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Apos, achros, and cool-down time

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#1 Aratus

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Posted 14 October 2020 - 07:44 PM

Hi all! Long-time, no post.

 

So, everyone seems to agree that triplets, naturally enough, take longer to come to thermal equilibrium than doublets of the same aperature.

 

But every now and then I hear or read the suggestion that achromatic doublets, made of "simpler", less dense glasses, cool faster than ED or fluorite doublets. Sometimes I hear that they also cool at a rate closer to that of common tubing/cell material - which improves the view - or maybe the two elements cool in greater "sync" with each other.

 

Is this true? Is there really a noticeable difference? Does anyone have experience with this?

 

I'm just curious. As someone that observes in Canadian winters, cooling time is something I think about a fair bit at this time of year!  I only own achromats, so I don't really have any means of comparing.

 

Best,

David



#2 Mitrovarr

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Posted 14 October 2020 - 08:22 PM

I don't think so.

I will say that longer scopes take more time to cool down than shorter ones, and look worse while cooling.

#3 sunnyday

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Posted 14 October 2020 - 08:26 PM

one trick I learned is to leave the diagonal cap open, this lets hot air come out faster.



#4 Ihtegla Sar

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Posted 14 October 2020 - 08:38 PM

I have two 4" apo doublets (f/7.4 and f/9) and a 4" achro (f/5) and I haven't noticed a difference in the cool down time between any of them. But they all cool really fast and I typically don't use the f/5 achro at high power so I can't say whether the achro cools faster or not since I've never worried about cooling when using the achro at low power. The 4" apos are ready for high power after anywhere between 5 and 20 minutes. Depending on the temperature differential. Usually closer to 5-10 minutes but it took a full 20 minutes when going from a 70 degree house to a 35 degree winter night before I could crank up the magnification. That was about the only time I noticed tube currents in any of them since the apos are typically cooled by the time I get my chair and eyepieces set up.
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#5 barbie

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Posted 14 October 2020 - 08:39 PM

My 72mm ED doublet cools relatively quickly.



#6 otocycle

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Posted 14 October 2020 - 09:00 PM

My achros cool faster because they have less mass....less glass and metal for a given aperture...my guess.   Maybe call it thermal inertia.  Regardless, I try to plan ahead and store the OTAs close to outside ambient temperature (in an unheated garage), using the case to moderate cooling.    I do not like taking fluorite doublets from a warm indoors to way below freezing temps outdoors.



#7 sunnyday

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Posted 14 October 2020 - 09:06 PM

tonight because of this thread I tried to watch without letting the telescope cool down, not very pretty at first but after 15-20 minutes it was fine.
lunt 102 mm ed.


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#8 Deadlake

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Posted 15 October 2020 - 02:55 AM

I own a 103 mm APO doublet. If I get the next size up, say 130 mm, what's the difference between a triplet which is air or oil gap spaced? I've been told oil spaced will cool quicker, but how much so?  



#9 Rutilus

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Posted 15 October 2020 - 04:03 AM

I've never really noticed a difference regarding Achro and Apo doublets, to me they seem to take the same for cool-down.

I certainly did notice a difference comparing  triplets to a doublet lens.  



#10 Mr. Mike

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Posted 15 October 2020 - 06:06 AM

I have a 102mm triplet and I dont think it takes that long to stabilise temp wise.  Now, I dont view during extreme heat or cold so that might have something to do with it.  For most viewing sessions i simply set up the scope on my patio/viewing area about an hour before its dark.  Issue solved. :)


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#11 John Huntley

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Posted 15 October 2020 - 07:31 PM

My Vixen ED102SS F/6.5 (ED doublet) takes around 30 minutes to cool down so that it is giving it's best images at high powers.

 

My Takahashi FC100-DL F/9 (Fluorite doublet) takes around 5-10 minutes but I have just carried out the house and used 200x with good results.

 

The Vixen has surprised me in the length of time that it takes. I thought I had a poor objective when I first got the scope but once cooled it's very good.


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#12 Jethro7

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Posted 15 October 2020 - 10:02 PM

Hello David,

I have timed my  4" ED F/ 7 and 4" Achro F/11  both  cool down pretty fast, about 20 minutes.  My C8 takes a hour to reach temperature equilibrium do to the size of the tube and mirror. It is interesting to view this change. The views are not gradual as I would expect them to be, it just seems to pop.  There are many factors involved with the size of the scope, density of the glass, ect... I need to add a few more minutes for the Baader Zeiss Prism Diagonal to reach temperature equilibrium. The mirror Diagonals are ready when the scopes are cooled down or warmed up. 

 

HAPPY SKIES AND KEEP LOOKING UP Jethro



#13 Deadlake

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Posted 16 October 2020 - 03:11 AM

My Vixen ED102SS F/6.5 (ED doublet) takes around 30 minutes to cool down so that it is giving it's best images at high powers.

 

My Takahashi FC100-DL F/9 (Fluorite doublet) takes around 5-10 minutes but I have just carried out the house and used 200x with good results.

 

The Vixen has surprised me in the length of time that it takes. I thought I had a poor objective when I first got the scope but once cooled it's very good.

John,

What about the 130 mm LZOS APM?

Thanks


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#14 gnowellsct

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Posted 16 October 2020 - 07:54 AM

one trick I learned is to leave the diagonal cap open, this lets hot air come out faster.


The downside is that in high humidity environments you are putting humid air into the tube where it can condense onto the objective. So that can lead to not only a DEW problem but a tendency for particles in the air to stick to the moist surface of the lens.

In my area of the United States I would not open the tube. But in Colorado I very well might.
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#15 Aratus

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Posted 16 October 2020 - 07:57 AM

Reflecting on the responses -- and particularly the generalized sense that most people experience cool-down times for their doublets of 10-20 min (obviously this would vary with temperature differential, aperture) -- it occurs to me that any difference between achro-apo cool-down would actually have to be quite pronounced for anyone to even notice.  I mean, let's say a doublet with specialized ED glass does take a third longer to get to an acceptable level of cool-down -- we'd still only be talking about five minutes, probably, not enough to really notice?

 

In practice, at least during the warmer months, I find that if I set up the scope first, by the time I have my chair, books, eyepieces, etc, out and I'm set to go, the vast majority of the time my scopes (100mm and 80mm) are fine. Only with my 4" f/15 might I actually wait a bit longer - using the diagonal-off trick.

 

That said - I do usually wait at least an hour before I do any planetary, just in case there are subtler effects on the lens. Maybe that's unnecessary... :)


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#16 gnowellsct

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Posted 16 October 2020 - 08:00 AM

I wonder whatever happened to the idea of putting your tube out to cool a little bit before using it. If you are at a star party you can set things up and then run around talking to other people. With social distance, of course.

If you're setting up a German equatorial that usually takes some time and you can get a head start on cooling just by bringing the tube out before you start and putting it somewhere where it's open but safe.

If you've got a quick setup altaz situation You can set up and if you don't want to go around visiting your friends and talking about their rigs then You can look at a star chart or other resource and see what's out there to take a look at. This might include checking at heavens - above to see if any telescopic comets are around.

#17 SeattleScott

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Posted 16 October 2020 - 10:47 AM

I had a 100ED that took 15 minutes to cool tops, even in middle of winter. But then it started taking over 20 minutes. How did the cool down time suddenly get longer? I switched to a 2” diagonal. At 4” or less, with a doublet and a 2” diagonal, your cooldown time is really diagonal limited in my experience.

So when I see a 4” Tak taking considerably shorter time to cool than a 4” Vixen, both doublets, I have to wonder if the Tak has a 1.25” diagonal. After all, the Vixen is fast and would require a 2” diagonal to do justice to its wide field capabilities. The Tak is F9, more of a planetary scope, and he has the Vixen for wide field. So maybe he just uses a 1.25” diagonal with the Tak.

It might be interesting to see if there is a difference in cooldown with mirror versus prism diagonals. I only have mirrors.

The 2” diagonal is a significant chunk of glass that is not exposed to outside air, other than when you swap eyepieces. The person who said they remove their diagonal cap to speed cooling has the right idea (but as pointed out this might not be the best approach in a humid climate, idk). Just to be clear, the issue isn’t taking the cap off the diagonal so the air inside the tube can escape. The issue is more allowing the diagonal to cool.

Scott

Edited by SeattleScott, 16 October 2020 - 11:10 AM.

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#18 Deadlake

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Posted 16 October 2020 - 01:33 PM

I had a 100ED that took 15 minutes to cool tops, even in middle of winter. But then it started taking over 20 minutes. How did the cool down time suddenly get longer? I switched to a 2” diagonal. At 4” or less, with a doublet and a 2” diagonal, your cooldown time is really diagonal limited in my experience.

So when I see a 4” Tak taking considerably shorter time to cool than a 4” Vixen, both doublets, I have to wonder if the Tak has a 1.25” diagonal. After all, the Vixen is fast and would require a 2” diagonal to do justice to its wide field capabilities. The Tak is F9, more of a planetary scope, and he has the Vixen for wide field. So maybe he just uses a 1.25” diagonal with the Tak.

It might be interesting to see if there is a difference in cooldown with mirror versus prism diagonals. I only have mirrors.

The 2” diagonal is a significant chunk of glass that is not exposed to outside air, other than when you swap eyepieces. The person who said they remove their diagonal cap to speed cooling has the right idea (but as pointed out this might not be the best approach in a humid climate, idk). Just to be clear, the issue isn’t taking the cap off the diagonal so the air inside the tube can escape. The issue is more allowing the diagonal to cool.

Scott

For instance Baader makes two sizes of diagonal 33 and 44 mm aperture in size in both prism and mirror flavour. Both can support 2” eye pieces. Would you group the 2” with the 33 mm aperture one? The prism is meant to be better at absorbing stray light, however cool down is longer where a mirror would take 5 minutes to acclimate.

 

What happens when you get to 130 mm for a scope presume that dominates over diagonal cooldown?



#19 SeattleScott

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Posted 16 October 2020 - 03:10 PM

Can’t speak for a 130mm Apo but most are triplet so definitely the refractor cooldown would exceed diagonal at that point with a triplet.

My 2” eyepieces have 46-47mm and 34-35mm field stops. So a 33mm diagonal would likely vignette slightly (or a lot for the 42lvw). A 44mm prism should be fine I think. My 2” mirrors take a good 20 minutes to acclimate in winter. Not sure if prism would be better or worse.

A prism is really best for an Apo. With a mirror scope you are already bouncing the light off two mirrors so adding a third doesn’t seem like it would be a big deal. Unless the point is to shave some mm’s off the back focus. But I hear they are nice for a premium Apo. Idk because I don’t have one, and likely won’t as long as I have my LVWs.

Scott
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#20 John Huntley

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Posted 16 October 2020 - 07:45 PM

John,

What about the 130 mm LZOS APM?

Thanks

Not as long as I feared when I first got the scope (2016). About 30-45 minutes. I'm in the SW UK so we don't really get it too cold here.



#21 Don Taylor

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Posted 16 October 2020 - 08:31 PM

I think there is something to this in theory - but perhaps it doesn’t make much difference in practice.

 

An example comparing optical glass properties of Ohara FPL-53 with BK7 (or the Ohara equivalent) shows the ED glass has about 2 times the thermal expansion coefficient of a “normal” crown glass. This means the shape of an ED lens element will be affected twice as much when undergoing a temperature change. Likewise the ED glass conducts heat only about 75% as well as the normal glass. This means it takes longer for the ED glass to reach thermal equilibrium.

 

When you expose the scope to a temperature change - some parts change temperature relatively quickly and other take more time. The outer surface of the objective feels the change quickly but the second surface, buried in the lens cell feels the change more slowly until the temperature change is conducted through the glass and the element reaches essentially a uniform temperature (and returns to its nominal shape).

 

The lens is distorted by the temperature change because not all regions of the glass change temperature at the same rate. Same goes for the second element.  Complicating things further is how heat is conducted through the lens cell from or into the lens elements.

 

This all means the shape of an FPL-53 lens element will be affected more, and the lens will take longer to return to its nominal shape than a dimensionally similar “normal” glass lens in an otherwise similar telescope when exposed to the same temperature change.

 

BUT - life is not so simple. The optical prescription (curves, lens thickness, airspace gap, etc.) of an achromat doublet and an ED doublet of the same aperture and focal length will not be the same due to differences in optical properties of the glasses used and the lens designer’s choices in optimizing the prescription. e.g. the ED lens and the “normal glass” lens would not be “dimensionally similar”.  Differences in lens cell design and fit, differences in tube baffles (affecting convection cells within the tube) and the thickness and material of the tube itself probably have some effect as well. Direct comparisons would be a challenge.

 

But in practice - based on the many field reports in this thread it seems any difference between an ED doublet and achromat doublet is small. How many would notice their ED scope took 21 minutes to settle down and their acromat only took 19?

 

I actually own two nearly identical Kunming 102mm F11 scopes, one a crown/flint achromat and the other an ED doublet. So I guess I could set up a small experiment.  But there are differences between the two scopes that might mask any difference in time to reach equilibrium.

 

So, while  it looks like there is a difference in theory, I think I’ll not worry too much about it and just observe.


Edited by Don Taylor, 16 October 2020 - 08:37 PM.


#22 Ihtegla Sar

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Posted 16 October 2020 - 08:37 PM

I'm mostly looking at planets and double stars in town, so I'm using a 1.25" diagonal and don't know if my scope might take longer to cool than my 2" diagonal. If I'm using the 2" diagonal then it's likely that I'm at a dark site since wide fields usually means im going after DSOs and there the scope will be set up at sunset so cooling is less of an issue and I never kept track of how long since it will be an hour before I use it. I keep my metal end cap and metal focuser cap on while cooling to protect the scope from dust, dew, and anything else. My 4" Tak doublet still cools incredibly fast, too fast to notice a difference between my Tak and my achro.

I wonder whatever happened to the idea of putting your tube out to cool a little bit before using it. If you are at a star party you can set things up and then run around talking to other people. With social distance, of course.

If you're setting up a German equatorial that usually takes some time and you can get a head start on cooling just by bringing the tube out before you start and putting it somewhere where it's open but safe.

If you've got a quick setup altaz situation You can set up and if you don't want to go around visiting your friends and talking about their rigs then You can look at a star chart or other resource and see what's out there to take a look at. This might include checking at heavens - above to see if any telescopic comets are around.


If I'm going out to a dark site, sure I set everything up at or shortly before sundown and it has an hour to cool before the skies get really dark. For me, having a scope that cools very quickly is more of a necessity for short sessions at home.

When I am trying to get in a short session and have work in the morning, and I'm dealing with the vagrancies of weather that can turn on a dime, having a scope that cools very quickly is imperative.

I'm not going to put a scope out in the yard at sunset any leave it exposed to possible rain on the off chance that the weather may clear up later or that I might get an hour of spare time later at night when I'm not too tired to observe.

I keep a GEM fully assembled in the garage that I can pick up and carry in one piece, along with my observing chair, and my Tak and eyepieces I keep handy inside in my spare room. I can be set up and ready to go in five minutes flat, if I get the opportunity to observe.

Sometimes I wake up in the middle of the night and can't get back to sleep. If I look out the window and it's clear, I will set the scope up for an hour before going back to bed.

For these types of short, opportunistic sessions at home you just can't beat a scope that cools quickly.

#23 noisejammer

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Posted 17 October 2020 - 04:56 AM

Hi all! Long-time, no post.

 

So, everyone seems to agree that triplets, naturally enough, take longer to come to thermal equilibrium than doublets of the same aperature.

 

But every now and then I hear or read the suggestion that achromatic doublets, made of "simpler", less dense glasses, cool faster than ED or fluorite doublets. Sometimes I hear that they also cool at a rate closer to that of common tubing/cell material - which improves the view - or maybe the two elements cool in greater "sync" with each other.

 

Is this true? Is there really a noticeable difference? Does anyone have experience with this?

 

I'm just curious. As someone that observes in Canadian winters, cooling time is something I think about a fair bit at this time of year!  I only own achromats, so I don't really have any means of comparing.

 

Best,

David

I don't know who 'everyone' is but it's not my experience. I commented on this in another thread...

 

Over the years, I've used a lot of refractors including triplets from AP (152 f/9 & 155 EDFS), APM (mine and a 152 f/8), Tak (mine, a TOA130 and an FS102), WO (FLT110) and probably several others. I've used achromat doublets from my first 60mm to a 19th century 6" & 7" to a 26" in the Johannesburg observatory.

 

Short version - I think this is an internet myth that keeps going around and around and around. It may be true for some scopes but it is certainly not an issue with mine.
 


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#24 Mark9473

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Posted 17 October 2020 - 05:29 AM

The difference in cooling between an ED doublet and an achromat of the same aperture, would be because typically the ED will be faster, requiring thicker lenses. More glass, longer cooling.
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#25 csauer52

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Posted 17 October 2020 - 08:26 AM

It's been my experience that specifically as it relates to AP, there's no noticeable difference whatsoever. Of course I'm not doing high power work so perhaps there's something in that regard but for AP alone, cooldown matters little, if even at all.




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