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How do refractors tolerate the cold?

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#1 cschopke

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Posted 08 October 2018 - 08:16 AM

Hi all,

 

I am fairly new to astronomy and have only had a telescope (Astromaster 130) for a few months. I have unfortunately (for my wallet) gotten hooked, and want to acquire some sort of apochromatic refractor. However, winter temperatures where I live in Norway have been known to hit -20C or even lower. Can these temperatures damage the precision optics in refractor telescopes? Any advice about the effects of low temperature on equipment is much appreciated.

 

Thanks,

Christian



#2 junomike

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Posted 08 October 2018 - 08:25 AM

Christian, Welcome to Cloudynights.  You'll be sure to get a slew of varying answers and depending on what size OTA you're consdiering, the answer may change.

Also, Its not only  the initial temp drop that can damage optics but the re-warming of then when you bring them back inside.

Some people slowly acclimate the OTA slowly by bringing it back inside the case before taking it out (if needed to dry).

Also, IMO air spaced lenses don't usually pose an issue although some owners of Fluorite take caution against shocking the lenses.

I for one have seen the issues of an oil spaced lens suffering "thermal shock" so I'm not a fan of those as my location gets rather cold as well.



#3 Richard O'Neill

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Posted 08 October 2018 - 09:18 AM

Typical airspaced doublets tolerate cold very well, much better than do I. As already stated, do take care to avoid rapid thermal changes and they'll be fine.



#4 diog

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Posted 08 October 2018 - 10:22 AM

Because they are much simpler devices. Only 2 lens, without the need to collimate and introduce errors my metals and other materials



#5 michael h

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Posted 08 October 2018 - 03:19 PM

Hello Christen

Welcome to CN.

 

I lived in Alaska for five years.   About 2003  I was on observing with a Celestron  100mm refractor on a Celestron Great Pacific mount.  The Celestron was a 2 element air spaced doublet made by Vixen in Japan.  About 10:00pm that night I noticed that the tracking was not working correctly.  I checked the temperature and it was -24F (-31C).

The scope was fine at that temp but the grease in the mount was getting stiff.  Should have changed that grease.  I used to cap it after observing and store it in an unheated shed.

 

 

Mike H



#6 Sketcher

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Posted 08 October 2018 - 03:21 PM

As long as "rapid" temperature changes are avoided, most refractors should do just fine.  They're designed to be used outside at night!

 

I've used cemented doublets, air-spaced doublets, and an oiled-triplet at fairly low temperatures.  If I recall correctly, all have been used at temperatures down to at least minus 20 degrees F (minus 29 degrees C).  My record cold observation was at minus 50 degrees F, but that was with 20x80 binoculars.  Very few people would want to set up and use any telescope when it's that cold!

 

I do take precautions when exposing my larger refractors to low temperatures.  "Thermal shock" is something that one should be aware of and take safeguards against.  On cold nights the telescope is taken outside while in its case.  The cased telescope is permitted to cool for a while before opening the case and removing the telescope.  At the end of the session, the telescope is returned to its (cold) case prior to bringing it back inside.

 

It wouldn't hurt to let the manufacturer know what temperatures you'll be exposing the telescope to -- and asking them for advice on how to safeguard the optics.  I did this for my oiled-triplet over 20 years ago.  That telescope is still as good as it was on its first day of use!

 

You should be able to find more information by Googling: "refractor telescope + thermal shock"



#7 leviathan

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Posted 09 October 2018 - 12:37 AM

+1 for slow cooldown and avoiding thermal shock. We've used 100ED at -20C here in Kazakhstan, as long as you care about optics and don't immediately jump from hot +25C hot room with telescope to -25C (50 dgr. delta !) - it should be fine.

 

My own record is around -30C for several hours observing mostly deepsky objects, but no refractors there, just big aperture dobsonians. wink.gif



#8 shredder1656

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Posted 09 October 2018 - 05:47 AM

I've asked similar questions before, but most of the equipment I used was fairly inexpensive and typical air-spaced doublets. I'm still using an older "classic", but it's a pricier one (in my opinion). It's a Celestron/Vixen C102F, and since someone mentioned Fluorite here, it gave me pause.

I'm in Indiana, and the temps get below zero F. I've been out in that weather, I actually enjoy at least short sessions in it, and have had a frosted tube for sure. The Fluorite is new to me, so it's never been taken out like that.

It sits on our unheated/uncooled front porch. So, while I wouldn't think the difference from inside to outside would be considered dramatic, I want to be certain. It's always capped when not in use.

Is Fluorite that much more fragile so that I should avoid subzero temps? I doubt I will participate when the temps are lower than -10* F, and that's a rare occasion anyway. But, zero can be an average for a few nights.

I guess the main question, after all of that and reading the earlier comments is, since the scope is typically set up in an unheated area, not in a case, it should be fine, correct? The difference might still be 30+degrees, due to ambient heat, so is that too "shocking" for Fluorite?

#9 SandyHouTex

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Posted 09 October 2018 - 09:39 AM

Hi all,

 

I am fairly new to astronomy and have only had a telescope (Astromaster 130) for a few months. I have unfortunately (for my wallet) gotten hooked, and want to acquire some sort of apochromatic refractor. However, winter temperatures where I live in Norway have been known to hit -20C or even lower. Can these temperatures damage the precision optics in refractor telescopes? Any advice about the effects of low temperature on equipment is much appreciated.

 

Thanks,

Christian

You should be cautious if you buy a refractor with a Fluorite front element.  Taking it from a warm house into a very cold environment with snow could shatter it.  Fluorite does not like being cooled quickly in a small area due to thermal stress.  If you want to buy one, just keep the lens cap on the front as it acclimates to the environment.



#10 mvas

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Posted 09 October 2018 - 10:10 AM

Hi all,

 

I am fairly new to astronomy and have only had a telescope (Astromaster 130) for a few months. I have unfortunately (for my wallet) gotten hooked, and want to acquire some sort of apochromatic refractor. However, winter temperatures where I live in Norway have been known to hit -20C or even lower. Can these temperatures damage the precision optics in refractor telescopes? Any advice about the effects of low temperature on equipment is much appreciated.

 

Thanks,

Christian

Read the section on Cold Weather 

http://www.astro-phy...tions070506.pdf

 

Setup ...

a) Move the scope and the case to the unheated garage, before taking outside

b) Leave the lens cap on, for 1 hour after setup

 

Tear down ...

a) Put the lens cap on, place the OTA inside case

b) Move the case to the garage first, then next day inside the home.

 

Your dew shield may not be enough, to prevent frost.


Edited by mvas, 09 October 2018 - 10:10 AM.


#11 Erik Bakker

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Posted 09 October 2018 - 11:04 AM

Many valuable tips here already.

 

As you may have seen from those tips, one of the problems is temperature difference beween storage and observing. 

 

Another problem is the difference between glass contracting and the material of the lens cell contracting differently in the (extreme) cold.

 

A quality refractor has the best cards per given aperture or number of lens elements.

 

Generally, medium focal ratio (f8 or slower) do best, especially doublets. And in smaller sizes, up to 5", preferably 4" or smaller.

Oil spaced triplets generally are the best of the 3 element designs in dealing with cold, but are already very expensive and/or rare.

 

4 element designs can take long, as can air spaced triplets. Especially when bigger than 5" of aperture.

 

All these telescope types have different strong points in observing. So decide what you like to observe first. That will narrow down your choices.

 

Then decide what your budget is, to pick the scope and mount to go with that. I consider convenience is a big plus in the cold.

 

With proper precautions and common sense, all my refractors from 55-130mm aperture, from doublet achromats to doublet fluorites to to oil spaced triplets have done well in the cold. In my case meaning from +20C inside down to -20C outside in the cold with strong freezing eastern winds. But no Norwegian winter conditions wink.gif



#12 spencerj

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Posted 09 October 2018 - 12:04 PM

On a cold clear winter night, I don't hesitate to set up my F6 oil-spaced triplet.  It is fine.  Been some talk already about shattered lenses from the cold.  I have been on this forum for a while now.  I have never seen a picture of a lens that was shattered because it was too cold outside.  Take care to acclimate the scope, but don't be too afraid to use it.  The winter skies are incredible.     



#13 cschopke

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Posted 09 October 2018 - 02:11 PM

Thanks everyone for the good insights! I will certainly keep all these tips in mind when I get around to acquiring a refractor.




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