Cool it down, heat it up .. Confused observer seeks help

[Orange Haze]

So with my new Dob, I know I need to put it outside first to let it cool before I start viewing. Check.

Last night after not that long observing, my mirror dewed up something rotten. So I read that a way to prevent that is to warm the mirror up.

So now I am a little confused. Cool it down, warm it up. What exactly am I cooling and why? And how best to warm up what has already gone cool?

I know the answer is out there......

Thanks

[itmo]

The optimum is that the mirror is exactly the same temperature as the air. Problem is that the mirror is radiating it's heat to cold space all the time so it will cool to a lower temperature than air, and it will start to condense moisture /ice up. One way to combat this is to have an insulating tube (dew shield) or to have a heater for the mirror. In newtonians the primary doesnt usually ice up, the secondary will. So one option is to insulate the back side of the secondary and add a dew shield. It seems to work for me ok.  

[Reeny]

So the primary will cool to ambient temperature.

And any dew will form on the secondary first - which may require the hairdryer treatment.

It makes sense now.

Also - I keep eyepieces in my fleece pocket on really cold nights to keep them warm.

It took me a while to learn how to breathe away from the EP when observing.

But I still need to rotate the eyepices every few minutes when the dew forms.

I guess this is normal.

[Northern Soul man]

Hi Lee, congrats on your new scope, Dew is a major problem with all scopes especially the SCT type scopes, it forms when there is a difference in temperature from the surface of mirrors/lens' temperature to the temperature of the air inside the tube assembly, the best way to prevent dew is to keep the scope as close to the outside temperature as possible, so, if you store the scope in an outside shed/garage, this keeps the mirrors clear - its only when you take the scope or move it from the cold to the warm - this creates the dew and all the cold glass surfaces get covered with it, some observers use hair dryers to blow  cool air currents in the direction of the mirror, but you have to be careful with electricity and water to avoid an electric shock.  gently warming the surrounding air by the mirror will evaporate any moisture and stop it settling on the surface of the mirror.

As said the best way is to keep the scope as close to the outside air temp as possible and when not observing try to point the scope to the horizontal position.  But dew will always be around when there's a difference in temperature, the SCT corrector lens' on the front of those scopes really suffer quite bad, you need a large dew shield and preferably a heater band around close to where the edge of the corrector is just to keep the temp slightly above the surrounding air mass to keep the dew away, but some nights its really bad, don't worry we all suffer with it.   Paul

[Orange Haze]

Thanks guys. My scope seemed to form dew on both mirrors at the same time last night. Should I be adding some insulation, such as a foam or polystyrene cut behind my primary when I go out as well?

I've only been out twice with it and both times the tube came back in almost dripping wet. Admit it did concern me.

[NGC 1502]

Thanks guys. My scope seemed to form dew on both mirrors at the same time last night. Should I be adding some insulation, such as a foam or polystyrene cut behind my primary when I go out as well?

I've only been out twice with it and both times the tube came back in almost dripping wet. Admit it did concern me.

With a Dob (Newtonian) the secondary usually dews up first.  On my Dob, I use a tube extension made from 'camping mat foam'.  It's good stuff, lightweight, readily available, and doesn't absorb moisture, although it does get wet of course.  If that doesn't do the trick, I remove the eyepiece, and point my 12 volt 'hair dryer' onto the secondary.  Don't wait till the secondary is wet (too late) but do it as soon as it mists up.  

Of course you would need a rechargeable battery pack and a 12v gun, but that's better than risking mains voltage outdoors in damp conditions.

Only had the primary dew up once whilst observing.   Maybe a cooling fan on the back of the primary could help, as moving air should lessen the formation of dew.

Hope you sort it, Ed.

[acey]

You don't need to leave a scope sitting unused while it cools down - just start observing, and the views will get better as the night goes on. Nor do you need heat to remove dew from optics - cold moving air is sufficient (you don't get fog when there's a breeze). A warm hairdryer does it faster, and has a negligible warming effect on the mirror itself during the short time you use it, so it's fine. Dew heaters may be another matter - I don't use one so can't comment.

Make a dewshield out of camping mat, cardboard, or anything that will hold its shape when damp. It works by physically shielding the optics from the sky: if glass can't see the sky then it won't dew (condensation forms mainly on the roof and bonnet of a car, not much on the sides, because they aren't skyward-facing.) So for a dewshield, insulation properties are irrelevant. Keep eyepieces covered when not in use, and lower your scope if leaving it unused for a while.

[Orange Haze]

Good stuff, thanks again all. I know it is obligatory to "mod" your Dob, something of a right of passage. Well after just a few days I may be on my way.

I'm still wondering tho, what is the benefit of cooling the scope in the first place? I assume it's mirror related.

Suffice to say, I will now cool with it to maximise viewing time.

[brantuk]

The idea is to remove any view disturbing air currents from the main tube. These are caused by temperature differences anywhere inside the scope and it can be quite a sensitive thing. On a very cold night just touching the outside of the tube with a warm hand can cause a small temperature change on the inside - some folks say just standing close to it, or breathing on it. Think what happens when you breathe on the eyepiece...... That level of refinement probably affects imaging more than observing.

Anyway - first thing is a dew shield to protect the secondary from dewing or misting up - cheap camping mat, scissors, and velcro - job done for peanuts. Next - if the dew shield isn't enough - a heat pad, or secondary dew band to keep it at or just above ambient will hold dew at bay - needs to be wired in with cable taped to the spider and requires a battery and dew controller.

Finally the primary - several solutions but the easiest/cheapest one that most go for is a fan on the back of the mirror which blows a cushion of air round the edge of the mirror and over it's surface. It has the benefit of moving warm air currents up the tube and out the end, and it works by keeping the air moving over the primary preventing temperature changes and subsequent dew formation.

Here's a couple of links - Hth:

http://www.fpi-protostar.com/bgreer/fanselect.htm

http://www.kendrickastro.com/astro/newtonian.html

[acey]

I keep my scope in the garage so it's always pretty much at ambient, but in the past when I've used scopes that have been indoors I have certainly noticed slightly poorer views to begin with - though only really noticeable by doing a star test or from seeing less detail on planets. In my opinion, cool-down is only really an issue for visual astronomy if you're viewing moon, planets etc (or maybe close double stars). For DSOs the most important thing is darkness, and getting the eye properly dark adapted, which takes about half an hour.

[Moonshane]

one reason mirrors might mist up is if you leave it to cool with the scope pointing at the sky, or leave it like that for a while when you go indoors for a brew or whatever. it's OK for a short period but if longer then the mirror tries to cool to the temperature of 'space' which is a lot colder than the ground of course hence the fogging.

best way is leave it approximately horizontal when not in use. it cools quicker this way too but I agree with Acey - observe almost immediately at low power.

my primary has only fogged once too, when observing Jupiter at the zenith in fog. it was basically raining!

[LukeSkywatcher]

Cooling/warming can be confusing. You initially cool it down to the ambient outside temp so there are no heat thermals etc crossing the mirror/lens which spoil the views. Then you keep it a couple of degrees above ambient outside temp while observing so no misting or fogging happens on the mirror/lens.

[Moonshane]

I just put mine out and observe with it at ambient. personally, I cannot be bothered with all the wires required for heaters etc - although I use dobs of course not an SCT/refractor/Mak.

[LukeSkywatcher]

I just put mine out and observe with it at ambient. personally, I cannot be bothered with all the wires required for heaters etc - although I use dobs of course not an SCT/refractor/Mak.

I honestly dont care for technology either. Any scope i use, i start observing straight away with and the views improve over the course of the night. 

Personally i dont pay much heed to cool down etc. 

What works for me........................works.

And that is what it is all about.