Refractor V's Newtonian Cool Down

[John]

I've been noticing in the colder weather that my 102mm refractor is taking much longer to cool from room to outside temperature than my 8 inch newtonian is. I've had to wait well over an hour this evening for the fine images that I know the refractor is capable of wheras the newt was doing pretty well after just 30 minutes.

When the temperature difference is less the refractor seems to be working well more quickly than the newt generally.

Having thought about this, I reckon that the newt cools quicker because it's an open tube design while the refractor tube offers no easy way for the warm air inside the tube to escape other than radiating away through the tube walls - which takes longer. Perhaps if I pointed the refractor downwards with the drawtube cap removed the warmer air could escape more quickly by rising up and outward ?.

Of course that only deals with the warm air in the scope tubes - we still have the issue of the lenses / mirrors cooling as well. One thing I'm not sure on is, which has the most detrimental effect on the images we see - currents of air within the tube or optical elements not fully cooled yet ?.

I've also noticed that above 90mm, refractors need significant cool down time while apertures less than this can be used almost "straight out of the house". I guess those with triplet lenses could be an exception to this as the mass of glass to cool will be significantly more.

What experiences have others had in cooling scopes and optics - any good tips for the rest of us ?.

John

[GazOC]

Reflector or refractor I always leave the 'open end' pointing upwards while I'm waiting for it to cool. I'm not sure if it does much good but it seems to make sense...

[aries]

I leave my Newt. pointing slightly upwards, at about the 10 o'clock point, with the Small cap on the main

lens cover removed at the 12 0'clock point on the OTA, Just in case of anything falling inside the tube, and leave it for at least 45 mins before I start......

Probably has no effect on things that much, but just following some advice read on other posts.

Nick..... :wave:

[nitram100]

I know i should leave the tube of my newt facing upwards but i can just imagine a bird flying past and laying one on my primary. My cool down time for the 10" newt was around 1hr, but i've reduced this to about 10 minutes by installing 3 fans behind the mirror, such a simple mod does wonders!

[thing]

Do you keep the scope indoors? I keep all of my stuff in the shed, I haven't really noticed any cool down problems.

[A McEwan]

Don't keep them in the house! Shed, unheated garage, etc = instant usability!

[John]

Don't keep them in the house! Shed, unheated garage, etc = instant usability!

Unfortunately we don't have a shed and our garage is full of junk and spiders !.

Thanks for the suggestions anyway

John

[supernova]

My Dob needs about 1 hour to cool down with the fan on. and I leave it horizontal with the caps off because seagulls fly at night.

I have noticed that the collimation changes as it gets cooler and seems to come into line, then when it is cold the colimation is not far out.

Bernie.

[orion69]

There is NO way I'll leave my refractor open when cooling down...

It's easy to clean reflector but try to clean refractor inside.

[davew]

Hi John,

Take a look at - http://media.skyandtelescope.com/documents/Adler-NewtThermal.pdf

This will give you an idea of what's going on in the Newt. I don't suggest you rip the side out of your tube however It's all good information. Some people put on a rear fan and then an inner ring in front of the mirror to direct the cold air across the mirror face. ( If you can understand what I'm saying ).

With my refractor I just leave it horizontal on the mount with the cell uncovered while I set up the rest of the gear. It's usually ok then. Mind you, it's only 78mm.

Hope that helps.

Dave.

[Astro_Baby]

Strange - I find the opposite - even with the newts monster fan providing a jet exhaust power of air up the tube the 8" newt is slower to cool down than my TAL 100 - no idea why because logic would suggest an open tube would cool faster than a closed one. I can only imagine theres some obscure effect (surface area, radiating area) type issue because the TAL can pretty much go great from the off.

[LukeSkywatcher]

I've never had to let my refractor cool down. Never even really considered that it needed it.

I can see the logic in removing the star diag from the tube and letting the warm air out but then dont you run the risk of the inside of the lens misting up?

[John]

My original post was over 2 years ago (!) and I've learned a lot since then (most of it from SGL ). I'd just got the ED102 and the 8" dob back then and I was still "finding out" about them

The 6" refractor I have now does need some cool down time and I tend to leave the scope "nose down" and uncapped at the eyepiece end to let the tube currents escape quicker.

[Astro_Baby]

:D whoops - didndt see the date - just saw it as a new post

[davew]

I was caught cold there ! Interesting first post

Sorry John.

Dave.

[LukeSkywatcher]

My original post was over 2 years ago (!) and I've learned a lot since then (most of it from SGL ). I'd just got the ED102 and the 8" dob back then and I was still "finding out" about them

The 6" refractor I have now does need some cool down time and I tend to leave the scope "nose down" and uncapped at the eyepiece end to let the tube currents escape quicker.

I spotted the date thingie but thought i'd join in anyways.

LOL.

[John]

Theres no problem - my posts are sometimes like the telly - the repeats are often better

[Peter Drew]

Never too late to air this topic as it's a perennial problem. The column of warm air inside a scope that has been kept in a warm environment should cool down quicker than the optics, so these are the main problem. Bear in mind that there is a whole lot more glass in an 8" mirror than a 4" object glass. Also worth remembering that refractors have only one converging pass through the tube, Newtonians two passes, the first very near to the tube wall and SCT's and Mak/Cass scopes have three passes!.

[Piero]

Sorry for re-opening an old thread (5 + 2 years!).

When I take my refractor (2.4") in my office for going outside straight after work, the difference in temperature can be quite significant (even 30 degrees in the winter). 

With such a difference, also this small refractor needs some time to cool down properly. 

As John said, I also leave the scope nose down and leave uncapped at the eyepiece and it seems to be a very effective method. 

To reduce dust or impurities entering in the diagonal, I generally "point" the refractor in the direction of the wind, so that, when the nose is left down, the diagonal opening is protected from the wind. 

(e.g. if the wind blows from west to east, I point the scope to the ground to east).

Doing so, cooling down takes about 5min with this scope. 

[MAN or ASTROMAN]

Strange - I find the opposite - even with the newts monster fan providing a jet exhaust power of air up the tube the 8" newt is slower to cool down than my TAL 100 - no idea why because logic would suggest an open tube would cool faster than a closed one. I can only imagine theres some obscure effect (surface area, radiating area) type issue because the TAL can pretty much go great from the off.

Maybe the Tal tube is full of cold Siberian air!