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Refractor and dew heaters questions

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#1 neptun2

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Posted 28 January 2013 - 10:22 AM

Hello. Now i also have refractor - the skywatcher 120ED f7.5 APO and have some questions about the usage of dew heaters with it.

First - where you put the heater strip to get best result? I see that some people ut it on the due shield and other on the tube itself just before the base of the due shield. My refractor has non removable due shield and the lens cell is placed at around 1/3 of it's length from the base. Considering this maybe the best position will be to put the strip on the dew shield directly above the lens cell. Is this correct?

I bought the following dew strip together with power cable but not hand controller.

http://www.teleskop-...info/p846_De...

I read many good things about the effectiveness of these strips and with their 8 watts power the really seem to be good choice. At this situation do i need controller at all? Will there be any problem if i run it full power all the time except the fact that it will drain my battery faster?

If you think that controller is needed should i get the much more expensive automatic controllers like the dew buster or you can achieve good results with the manual ones without too much hassle. If you use manual controller how frequent do you typically make adjustments of the power during the night? Does it require only several tweaks or it is constant process of adjustments? Thank you in advance.

#2 CJK

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Posted 28 January 2013 - 10:27 AM

The dew heater strip should be wrapped around the OTA just behind the primary lens cell. That way it warms the air behind the primary (and the primary itself) rather than futilely trying to heat the ambient air (which will obviously then simply rise up out of the tube).

-- Chris

#3 GlennLeDrew

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Posted 28 January 2013 - 10:30 AM

If the dew shield is of the sliding variety, with a soft material which acts like an insulator against hear transfer, the heater should be wrapped around the dew shield, perhaps at about the middle. This is to warm the dew sheild and thus compensate for the loss via IR radiation of the objective into the cold heat sink of the sky.

If the dew shield is fixed in place and has thermal contact with the cell, it doesn't so much matter exactly where the heater is placed, for thermal conductivity will distribute the heat fairly quickly. Myself, I prefer to always wrap around the dew shield. Some will choose to wrap around the cell region, and others just behind the cell. No big difference when all parts are metal and thus conduct heat readily.

One is not warming air; that would be inefficient anyway. Rather, the heater is redressing the radiative imbalance. Whether by adding energy to warm the objective conductively, or to irradiate the objective with sufficient IR to compensate for radiative loss to the sky, the heater is keeping the glass surface at a temperature above the dew point temperature.

I make my own heaters, and run them full time when deemed necessary (no duty cycle controller.) These heaters are of lower power consumption than commercial units, being 1/4 to 1/2 as energy hungry. I increase efficiency by making them slim and hence with good thermal contact, az well as wrapping in a thin insulating foam.

#4 neptun2

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Posted 28 January 2013 - 10:35 AM

My shield is fixed, metal and have thermal contact with the cell. In this case most probably if i wrap the strip around the dew shield directly above the lens cell it should be ok i suppose.

#5 CJK

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Posted 28 January 2013 - 10:35 AM

In answer to your question about controlling your heaters, here's my thinking (I am a newbie, though, so take any of my advice with a grain or ten of salt): a smart controller like the DewBuster allows you to keep the surface just a bit above ambient, which is what you want to prevent dew yet not too much so as to create air currents in front of the optics which could affect your viewing/imaging. But there really isn't any reason you'd have to use a smart controller -- a manual one will just require tweaking as you mentioned yourself. (I've got a DewBuster, so I can't speak to how best to adjust a manual one.)

-- Chris

#6 neptun2

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Posted 28 January 2013 - 10:37 AM

Thank you. The dew buster is definitely the best option but here in europe i can't get it easy. There are alternatives but they cost around 300 eur while manual controller cost around 70 eur. This is quite a big difference.

#7 George N

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Posted 30 January 2013 - 04:56 PM

The dew heater strip should be wrapped around the OTA just behind the primary lens cell. .......

-- Chris


Last year at NEAF I asked Scott Roberts of Explore Scientific this very question, and his answer was the same as yours: place it on the tube behind the cell.

#8 mikey cee

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Posted 30 January 2013 - 06:23 PM

If the dew shield is of the sliding variety, with a soft material which acts like an insulator against hear transfer, the heater should be wrapped around the dew shield, perhaps at about the middle. This is to warm the dew sheild and thus compensate for the loss via IR radiation of the objective into the cold heat sink of the sky.

If the dew shield is fixed in place and has thermal contact with the cell, it doesn't so much matter exactly where the heater is placed, for thermal conductivity will distribute the heat fairly quickly. Myself, I prefer to always wrap around the dew shield. Some will choose to wrap around the cell region, and others just behind the cell. No big difference when all parts are metal and thus conduct heat readily.

One is not warming air; that would be inefficient anyway. Rather, the heater is redressing the radiative imbalance. Whether by adding energy to warm the objective conductively, or to irradiate the objective with sufficient IR to compensate for radiative loss to the sky, the heater is keeping the glass surface at a temperature above the dew point temperature.

I make my own heaters, and run them full time when deemed necessary (no duty cycle controller.) These heaters are of lower power consumption than commercial units, being 1/4 to 1/2 as energy hungry. I increase efficiency by making them slim and hence with good thermal contact, az well as wrapping in a thin insulating foam.

I have mine just in front of the lens. I too have a slide on dewcap with wide strips of adhesive felt. ;) Mike

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#9 neptun2

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Posted 30 January 2013 - 08:01 PM

Thanks to all for the help. As i understand the correct answer is that the heater should be as close to the lens cell as possible. For some people this will be on the tube itself and for other like me at the dew shield.

#10 microstar

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Posted 31 January 2013 - 09:29 PM

Could I expand this interesting discussion? What about dew heaters on carbon fiber versus aluminum tubes? Would you have to change the settings because of the lower thermal conductivity of CF tubes? Or are dew heaters even effective on CF tubes?
...Keith

#11 rigel123

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Posted 31 January 2013 - 09:35 PM

Could I expand this interesting discussion? What about dew heaters on carbon fiber versus aluminum tubes? Would you have to change the settings because of the lower thermal conductivity of CF tubes? Or are dew heaters even effective on CF tubes?
...Keith


I use the Orion Dew Zapper system with my ED80T CF and it works well. I put the dew strap on the dew shield near the objective.

#12 microstar

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Posted 31 January 2013 - 10:47 PM

Thanks Warren. Do you find you have to adjust the heat higher because of the lower thermal conductivity, or is it just business as usual?
...Keith

#13 neptun2

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Posted 01 February 2013 - 08:50 AM

Well interesting question. The thermal conductivity of the carbon should be less than that of metals but the usage of heaters should still be possible. If it will require more power - well i think that it should but it will be better if someone with CF tube scope and heaters share first hand experience.

#14 rigel123

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Posted 01 February 2013 - 11:40 PM

Thanks Warren. Do you find you have to adjust the heat higher because of the lower thermal conductivity, or is it just business as usual?
...Keith


Good question, since I only use the Dew Heaters on my ED80T CF, it is business as usual for me! :grin: I run it with the dial about 1/2 from full. I often leave it on when I'm finished and cover the scope if I plan to leave everything set up so it doesn't frost up later (earlier?) Typically the only place I get frost is right along the edge of the dew shield if it is really cold, and the heat strap is probably about 4" away from that.

#15 microstar

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Posted 02 February 2013 - 12:42 AM

Thanks Warren. That's a higher setting than I use with my Kendrick system on my aluminum tube Megrez 72, so although the different dew systems aren't necessarily comparable, I'll expect to run the settings a bit higher.
...Keith

#16 GlennLeDrew

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Posted 02 February 2013 - 01:21 PM

If anything, I should expect a wee bit less power to be required on less conductive materials, since they do not radiate as readily into the sky. Metal should tend to cool farther below ambient than carbon fiber, methinks, thus requiring a bit higher energy input.

In any event, other things being more or less equal, aluminum vs carbon fiber should not require any *significant* difference in power requirements.

#17 microstar

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Posted 02 February 2013 - 02:52 PM

Thanks. I'm trying to get my head around this. Seems to me like you are wrapping your dew heater strap around an insulated cooler box, so it seems intuitive to me that you would need more heating to warm the inside of the tube. It didn't occur to me that you might need less because less heat will be radiated away by the tube. Undoubtedly I'll have to experiment a little.

Thanks for the replies,
Keith

#18 GlennLeDrew

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Posted 02 February 2013 - 03:52 PM

Remember, heaters need not work only via conducting their warmth directly to the optic (via its cell.) They can work radiatively as well. For example, consider a sliding dew cap which does not thermally conduct so well due to the soft material separating cap and cell. It the heater is wrapped around the dew cap, the slightly warmed cap will 'bathe' the objective with sufficient IR to compensate for that lost radiatively through the cap's opening.

This is why a dew cap by itself is the first line of defense. By restricting the solid angle of cold (-30C or so) sky into which the objective radiates, the cooling rate is much reduced. Furthermore, the walls of the dew cap are at or just below the ambient air temperature. This is (usually, for those not out in Arctic conditions) warmer than the clear sky's -30C heat sink, and so does its part to present a warmer heat sink which covers that portion of the near-hemisphere blocked and into which the objective would otherwise radiate more vigorously.

In short, dew prevention is essentially about redressing a radiative imbalance.

#19 microstar

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Posted 03 February 2013 - 01:17 PM

Thanks Glenn, I certainly wasn't thinking about it in a very sophisticated way. Your explanation really helped. I'll have to experiment to see what works, as now I'm not sure which it would be - more or less. Funny how often things turn out to be counter-intuitive to the way you are thinking about the problem.

Thanks again,
Keith






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