I find this interesting. Reflective tape would reflect heat *away, wouldn't it? I would have thought it would *accelerate the cooling of the scope past the dew point? I believe you that it works, I'm just grappling with the how? (and then thinking of doing something similar :-)
Markus
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Originally Posted by Wilso
Probably works as a blanket,
keeps the heat in reducing thermal loss.
Works both ways.
Warmer in winter, cooler in summer.
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Originally Posted by glend
I recall that Allan L had a theory that using one of those reflective "Space Blankets"/ survival blankets, would work very well at keeping condensation off of his dob. I think the idea of slowing thermal loss to the cold night sky is exactly right. Keep the scope warm and cosy. I invisage a new must have accessory, a "Telescope Cosy" - a heated reflective shroud that keeps just enough heat in your setup to avoid the dreaded Dew. Dew Not be without one.
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Originally Posted by multiweb
As long as you're shaded from the night sky you'll be alright. The only time dew is an issue for me is when I point the scope right up. The C11 with its larger glass corrector is the worst ...
the reason that dew forms on scopes in such profusion is that a scope gets significantly colder than the surrounding air. That is because any surface other than clean metal will radiate heat into the very cold night sky (clear sky is at -20C or less) and the scope tries to get to thermal equilibrium with the sky on upwards pointing surfaces. Any type of paint, carbon fibre, glass, plastic etc all work well at trying to freeze the scope.
Radiative cooling can be stopped by a shiny metal surface and the scope will stay ~at the ambient air temperature with a little bit of forced air flow over a very small heater to mop up the cooling due to the open aperture. Reflective aircon tape, alfoil or a space blanket work well as a low emissivity surface coating. Dew will still sometimes form, but only if the ambient drops below the dew point. Most nights it is not a problem in my seaside location.
reflective exterior surfaces work well on a tube Newtonian or RC, where the glass is all pretty well hidden from most of the sky. The idea still works on refractors or anything with a corrector plate, provided that a long dew shield (with reflective outer surface) is used to restrict the sky angle that the front glass sees. Truss scopes will also need a shroud with a reflective outer surface.
Planetary imagers go to extreme lengths to ensure that their optics are no warmer than the surrounding air. Even a 1C temperature difference can cause a noticeable drop in resolution. The situation will not be as bad with DSO imaging, but even so, there is the potential for excessively heated optics to degrade the scope resolution.
Edit: to see how cold the top of a scope gets from radiative cooling, have a look at Clive Milne's last image in the first post of http://www.iceinspace.com.au/forum/s...ad.php?t=83503 The top of the scope and shroud is much colder than ambient. A little bit of that may be sky reflection, but not much.
If you wanted something semi-permanent, you can buy these tent domes from the USA. Should help with keeping equipment dry. It could be light enough to pick it up for rotation. Probably wouldn't handle a heavy wind storm though.
What is not being given enough consideration is site selection.
Giving your site more consideration than just "this open grassy field will do" will not only just about eliminate your dew problems, but also improve the quality of your sky with improved transparency, and if even more consideration is given will see a laminar airflow over your location that will go a long way to improving seeing conditions too.
My friends and I have three locations available to us, which we have spent many years researching and settled on because these sites provide the very best astro locations within a two hour drive from Sydney. We've seen M33 naked eye from these sites and the Milky Way arches from horizon to horizon without it fading out visually along the horizons. And we've experienced light dew form on our optics only three times the last seven years, and never has our gear been packed up sodden with dew.
If you are already on a rural property, you may unfortunately have no choice but to deal with dew every day, unless you are then willing to travel.
It isn't hard to find these locations. Professional observatories give us all the clues we need to identifying the conditions as to why they are located where they are, and just being far from light pollution is only one aspect of several.
It takes studying the geography and how yearly weather patterns all interact, along with the local surrounding vegetation and landscape. That big grassy field is actually the very worst place to set up a scope. Being dew free may also only be seasonal, but better this than nothing. Other times by reading the weather system at play on any given day will tell you if your location will be optimal or just give it a miss. This is important as it will tell you if you will experience dew and/or cloud cover. For even professional observatories are not exempt from poor weather, but by giving due attention to where you set up, you maximize your productivity and reduce your complications.
It also means giving attention to the colour of your gear too. Black is the very worst as it sucks up any heat like no ones business and at night any black surfaces will be several degrees below ambient. While the temp will say 3°, frost will however be forming on your black vinyl seat, shroud, black painted OTA and mount, and your car's windscreen. This is why some of you are talking about wrapping your gear in essentially brighter materials, such as blankets, in order to not have the temperature on your gear drop too far. Introducing heat to your gear needs to be done very carefully - you've just spent a whole heap of time waiting for your gear to cool, and then you introduce a heat source... Careful here. Our friend Mark Suching made me aware of this here many years ago.
Agricultural land needs to be given a miss. Inside valleys too. Expansive plateaus are problematic as well.
Yet great locations can also be found at sea level if attention is paid to the geography and weather patterns at play. Florida has one of the most fabulous astro locations around with laminar airflow at one location being a regular occurrence meaning you can max out the magnification on a regular basis. You may not get a dew free environment, as it is all about compromise somewhere along the chain.
I have a couple dew migitation systems on my open dobs. At the locations my friends and I use I've switched one system on just three times in seven years. Yet at other locations, most times it doesn't matter what you do, dew and the associated mist that comes with it just kills dead all your efforts on a regular basis.
I hate dew just like everybody. My niche in astro, sketching, has made me very acutely aware of it. It has also been the driving force behind looking for and identifying those location that mean I don't need to contend with dew or it is very easy to deal with. These locations do exist, and it is not difficult to identify them.
Alex.
Last edited by mental4astro; 29-05-2018 at 12:02 PM.
the time the secondary dews up you'd be in for a pretty uncomfortable session anyway
If the seeing is great I’d be there till I froze regardless of dew. I typically use THREE sites. The nearest is an oval 5 mins from home, good seeing but not ideal dew-wise. The second is 15 mins from home, quite good dew-wise as it is on a sandstone ridge and dry and gets laminar airflow. The third is a dark sky site in sandstone country behind a ridge at 1100m, it is excellent, but a 2h drive.
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I'm playing with a couple of new scopes now and I'm trying to gauge whether I've just been lucky.
As per previous the factor you’re ignoring is the microclimate at your site. Avoid black soil, farming country, ovals. Look for an area with pale coloured terrain, preferably sandy or sandstone - which doesn’t hold water. On a calm night 100 metres can mean all the difference between no dew, or terrible dew.
The other cause of dew is a sea breeze (I'm thinking Great Ocean Rd, Geelong, or Mornington), loaded with moisture and salt off the sea. The only solution to that is to go somewhere this won't be an issue, usually a long way inland.
Ideally on or just back from a high ridge facing into the prevailing breeze to get laminar airflow over the top which maximises your chance of good seeing. Sites downwind of sources of turbulence will have rotten seeing most of the time - starting with homes, buildings etc.
Melbourne is a flat pancake city on a VERY FLAT plain, so I'd say you're stuck with crap seeing unless you head to Mt Macedon, the Grampians, the Dandenongs or the high country around Bright, ie Mt Buffalo.
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never on my optics
I’ve seen dew on primary mirrors AND the secondaries of dobs - especially the open truss type. Old-school full length closed tube has a lot of merit IMHO. And the old Sonotube (cardboard) tubes offered a lot of insulation. While it looks like the cheap crap that it is, it worked well - a lot better than metal tubes.
Refractors and SCTs form dew on the corrector even worse than car windscreens.
I’ve even had dew form on the inside of my corrector plate.
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the important surfaces to protect
1. Refractors and SCT Corrector plates. Insulation around the OTA and heater strap... a WHITE OTA is by far better than dark colours (theres a hint, Synta, Celestron, Meade and Tasco, if you read this). The old Unitron refractor tubes were always white, for a REASON. As a workaround, a 12V heated airblower will clear dew and warm the corrector enough for an hour or more.
2. Exposed mirrors (dobs) - fans and a little heat, 2W is likely enough.
3. Eyepieces - keeping them warm inside a jacket does the trick
I had spent a lot of time looking for spots via googlemaps with some success, but I found the requirements for astronomy were often self-defeating.
1) Places with open access to the public after dark where the public don't actually *go after dark.
2) Places with low horizons and car access via roads no-one actually drives on (low horizons means you will be exposed to light for miles in any direction; being sheltered from light means your horizons are unlikely to be low).
and 3) is the big one (no pun intended) - Places with toilets but no lighting.
If you've ever been stuck at 1am needing a loo and you're thinking about how long it takes to pack up your gear and set it up again versus the risk of leaving it while you drive 20 minutes to the nearest loo, versus, trying to dig a latrine in the dark on a rocky ridge...it's not fun.
TBH it was this last point that made me join the ASV, I kid you not.
On the laminar airflow point - does it matter so much if conditions are generally calm? How high does this mythical ridge have to be in order to make the air flow laminar? I guess there's also a danger that the effects of orographic uplift will give you more local clouds too?
Why are you limiting your search to just public areas?
Remember the saying "ask and you shall receive"?
Not all private rural property owners are homicidal maniacs.
If you are prepared to ask people, you can access a lot more territory, and make some great friendships in the process. We have.
This may also help you with your belly situation too as many places have outdoor loos too.
Can't say this has been a concern for us, the toilet situation. Sometimes you just need to use the shovel AFTER the fact. Like you said, sometimes there isn't enough time to dig first. So, dig afterwards.
Laminar air can occur anywhere from sea level to mountain tops. It is a function of heat, wind direction and terrain.
Remember those images of fog filled valleys? The mechanism that drives the fog to accumulate in the valley also generates the laminar airflow that's coming off the ridgetops and cascades down into the valley below. This airflow is not just laminar, but it is also this movement of air is that helps keep your gear dry from dew.
The images below are of the Megalong Valley from Hargraves Lookout in the Blue Mountains. You don't want to set up in the lovely green fields down in the valley. The ridgetops is where you want to be...
The orographic uplift you mention is important too. It forms part of your reading of the weather forecast to tell you if you go to your chosen dark site or go somewhere else or stay home. This is why we have a few different places we can use. This means we are able to keep our gear dry, and not needlessly subject it to dew.
Wavy mentioned that 100m can mean the difference between terrible dew and no dew at all. We've had brilliant nights up in Medlow Bath in the Blue Mountains, bone dry and awesome sky, and the drive home through Katoomba everything was soaked like it had rained the whole night. That was just a 50m difference.
Different geographic locations will present different conditions, and these conditions will change with the prevailing weather of the day. But a bit of homework goes a long way. The Obs at Coonabarabran was not a fluke selection. A lot of freaking work went into selecting that site. Thing is it wasn't professional astronomers who did this work... Sadly however, when it comes to amateur astronomers, this part isn't noticed. And just pitching a tent blindly just anywhere in Coonabarabran will not give you the same sky conditions as the AAO has. You may actually still get a crap sky even though the AAO is tearing it up. For that matter, I also don't know of any club or society that has also undertaken the necessary feasibility studies for selecting the dark sky site they use. There may be a society or two that has in Australia, but none in NSW.
I had actually submitted an article I wrote exactly about this to IIS 2 years ago. Everything I mention here is in that article. For whatever reason nothing happened and it was never posted in the Articles section. Things just don't work here on IIS as they used to.
All the same, the article is on my astro blog site:
Katoomba Airfield closed down at the time I wrote the piece as the Airfield manager was killed in an aircraft accident - I miss my friend. What is important is the research exercise in finding locations.
Alex.
Last edited by mental4astro; 31-05-2018 at 07:22 AM.
The terrain I was referring to is Shipley Plateau west of Blackheath in the Blue mountains. This is a Plateau at approx 1000m delineated by a long line (49km) of west-facing cliffs, where the top is a sheer vertical drop typically 300m high, with a further 300m of inclined scree to wide valleys that are typically 200-400m above sea level. The prevailing wind is SW to NW.
The reason I know the air is often laminar in the evenings is from flying a paraglider in this same area, in which you feel the air just as a bird does- every slightest ripple, every microthermal is felt through the fingertips.
A common pattern in the afternoon is that the sun warms the cliff faces and the valleys below. In the middle of the day the thermals can be savage - as fast as 40m/s going up. As the sun sets the thermals die down but the residual heat in the ground causes the air mass in the valley to gently rise - with enough lift for the paragliders to fly well after sunset. In these conditions the air is as smooth as you could wish for and flying in it is magical, in silence.
With a gentle breeze impinging on the ridge facing west the other phenomenon is compression over the top edge of the ridge - the air accelerates and any turbulence vanishes.
Same would happen on a high ridge in the Grampians, mt Macedon, or on the plateau at Mt Buffalo.
The compression phenomenon also occurs on a decent sand dune or coastal ridge facing into a sea breeze - even just 20 metres is sufficient. Examples of that occur all along the coast here and particularly at Long Reef, Stanwell Park and Mona Vale which would be good for observing except that (a) the sea breeze is saturated with water vapour (b) it’s salty (no good for scopes) and (c) badly light polluted. In Victoria the analogy is the top of Mornington peninsula which also has the same negative aspects.
Where I live dew is horrendous. On about 75% of my viewing/imaging nights
it would be utterly impossible to continue without heaters on both the primary and secondary; the secondary dews up almost immediately.
I have had to modify one of my heaters to use on my widefield camera lenses.
I did an experiment the other night. After being outside for about 30mins
I wiped the lens and waited to see how long it would take to mist up again.
To keep the unheated lens clear I would have to wipe it every seven seconds;
a bit impractical methinks.
raymo
- farming country (plenty of vegetation = moisture),
- dark soils,
- on a flat coastal plain, barely above sea level,
- faces west into a seabreeze = saturated air.
Again the dual concerns of Astronomy are confounding - cleared land is usually cleared for a reason (farming) so there will likely be either moisture from irrigated crops or moisture from natural vegetation. Higher elevations do tend to attract more rain too. It's crazy hard to find a site that satisfies all criteria.
The LMDSS is pretty dry. So dry that dust on the track is a year-round issue, but it settles. The only crops nearby are vineyards which, as far as I understand are not a heavily irrigated crop. We don't have a ridge for laminar airflow, but hey, did I mention we have toilets?
Oh, and running water and electricity, etc, etc. I'm happy enough not to suffer the creature discomforts of observing by myself, but this conversation has got me curious enough to start searching again just to see...
I know what laminar airflow is, I just don't know the conditions that are favourable to it.
Could the experts enlighten me;
Which is more conducive to laminar airflow - the tallest hill in a range, or a single hill in an otherwise flat plain? I'm finding elevations are lower with the latter.
(You can tell me because I'm in Melbourne, so I'm not going to steal your observing spots!) :-D
Higher elevations also tend to have poor soils that drain very easily and do not hold water. Cultivated land has the soil improved, so even if there are vineyards, the soil will be richer and harbour more moister than say gravel or sand.
This is what made Katoomba Airfield so good. Not only its location on a narrow plateau and also close to the cliff edge, but that the ground there was all highly compacted gravel/sandstone. When it did rain the water did not stick around.
One location we use now, the site's situation is very similar to that of the Airfield. However, the main difference is the soil profile of the property and the surrounding properties. The soil is significantly richer, all with lush turf and with farming and orchards all around. The result being that despite the laminar air, the moisture content of the air is still much higher, so we do get a light dew sometimes form. The microclimate difference between the two sites is what distinguishes the two. We never had dew form on our gear during our observing sessions at the Airfield.
Both Wavy and the article I wrote mention the importance of the microclimate/surrounding vegetation when it comes to site selection.
As to which is better, the highest peak in a ridge or a lone peak on a plane, well, both, and it depends...
The lone peak will keep you higher above the more moisture rich lower plane. But can you access that peak and does it have a suitable area to work on?
A valley or mountain range will be influenced by the direction of the prevailing seasonal winds, and this is important to take into consideration.
One major clue is given by Wavy - where do you find hang gliders and paragliders?
Remember, a totally exposed ridge top can also be more trouble than help. While it may remain totally dew free, it will also mean you are totally exposed to all the wind, and without any relief. A sandy/rocky clearing may be a better option with the trees offering a much needed windbreak without totally killing the breeze.
For all the suggestions and guidance we can give, you will still need to do much leg work to get out to promising locations and suss them out in person. Only this way can you determine the true soil condition, surrounding fields/land use, site exposure, accessibility, security, safety, facilities, fireplaces, etc. All the same leg work that was done for the AAO. Wavy has already given some locations you may like to start exploring.
The rewards for having done all of this ourselves now means our gear is not constantly and needlessly soaked, we are not burdened with all the complications that dew mitigation brings with it, and we can just concentrate on astronomy under the very best quality sky.
I think what you say is true. I spent a lot of time finding a spot in the Lerdedergh forest thinking it would be secluded and relatively free of LP and close to Melbourne. It was a clearing on a ridge, and did have rocky gravelly soil (I know, I checked! :-P), but the surrounding forest made it a dewy, foggy nightmare.
:-(
Laminar flow does form over the a dead flat surface (the sea) so in principle it can over a flat plain too, if you’re on a decent ridge facing into the wind stream and provided it doesn’t have things that will kick up turbulence upwind of the observing site, such as
- lower hills and ridges upwind (this is what spoils the seeing on Siding Spring Mtn)
- sources of strong thermals such as buildings, car parks, large expanses of tar...
Best you can hope for would be a National Park or large expanse of bushland upwind ...
A few clues are available regarding thermals - smoke from a fire or chimney, dust devils, and if they’re low, bear in mind that cumulus clouds form where thermals reach the dew point. On a few je day they often form in lines blown by the wind downwind of whatever on the ground is triggering the thermals. If that’s happening in the late afternoon the seeing will be crap.
We had one extraordinary experience with frost one night at the Airfield. The night started brilliantly, clear, good breeze without being too strong, when few hours in a cloud bank rolled in. Curious thing was the breeze stopped dead while the cloud went overhead. That's when frost started forming on everything. The cloud cover didn't last long, and it was an opportunity to warm up by the combustion heater and chew the fat a while. A short time later as the cloud disappeared the breeze pickup up too, and the frost evaporated away! Didn't melt first, just evaporated. Amazing really.
Another night, while the forecast was iffy, we were greeted by a massive thunder storm while heading up. We all stopped for pizza to wait out the storm. The rain was so heavy we couldn't see the across the street. The storm passed and a most spectacular sky revealed itself. The gravel surface of the Airfield drained so quickly by the time we got there we were able to set up straight away. No dew, no fog. Just a spectacular sky
All comes down to carefully selecting your site and understanding its weather patterns.
