Heating the tube to keep dew of a Newtonian mirror

[strongmanmike (Michael)]

Just wondering what experienced people thought of the idea of heating the tube via a heating wire moulded around the inside of the tube say 100mm above the mirror and then the heat just warms the tube slightly and the fans can then create a vortex in the tube to help reduce condensation..?

Anyone used this method? Pro's, con's?

Mike

[Gama]

You may be better off getting a mirror warming pad.
It sits under the entire mirror and you can control the temp.
I had this on my 20" Obsession, and worked very well.
I also had one on my secondary too.

Theo

[AlexN]

The only concern I would have would be thermal currents within the tube caused by the warm mirror vs the outside temperature.

I wonder if it would not be better to modify the scope in such a way that you had fans moving air across the face of the primary.. This may well increase the ingress of dew, as outside air would be damp and blowing it onto the mirror would cool the mirror, making it more susceptible to dew..

Another thought...

Wouldn't having rear fans on the ota, blowing air into the tube (provided the rear end of the tube was sealed apart from the openings for the fans) help by pushing airflow up the OTA, stopping moisture from entering the tube...

These are all things that I considered with my 8" RC. as its primary mirror was prone to excessive dewing due to its short tube and relatively small mirror. The thoughts on thermal currents were highlighted to me by Paul Haese, As I was considering a dew heater for the secondary mirror. His testing had shown him that use of heaters on his SCT when planetary imaging had a detrimental affect to image quality. Obviously, for his setup, when imaging the planets at 8000~14000mm focal length, these affects would have been heavily magnified. Still I wonder how much of an affect it would have if you were taking very long exposures at short focal lengths (700~900mm) vs very short exposures at very long focal lengths..

I suppose there is only one way to find out...

Ps.. What's all this about Newtonians? Don't tell me you've been pulled down the aperture of a big fast newt again! (I'm working ever so slowly on something rather similar myself..)

[iceman (Mike)]

A fan blowing up from under the mirror keeps dew off the mirror.

But you may need a heater for the secondary.

[Garyh]

Hi Mike,
I find a fan under the primary does a excellent job, never had dew on the primary even on a real dewy night.
Secondary mirror heater is recommended but as I have had this fog up occasionally without the heater on. Just a 1 watt or so.
Gary

[Doomsayer]

hi mike
Blowing air from the rear will help as others have said. Having an idea of the relative humidity on ths night and a temp sensor on the primary blank can also help you understand what is going on.

There were some fairly detailed articles on mirror fans in US S&T some years back. Blowing air across the front surface of the mirror may also help with preventing dew, and front fans do specifically help with preventing local seeing effects due to pooled warm air etc.

I assume this is in a cloeed steel tube? Is this to prevent dewing on the mirror while it is 'parked' in the wee hours or just while shooting? (the high humidity at the moment is a challenge)

Wrapping a dew heater strap around the mirror with a low pulse going through it might help here - an intelligent dew controller with a temp probe could also be a good option. Testing would be required to see if it would work positively while actually shooting with the scope.

I'm considering this as well as I'm milling a primary mirror cell for a mini 6"newt CF astrograph at the moment. It will be the first solid CF tube scope I have worked with so I'm not 100% sure how it will deal with dew etc. In its former timber tube, it never ever dewed up.

guy

[multiweb (Marc)]

Originally Posted by strongmanmike

Just wondering what experienced people thought of the idea of heating the tube via a heating wire moulded around the inside of the tube say 100mm above the mirror and then the heat just warms the tube slightly and the fans can then create a vortex in the tube to help reduce condensation..?

Anyone used this method? Pro's, con's?

Mike

I had a nichrome wire in a spiral pattern at the back of my newt once and as soon as I switched it on I'd see the focus go off in realtime so I dropped that idea real quick. A fan at the back of the mirror seems to be a sensible idea. A long dewshield goes a long way keeping the moist from falling into the tube too so you keep your secondary relatively dry.

[gregbradley]

Roland Christen has posted about tubed telescopes several times now and how bad tube currents can be and how badly they affect the scope.

As a general principle anything that generates tube currents would wreck your scopes performance.

Hence the temp controls on the RCOS's with the telescope command centre.

I did see a post by Roland about dew control not that long ago. As I recall it was as posted here by others - a fan blowing air up keeps the dew from falling in? As I recall a very small temp difference can prevent dew from forming. A fan across the surface of the mirror sounds good as the thermal layer that is maybe only 1mm thick on the surface of the mirror affects mirror performance.


Greg.

[peter_4059 (Peter)]

Mike,

I tried the fan blowing air onto the back of the mirror and up the tube last year at astrofest and it made no difference - you are still introducing wet air into the tube and if anything the fan is going increase the rate of heat loss from the mirror. I think the only way to stop this is to keep the mirror above the dew point temperature.

I'm going to try a Kendrick primary mirror pad with an inside/outside temperature probe to measure the temperature differential. It would also be possible to use the Digifire 10 controller that has this temperature measurement and control functionality built in however I've already got a Thousand Oaks unit.

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

This is the same principle that is employed in the Kendrick Dewguard secondary mirror heaters that are installed on quite a few SDM's. These are programmed to keep the mirror 1-3 degs above ambient.

I reckon dew formation is going to have a far worse impact on the scopes performance than any slight current that is generated. I've had the 2024 10" Primary Mirror Heater and DG1 secondary heater on order for a few weeks.

Peter

[adman (Adam)]

Sorry in advance for the slight thread hijack, but it is very much on topic, even though there are more questions than answers:

Can anybody please let me know whether my thoughts about dew formation are correct. I have an 8" newt and have not (so far) been in conditions that warrant any dew amelioration measures - but can see the day approaching - especially with the current humidity.

From what I understand, all air contains water vapour, and that is quantified by talking about the relative humidity of the air. It is relative because the percentage gives how much water vapour the air holds relative to the most it could potentially hold at that temperature.

Air also has a dew point which is a relationship between temperature and relative humidity and is expressed as a second temperature. So air at 25 degrees C with an 80% relative humidity has a dew point of about 21.5 degrees C. This means that if this air comes into contact with a surface (or even another air mass) that is below this dew point temperature, then the water vapour will condense. If it contacts a surface, then dew forms, if it is another, colder, air mass, then clouds form.

So from my understanding, what is required to keep dew from forming on a mirror, is to simply keep its temperature above the dewpoint.

I can't see how blowing air across a mirror, or even up the tube would prevent dew formation, as the air you are blowing is still at the same temp and relative humidity, and if your mirror is at a temp lower than the dew point then you will still get dew formation. Unless, maybe the air movement just results in evaporation once the dew has formed. If this is the case then the evaporation will lead to greater cooling of the mirror (from the latent heat of evaporation), and you will eventually get dew anyway.....

Likewise, dew shields can't work by stopping the moisture from falling into the tube as the condensation happens at the glass/air interface as a result of the cooling of the air at this boundary....

Do the dew heater controllers have sensors built in that measure ambient and mirror temps as well as humidity and then adjust their heat output accordingly - or are they just dumb heaters? And if the latter is the case, don't they create more problems than they solve with all that heat wafting around? I mean what is the point of letting your mirror come into thermal equilibrium with its surroundings for a few hours, only to whack on the heater??

Also it seems that you should never have to heat up the mirror to greater than ambient temp to stop dew formation - as dew point should always be equal to or less than ambient. At 100% humidity the dew point equals the ambient temp, and as relative humidity drops at a particular temp, so does the dew point. So there should be no issue with thermal currents unless you are cranking up the heat above ambient.

The more I think about it the less sense it makes - but like I said I have no experience of trying to deal with dew, and have no idea what works in the field. But at the moment what I understand about water vapour and condensation doesn't make sense when I look at some of the methods of controlling it on a telescope.

Anybody??

Adam

[peter_4059 (Peter)]

I agree with you Adam - if you keep the mirror above the dew point temperature then it isn't a problem. The dew shield works by keeping a pocket of warm air above the mirror or corrector plate however as the night progresses this pocket also cools and in my case dew forms on the primary.

Most heaters don't account for the relative humidity so are as you call them "dumb" however you'll know from your dew point calculations that if you keep the mirror slightly above ambient you won't get dew formation no matter what the relative humidity is and thermal currents are not going to be an issue unless you overdo the heating. This principle is successfully applied to refractors using heater straps on the dew shield however in a big Newt you'd have to apply a fair bit of heat to the tube to heat the air across the mirror face and it would be difficult to get the temperature uniform. If you set up hot and cold zones in the tube you are going to get thermal currents forming. You also want to ensure you apply the heating to the mirror in a way that won't cause it to deform.

Peter

[Doomsayer]

I use a Kendrick premier controller plus temp probes for my RC mirror which allows me to set the heat strap to kick in a certain temp above ambient to avoid the dewpont. I usually only need this for the guidescope however. The RC primary has only dewed up once so far.

Same reason that I have a thermostat controlled water chiller for my STL water cooling system to keep the water at a set temp - if the humidity is high, chilling the water too far (sometimes just a few degrees) below ambient can make it 'rain' inside the camera - not good. Of course you can then add heaters to avoid this.

[multiweb (Marc)]

Originally Posted by peter_4059

I agree with you Adam - if you keep the mirror above the dew point temperature then it isn't a problem.

This is correct. Any part whether it's glass or aluminium left uncovered open to the night sky (clear sky) will drop in temperature quicker than the ambient air. The sky will suck up any heat out of it, the same as the sun would heat it up if left uncovered during the day.You'd burn your hands touching an alu tube that's been left in the full sun although ambient temp might be mild. A fan will blow ambient air on the surface to minimise the temperature difference. A dew shield will in most case also cover the secondary and primary unless you're imaging right up at the zenith and then it's pretty much back to square one. I also wrap my mount with rags at night so it it doesn't get too cold because as soon as the air warms up dew will form on it as the aluminium is freezing.

[Garyh]

Well on a clear night the air temp is always above the dew point and by blowing that air over the mirror it will be close to equalization with this temp. If this was below dew point a mist or fog would make you pack up anyway. Different story if we are talking about a corrector plate that will loose heat directly into space than in a closed tube Newt.
I have had the newt from sunset to sunrise in high humidity with no dew on the mirrors, mind you the tube was dripping everywhere!

[alan meehan (Alan)]

Not sure about the heating method Mike ,i know iam always trying to cool down the 10"newt,but a quick and easy method i found to stop condensation on the primary was to put a shower cap over the back of the scope while iam imaging works a treat.

[strongmanmike (Michael)]

Thanks for all the replies guys but I'm not excatly sure what most of you thought of the original question..?

I was keen to hear opinions, for and against, for the practice of placing a heated strip within the tube wall and slightly in front of the primary as a means of heating the tube wall near the primary, in order to prevent dew formation on the primary mirror in a closed rear-end scope that also has rear cooling fans..?

Wadaya recon..?

MIke

[AlexN]

So the heating element inside the tube would gently warm the air/area within the tube, above the mirror, and the fans push said warm air up the tube in order to stop the ingress of dew into the tube? I don't see why that wouldn't work? although the same issue with regards to thermal currents still apply I would think, probably to a lesser degree, as the mirror may well reach ambient temps, and you may not suffer the boundary layer of warm air associated with heating the mirror itself...

I've seen a photo on the forums somewhere.. Someone wrapped a dew heater band around the outside of the OTA 2~3 inches further up than the mirror... I think it was in the "Show us your equipment" thread... I suppose his thoughts were the same, although, implemented in a much less efficient manner.. Having the heater inside the tube would make it much more efficient I think.. especially if the tube was CF..

[iceman (Mike)]

Well I suggested the fan because anything introducing heat in the tube like that has the potential to introduce tube currents and will kill your local seeing.

[adman (Adam)]

Sorry Mike - I know I hijacked your thread there .

Adam

[multiweb (Marc)]

Originally Posted by strongmanmike

for the practice of placing a heated strip within the tube wall and slightly in front of the primary as a means of heating the tube wall near the primary

It the heat is not transfered from the tube to the mirror itself via the mirror cell I don't think it'll work. I see what you're getting at. Warming the air in front of the mirror? I think it would make things worse.