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Dew!


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HOw on earth do yoy deal with it without putting a heat source on it? Its ruining my viewing pleasure. By the time I set up my scope and spend time on my XBOx360 waiting for the telescope to chill out, Its all covered in dew. With winter around the corner, its sure to be icicles!

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HOw on earth do yoy deal with it without putting a heat source on it? Its ruining my viewing pleasure. By the time I set up my scope and spend time on my XBOx360 waiting for the telescope to chill out, Its all covered in dew. With winter around the corner, its sure to be icicles!

Dew controller and dew straps, plus a dew shield works most of the time :shock:

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Definately get a dew shield first - ne, NOW. It WILL help alot. The science of that statement is knocking around on SGL somewhere, but trust me - get one.

I haven't got round to getting the rest of the stuff yet (though I probly should) but if it gets so bad that it's past the dew shield capabilities - a few seconds with the wifes hair dryer sorts things out for a while - bit noisy though.

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A dew-shield works by holding a pocket of air in front of the lens. You can improvise by wrapping some black card or stiff paper around the tube. To be effective, it must project forward by at least the telescopes aperture. Best to try the dew-shield first before investing in heater tapes & controllers.

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Depends what you are using really. A refractor will hold out for a quite a while with the dew shield alone. But an SCT, in an area with dew problems, will last no more than 10 minutes even with a dew shield.

I wouldn't bother setting up the SCT or maksutov unless I knew my dew heater was going to hold up.

Russ

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Yeah, I intend on making one at some point. i like your idea of using weed control fabric as i have a roll spare. Need to sort out some hoops though as the 6 tube truss means that the shroud would impinge into the light path. Again no idea why meade didn't use the traditional 8 tube design but hey ho

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Be careful with the hair dryer approach. I used to use one and found that it "baked" particles and dust onto the corrector plate making them very hard to remove later. Best approach is to bite the bullet and either make or buy a dew heater in addition to using a dew shield.

bern

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... maybe. I've yet to have the main mirror dew up on me but the secondary has, hairdryer sorted that OK. I think direct on optical surfaces is a nono but don't ditch them just for that. you can always heat up your tube from the side and get the same effect. Might take a couple of minutes longer but it *will* get rid of the dew. Anyone that even *considers* directly blowing hot air onto cold optical surfaces should go back to their physics classes! (Or go an watch the tailend of Terminator III :D )

Arthur

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The cord on the one I use don't reach far enough towards the scope for me to be actually heating up the glass - my setup is a couple of yards from the back door and the HD is plugged into the kitchen socket. A gentle warm breeze (dryer on low setting only) in the general direction of the shield is what I get - which works A TREAT!!! - and looking at my nice clean objective lens no harm has been done WHATSOEVER(phew).

To be honest I only got the idea from a certain well known ammatuer astronomer on a sky at night special - ok he was on about his reflector but if it's good enough for him, it's good enough for me.

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So you get the 1 st clear night for weeks, just a bit damp. You have a dewnot strap going full pelt and a dew shield in place. You are half way through your capture and the dreaded dew appears. My hairdryer is coming out prompto. My optics cost too much not to use them, they are bog all use in a cupboard!

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Yes the Dewnot is brill but truth be told even that and a decent dew shield still isn't 100% on those really drippy nights and in this country we have to grab what we can when we can so the hair dryer stays :D

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My main dew problem is with the secondary.

I looked up a few web sites and made a heater using resistors but I'm still a bit nervous about using it!

In total it only generates about 1.5W but it reaches a temperature of about 30 degrees C. That sounds a lot but it is only warm to the touch - not scolding hot.

Still, I'm a bit nervous about putting it on the back of the secondary because I don't want to damage it.

Anyone got any idea if this sounds excessive?

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Understood - it does sound a lot.

However, I thought that to keep the dew off you needed the secondary to be a few degrees above ambient (rather than the heater). Presumably the secondary will be a lot cooler than the heater given that the heater only generates 1.5W and that has to set against the heat lost by the secondary??? I'm sure I saw a figure of 2W for a heater for a secondary - which is roughly what I based this on. I was surprised that this produced such a high temperature, but since temperature doesn't appear in the equations used to design this it's hard to know how to calculate this. TBH I'm out of my depth here, hence the nerves!!

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My main dew problem is with the secondary.

I looked up a few web sites and made a heater using resistors but I'm still a bit nervous about using it!

In total it only generates about 1.5W but it reaches a temperature of about 30 degrees C. That sounds a lot but it is only warm to the touch - not scolding hot.

Still, I'm a bit nervous about putting it on the back of the secondary because I don't want to damage it.

Anyone got any idea if this sounds excessive?

Bit late jumping in here, but here's how it works. The actual heater thing generates 1.5W of heat and theretore gets warm. It supplies the heat to the secondary and because the flow of heat is towards the secondary it has to be warmer than the secondary (heat always goes downhill towards the colder part of anything 1st law of Thermodynamics). The heat generated is dissipated (lost) to the surrounding air at a rate equal to the heat added so the secondary cannot get hot with a supply of heat rated at 1.5W, it's just not enough.

The heat goes (from the resistor heat source) in every way that involves heat travelling towards a colder object. Heat is lost to the air around the heater, is transferred by conduction into the secondary and then comes out of the secondary and warms the surrounding air. The heat has to transfer between the heater and the secondary through a small restriction ( the conductive path between the heater and the secondary. the amount transferred is equal to the amount of heat lost from the secondary, or the temperature of the secondary changes because the heat gained is greater than the heat lost or vice versa. As the surface area of the secondary is much bigger than the surface area of the contact patch between the heater and the secondary then it follows that the heat flow density has to be bigger for the smaller heater to secondary area. (Remember that the heat transferred is exactly the same but the area is significantly different.).

Let's try a worked example. The suggested heater consists of three resistors each supplying 0.5W of heat to the secondary. these are 15mm long and the contact patch is 2mm wide. The total area available for heat transfer is therefore 15mm X 2mm (=30 square millimetres) per resistor, or 90 square millimetres. The surface area of the diagonal is much bigger than this, even if you just count the shiny bit. As an example, lets suggest that the secondary is (a very small) ellipse equivalent to a disc 40mm diameter. This would have an area of pi X 20 X 20, or about 1250 square mm.

The temperature differece to give the same heat output from the secondary would be 90 / 1250 (=0.072) times as great as the temperature difference between the heater and the secondary. If the heater does run at 30C and the ambient temperature is a balmy -40C, then the temperature difference would be 70C betwenn the heater and the outside air. The heater has a temperature difference of 70C which would be split between the two resistances to the flow of heat so that the secondary temperature is 70 / 1.072 + (-40). This gives a secondary temperature of 0.072 X (70 / 1.072) = near enough, 5C. The secondary will be at 5C above ambient.

HTH

Captain Chaos

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