Dew heater placement

[+bryand]

A simple question, but probably a complicated answer:

Given that I need dew control on an aluminium-tube refractor, where is the most effective position for a dew heater tape?:

1) Around the dew shield, directly over the outermost objective lens (heat where it is needed)

2) Rear of dew shield, as close as possible to where dew shield makes contact with main tube (maximum conduction to main tube), or

3) Around the main tube as close to dew shield as possible (closest to lens cell).

... and I guess there is even a position 0: halfway along the dew shield (warms air ahead of the lens).

Any thoughts?

 

[+RonC]

I use '1', never had a problem. I put it over the end lens placement.

 

Ron

[+Bino-viewer]

I always thought 'position 3'  : its an air spaced triplet, so you really need the heater surrounding the cell,

which is probably wider than you think.

Can the glass surfaces between the elements or rear most element inside the scope dew up ???

On some nights, when its really dewey, quite possibly yes. If you put the heater in 'position 1' you're warming up the surface inside the dewshield

and above the front facing lens. I would want to concentrate it over the cell, which is exactly where 'position 3' is.

 

This is a question thats been debated at length on this and various other forums.

And to be honest, i think any of the 3 positions will do the job if you're only having a 3 or 4 hour session.

 

On my old WO scope i always used 'position 3' ; another reason for that is it kept the dew shield extended !!

It often used to slide down on its own when it got cold when the scope was pointed high up.

One night it did it i didn't notice, and with the dew shield down it completely dewed over in no time bringing an end to that nights session.

It looks like they've added a thumbscrew now on your scope to stop that happening, so thats not an issue there.

 

If it was me though, i'd opt for position 3.

 

My £0.02p

 

 

 

[+RonC]

Not sure how dew which is condensed air can reach the cell? It forms on cold surfaces such as the front lens surely?

 

Ron

[Bottletopburly]

I put mine in position 3 quick google and that seems correct, in high humidity areas another band in position 2 also helps 

https://www.cloudynights.com/topic/611533-dew-heater-strip-placement-for-widely-spaced-elements/

[+RonC]

Only having an ED80 explains it then...

[+bryand]

I must admit I'm tending to the 'radiative heating' camp, concentrating on replacing the heat lost from the outermost glass (position 1-ish), rather than the conductive heating camp trying to heat the front glass by heating the tube and the back of the lens (position 3-ish).
This is backed up by the responses from William Optics (who supplied the scope) and Dew Control (who supplied the heater tapes and controller).  
WO point out that there is no lens at position 3 (it's farther forward) and suggest a position best described as 1 1/3rd.  (The lens cell looks to be deeper than the width of the tape, so one tape won't cover the whole cell wherever it is placed.)

Dew Control suggest that the dew shield is best provided there isn't too big a gap between shield and main tube.

 

[Nightspore]

I've always wondered about this. I suppose it depends where the objective surface is within the OTA/dew shield. 

 

 

I thought that it was important to keep the objective surface glass at a temperature slightly above the dew event horizon.

 

 

The size of the scope and the powertank output is important. I can just about heat my Starwave or ST102 with a small tank. The finder is also heated.

 

 

Personally I'd prefer the heater element as near the objective surface as possible.

 

 

So position '2' looks about right to me.

 

 

With the small powertank the 72ED and finder is pretty easy to keep dew free.

 

 

Although I had no dewing with the ST102 last Thursday, I didn't even take the powertank out. It wasn't a bad night visually. The ST102 didn't have a chance to dew, the frost stopped it lol.

Edited October 9, 2019 by Nightspore

[+Bino-viewer]

3 hours ago, bryand said:

I must admit I'm tending to the 'radiative heating' camp, concentrating on replacing the heat lost from the outermost glass (position 1-ish), rather than the conductive heating camp trying to heat the front glass by heating the tube and the back of the lens (position 3-ish).
This is backed up by the responses from William Optics (who supplied the scope) and Dew Control (who supplied the heater tapes and controller).  
WO point out that there is no lens at position 3 (it's farther forward) and suggest a position best described as 1 1/3rd.  (The lens cell looks to be deeper than the width of the tape, so one tape won't cover the whole cell wherever it is placed.)

Dew Control suggest that the dew shield is best provided there isn't too big a gap between shield and main tube.

 

You make some good points there Bryan.

I think i got into the habit of 'position 3' to help keep my dew shield extended on my old scope.

 

Position 1 seems a better option. However.....

 

With a big triplet design (some of the air spaces are wider than you may think) i still think they'd be vulnerable to dew on bad nights with a position 1 dewband.

Don't think the 2 gaps in an air spaced triplet are completely immune from dew because they aren't.

 

On a doublet design with only the two glass to air surfaces, maybe not quite so much ?

 

[+bryand]

1 hour ago, Bino-viewer said:

With a big triplet design (some of the air spaces are wider than you may think) i still think they'd be vulnerable to dew on bad nights with a position 1 dewband.

Don't think the 2 gaps in an air spaced triplet are completely immune from dew because they aren't.

As you said in an earlier post, there probably isn't too much difference between the options.

I'm sure I saw somebody advertising 2" wide heater bands, but I can't find the ad. 

I might instead go for a very long (106 cm) normal-width band and wrap it twice round the dew shield to give the desired coverage.  That would draw 2 amps at full strength, so would need a decent battery for an all-nighter.

[+bryand]

I got the chance to get some more FLIR (heat)shots during this evening's session.  Ambient temp was about 2°C.

The 50 cm heater strip round the dew shield (approx Pos 1, @ 100%) was working well:

 

A view inside the dew shield showed it is doing its job (no dew evident all night):

 

 

Surprised how much heat the mount electronics generate...:

 

Edited November 18, 2019 by bryand
added pix

[philjay]

Having had one of the biggest dew collectors around a few years back, 11" SCT, the corrector was a dew magnet, I investigated this to great length.

Dew forms on a surface which has reached the dew point of the current atmospheric conditions. So if the front element is kept above the dew point all is well.

How you achieve that temperarure doesnt really matter, all 3 positions will work BUT

 

Position 1 will get the front element to temperature faster and the air infront of the element

Position 2 needs to heat the cell first then the front element will eventually follow suit then the air in front

Position 3 heats the tube, then the cell, then the lens then the air in front.

 

So they all do the job but take different times to do their job.

From years of experience and ruined observing sessions, I prefer, position 1, just forward of the lens, it achieves a heated volume of air in front of the lens first, thus keeping dew at bay, then the lens etc follows.

 

 

[+Bino-viewer]

Good points Phil ; thanks for your input.

 

Do you miss the 11" SCT  ?

 

[philjay]

Funnily enough I dont Rob. I had it some years ago for about 4 years, visual it was bosting, planetary imaging it was spot on but I wasnt in ti that, I found I was using a zenithstar fd80 more for imaging. The c11 needed coaxing to get good images with, refractors were better behaved.

So I got a Meade 127 ed triplet and did a few months of comparison and found I really preferred the Meade frac. So the c11 went, I havent looked back, still got the Meade.