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Primary Mirror Dew Prevention


Guest sharpie78
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Guest sharpie78

I had some dew problems on my primary a couple of nights ago and was wondering if someone could point me in the right direction for some dew prevention please?


 


I have an astrozap dew shield which is working well for my secondary mirror but this is the first time I've had it on my primary.


 


I have a Skywatcher Quattro 8S and the back side of the mirror is exposed to the elements. Is that why it got dew'd up ?


 


I have seen info saying a fan would help so was thinking of going down that route seeing as I have an 80mm pc fan kicking around. I've also thought about making a dew heater strip using either nichrome wire or resistors (leaning towards nichrome wire).


 


I'll be making a dew heater strip for my guidescope anyway but would like some clarification on the best option for the primary mirror please.


 


Thanks in advance :)


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IMO a dew strip for the primary is no good, you would just be heating the metal tube and not the actual mirror cell. Instead, I have gone down the primary mirror fan option, here is mine prior to installation....

68C00D16-B3CC-4B6F-96B6-26A298A6308A_zps

It cools the scope quicker, then you can even leave it running on low while imaging/observing and some say it even improves the view.

Lots of people on here with newts have done this, I'm not sure of anybody using a dew heater.

Edited by Daz Type-R
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Guest sharpie78

Thanks for your advice guys. Seeing as I've got a fan kicking around I'll do a trial with that and go from there. I think I'll order enough nichrome wire to do both the guidescope and the primary just in case. Fingers crossed the fan will do the job though and I won't have to have to add the heater, keeping down the amount of cables down etc. My only concern with the fan was vibration but I'm guessing that as long as the fan is a well balanced one this would not cause any issues.


Thanks again


 


ps. anyone know the theory behind how the fan method works ? (I like to know why I'm doing mods)


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The fan pulls cold air from outside of the telescope and draws it into the OTA, as you want your scope to be at the same temperature as the outside air temp to reduce thermal currants which will distort the image you are trying to observe/image. The next benefit is that the fan somehow while blowing the ambient air up and around the mirror, creates at really thin layer of air that sits just above the primary surface and acts as a shield, preventing moisture from forming on the optics.

So a fan is win/win really, faster cool down times and no dewed up primary.

With regards to vibration, obviously the better the fan the lower the vibration but set to its lowest speed, nobody has complained on here yet about wobbly pictures.

Edited by Daz Type-R
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Also you will need to make sure you seal the back of the OTA, otherwise the fan will blow the air onto the back of the mirror cell which will then bounce off back into space.

If you look at the picture of mine above, the 3 lugs will stick onto the mirror cell housing and the inner circle (the bit the fan is attached too) completely fills the back of the OTA and seals it, therefor the cool air can only go up the tube.

Just read that link Mick, cracking good explanation, bookmarked that!

Edited by Daz Type-R
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Guest sharpie78

Nice one. That website along with your explanation really helped.


I'd been scratching my head trying to work out how blowing cold air at the mirror would work. I'd been thinking along the lines of heating it above the dew point and hadn't even considered possible degradation of the images produced.


The whole thin layer of air above the primary concept makes perfect sense.


 


Time to pick up a few bits n bobs for mounting the fan I've got and figure out a way to control the speed of the fan. My list of mods n jobs for the weekend is starting to build up a bit now. Ah well, keeps me busy while the kids are at their mums house this weekend


 


Thanks again :)


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Guest sharpie78

speed controller ordered - £4. Happy days. I'm pretty sure it's the same one you have Daren. I'll be honest, I'm gonna try and replicate your design too. I hope you don't mind and I hope you take that as a compliment. It's meant as one.


Right......now I've buttered you up a bit..... ;).......just wondering what you mounted the fan on ? plastic or metal ?


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Guest sharpie78

on the other method........Did you attach your dew heater to the OTA Andy or the primary itself ? I've been doing a bit of reading up on the whole dew control issue over the past couple of days and have read that people do both.


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Ha, confession time, I didn't make it!

While most of my mods are DIY, this one is not, check this out for all the info you need.....


>http://youtu.be/tON69sEH-AY

Dion from Astronomy Shed is awesome!

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Guest sharpie78

I just checked the video and he's got it on a 200P which has very similar dimensions to my Quattro. Looks like he's used 2-3mm plastic to mount the fan on. I now have all the info I need to get making :)


Thanks again. Once again EMS and it's user knowledgebase has helped me out. Happy days :)


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on the other method........Did you attach your dew heater to the OTA Andy or the primary itself ? I've been doing a bit of reading up on the whole dew control issue over the past couple of days and have read that people do both.

Just slung it round the ota level with the primary.

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Guest sharpie78

Cool. Cheers. Handy to know if the fan doesn't sort the problem. I don't like the idea of putting the heater around the mirror itself. Don't know why, just seems wrong to me. I'm sure the stuff I've read and the people who wrote it have their reasons but I tend to trust the advice on here rather than blogs etc. EMS members have never steered me in the wrong direction :)


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Thanks again. Once again EMS and it's user knowledgebase has helped me out. Happy days :)

No worries, that's why we are here.

Keep us informed of how you get on.

Edited by Daz Type-R
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