Tag Archives: condensation

Dew Defeated

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In an earlier column I reported my frustration with dew when I attempted to capture meteors along the Pacific Coast. I have had problems with condensation before – notably when in the North Coast of California on a vacation with my wife.  The skies were relatively clear, but the humidity was very high and there was so much ground level moisture the lens was covered in dew within minutes. Indeed, the moon above and the streetlights below led to this:

Earth vs Sky [5_020297]

Photo 1: Dew destroys an exposure.

Interesting, but not at all what I wanted. I was so desperate to get a good star trail while we vacationed near Point Arena, that we drove up to a Fort Brag, California,  Radio Shack and an Ace Hardware store so that I could purchase all the items I needed to build a  heater.  I had done all the math, so I knew that I would need at least a dozen resistors (200 Ohm, 1/2 Watt) for a heater powered by my big lithium battery (7.4 volts), or about a dozen 1/2 watt 390 Ohm batteries if powered by my 12 volt battery. I bought both plus wire, weather stripping, some tools, duct tape and velcro.  Unfortunately I also bought a thoroughly underpowered soldering iron and was unable to build the unit in the field due to flimsy cold solder joint.  My total outlay was about 40 dollars for all the components, tools, etc.   When I returned home I got out the GOOD soldering iron and completed the job.

BUT, you knew there would be a but, right? I was skeptical about using my home brew heater without something to protect my battery from over discharging. Lead acid batteries hate being completely discharged. Lithium batteries are not happy about that either. So I did TEST my unit briefly and was happy that it could work… but I never actually used it and even forgot to bring it on the night I needed it most.

So I BOUGHT a solution instead. The “Dew-Not“.  All that trouble I spent building a heater element was needless as the $22, 13 inch long DN004 heater does the job quite nicely and it fits my 82mm lens quite well.  I also ordered the priciest part of the solution, a $135 DNC2 controller which has under and over voltage protection and can be used to adjust the amount of heating (over heating is just wasting energy). I also got a DN003 (shorter) heating strip for my smaller barrel lenses.

I tested the Dew-Not unit out in my back yard during the following meteor shower – literally on the night the Dew Not arrived. I set the unit up at about 2 am and let it run until 7 am on medium. The ground and the camera were soaking wet from dew but the lens was clear and dry and it drained less than half of my 12V battery.

Photo 2: Dew Not 2-Channel Controller

The down side to this unit is that it does require a 12 volt supply – so only my HEAVY battery solution really works well with it.  And of course there are extra cords and such.  With a 12 volt battery it produces 6 watts of heat. With an 8 volt battery it would produce almost 3 watts which is still plenty except for the most humid nights. But when the humidity is close to 100% no solution will work!

Happy photons!

Damnable Dew!

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First a word from our sponsors: the Star Circle Academy has scheduled a reprise of the Alabama Hills Star Circle Workshop in November, 2011. Signup is available now.

The annual Geminid Meteor shower peaks December 13/14th (that’s after midnight on the 13th). The weather forecast for the San Francisco Bay area was not promising for 2010. Fortunately less than two hours to the south in Big Sur the forecast was for clear skies. And the skies were clear!

Image 1: A slightly out of focus image reveals a meteor from the Gemini radiate point. The radiant point  is above and to the left of this image.

The best weather forecasts for night photographers are not found by watching the news or looking at the various weather related web sites. Those sites concern themselves with day time conditions and treat night time hours as though rain and temperature are the only important things to know about the night. I prefer forecasts that are hourly or nearly so and include things like the expected cloud cover, humidity, and temperature. Wunderground does what I would like quite well. Especially the “hourly forecast” – which is a bit of a misnomer because the forecast is in blocks of 3 hour periods. This is part of the forecast that drew me to Big Sur (Figure 1)

Figure 1: Sweet. Clear, no clouds, no rain, and humidity not too high. But this area is a bit inland and we’ll be at the coast.

Notice the 0% cloud cover and the 0% chance of precipitation from 1 AM until 7 AM.

Another important factor is the dew point. With an air temperature of 54 degrees and a dew point of 50 degrees (humidity of 89%) there is a pretty good chance of avoiding both fog and dew.  But remember that these are forecasts. Like all forecasts they may be wrong.

Dear Mr. Weatherman,
I dropped you this note to let you know I just finished shoveling 3 feet of “partly cloudy” from my driveway.

The forecast wind direction is also favorable. A light off shore wind from the north is perfect for the location I had in mind.

For comparison, Figure 2 shows the forecast for a location much nearer to home.

Figure 2: Lots of clouds.

Cloud cover 90% and by 4 AM the dew point is pretty close to the air temperature. But the forecast for the night of the meteor shower peak is even worse (Figure 3):

Figure 3: Peak forecast. Rain, Humidity, Clouds… oh my!

Notice that the prediction is 51% or more cloud cover, and very high humidity. High humidity and low winds means fog and lots of dew on the lens.

An ideal star shoot has few or no clouds, low humidity low wind and no precipitation.  An even better hour by hour forecast can be found with the tool “Clear Sky Chart” (aka Clear Sky Clock) which I described in the article Mostly Free Tools. But Clear Sky Chart does not cover every location, and provides at most 36 hours advance forecast. It is, however very accurate and very applicable for night sky observing.

And Now A Word About Dew

Dew is definitely a problem. Dew is moisture that condenses and falls onto or forms on a surface. Think cold drink with a damp wet smog forming on it.  An empty glass, or a glass of room temperature water will not collect condensation – but one below room temperature does.

I got the double whammy at Big Sur. Just near midnight condensation was forming on my lenses.  Periodically wiping the lens with a cloth certainly helps but the camera better be rock solid and it is best to use a dark rag so as not to ruin an exposure in progress. The rag also should be clean (no sand/grit) so as not to scratch the lens and lint free so it does not leave detritus and it must be dry so it does not leave smudges. That is a lot to ask of one little cloth – better have two or more. Creative alternatives to the dew rag include using a blow dryer periodically. If a nearby power outlet is available you are in business. There are 12 volt dryers that can be powered from a car battery – but these are not completely practical solutions; who wants to be tethered to a car or a building? Another strategy is to employ an anti-dew heater. Sometimes these are called “Dew Heaters” but in fact they are not designed to ‘heat the dew’ but to keep the lens just a skoch warmer than the atmosphere so that condensation does not form in the first place. Powered dew deterrents can be purchased – usually from astronomy shops and sites. Building your own heater is not terribly difficult if you know which end of a soldering iron to hold. Doing a good job of making a heater is not trivial. Another alternative is to use the chemical hand warmers that are air-activated. One caution is that these packets contain charcoal and iron filings – definitely not things you want to get into or on your lens. On the up side, however the chemical warmer packs are light weight and pretty effective – and they will help with cold hands and feet, too.  You can find hand warmers at many department stores and sporting goods stores, or online.

Another deterrent is a nice big lens hood. Something I ALSO forgot to bring for my workhorse lens. But it may not have helped all that much because I was pointing my lens up at a sharp angle – about 45 degrees to try to catch as much in the sky as possible.  At that angle the lens is completely pointed at the sky and it quickly loses heat. Don’t believe me? Try this experiment: park your car overnight where one side is near a tall wooden fence or thick hedges and the other side is exposed to open air.  In the morning a lot more frost and dew will appear on the exposed side of the car.

On the night of my expedition to Big Sur I neglected to bring any dew deterrent – not the hand warmers and not the heater, not the lens hood. Collecting star trails or attempting to capture meteors while also having to frequently wipe a lens is both aggravating and tedious and a bit scary since at one point I dropped the rag in the sand. Knowing that the dew was forming on my lens because the lens was colder than the air I took the camera off the tripod and grasped the barrel of the lens in my nice warm hands – under a blanket – for about 10 minutes. This quite successfully eliminated the dew from the outermost lens (the objective) but because the lens had been in a moist environment for quite a while my action created a worse problem… condensation on one of the inner elements.

A warm, dry environment, especially one with a desiccant like silica gel is the only solution for condensation inside the lens… and you do not want it TOO warm because it might encourage fungus to grow. The key is dry.  With my batteries spent and my fight against the dew a losing one I licked my wounds and called it quits.  Fortunately the night was not a complete loss as I did collect one meteor (above), I saw about two dozen others and I came away with some appealing shots of the beach, rocks, surf and stars – as well as a smug grin knowing that the horribly overcast conditions two hours north of me was confounding my friends who stayed in the Bay Area.

A Night at the Beach [C_019477-9]

Image 2: Rock & Stars witness the setting Moon and Jupiter – flash filled.

A Flash at the Beach [C_019464]

Image 3: Eroded Cliff and Textured Sand – flash filled.

 

By the way, you might be interested in my “Dew Defeated” article which discusses the Dew-Not system.

Trouble with Long Exposures – Part 2 of 2

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In the previous article I discussed 4 of the 6 most common problems that occur with long exposures.  Those problems are:

  1. Poor Focus
  2. Dim Stars (low contrast)
  3. Strange Colors
  4. Purple or Pink Glow

In this installment we tackle these two issues

  1. Gaps in Star Trails
  2. Lots of Noise (Colored Speckles)

Gaps in Star Trails

To oversimplify a bit there are four causes for gaps in star trails created from successive exposures:

  1. Camera limitations
  2. Camera or intervalometer misconfiguration
  3. Processing choices
  4. Weather conditions

Camera limitations: I described this issue in my article “How long does a 30 second exposure take?”  All the Canon cameras I own – including the top of the line 5D Mark II require 32.8 seconds to complete a single 30 second exposure. Well there you go: almost 3 seconds of time where there is no exposure. This problem can be compounded by two common misconfiguration blunders:

  • Failing to allow enough time between exposures when using an intervalometer. Or using the wrong drive mode on the camera.
  • Failing to turn off long exposure noise reduction.

To avoid intervalometer misconfiguration I operate in either continuous exposure mode or bulb mode. I use continuous exposure mode when my exposures will be many and a maximum of 30 seconds – e.g. when trying to capture meteors or planning for a time-lapse animation. In continuous exposure mode I set my intervalometer with a start delay and then program an exposure time of several hours… AND I put my camera in Manual, high-speed continuous exposure mode with a typical exposure of 30 seconds. You do not really need an intervalometer for this – a locking cable release is sufficient.

When I operate in bulb mode, I try to get a moderately long exposure. Usually in the 4 to 10 minute range depending on the sky conditions. In this setup it is very important to put the camera in Bulb exposure and program the intervalometer to leave a 3 second gap between one exposure and the next. I have recently discovered, however that the Canon 5D Mark II will work with my intervalometer set to 1 second intervals. That’s goodness. I am still trying to work out whether the problem is due more to the timer or the camera. I do know that in continuous exposure mode all my cameras require 32.8 seconds per each 30 second exposure. Failure to allow a long enough pause between exposures can cause unexpected results.

Photo 1: For the first half of the evening I mistakenly left long exposure noise reduction on. The result was that half of my shots occured at every-other eight minute intervals.

The “dotted lines” in the circle above were caused by leaving on long exposure noise reduction. The result was that the intervalometer timed an 8 minute exposure, waited three seconds and then pressed the shutter for the next 8 minute exposure. However 3 seconds after the exposure completed it was still doing long exposure noise reduction so that cycle was skipped until the intervalometer released the shutter for the next 3 second “off” interval.

I have gotten into the habit of setting my exposure length to 3 seconds less than what I want… e.g. 9:57 for a 10 minute exposure. I then set a 3 second inter-shot interval. I used to set a 10 minute exposure plus a 3 second gap – but the predictability of starting a new exposure every 10 minutes makes it easier to monitor what is going on.

Another cause for gaps: changing the battery. I can offer the following important tidbits when you need to change the battery.

  • Do not wait for your battery to be exhausted. A partial exposure may not stack well or be completely written to your card. Battery exhaustion will likely occur at an inopportune time.
  • Have everything at hand in advance of the change. For example, keep the battery in your front pocket where your body heat will keep it warm.
  • Practice a battery change BEFORE you start your exposures. Only by practice beforehand will you be able to discover that the battery compartment is blocked by your tripod, or impossible to reach, etc.
  • When you DO change batteries beware! Your camera settings may change dramatically!

Processing choices you make when stacking the star trails also affect whether your gaps will be inconspicuous. Do not do any sharpening until you complete your stacking – and even then avoid sharpening the star trails themselves. The method used to stack trails is significant. However, I have observed that people do not notice gaps even in this image of 19 8-minute exposures printed out at 20×30 inches.

Photo 2: Even though it is composed of 19 eight minute exposures the gaps are never noticed even when printed at 20x30.

Weather conditions can also introduce gaps. In a truly dark sky where clouds are not lit by city glow, moonlight or twilight, clouds become “black holes” and block starlight. Low or fast moving clouds can obscure some, most or all of one or more images in the set. This can be perplexing if you happen to be sleeping during exposures which started and ended with clear skies.  Another problem is dew which may form a fog that diminishes or eliminates some or all of the exposures. Vigilance with a rag, the use of a hood or a dew heater are your only weapons against dew.

Lots of Noise (Speckled Colors)

I purposefully left the noise in Photo 1. It’s quite noticeable in the rock silhouette at the lower right and appears mostly as red specs. Annoying? Well, yes, but it is not the end of the world.  In order of effectiveness here are your best approaches to keep the noise manageable:

  1. Shoot at a lower ISO (100 or 200)
  2. Shoot and stack shorter exposures – longer exposures generate more noise.
  3. Capture the foreground and the star trails separately. A better lit foreground will exhibit less noise.
  4. Shoot during colder seasons – lower temperatures result in lower noise.
  5. Control stray light with a lens hood – and close or cover your viewfinder while exposing.
  6. Use high ISO noise reduction
  7. Use noise reduction post processing tools. Chrominance noise is usually most in need of correction.
  8. Use long exposure noise reduction.

Hopefully you noticed that long exposure noise reduction (LENR) is last on the list. If you are trying to stack star trails it is impossible to get continuous trails with LENR on. It is also the least effective unless you are only going to shoot one shot.

Before we go much further, it is worthwhile to note that there are 4 causes of “noise” and each has a different source. The random speckles are usually what is meant by noise. Those random speckles are created by heat, limitations in the electronics, and things as bizarre as electromagnetic phenomenon like sunspots. No kidding. True noise is by nature random and LENR can not do a thing to combat random noise except to diminish it by reducing the luminance of the offending pixels – which also reduces the sharpness of your image. But there are 3 other kinds of noise that are not random though often lumped into the same general category: hot/stuck or degraded pixels, local heat noise (sometimes called amp glow), and high ISO noise. LENR is effective for these because they are not random.

Hot or stuck pixels usually appear as bright pink, red, blue, green, white or purple spots. They are caused by either electronic problems on the sensor chip or by the dyes used to detect the color.  A pixel detects the intensity of the color red by use of a red dye (inkjet droplet) over a sensor site. If that red dye is insufficiently thick, or missing altogether then that pixel location will always read hot if there is any light falling on it – and if the problem is electronic it may read hot even if no light is striking it. Dead or degraded pixels are just the opposite. Too much dye or dead electronics at a pixel site. Degraded pixels are stuck black or darker than the surrounding pixels and are seldom if ever noticed in night photography.

Locally caused heat noise is noticeable in some cameras and is due to the heat of electronics in proximity to the sensor. In my opinion this problem is a design flaw in the camera. However this kind of noise is repeatable so LENR can help correct it. The “Pink or Purple Glow” that results from this flaw was discussed in Part 1.

High ISO noise has an understandable parallel in the world of audio. Take nearly any cheap radio. Turn it up. At some point the sound will become distorted and harsh. This harshness is because there are limitations in the signal, the amplifier circuitry and the speaker used to produce the sound.  Increasing the ISO in your camera is the photographic equivalent of the audio scenario.  At some point amplifying the light measurements made at each pixel makes the noise more obvious.