stop dropping and start rising. Then the 20" gives outstanding planetary views.

The 20" can also give good planetary views after a few hours of cooling during seasons other than summer when the nights are longer and the temperature swing is less.

Since it takes unique circumstances for the 20" to give good planetary and double star views, I use it almost exclusively for DSOs since most DSOs aren't as affected by by cooling as planets and double stars. And I can use my 4" refractor in town for planets and double stars since those aren't as affected by light pollution.

So for me, cooling issues are the main factor that determines where and when and what scope I can use for which targets.
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#17 SteveG

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Posted 30 October 2021 - 03:12 PM

Cool down for my 10" dob is at least 1 hour, and up to two hours depending on spread. It's coming out of a 74 deg house, and low 40's outside. I don't view below 40 deg F.

My 4" doublet refractor, about 15-20 minutes.

My 60 mm refractor - no cooldown required.


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#18 GOLGO13

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Posted 30 October 2021 - 03:24 PM

One thing I've noticed with the TSA-120 is the star test is very different early in cooling as opposed to when it's cool. So with this scope I think cool down is quite important. Depends on the scope and the size, but I always feel cool down is an important factor with most telescopes. 4 inch doublet and below, not bad at all. But most other scopes it's pretty huge.


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#19 Lentini

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Posted 30 October 2021 - 04:59 PM

One thing I’ll mention with respect to cooling reflectors… If the temperature is dropping, don’t turn off the fan. The mirror won’t keep up with any delta in temperature by itself.

 

I am looking forward to my first winter with my two refractors… The very quick cool down will be interesting to monitor.



#20 Redbetter

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Posted 30 October 2021 - 05:37 PM

Okay, so you don't live in a "mild climate" but you think cool down issues are overemphasized because seeing issues often overwhelm cooling issues where you live due to thermals and your scope has plenty of time to cool because you don't start observing for several hours after sunset because of all the novices and traffic at your observing site?

How thick is your mirror? Mine is 2" (Obsession).

There is no one ever at my observing site after dark, so I don't have to worry about that. And there aren't any thermals nearby as I'm out on a small steppe and it's quite a few miles to the nearest mountains. But I do have to contend with high altitude seeing issues as I'm right under the jet stream.

I too often set up a four inch refractor (FC100DF) next to my 20" dob. Many (maybe most) nights the refractor does better than the 20" on planets and double stars due to cooling issues with the big mirror that renders planetary views complete mush and makes tight double stars impossible. I installed corner boundary fans but that didn't seem to help. The only time I've gotten good planetary views out of the 20" during the summer planetary season is early morning right before the sun comes up when temperatures stop dropping and start rising. Then the 20" gives outstanding planetary views.

The 20" can also give good planetary views after a few hours of cooling during seasons other than summer when the nights are longer and the temperature swing is less.

Since it takes unique circumstances for the 20" to give good planetary and double star views, I use it almost exclusively for DSOs since most DSOs aren't as affected by by cooling as planets and double stars. And I can use my 4" refractor in town for planets and double stars since those aren't as affected by light pollution.

So for me, cooling issues are the main factor that determines where and when and what scope I can use for which targets.

You misread what I wrote.  I start observing about an hour after I arrive at the site most of the time.  About half of that time is set up, with the mirror running during most of that.  The rocker box is wrapped in a blanket for protection on the way to the site, so it isn't cooling much until unloaded and the fan is connected.  Lots of disruptions, a fair amount of planetary observing early associated with those.  It isn't my most effective observing time, so I don't worry about it, and instead work around it, often arriving later.

 

Mine is a 2" thick Galaxy mirror in an Obsession.   I don't know why yours has so much trouble cooling sufficiently or keep up, but mine does not.  "Summer planetary season" in the Pacific Northwest might be part of the problem.  The ecliptic is rather low in the Summer that far north (it isn't well placed here either), so the major limitation to seeing is unlikely to be the cooling of the mirror, but instead the problems of observing planets low in the sky which you won't have control over.   As I have said before, this lower elevation seeing favors smaller apertures and refractors mounted up off of the ground.  

 

My 20" trounces the refractor's views any time the seeing is good enough for planetary.  I see that near sea level in the backyard, and I see it up in the mountains.  I don't use the scopes much pointed at planets particularly low in the sky because of the seeing disruption (and to a lesser extent atmospheric chromatic dispersion.)

 

I find the jet stream far less of a factor here than local thermal mediated weather patterns.  Most of the disruption appears to be relatively close to the ground, rather than the high atmosphere.   Breezes have been the best indicator.  When the surface layer becomes calm the seeing is sharp.  When the diurnal patterns pick up, or some sort of change is happening (where I can actually feel the temp delta's in the mixing air), the seeing gets bad.   Mush in the 20" is not great in the refractor either...that is what I see side-by-side when this happens.

 

This can be particularly bad up in the mountains where thin layers of katabatic winds can dominate.  It is a major problem at my darkest site at over 8,000 feet.  DSO observing isn't even worthwhile in such conditions when the disruption is severe.   I can have some wonderfully dark and transparent sky, and pack up because the seeing is so poor that 15 mag stars are little pseudo-galaxies.  I make the call whenever I conclude that I would rather not record observations of blurred galaxies that won't reveal characteristic details.  Sleep becomes a better use of the time.

 

Smoke in the air during recent summers/and early fall helped transform my impressions of the major sources of poor images in scopes.  I had been puzzled in prior years why even lower jet stream speeds did not reduce naked eye twinkle in summer, and how the seeing was chronically poor in all of the scopes in the backyard.  With smoke preventing me from observing DSO's, I was doing more planetary in the backyard with the 20".  That revealed a surprising benefit to the smoke:  thermal stabilization of the air.  Twinkle would subside (for stars that could still be seen) and the images would become still in all the scopes.  And as soon as the air began to clear, the seeing began to deteriorate in all the scopes.  It wasn't thermal problems with the mirror (and I knew that already from the twinkle) but roiling layers of cooling ground level air that was the source of the problem.



#21 RichA

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Posted 30 October 2021 - 05:38 PM

My 6 inch cat and 12 inch dob easily trump my 4 inch refractor.

But sometimes they aren't ready to go when I am.

It's a good thing.  I once put my 12 inch LX200 out back to cool down...and my father turned on the sprinkler system.



#22 Ihtegla Sar

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Posted 30 October 2021 - 06:26 PM

You misread what I wrote. I start observing about an hour after I arrive at the site most of the time. About half of that time is set up, with the mirror running during most of that. The rocker box is wrapped in a blanket for protection on the way to the site, so it isn't cooling much until unloaded and the fan is connected. Lots of disruptions, a fair amount of planetary observing early associated with those. It isn't my most effective observing time, so I don't worry about it, and instead work around it, often arriving later.

Mine is a 2" thick Galaxy mirror in an Obsession. I don't know why yours has so much trouble cooling sufficiently or keep up, but mine does not. "Summer planetary season" in the Pacific Northwest might be part of the problem. The ecliptic is rather low in the Summer that far north (it isn't well placed here either), so the major limitation to seeing is unlikely to be the cooling of the mirror, but instead the problems of observing planets low in the sky which you won't have control over. As I have said before, this lower elevation seeing favors smaller apertures and refractors mounted up off of the ground.

My 20" trounces the refractor's views any time the seeing is good enough for planetary. I see that near sea level in the backyard, and I see it up in the mountains. I don't use the scopes much pointed at planets particularly low in the sky because of the seeing disruption (and to a lesser extent atmospheric chromatic dispersion.)

I find the jet stream far less of a factor here than local thermal mediated weather patterns. Most of the disruption appears to be relatively close to the ground, rather than the high atmosphere. Breezes have been the best indicator. When the surface layer becomes calm the seeing is sharp. When the diurnal patterns pick up, or some sort of change is happening (where I can actually feel the temp delta's in the mixing air), the seeing gets bad. Mush in the 20" is not great in the refractor either...that is what I see side-by-side when this happens.

This can be particularly bad up in the mountains where thin layers of katabatic winds can dominate. It is a major problem at my darkest site at over 8,000 feet. DSO observing isn't even worthwhile in such conditions when the disruption is severe. I can have some wonderfully dark and transparent sky, and pack up because the seeing is so poor that 15 mag stars are little pseudo-galaxies. I make the call whenever I conclude that I would rather not record observations of blurred galaxies that won't reveal characteristic details. Sleep becomes a better use of the time.

Smoke in the air during recent summers/and early fall helped transform my impressions of the major sources of poor images in scopes. I had been puzzled in prior years why even lower jet stream speeds did not reduce naked eye twinkle in summer, and how the seeing was chronically poor in all of the scopes in the backyard. With smoke preventing me from observing DSO's, I was doing more planetary in the backyard with the 20". That revealed a surprising benefit to the smoke: thermal stabilization of the air. Twinkle would subside (for stars that could still be seen) and the images would become still in all the scopes. And as soon as the air began to clear, the seeing began to deteriorate in all the scopes. It wasn't thermal problems with the mirror (and I knew that already from the twinkle) but roiling layers of cooling ground level air that was the source of the problem.


You misread what I said about the "summer planetary season." In the same paragraph I mentioned both planets and double stars as being problematic in the 20" due to cooling. There are plenty of double stars that are not low in the sky where I'm at (45 degrees north latitude). When I am looking at double stars I am looking at stars as close to the zenith as I can. I have the same cooling issues with both planets and double stars in bigger scopes, and only the planets are low in the sky.

I've set my four inch refractor next to both my 20" dob, my 8" SCT (and also the 12" dob I used to own) and the refractor almost always wins on both planets and double stars.

I'm not sure why your 20" doesn't have the cooling issues mine does. I set mine out just before sunset and out a camping fan on it and turn on the rear fan and the four corner fans. Once I start observing I turn off the big camping fan but keep the others on. It still doesn't completely cool during the summer, until just before dawn when the temperture stops dropping.

Since I've had similar issues with my C-8 and the 12" Skywatcher I used to own, I assume that my location has a lot of temperture drop throughout the night. I should get a portable thermometer and track the temperature throughout the night.

Dobs are on the warm ground so they will struggle more with cooling issues and heat rising from the ground than a scope on a tripod. But why would having a scope on a tripod make a difference with respect to seeing?

Smoke. That's one advantage of my location. The wildfire smoke rarely interferes with my observing. Last year I only got smoked out for about three weeks and I didn't have any issues at all with smoke this year. There are some very dark skies a few hours south in eastern Oregon but looking at the smoke maps convinced me not to even bother with a summer camping trip there.
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#23 alnitak22

alnitak22

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Posted 30 October 2021 - 06:53 PM

My 6 inch cat and 12 inch dob easily trump my 4 inch refractor.

But sometimes they aren't ready to go when I am.

An insulating wrap like Reflectix totally eliminates thermal issues in a Mak.  I have the Orion 6” Mak and went from my 70 degree apt to my rooftop where it was 42 here recently. The Mak showed a perfect star test on Deneb at 200x immediately, and took 250x on Jupiter and the moon 5 minutes later with excellent results. We’ll  see how it does in the dead of winter, but getting the tip about Reflectix was a game changer for me. I resisted getting a Mak for years due to tales of thermal issues. This is now a non issue for anyone who wants a Mak. Even my TV85 needs around 20 minutes in winter to be best at high power, though it can be used at low power right out the door. My 6” Newt needs about 30 minute of cooldown in winter to give best images. The Reflectix wrap will make my Mak the least affected by thermal issues instead of the most! 

 

p.s.,..I know you wrote 6” Cat instead of Mak, but I see no reason why the Reflectix wouldn’t work as well on a Cat as it does on a Mak.


Edited by alnitak22, 30 October 2021 - 07:24 PM.

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#24 Ihtegla Sar

Ihtegla Sar

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Posted 30 October 2021 - 07:59 PM

An insulating wrap like Reflectix totally eliminates thermal issues in a Mak. I have the Orion 6” Mak and went from my 70 degree apt to my rooftop where it was 42 here recently. The Mak showed a perfect star test on Deneb at 200x immediately, and took 250x on Jupiter and the moon 5 minutes later with excellent results. We’ll see how it does in the dead of winter, but getting the tip about Reflectix was a game changer for me. I resisted getting a Mak for years due to tales of thermal issues. This is now a non issue for anyone who wants a Mak. Even my TV85 needs around 20 minutes in winter to be best at high power, though it can be used at low power right out the door. My 6” Newt needs about 30 minute of cooldown in winter to give best images. The Reflectix wrap will make my Mak the least affected by thermal issues instead of the most!

p.s.,..I know you wrote 6” Cat instead of Mak, but I see no reason why the Reflectix wouldn’t work as well on a Cat as it does on a Mak.


My C-8 has worse thermal issues than my 20" dob. I bought some insulation and reflectix a couple months ago but haven't gotten around to crafting a jacket for it yet. Definately on my To Do list. I don't take it out much anymore because of the cooling issues.

It's an older scope that has been in storage for a long time. I took it out for the first time in years on a warm summer night in 2020 when the temperature never dropped below 70 and it gave some incredible views of Saturn that I still remember, easily besting my four inch refractors. But the next few times I had it out were a big disappointment. I set it up next to one of my four inch refractors for several hours and went back and forth looking at Jupiter, Saturn and Mars, and ended up spending most of the time with the Tak since it was giving much better views due to the thermal issues with the C-8. After that, the C-8 has stayed in the case.

If the reflectix trick works I will be very happy.
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#25 alnitak22

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Posted 30 October 2021 - 11:06 PM

My C-8 has worse thermal issues than my 20" dob. I bought some insulation and reflectix a couple months ago but haven't gotten around to crafting a jacket for it yet. Definately on my To Do list. I don't take it out much anymore because of the cooling issues.

It's an older scope that has been in storage for a long time. I took it out for the first time in years on a warm summer night in 2020 when the temperature never dropped below 70 and it gave some incredible views of Saturn that I still remember, easily besting my four inch refractors. But the next few times I had it out were a big disappointment. I set it up next to one of my four inch refractors for several hours and went back and forth looking at Jupiter, Saturn and Mars, and ended up spending most of the time with the Tak since it was giving much better views due to the thermal issues with the C-8. After that, the C-8 has stayed in the case.

If the reflectix trick works I will be very happy.

I think the Reflectix will work! Give it a try and see. A C-8 is a nice scope and would be a shame to only be able to use it in summer.




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