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Old 06-07-2018, 06:12 AM
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Primary dew fogging causing gradients

I've noticed some strong gradients in my images towards the end of my image run last night with my truss newtonian. I'm using a shroud and have the three small fans running on the back on the mirror cell. I was imaging in a dome pointing close to the zenith.

Early stages of dew formation on one half of the primary seems to be the cause of the gradients.

Any ideas on how to avoid this. Introducing a side fan to blow across the primary or perhaps a primary mirror heated dew system ?

Thanks Dave
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Old 06-07-2018, 08:02 AM
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That is quite interesting Dave, as I had gradients a lot with the GSO Newt too. I put that down to more light in the atmosphere from Adelaide and suburbs or because the there was not a front shroud. I never once considered this might be a problem because the fans were on and drawing air over the primary. That is supposed to keep dew from forming.

Perhaps the primary being a conical shape contributes with excessive cooling which pushed some parts of the primary below the dew point after a few hours with the fans operating. It might also only occur in areas of high dew too. What is your dew like each night? We have heavy dew generally each night at Clayton and moisture laden air would perhaps contribute.

I'm reluctant to put any heat into the primary as this can set up tube currents and movement of the primary if the heat is too high, not to mention how do you use a primary heater on a conical shaped mirror? Perhaps just turn the fans off after a few hours and see what happens. Have you tried that?
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Old 06-07-2018, 08:28 AM
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Originally Posted by Paul Haese View Post
That is quite interesting Dave, as I had gradients a lot with the GSO Newt too. I put that down to more light in the atmosphere from Adelaide and suburbs or because the there was not a front shroud. I never once considered this might be a problem because the fans were on and drawing air over the primary. That is supposed to keep dew from forming.

Perhaps the primary being a conical shape contributes with excessive cooling which pushed some parts of the primary below the dew point after a few hours with the fans operating. It might also only occur in areas of high dew too. What is your dew like each night? We have heavy dew generally each night at Clayton and moisture laden air would perhaps contribute.

I'm reluctant to put any heat into the primary as this can set up tube currents and movement of the primary if the heat is too high, not to mention how do you use a primary heater on a conical shaped mirror? Perhaps just turn the fans off after a few hours and see what happens. Have you tried that?
Hi Paul.

Yes, the nights have been of high dew although the mirror is never heavily dewed up. In fact the fogging is easily missed unless you shine a light on the mirror. I might try the easy option first - Turn off the fans and see what happens.

I noticed when only half the mirror was lightly fogged that this corresponded to an image that had a light background on one side of the image and a dark background on the other half. The image beforehand less so and the images before that were fine. So as the fogging increased the gradient increased.

Yes, I dont really want to apply heat to the main mirror and a conical mirror would be a challenge. I know AG Optical use such a system on their iDK scopes.
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Old 08-07-2018, 11:04 AM
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Yesterday as I started stripping down the Newtonian I discovered some things that might be affecting your dew problem.

See attached images.

The first things is that the primary is fixed with a threaded plug. This is glued into position with that silastic goo they use. I noted that the mirror was flexing with a bit of pressure and that might be contributing to star shapes. It takes a bit of pressure to remove the mirror but go in an anti clockwise direction and it will come off eventually.

Secondary and hence the reason why you need to remove the mirror, is that the fans blow air onto the mirror not suck it down and past the mirror. This would also create astigmatic star shapes early on with cooling and caused partial dewing of the primary too. I reversed mine, so you might want to check the direction of flow. In the pictures the fans are now correctly facing outward and you can see the dust marks on the back of the mirror too where air was being pushed into one spot before.
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Old 09-07-2018, 04:25 PM
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Originally Posted by Paul Haese View Post
Yesterday as I started stripping down the Newtonian I discovered some things that might be affecting your dew problem.

See attached images.

The first things is that the primary is fixed with a threaded plug. This is glued into position with that silastic goo they use. I noted that the mirror was flexing with a bit of pressure and that might be contributing to star shapes. It takes a bit of pressure to remove the mirror but go in an anti clockwise direction and it will come off eventually.

Secondary and hence the reason why you need to remove the mirror, is that the fans blow air onto the mirror not suck it down and past the mirror. This would also create astigmatic star shapes early on with cooling and caused partial dewing of the primary too. I reversed mine, so you might want to check the direction of flow. In the pictures the fans are now correctly facing outward and you can see the dust marks on the back of the mirror too where air was being pushed into one spot before.
Yes, my fans are the same. I noticed this a couple of days ago. I will swap them around.

So there is a bit of play with the silicon goo ? Can you reduce the amount of goo to get around the flexing ?
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Old 09-07-2018, 06:25 PM
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Yes, I found my mirror had a bit of flex, I suspect GSO put a bit of the silastic used that glue just to stoop the mirror from unthreading in transit but I think this allows for some movement and rubbery flex. I would not bother with using glue again and just thread it back into the female thread and just nip it up.
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Old 10-07-2018, 10:27 PM
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I don't think that a fan can cool anything under ambient temperature, as the air is at ambient temp. It's the mirror itself that is cooling, by radiating it's heat to space.
Maybe more air is the solution to keep the mirror at ambient temperature?
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Old 12-07-2018, 08:27 PM
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I had an 18inch plate glass closed tube newtonian.( also on a Titan) I did find that I could induce primary mirror dewing with the cooling fan and that switching off the fan was the solution. I did have a fan to blow across the primary front surface
but didnt use it much. It did help a bit with seeing. Such dewing was a rare thing.
GlennB
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Old 13-07-2018, 05:33 AM
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I had an 18inch plate glass closed tube newtonian.( also on a Titan) I did find that I could induce primary mirror dewing with the cooling fan and that switching off the fan was the solution. I did have a fan to blow across the primary front surface
but didnt use it much. It did help a bit with seeing. Such dewing was a rare thing.
GlennB
Well, dewing occurs because hot air can transport more humidity than cold air. So, if hot and humid air hits a cold surface, such that the air is cooled to a temperature which cannot hold the humidity it carries anymore, it will release the extra water as droplets on the cold surface. I can see how a fan can be a problem if the air begins to dew over the mirror, as you're forcing more air to hit the mirror. Up to that point, the air should heat the mirror slightly, or to be more precise, stop the mirror from falling too much under ambient temperature, but if it fails to keep the temp over the dew point, then it's blowing air at ambient temp over a mirror under the dew point.

So, it seems that the options are: use a dew cap, if the scope has a lens on the front; keep the dew cap and the scope from loosing too much temp; keep the mirror/lens over the dew point by heating it.

I found this article about dew in scopes. Very informative, and good ideas: http://www.asterism.org/tutorials/tu...ngOfOptics.pdf
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Old 18-07-2018, 09:00 AM
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Originally Posted by fsr View Post
I don't think that a fan can cool anything under ambient temperature, as the air is at ambient temp. It's the mirror itself that is cooling, by radiating it's heat to space.
Maybe more air is the solution to keep the mirror at ambient temperature?
The direction of flow is the critical aspect with this problem. If the fans pull the air past the mirror then dewing is less likely to occur.

I have found that at my site which nearly every night has a humidity content of nearly 100% and very still conditions, that dew forms easily on everything. Most years I can gauge nearly 100mm of dew alone at a site that gets less rain that the Mt Lofty ranges.

These scopes have conical mirrors which cool very quickly and can come very close to ambient in these conditions. Some morning I have visited the site in winter and seen dew on the primary mirrors of the scope.
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