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Low temp, high Rh = interesting time. 6SE v ED80

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I have been travelling a lot recently, Korea and China, and only got back last Sunday night. It was great that last night was particularly clear and bright. So I had both 'scopes out.

The seeing was superb and I spent quite some time hopping through Scorpio which at 8 pm down under is nice and high in the sky.

The temperature was 13 C (which is cool for WA) and the Rh a surprising 81%, though with the low temperature it did not feel like it was humid. And there lay the problem....too much vapour in the air looking for something to do mischief with.

The RH/temp meter I used is a piece of demo kit from the company I work for, I know it is very accurate as I have calibrated it myself.

I have a Celestron 6SE and a SW ED80 on an EQ5 goto mount. I had only been out an hour or less when the 6SE just clouded up with dew so of course I then had to do all my observing with the ED80. The ED80 stayed dew free for hours, mainly because it has a dew shield which works very well.

So now I need to get a dew shield for the 6SE..or make one.

The more I use the ED80 the more I like it. Though it does not have the apeture of the 6SE even under high power I got some very nice views of Saturn, crisp and sharp. On star field it is a knock out. The Jewel Box was lovely and the M6 and M7 in Scorpio like crystal.

Tonight if it remains clear I will be out again testing the ED80. It really is a lovely 'scope. I am going to do some wide field imaging tonight now I have Backyard EOS which is so easy to use with the Canin 650D I have.

I was not dissapointed too much with getting dewed up on one 'scope as it made me use the other in a better manner and I learned a couple of good lessons.

The 6SE definately needs a dew shield.

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Have the 5SE and even with an astrozap dew shield, think all that you do, is delay the inevitable. Last night at Seething after a baking hot day, was dewed up by 1am. Short of running dew heaters...must remember to try pointing the scope down with the visual back removed next time prior to starting to observe; read somewhere that may help.

But after an iffy start to summer, will take a few hours observing here and there.


Edited by Cjg

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Cut a section of camping mat and wrap it around the ota, about the same length as the ota, and that should do the trick!

Out east (Sydney) it was pretty humid last night, temperature in the single digits but the dew shield kept my C8 clear for 3 hours before dinner (without heating) and it was still clear when I packed it away. There was plenty of dew on the outside of the ota, mount and EP case though, so it was definitely doing its job :cool:

Putting the shield on is habitual for me, originally being from the UK :D I'm sure it'll need it most of the time but I needed the heater more often in the UK.

Btw, Saturn was fantastic last night, probably the best I've seen it to date :cool:

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Dunkster, thanks for that tip, just to be clear, you 're suggesting the camping mat insulates the ota from temperature changes? Will have to give that a go. Thank you.


No, the idea is not to insulate the tube but to limit the amount of sky that the corrector plate is exposed to.

Have a read here for the full explanation http://www.sctscopes.net/SCT_Tips/Need_to_Have/Dew_Prevention/dew_prevention.html


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