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Which acclimates faster, a 150 mak or an 8” SCT

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#26 Sarkikos

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Posted 10 June 2021 - 07:19 AM

Mike, the Evolution 8" on the Evo Mount comes in BOTH the 'Standard' SCT configuration AND the EDGE configuration.  I have had BOTH, and had them several times over.  I am VERY familiar with them.  The 'standard' Evo-8 OTA is simply a 'standard' C8, same as all the others.  The only difference is the StarBright/XLT Coatings which came standard on all  models produced after 2005, (and a few before that, notably some of the NexStar GPS models) as opposed to just the Starbright coatings.  Of course, if you wish, we can go back to the 70's, when there was no mention of a type of coating, save for the 'Special Coatings', which were a purchased option.  Anything else?

Why the attitude?  Why so defensive?  I meant no offense.  shrug.gif

 

I just think it's quirky how the Evolution 8" is named by the mount rather than the telescope that's on the mount.  I would expect that from Meade.  Now Celestron is doing likewise?

 

Yes, one more thing, a clarification.

 

You said you tested "the Orion 6" Mak (Synta) from 2016, (with the 'standard' SCT Threads on the Rear Cell); and the Celestron (Synta) 'Evolution' 8-Inch SCT OTA."  And you said the Evolution 8" comes in standard C8 and EdgeHD 8" flavors.  Which type of 8" SCT did you test against the Orion 6" Mak?  You were so busy giving a history of Celestron 8" SCTs, you did not make that point clear.

 

Mike


Edited by Sarkikos, 10 June 2021 - 07:28 AM.


#27 Bean614

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Posted 10 June 2021 - 07:32 AM

The 'standard' 8" Synta SCT, same as the one the OP inquired about.

 

"Why the attitude?  Why so defensive?"...???  No attitude, not being 'defensive', just trying to provide as much correct information as possible.  No offense taken, and I hope you realize that none was given.  


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#28 Sarkikos

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Posted 10 June 2021 - 08:52 AM

Thanks guys for your input.  There seems to be no definitive answer so far.  I have both a 150 mak ota and a Celestron 8 inch SCT ota.  Was thinking of selling the one that takes the longest to acclimate since both have really good optics.  I only recently acquired a second mount, but have not had a good night to compare one to the other.  Tough call....

I've often thought of selling my Bosma 150 Mak, because I seldom use it.  It has great optics, is especially good for lunar viewing and double stars.  But it is heavier than my EdgeHD 8" and in my experience takes longer to acclimate than the EdgeHD 8".  So guess which one I use more often?  The EdgeHD 8", of course.  Much more often.  In fact, I can't remember the last time I took out the 150 Mak.  

 

But in my case, I have a Bosma 150 Mak and an EdgeHD 8".   

 

If you have an Orion / Synta 150 Mak and a standard C8 SCT, and they acclimate at about the same rate, I'd keep the C8.  (I doubt if the Mak would acclimate faster.)   2" more of aperture counts for something.

 

Mike


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#29 Auburn80

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Posted 10 June 2021 - 09:03 AM

The Evolution 8" is an EdgeHD 8" on an Evolution mount. It irks me when the companies name their scopes by the mounts they ride on, rather than by the OTA. fingertap.gif

Mike


Been going on a long time. Celestron was doing it 30 years ago - sorta. Don't remember the other lines but Celestron sold the
SPC80 - An 80mm achro on the Super Polaris mount
SPC8 - 8" SCT on said mount
SPC6 - 6" newt on said mount.
Etc.
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#30 Sarkikos

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Posted 10 June 2021 - 09:23 AM

Been going on a long time. Celestron was doing it 30 years ago - sorta. Don't remember the other lines but Celestron sold the
SPC80 - An 80mm achro on the Super Polaris mount
SPC8 - 8" SCT on said mount
SPC6 - 6" newt on said mount.
Etc.

At least, though, the aperture size was somewhere in the name.

 

Mike


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#31 Old Speckled Hen

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Posted 10 June 2021 - 10:29 AM

I've often thought of selling my Bosma 150 Mak, because I seldom use it.  It has great optics, is especially good for lunar viewing and double stars.  But it is heavier than my EdgeHD 8" and in my experience takes longer to acclimate than the EdgeHD 8".  So guess which one I use more often?  The EdgeHD 8", of course.  Much more often.  In fact, I can't remember the last time I took out the 150 Mak.  

 

But in my case, I have a Bosma 150 Mak and an EdgeHD 8".   

 

If you have an Orion / Synta 150 Mak and a standard C8 SCT, and they acclimate at about the same rate, I'd keep the C8.  (I doubt if the Mak would acclimate faster.)   2" more of aperture counts for something.

 

Mike

Insulate them both, live long and [neverbuyanotherscope] prosper!


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#32 gamma_ari

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Posted 10 June 2021 - 05:43 PM

Most commenters succumb to the fallacy "scope mass ~ cool-down time". This assumes that heat plumes result from different temperatures inside and outside the tube. Wrong. They result from an uneven cool-down of the tube, resulting in a temperature gradient inside the tube. This is why insultation works, which provides for a more even cool-down of the tube.

 

Only controlled experiments can show whether a particular scope "cools down" (i.e. reaches thermal equilibrium inside the tube) faster or slower than another. The presence of a thick corrector plate, like in a Maksutov, doesn't allow premature conclusions.



#33 Asbytec

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Posted 10 June 2021 - 09:03 PM

Yes, a temperature gradient forms inside the tube causing thermal convection. I believe the visible heat plume appears the emanate from the center of a defocused star image and primary baffle because the massive primary mirror retains heat. As evidence, I've observed cold plumes falling through the defocused image when I applied a cold pack to the primary mirror assembly.

Edited by Asbytec, 10 June 2021 - 09:05 PM.

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#34 luxo II

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Posted 11 June 2021 - 01:36 AM

Well, since you are asking for an opinion, I doubt there would be a great deal of difference. To my knowledge, the corrector in the Mak is thicker than that of the SCT, therefore even with the smaller aperture, it might not cool as quickly as you expect.

The thickness of the corrector is irrelevant - what drives the thermal plume inside the OTA is the temperature gradient between the outside of the OTA (cold, conductive metal which will rapidly cool below the ambient air temp) and the central baffle inside, which is supporting a large warm thermal mass (the primary mirror) - and that continues to supply heat to the central baffle.

 

And as Asbytec notes, you can even create a cold plume by applying an icepack to the back of a scope, rapidly chilling the backend and mirror well below the ambient air temperature.

 

Reducing the thermal gradient - one way or another - is what reduces or even eliminates the thermal plume. You can achieve that by reducing the temperature gradient (wait for the scope to cool, passively or with forced cooling), or better, by applying insulation around the OTA to increase the thermal resistance of the OTA and reduce the flow of heat - just like wearing a thick pullover or insulating your house, this works surprisingly well, takes effect in minutes, is low-cost and  best of all, no electrical power required.

 

The thickness - or thinness - of the corrector primarily influences how fast the glass cools and the time to dew over. Having had SCT's many years ago I first noticed this years ago with my first 7" mak which, without any effort on my part, remained dew free long after the SCT's had dewed over, though eventually the mak would dew over too. The corrector on my 10" is a massive piece of glass 30mm thick, and simply its thermal mass usually means dew-free for several hours, though it will eventually dew unless I have fitted a dewcap, or switched on an external fan. I have a heater strap but have only ever needed that once in 5 years.


Edited by luxo II, 11 June 2021 - 01:48 AM.

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#35 gamma_ari

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Posted 11 June 2021 - 02:57 AM

Yes, a temperature gradient forms inside the tube causing thermal convection. I believe the visible heat plume appears the emanate from the center of a defocused star image and primary baffle because the massive primary mirror retains heat. As evidence, I've observed cold plumes falling through the defocused image when I applied a cold pack to the primary mirror assembly.

Did the cold pack help with reaching thermal equilibrium?

 

The thickness - or thinness - of the corrector primarily influences how fast the glass cools and the time to dew over. Having had SCT's many years ago I first noticed this years ago with my first 7" mak which, without any effort on my part, remained dew free long after the SCT's had dewed over, though eventually the mak would dew over too. The corrector on my 10" is a massive piece of glass 30mm thick, and simply its thermal mass usually means dew-free for several hours, though it will eventually dew unless I have fitted a dewcap, or switched on an external fan. I have a heater strap but have only ever needed that once in 5 years.

That is interesting! I feel encouraged to get a larger Mak myself grin.gif



#36 Asbytec

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Posted 11 June 2021 - 05:56 AM

Did the cold pack help with reaching thermal equilibrium?

That is interesting! I feel encouraged to get a larger Mak myself grin.gif

Most definitely it helped, after 30 minutes of cooling it gave textbook stars right out the door. At least in our modest climate. I have no data on colder climates. Observing the cold pack was interesting. I forgot to cool my MCT prior to observing, so a heat plume was evident rising through the defocused star. When I applied the cold pack, almost immediately I saw three cold plumes falling through the defocused star pattern. When I removed it, the heat plume re emerged. Each time it was dampened noticeably. It took several iterations until the heat plume was gone.

I agree with Luxo, the thicker meniscus does seem to be far less susceptible to dew. I experienced dew only once is about 8 years of use, whereas my C11 dewed over frequently. I presume it is the thickness and type of glass that retains some residual heat. And of course climate or dew point matters. Fortunately, it was not enough heat to cause any noticeable degradation to the defocused and focused image. The tropics generally has very good seeing, so any affect of a warm meniscus should have been visible.

Edit: I agree with you. The best way to know for sure is to test it. Set both up and watch. Or, maybe take the temperature at specific points at some time interval. Since the pandemic, maybe use one of those handheld thermometers we see everywhere.

Edited by Asbytec, 11 June 2021 - 08:58 AM.

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#37 Echolight

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Posted 11 June 2021 - 05:06 PM

I'm about to get one of them telegizmos for my C8 so I can just leave it outside for the summer. And maybe a couple of wheels for the AVX so it'll be easier to move to a spot that will be shaded in the daytime. Because here, it's about bringing a scope out of the AC into the sauna and it fogging over.



#38 Cali

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Posted 11 June 2021 - 05:19 PM

Do we get a prize if we guess correctly?

Yeah, a thermally unstabilized 150mm Mak or 8” SCT.

 

- Cal


Edited by Cali, 11 June 2021 - 05:36 PM.

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#39 rmollise

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Posted 12 June 2021 - 06:35 AM

It depends on the design and construction of the OTA, not the optical design. I used to think MCTs naturally took longer to cool what with that corrector. Roland C. schooled me on that a long time ago. lol.gif


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#40 Sarkikos

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Posted 12 June 2021 - 07:45 AM

How closely related are dewing on the optics, acclimation of the optics and tube currents?  Maybe we shouldn't be so quick to conflate them all and expect to prevent all three by the same means.  You can prevent dewing on the optics by keeping the optics warm and / or attaching a longer dew shield.  But how is that going to affect acclimation of the optics and tube currents?  And how is wrapping the OTA going to affect dewing versus acclimation and tube currents?

 

I know by experience that warming the optics and a long dew shield will prevent dewing.  So far the only method I've used to acclimate the optics and remove tube currents is time sitting in the field.

 

Mike


Edited by Sarkikos, 12 June 2021 - 07:49 AM.


#41 Old Speckled Hen

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Posted 12 June 2021 - 09:17 AM

How closely related are dewing on the optics, acclimation of the optics and tube currents?  Maybe we shouldn't be so quick to conflate them all and expect to prevent all three by the same means.  You can prevent dewing on the optics by keeping the optics warm and / or attaching a longer dew shield.  But how is that going to affect acclimation of the optics and tube currents?  And how is wrapping the OTA going to affect dewing versus acclimation and tube currents?

 

I know by experience that warming the optics and a long dew shield will prevent dewing.  So far the only method I've used to acclimate the optics and remove tube currents is time sitting in the field.

 

Mike

Some chaps are using a fan to keep dew off and no heating just insulation to reduce tube currents below detectable limits. Yes I said detectable on purpose!!

 

Edited for spelling. 


Edited by Andrew Brown, 12 June 2021 - 09:22 AM.

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#42 Auburn80

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Posted 12 June 2021 - 09:31 AM

How closely related are dewing on the optics, acclimation of the optics and tube currents? Maybe we shouldn't be so quick to conflate them all and expect to prevent all three by the same means. You can prevent dewing on the optics by keeping the optics warm and / or attaching a longer dew shield. But how is that going to affect acclimation of the optics and tube currents? And how is wrapping the OTA going to affect dewing versus acclimation and tube currents?

I know by experience that warming the optics and a long dew shield will prevent dewing. So far the only method I've used to acclimate the optics and remove tube currents is time sitting in the field.

Mike

Good question. Given the typical situation - warm ota taken to cooler surroundings - my understanding is that insulating the ota would retard dewing until the eventual condition where the ota components aren't warm enough to keep dew off the corrector.
Those who regularly practice insulating the ota could probably answer it best.
This time of year, we in the South have to deal with the opposite - cool ota taken to a warmer (and typically more humid) environment.
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#43 jeremiah2229

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Posted 12 June 2021 - 12:12 PM

Good question. Given the typical situation - warm ota taken to cooler surroundings - my understanding is that insulating the ota would retard dewing until the eventual condition where the ota components aren't warm enough to keep dew off the corrector.
Those who regularly practice insulating the ota could probably answer it best.
This time of year, we in the South have to deal with the opposite - cool ota taken to a warmer (and typically more humid) environment.

It is like this here now. If indoors is 78 and outdoors is 85 for example the corrector on this C8 is a water producing machine lol. Maybe 20 minutes before dripping. The Maks will last a couple of hours before dripping. And the sweat is running off of my nose while I'm setting up.  ;)

 

 

Peace...
 


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#44 Sarkikos

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Posted 12 June 2021 - 02:38 PM

Some chaps are using a fan to keep dew off and no heating just insulation to reduce tube currents below detectable limits. Yes I said detectable on purpose!!

 

Edited for spelling. 

I have more experience in experimenting with acclimation and dew control in Dobs rather than Cats.

 

Fans are used on primary mirrors in Newts to acclimate the mirror and reduce tube currents, depending on how the fans are situated.  In a solid tube Newt, the long OTA tube should be enough to keep dew off the primary.  A fan behind the primary acclimates the primary, and to some extent the OTA, depending on how well air flows around the primary.  In an open design Dob, fans directed toward the side and top surface of the primary can help break up the boundary layer and acclimate the primary. 

 

I've never heard of a fan at the secondary on a Newt.  Some attach a warming strip to the secondary to prevent condensation.  All my Newts have been solid tube, so I've never had to do that.  A dew / light shield extension at the top of the OTA is enough to keep dew off the secondary in my solid tube 10" f/5 Newt.  It also helps shield from ambient light.

 

On the Cat side of the ledger, I installed two fans in the OTA of my EdgeHD 8".

 

Mike


Edited by Sarkikos, 12 June 2021 - 02:40 PM.


#45 Sarkikos

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Posted 12 June 2021 - 02:42 PM

Good question. Given the typical situation - warm ota taken to cooler surroundings - my understanding is that insulating the ota would retard dewing until the eventual condition where the ota components aren't warm enough to keep dew off the corrector.
Those who regularly practice insulating the ota could probably answer it best.
This time of year, we in the South have to deal with the opposite - cool ota taken to a warmer (and typically more humid) environment.

I live on the Eastern Shore of Maryland.  Often here at night it's dewy in the warm months and frosty in the cold.  Keeping the dew / frost off is usually the top priority among observers.  The slackers don't use any prevention.  They wait until the optics dew over, then blast them with a portable hair dryer, or wipe them off (objectives, eyepieces, Telrads and correctors).  How would you like to repeat that routine every 15 minutes?  Or they just go home.  The more prepared use dew shields and wrap warming strips around the optics prone to dewing, and position fans around the primary mirrors.  

 

Mike


Edited by Sarkikos, 12 June 2021 - 02:49 PM.

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#46 luxo II

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Posted 13 June 2021 - 12:06 AM

 Ok… back to the original question I’ll put my money on the 6”. Even an Intes MK66.. Thinner and smaller primary with less thermal mass, shorter central baffle. 

 

The cheap Chinese maks are possibly better.


Edited by luxo II, 13 June 2021 - 12:10 AM.



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