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I was thinking of buying an AstroZap Dew Heater so I wouldn't have to worry about dew on my Dobs primary mirror. (Last night I stayed up nearly an Hour and a Half waiting for the dew to evaporate and had to go to bed with Dew still on the primary!)

I can easily afford the dew heater but I don't know much about them. So;

  1. How do they work?
  2. How are they used?
  3. Would I need to buy a power pack? If so, which one and how do they work?
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I'm not sure it is necessary to get anything. With a solid tube you shouldn't be getting dew on the primary while you are outside observing. Once you bring the scope back inside the mirror will be cold and you may well get moisture condensing on the mirror then. Once the mirror has warmed up a bit the water will evaporate. The primary mirror is the part of your scope with the highest thermal mass so it is last to cool down when you take it outside and also last to warm up when you bring it back in. Can you not just leave the scope tilted over so there is no dust settling on the mirror? Alternatively you could just open the small hole on the lid and leave the focusser open too. That is probably enough.

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for the first time ever my primary (16" dob) dewed up at the Peak Star party. It was really foggy but Jupiter was just about visible still (with full aperture). I then walked away for about an hour with the scope tilted at about 70 degrees (pretty foolish really, don't ever leave your scope pointed at space when not in use) and dew took hold. once the mist cleared (it was almost raining out of the air) and a light breeze got up, the mirror cleared in a few minutes.

I personally think the only way to prevent dew in these circumstances is to point the scope lower (which should work) or add a fan blowing across the surface (needs a hole in the tube wall). once dew has started nothing will stop it apart from a blast with a warm air gun (which I don't like the idea of) or time in a breeze or inside the house in a warmer environment.

if you are talking about dew when you bring the scope in, this is unavoidable. just tilt it at 45 degrees and go to bed. it will be fine and the dew gone in the morning.

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Like Moonshane, the only time I've ever run into issues with dew on any of my Dobs/reflectors... has been in leaving the scope at an attitude near zenith for an extended period.

I'm not sure what the English interpretation of a 'warm air gun' is :) ... but there's certainly nothing wrong with using a 12-volt hair dryer... they work instantaneously. I've never had to use one on a primary... but I use it all the time on my Telrads/key pads/etc. Keeping in mind, you're all done once it turns to water... I'd never suggest using it to blow moisture OFF of anything... but if you stay on top of it, a dryer will evaporate a thin film of dew quickly, when it first shows up.

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or add a fan blowing across the surface (needs a hole in the tube wall).

How about keeping the rear fans running? I know they wont help as much as a side fan but doesnt it help at all?

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Ant - When there is moisture in the air, any object that is cooler than the air temperature in that moist air will cause the moisture to condensate on the object's surface causing a fogged surface. In most cases, when you are outside in such moist air, the parts of the scope that cool the fastest are the ones that the dew fog forms on first. Usually, because of its large mass, the primary mirror will be the last thing to fog up since it takes longer to reach the temp. where dew forms on it (the "dew point").

So the first thing you should notice to form a dew on is the optical tube's outside surface. Next will be the EPs (keeping them in your pocket until use will slow down the dew forming process). Then the secondary will fog up next (the dew zapper you mentioned would keep this mirror from fogging up). The last thing to form dew is the primary mirror. When this happens, you can use the dew zapper (wrapped around the outside edge of the mirror) to extend your viewing time.

But usually only the EPs and occasionally the secondary mirror fog up in very moist nights. Most nights only the outside of the OTA produce a film of dew. As an example, I usually find the OTA dripping with dew while the rest of the scope stays clear of dew without any dew removing devices.

So that's how the dew forms and which component fogs up first and last.

The important question regarding your post is, was the mirror still outside when it fogged up or when it was brought back inside? In the first case, there is not much you can do to prolong the viewing session but in the second, the dew will evaporate in a short while when the scope is brought back inside.

As for the power requirements of the dew zapper, it will use a 12V battery of the proper amp hour capacity to operate the zapper for several hours before the battery runs down. Or, you can hook up a power pack that runs on house current using an extension cord. The power requirements of the zapper (voltage and current draw) must be below the power pack's maximum ratings.

One final note. If too much heat is applied to the mirrors, the heat can cause thermal eddies inside the OTA and the scope will never reach a cooled down level.

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My telescope doesn't have fans. Its just that when moisture forms on the primary mirror it always takes ages to evaporate and I don't have the time to sit by my scope until the moistures gone.

Dew and condensation are the same thing. It forms when warmer moisture laden air comes into contact with a colder surface. The moisture in the warm air condenses out onto the colder surface.

You don't need a dew heater for a primary mirror on a solid tube newtonian, nor can you avoid condensation forming when you bring it back into the house at the end of your observing session - it's just a fact of life. Also, you don't have to sit with the scope until the condensation has cleared either, just tilt the tube so that it's at around 45 degrees or even 90 degrees if room allows to stop dust from settling onto the damp mirror and go to bed. Cap the scope and put it back upright in the morning.

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Some people get really worked up about using a hair dryer on scope glass, thinking about cool down issues.

I use a mains dryer regulalry in the shed without problem.

A quick blast on the offending glass creates an air flow to encourage evaporation, and puts some heat into the water to further encourage evaporation.

This works for eyepieces, mirrors, corrector plates, etc.

A few seconds is all it takes. The air currents over the glass vanish in a second or two. There is little residual glass heating as the dryer is running for only a few seconds. There are therefore no cool down issues.

At the end of the night, after replacing the shed roof, I often spend time deliberately warming both glass and metal parts to remove condensation. As well as leaving the dehumidifier running. But that is a different story.

Hope this is useful.

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So there is no way to prevent the condensation on the mirror when I bring it in and my best solution is a hair dryer?

Correct, it will always do it when you bring it in. You dont need to do anything about it, it will evaporate naturally by the morning. I usually cap my scope before bringing it in to minimize the fogging, and leave it capped until next use (by then, the mirror is clear and cool). This really isn't anything to worry about at all.

Consider a shower cap or wooly hat for the mirror end of the tube, it prevents warm house air getting straight onto the mirror, with a shield like that in place the heat will slowly creep through and shouldnt fog up at all.

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