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Effects of a hair dryer on Jupiter

Space Cowboy

By Space Cowboy
in Imaging - Planetary

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Space Cowboy

Space Cowboy

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Rather than use a dew shield or heating straps I rely on keeping the secondary mirror clear of dew with a hair dryer which is very easy with an open flextube OTA. At the end of my last imaging session I decided to record Jupiter as the hair dryer was used to see how quickly the image returned to normal.

I must add there was no dew on the mirror when I did this.

The avi duration was about 45 seconds not the figure shown on youtube as the compressed video came out at 15fps.

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Jupiterholic

Jupiterholic

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Yep, add me as a plus one for the hairdryer crew as well. I use mine copiously as I live in the more high altitude hilly regions of Bristol so its always insanely dewy. I sometimes worry what the neighbours must think though when they hear a hairdryer blasting out in the garden next door at 3am. Really does the job though.

Thats a great vid. Looks like the seeing was pretty good that night as well, at least... compared to recent times. When was this? wish I'd been out then. Do you have any images from that night?

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Space Cowboy

Space Cowboy

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Thanks for your comments guys. Personally I think the less material wrapped around the scope the better as this causes thermal turbulence and if you combine dew shields with heater tape then you've got a perfect chimney effect.

There is a very strong case for having a trap door cut into a solid Newt OTA below the secondary allowing hair dyer access.

Cheers Steve, yes the seeing was pretty good on this morning after 6am. I'm never out with the dryer at 3am as the local police have been cracking down on moon lighting hair stylists so its just too risky! :cool:

The avi was taken back in November, here is the thread for the best image I got a few mins earlier :

http://stargazerslounge.com/topic/200390-jupiter-callisto-nov-23rd/

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neil phillips

neil phillips

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When I had SCTs and Maks it was hairdryers for me too. No messing with those heating strips. when a quick blast cures the problem.

My only problem is getting a trap door  cut into place. Not easy for me. Or likely that cheap. The door would have to be airtight I suspect. Because of the inbalance the closed tube would cause if it was not. The door would also have to be in the right place to allow easy acess to the front of the secondary. While keeping the direction of the heat well upwards. As any leakage downwards would result in dewed primary. Do you know of anyone that does this with a closed tube Stuart. Was it Bird who did this with a closed tube. If I remember your link.

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JamesF

JamesF

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I must admit that whilst I am making up some heater tapes for my C9.25 I have been wondering if it doesn't make much more sense to remove the dew shield when using them.

I have also wondered if it might help just to have a fan blowing (ambient temperature) air across the face of the corrector all the time (no dew shield, no heater strips).  I was thinking that a free-standing fan might make more sense.

James

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neil phillips

neil phillips

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I must admit that whilst I am making up some heater tapes for my C9.25 I have been wondering if it doesn't make much more sense to remove the dew shield when using them.

I have also wondered if it might help just to have a fan blowing (ambient temperature) air across the face of the corrector all the time (no dew shield, no heater strips).  I was thinking that a free-standing fan might make more sense.

James

That's a interesting question James for SCT users. Because ive often read a fan blowing on mirrors at ambient. will prevent dew. Yet John H told me the opposite. In the field it seems to actually help the dew form. As ambient air will not prevent dew. From what ive seen.

The Temp needs to be slightly above ambient. which we can not do. I suspect disturbing the air cells directly in front of the Corrector plate, would be bad news. It just seems to make little sense to disturb the air the light is passing through.

Having said that, I am doing something similar with a back blowing fan on the primary. Though there are big differences. The air is not being blown sideways. and is behind the mirror rising in front of it. What effects all these different things will do is a interesting Question James. Ive not heard of that mentioned before. For example would the strength of the fan influence its effect on the air cells directly in front of the corrector surely it must ?

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neil phillips

neil phillips

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You're welcome Sammy.

Yes Neil, Bird is the man. I don't know whether he still uses this method but here is the link where he explains how it works :

http://www.iceinspace.com.au/forum/archive/index.php/t-19111.html

It does seem a very good solution I agree. Its not easy for me to get the job done is the only problem I have

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JamesF

JamesF

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That's a interesting question James for SCT users. Because ive often read a fan blowing on mirrors at ambient. will prevent dew. Yet John H told me the opposite. In the field it seems to actually help the dew form. As ambient air will not prevent dew. From what ive seen.

My thinking here is that dew seems far more common when the air is still.  Certainly with my Mak it seems to be the case that on nights when the air is still dew forms far more quickly than when there's a breeze, even a very small one that's almost beneath notice.

I'm guessing rather than having any sure reasoning behind this, but I'm assuming what happens is that the corrector gets very cold -- certainly colder than ambient, it being easy to radiate heat away when pointing at a cold dark sky.  If the air is sitting in front of the corrector with little reason to go anywhere (as perhaps it can when a dew shield is in place) then it will be chilled by the corrector to the point where it reaches 100% RH and starts giving up moisture that ends up on the corrector.  By keeping the air moving then air doesn't get much of a chance to chill in front of the corrector and therefore shouldn't reach dewpoint.

There are a fair few variables here, I know.  It's presumably less likely to work (if at all) if the air is already below dew point anyhow as the moisture has to come out of the air anyway.  In that case I think only heat can ever help.  It hasn't skipped my attention however that if the corrector is below ambient then blowing ambient air onto it is effectively heating it :)

Is it likely to measurably alter the image quality?  I have no idea.  I'm not even sure how to properly go about measuring it.  I suspect that if implemented carefully then many of us have far larger sources of "optical interference" from our surroundings.

Perhaps one day if I have two similar telescopes so I have one to use as a control I can set up some experiments to see what happens -- just in terms of how long it takes dew to form in each case.  Or perhaps I could do it with the ST102 and ST80, allow one of them to dew up and then swap them to repeat the experiment.

James

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JamesF

JamesF

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Sounds good theory James. So if I sat with my hair dryer on cool pointed at the secondary it wont dew up......hmmmm....or a small fan fixed to the side of the OTA.  Many possibilities!

I just don't have a clue if it will work or not, to be honest.  At the moment I'm just unable to find reasons to definitely expect it not to work :)

James

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JamesF

JamesF

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Sounds good theory James. So if I sat with my hair dryer on cool pointed at the secondary it wont dew up......hmmmm....or a small fan fixed to the side of the OTA.  Many possibilities!

Oh, I was thinking of some sort of fan without a physical connection to the OTA to avoid transmitting vibrations to it -- probably on a separate stand.  Though it has just occurred to me that fixing it to the mount somehow might be adequate.

James

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Space Cowboy

Space Cowboy

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Yep vibration is a problem as is falling over a stand lol I doubt I could persuade my lady to hold the hair dryer whilst we spent a romantic evening under the stars.....

I wonder if a cooling fan on the secondary would also improve the image in the same way a primary fan does by equalizing the surface temp?

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cantab

cantab

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It's presumably less likely to work (if at all) if the air is already below dew point anyhow as the moisture has to come out of the air anyway.

If the air temperature is already below the dew point then isn't that going to make it foggy???

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neil phillips

neil phillips

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Found this thread on the CN forum which backs up James's ambient air blowing theory :

http://www.cloudynights.com/ubbthreads/showflat.php/Number/4334022

If this was truly the case, when I cool with two fans running. One at the back, and one at the front. surely dew would not form. So here's the rub I can assure everyone. I cooled like this once for perhaps nearly two hours. When it came to image, My secondary died within 20 Mins of starting ?

 One of the fastest times its ever died on me.

Yet when I cool with a bag protecting the secondary. I can image for at least a couple of hours. This behaviour is making no sense at all with the theory that I keep reading. I would like to talk with John H about this, as he was the first to mention to me that blowing air on optics can actually speed up dew formation. I am not sure if like me he has direct effects of it doing so on hes 14" Newt. But that's what he told me. And it certainly appears true to me in the field. I can not see how the fan is not making the situation worse. When the scope is dying about 4 times faster than if I protect the secondary during fan cooldown ? Any explanation lads ?

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