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Instant cooldown small refractor?

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#1 betacygni

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Posted 13 August 2020 - 10:50 PM

Wondering if this concept is a proverbial unicorn.

Does a small refractor exist that requires zero cooldown for moderate to high power observing? I realize this will always depend on what type of temp differences we are talking about, in my case it would be worst case in dead of winter maybe a 30 degree difference.

I’m obviously thinking the best candidates would be 60mm refractors like the AT60ED, TV-60, tak foa-60, etc. But maybe larger 80mm class doublets too? Or maybe something like the oil spaced CFF 80mm triplet (oil spaced provides better image before complete cooldown I’ve read?)?

Basically I’m thinking about getting a grab and go setup, but for me grab and go would be a 30 minute impromptu session with my primary interest being double stars. If it couldn’t provide steady clean splits it wouldn’t really meet my grab and go criteria. Not sure if this is a viable concept, even with a 60mm. What are others experiences on this?

#2 BlueMoon

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Posted 13 August 2020 - 11:00 PM

I'm using my 100ED apo in such a "grab n go" role and it works just fine. 5 minute setup and practically zero cool down. My experience thus far: I'm using it to learn how to split doubles and so far so good. I've been primarily working in Cassiopeia as it's in a favorable position and using Stellarium to choose targets in my magnitude range which appears to extend to about magnitude 14 with current seeing conditions. I've managed about 180x mag with a 5mm "no name" (plossl I think) so far. I'm looking forward to seeing what a Tele Vue 5mm will do.

 

Cheers.


Edited by BlueMoon, 14 August 2020 - 04:22 PM.

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#3 clusterbuster

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Posted 13 August 2020 - 11:35 PM

FYI...

 There are a LOT of doubles in LYRA, also LYRA is the home of EPSILON LYRA, the DOUBLE DOUBLE...

 Mark


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#4 Astro-Master

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Posted 13 August 2020 - 11:51 PM

A 80mm doublet should cool down very quickly, and would be ideal for a quick grab and go scope, and split tighter doubles than a 60mm.


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#5 Hesiod

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Posted 14 August 2020 - 01:45 AM

A 60mm has its "best" asset in the very low resolving power and therefore can dodge very well both seeing and thermal issues.
A zero cooldown does not exist but in my agenda if the cooldown time is less than the time required to carry out and set up the telescope, then in practical term is if as were a "zero cooldown telescope".
I have a 66/400 and 55/300 and never had thermal issues with them even in deep winter; on the other hand my 100mm refractors may take a while to stabilize under large thermal gradients (>20°C), between 15' and 30' befote getting the best results.
It may be worth mentioning however that even when still cooling can split the 2" or so stars which are more or less the upper limits for 60mm ones so, if are interested in these somewhat loose systems both a 100mm and a 60mm behaves exactly the same with tegard of waiting time.
The only real difference is the much greater bulk of the 100mm (and that its tighter diffraction pattern looks less "clean" while cooling)
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#6 Kunama

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Posted 14 August 2020 - 01:54 AM

A Unicorn it is, even the diminutive FOA60Q, which is fine at low and medium powers ( < 100x) while cooling, really only gives its best once fully acclimatised to ambient ( =/> 180x).

 

Here in winter my scope goes from 22C to about 0C and takes 20-30 minutes to reach sub 5C.



#7 areyoukiddingme

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Posted 14 August 2020 - 02:10 AM

My smallest is an 80mm triplet with a carbon fiber tube. When there's a pretty good delta, say 20+ degrees F, I find often takes more than 1/2 an hour to be at its best. 

 

It's often slower than my Televue 101, which is a lot bigger, and with lenses in the back, I'd have through it would be slower, but it's usually a bit quicker than the 80.

 

FWIW, the fastest cool down I've found for any scope is my 8"  F7 reflector. The primary is quartz and is ready to go in a few minutes with a fan blowing on the back. I suspect it would be even quicker with a fan blowing horizontally across the primary, as apparently all that is needed is to remove the boundary layer.

 

But I don't have much experience with big temperature deltas on that one yet, so this may change. . . 



#8 Jon Isaacs

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Posted 14 August 2020 - 08:42 AM

I have an William Optics 80 mm Megrez ll FD, it's an F/7 FPL-53 Doublet. I use it as a grab n go doubles scope. It seems to have essentially zero cool down and sharp optics.

 

I'd choose an 80 mm over a 60 mm because it will split closer doubles even during any cool down.

 

Jon


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#9 Terra Nova

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Posted 14 August 2020 - 08:51 AM

Yes, Zero Cooldown does exist! It’s not Unobtanium or fairy dust!

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Edited by Terra Nova, 14 August 2020 - 08:56 AM.

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#10 Simoes Pedro

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Posted 14 August 2020 - 03:15 PM

My 102 taka triplet takes 10 minutes max. 



#11 Kunama

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Posted 14 August 2020 - 04:32 PM

My 102 taka triplet takes 10 minutes max. 

I suspect that would only be the case if the temperature difference from inside to outside was 5 degrees. 

 

Having owned three Tak TSA102S scopes as well as three FS102 scopes, I found the TSA102S can take up to an hour to reach ambient if the temperature delta is 20 degrees or more.....


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#12 gnowellsct

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Posted 14 August 2020 - 04:51 PM

In summer more of an issue about heat up time.  Take a scope out of an air conditioned house and watch the objective dew up as soon as you uncork it outside.  Can happen to your glasses too.

 

Greg N



#13 John Huntley

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Posted 15 August 2020 - 06:40 AM

My Tak FC100-DL F/9 performs really well at high magnifications straight out of the house. My Vixen ED102SS F6.5 doublet however needs at least 30 minutes to give it's best star images.

 

Not quite sure why this is but there you go hmm.gif


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#14 Rutilus

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Posted 15 August 2020 - 06:51 AM

I suspect that would only be the case if the temperature difference from inside to outside was 5 degrees. 

 

Having owned three Tak TSA102S scopes as well as three FS102 scopes, I found the TSA102S can take up to an hour to reach ambient if the temperature delta is 20 degrees or more.....

This was also the case with my TSA-102.  I could still use the scope straight out of the house, but only after it

had cooled down properly would it give the best views.


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#15 Daniel Mounsey

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Posted 15 August 2020 - 02:38 PM

Instant cooldown small refractor?

 

If you are just trying to have a good casual evening observing, then don't worry too much about this, otherwise you may encounter analysis paralysis. You can obviously see by the responses throughout many refractor forums, nothing appears to be an issue with refractor performance to a reasonable degree as others have claimed. It's true to some extent but it's a bit more complicated. This is the main reason I use refractors quite often for what I enjoy to study in the sky. I'm extra careful when it comes to acclimating because I mainly observe variable stars, double and multiple stars, planets and the Moon from my preferred light polluted skies.

 

More often with refractors, cooldown is taken for granted. Refractors actually take just as long as reflectors and cats and in some cases even longer to acclimate which can literally be several hours into the night. So why doesn't this appear to be a problem? Why do so many believe their refractors cool so quickly, say 10 to 30 minutes?

 

Here's what actually happens and it can be studied and scrutinized if you understand what you're looking at. In most of these smaller refractors it's not a a big issue because the lower mass of the objectives in these simpler doublets and even some smaller triplets hardly shift or change shape. When they do, it's pretty minimal, particularly longer focal ratios where you typically have two elements that work in harmony with each-other. When you get to triplets, it can get a bit stranger.

 

As the objectives in these refractors acclimate, the light rays obviously converge but something interesting happens. As the objective acclimates, the light rays from the outer perimeter of the objective converge inward faster or closer than light rays from the central portion of the objective, also known as under-correction. This is not as much an issue with small 60mm and 80mm objectives but the faster the optics, particularly with triplets, the more evident it may become, especially as the lens objectives increase in size and mass. An 8" or 9" triplet alone can easily require 3 hours for the objective to finally reach a null. 

 

If you study and scrutinize the fresnel pattern when the focuser is racked slightly outwards from perfect focus in good seeing conditions, you will typically notice that the outer-most diffraction ring in the fresnel pattern will appear more faint and ill-defined while the central airy disc will exhibit more energy and a brighter image at its center. When the focus is racked slightly inward from perfect focus, you will typically see just the opposite and/or see a well defined fresnel pattern. This is completely normal and what you want to see at first and it's common with the exception of just a few brands of refractors. As the optic acclimates, the spherical correction will slowly shift to a null and when the objective reaches that point, that's how you know the optic is settled well enough to perform quite good for most applications, but it's still not good enough for the best results. This is why all the purists I observe with often open the back of their tubes and point to objectives down to let the OTA ventilate and I do the same.

 

So once again, why do so many believe their refractors cool so quickly, say 10 to 30 minutes?

 

The answer is simply because the photons are being bent away from the wall of the tube rather than reflected back and forth which in turn, scatters the images with cats and reflectors. The observer sees this and instead, interprets that their refractor appears to be cooled down but it's actually not cooled down in fact in most cases just the opposite. To prove it to yourself, after you've been observing for a couple of hours (depending on the size of the objective), point the optical tube downward toward the ground and remove the diagonal and place your index finger inside the OTA. You will immediately feel heat funneling up and it will warm your finger quickly. With larger refractors it will usually last for several hours into the night. If that's not enough, take an infrared thermometer and point it into the walls and baffles of the OTA and you will find out very quickly.

 

As the objective is pointed upward, the heat inside the OTA rises and cooks the objective from below like a pressure cooker for a few hours and as the objectives get larger and faster (elements having to work in perfect harmony) it often takes longer for the spherical correction to reach a null. Bad seeing conditions can also mimic this effect so the observer has to be very careful distinguishing the difference. 

 

I don't recall which magazine article itis but several individuals in the forums have posted it as a testimonial, but that too is more complicated than most realize. It was a test regarding an AP155EDF if I recall correctly. The author said the fresnel patterns appeared nice and uniform just 30 minutes into the session. To the best of my knowledge, no other comments were made about the spherical correction 1 or 2  hours later which is more important to me in some areas where others observe in strong deltas or extremely cold environments

 

Most AP155's I've tested (which were 7 in total samples over the years from friends of mine who owned them or loaned them to me), I would notice that the fresnel patterns were often slightly over-corrected after their objectives were finally at a null. I discussed this with Roland Christen in person several years ago at NEAF to ask him why he does this. He did in fact acknowledge this and his response was that he doesn't mean to do this and tries to make his objectives behave as perfect as possible and BTW, don't bother posting his star testing premier, I discussed that with him as well and there's some things about that others are not fully aware of. 

 

So why would this even be an issue in the first place? When a refractor acclimates, what you always want to see is some degree of under-correction, not overcorrection. Why? Let's say you take your refractor out from your warm house and into the cold and you observe the fresnel pattern just 15 or 30 minutes into the observing session like that review. What you can expect to see is a nearly perfect fresnel pattern as was described in that review just 30 minutes later. 

 

Normally an observer who has done this countless times knows you always want to check the spherical correction later than 30 short minutes into the session with larger refractors, particularly triplets, even when oiled because whenever you see perfect fresnel patterns out of the gate, that usually spells doom if it's too extreme because the objective will always sway over and unfortunately it's a one way ticket. With under-correction, the objective starts out under and works its way forward till it finally comes to a rest and that's normally the way you want the objective to settle.

 

This is by no means meant to imply AP155EDF's are an issue. I was not present at that reviewers observing session and I may not have all the facts, but that's the way the review appeared to me from what I gathered. BTW, AP155's are some of my favorite planet scopes (even more than their AP160's) because I absolutely love the images their oil spaced objectives produce on planets and the degrees of spherical correction I'm referring to are minimal and not much to be concerned about but it's interesting nevertheless. I have an NP101 that does this too. 

 

What about carbon fiber and cooling? In a "closed tube", carbon fiber is the absolute worst material you can use. It has been marketed as a stiff to help prevent change in focusing while imaging in temperature extremes. The truth is that what's really causing the change in focus is the fact that nearly all objects exhibit under-correction. If you focus early in the night and the focus changes after and hour or so, the main reason this is happening isn't because of your tube, it's because the objectives and their spherical correction are still settling to a null. As a result, the focus shifts. This is the real cause. 

 

Steady skies!

---dr:D

 

 

 

 

 

 

 

 


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#16 Echolight

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Posted 15 August 2020 - 04:03 PM

My grab and go is a 6 inch f8 that stays in an outdoor storage bin with the Unistar mount already attached. Instant acclamation.

 

Just four thumb screws for the spreader on the tripod and slip the mount in place and snug it down and I'm in business.

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