Effects of a hair dryer on Jupiter

[neil phillips]

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?

It probably would. But at the sizes secondary's are, getting the secondary cool ( getting all the heat out of the secondary ) is not nearly as problematic as a large primary. And should cool fairly quickly in comparison to a thick large primary.

The only real advantage I can imagine is when there are falling temps. in those situations keeping the secondary falling with the night time air could possibly help. But by how much I haven't a clue. I wouldn't underestimate these kind of problems. with falling temperatures, and there effects on secondarys. Its a interesting idea. But there might be other considerations like affecting thermal stability of a closed tube. Open tubes would likely suffer less problems. But as with James I am really guessing all this ?

[neil phillips]

If you decide to get the jig saw out, take care Neil :

Well that's swung it for me, just too dam risky. Only a fool would contemplate this idea.

[JamesF]

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 ?

I appreciate what you're saying, Neil.  If dew control is well understood (ie. at better than "fit a dew shield" or "fit heater tapes" level) then I don't think it's very well explained anywhere that I've seen.  I'm not at all convinced that it is that well understood though, at least at an amateur level.  It seems to me that there are a lot of variables, some of which may be far from obvious.  I wouldn't be in the least surprised if open-tube OTAs behave differently from closed-tubes and both are different from truss-tube systems.

I'm also very much aware that it doesn't matter how nice a theory you have, if it doesn't work in practice then you need a new theory.  That you and JonH have had issues with dewing very quickly when blowing ambient air over the optical surfaces certainly suggests to me that we don't understand something somewhere or that some of our assumptions are invalid.

I don't know why this didn't occur to me earlier, but I guess some experimentation can be done without clear sky (though I don't think we can guarantee that the results with also apply given a perfectly clear sky), so if I have time I might try to do some testing over Christmas.

James

James

[neil phillips]

I appreciate what you're saying, Neil.  If dew control is well understood (ie. at better than "fit a dew shield" or "fit heater tapes" level) then I don't think it's very well explained anywhere that I've seen.  I'm not at all convinced that it is that well understood though, at least at an amateur level.  It seems to me that there are a lot of variables, some of which may be far from obvious.  I wouldn't be in the least surprised if open-tube OTAs behave differently from closed-tubes and both are different from truss-tube systems.

I'm also very much aware that it doesn't matter how nice a theory you have, if it doesn't work in practice then you need a new theory.  That you and JonH have had issues with dewing very quickly when blowing ambient air over the optical surfaces certainly suggests to me that we don't understand something somewhere or that some of our assumptions are invalid.

I don't know why this didn't occur to me earlier, but I guess some experimentation can be done without clear sky (though I don't think we can guarantee that the results with also apply given a perfectly clear sky), so if I have time I might try to do some testing over Christmas.

James

James

Good idea James, testing is needed. The only other variable I should mention is I often put the scope in the shed hours before I image. But I am careful to make sure dew can not enter the scope.

With a large bag over the focuser end, and sealed with tape. The sealing definitely is airtight. No question.

The primary end is standing upright so a certain amount of sealing  will be inherent doing that. But even if air can get in there, and I agree it likely can.

Because of the long tube length of where air can get in. It must act like a 4 foot dew shield. Rather like the primary not ever dewing up because its so far from a open end.

So the secondary should behave the same.

The amount of dew that gets in the shed compared to the garden is strongly reduced. Although the shed is certainly not airtight. When I feel the tube prior to setting up on the mount. The tube has not even felt wet. let alone dripping, like would happen after two hours in the open.

So this is the only variable I can think of. And I am pretty certain this is not the cause of what I am discovering. Though to be absolutely certain, ( lets say scientific ) about this, testing would have to be performed without cooling in a shed.

Its possible I am missing something. But I don't presently believe I am. If dew was getting in with a sealed Focuser and cap end. And the scope doesn't even feel wet. That would be surprising to me beyond belief.

But I do have to mention this as a variable, that is the only other possible cause of what I have described. We have to be transparent when discussing everything don't we.

Your Thoughts James

[Space Cowboy]

I guess experimentation is the only option but I would think it can only be done under a clear sky to get a true result and alas we don't get many opportunities in the UK which makes it so frustrating.

[neil phillips]

I guess experimentation is the only option but I would think it can only be done under a clear sky to get a true result and alas we don't get many opportunities in the UK which makes it so frustrating.

Agreed Stuart. I will continue to protect the secondary though, at some point I may test cooling again. And see how fast the secondary dies.  I wonder if others have any  direct experience with this issue. Meaning in the field.

One thing that doesn't make any sense to me is why the primary hasn't dewed over with the front fan, when I cooled for a long time. But it didn't.

Maybe the primary being larger held more heat and hence would take much longer to dew ?

Its a interesting subject

[Kokatha man]

I'm a hair dryer man myself Stuart...got to look your best out in the bush in the middle of nowhere - who knows when a large saltwater croc might've come along during our recent trip up North.....they're fussy eaters..!

Seriously though, the HD I have (circa 1950's) not only does a nice "Rockers" style blow-wave but set on "cool" it actually blows slightly warmish air and quickly rips dew/fog off the corrector with minimal cooldown time.....although I'm also known to fan the c/p for about 30 seconds with last years ephemeris to hasten the cooling...a [removed word] if I hit the scope lip with the mag though, sometimes I need to re-find the planet onscreen when that happens!

[Space Cowboy]

Never knew those Aussie croc wrestlers were so fussy about their appearance Darryl lol

[Kokatha man]

Never knew those Aussie croc wrestlers were so fussy about their appearance Darryl lol

...well, I did carry a very sharp hatchet in my belt everytime we went down to the river to pull our prawn nets Stuart - and Pat always stayed further up the bank and no doubt would've whipped her iphone out for some pikkys if I had to do battle..!

[Kokatha man]

ps: Kenny would've looked a definite picture of health in comparison should that have occurred!