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hi guys firstly I would like the pros and cons of using a dew heater on a reflector scope .

also me being a complete novice , where is the best position to wrap these around the scope ? front or rear by the primary mirror .

may seem like a dumb question but on reading different posts I get the impression that the mirror needs to be at ambient temp or even cooler . putting the dew heater near the mirror would this effect viewing on a cold night ? . many thanks Dave

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I have only needed to use a dew heater on the eyepiece and finderscope.  The scope's primary and secondary mirrors are deep inside the scope, especially if you use a dew shield, and don't seem too prone to dew.

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Hi Daveyboy.

I'm a complete novice myself although I had a similar experience myself the other night and by the time I went out to view the wonders of the universe everything was so fogged up that I had to come back inside and read the forums.

The basics I got wrong were:

I had the eyepieces outside cooling with the scope, many people speak about keeping them in their pockets when not being used.

I had all the covers off, OTA cover, eyepiece cover and finder scope covers. I have since learned to keep everything covered until you are using it and even cap the finder scope as soon as you are in the area of sky you want.

I pulled out a hair dryer (not my own I should maybe add) and warmed the OTA and it was surprisingly easy to remove the dew. I took the finder scope and eyepieces inside until they cleared and I made a cardboard dew shield for the end of the scope. I left it out for a couple of hours after that (unfortunately by then the clouds had rolled in) and it never fogged up again, whether it was the caps or my cardboard dew shield I don't know but I will experiment in future and I might make a 'proper' dew shield

The dew was a surprise to me and spoiled my whole evening but I felt I learned a little more about my scope. Looking into it a little further it might be worth making a small dew shield for my finder scope but I'll wait and see how I get on when / if this rain ever clears up.

Other people on the forum told me a dew shield can help prevent dew on the secondary mirror though, the primary should be alright with the tube acting as a dew shield.

Hope some of this can help you.

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hi guys thanks for the replys

 typical really lol I buy something I don't reall need by the sounds of it . so would you advise I make a dew shield for mine ? I thought they were mainly used on cassergrain scopes to stop the front lense dewing up . maybe im far off . cheers Dave

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I observe with a number of dob/reflector owners and they have stuck a basic ceramic resistor to the rear of the secondary as that is more prone to dew as it is more exposed - your primary is unlikely to ever dew up.  Then finderscope/telrad etc need a dew shield or a heater - I have a Rigel Quickfinder so have a dew band on that (and on my corrector), and then I also use an eyepiece dewband.

Depending on what scope you have and what mount adding a dew shield to a reflector on a GEM mount makes it more of a sail in wind

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I've used a dew shield but never a heater on my Newt. Only once when I didn't use the shield has the secondary ever dewed over. My Newt is also fitted with a plate carrying 3 fans below the primary mirror cell to cool and ventilate the tube and find that a more useful accessory than dew heaters.

ChrisH

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hi guys thanks for the replys

 typical really lol I buy something I don't reall need by the sounds of it . so would you advise I make a dew shield for mine ? I thought they were mainly used on cassergrain scopes to stop the front lense dewing up . maybe im far off . cheers Dave

From my experience of using different scopes and dew shields/heated strips etc.....................

It is mainly reflectors (lets call them Newtonions) that need/use dew shields. Its scopes such as refractors and SCT's that have a main objective glass lens at the front that need the use of heated dew strips. I bought one for my 8" SCT a couple of yrs ago and amazingly i have only really needed to use it a couple of times.

Edited by LukeSkywatcher

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