Weather, as I mentioned, is a key ingredient to this type if photography. If you've never done amateur astronomy or anything, you might get some surprises. Weather does weird things at night, and humidity does even weirder things. Optics like camera lenses will fog over with dew, tripods collect condensation, etc. Some basic meteorology information is essential. You need to check satellite images before leaving the house, use the IR and WV (water vapor) images to see how clear the sky is. WV will tell you just how much moisture will be absorbing light and distorting it, interfering with your exposures. I always use the National Oceanic and Atmospheric Administration's geostationary satellite server NOAA GOES which is USA based, but has images from different parts of the world as well. Watch the time lapse videos associated with the image you're viewing in order to see how things are changing and to predict what will happen later in the night. Check temperatures, wind, humidity, and dew point temperatures where you will be. Understand that in rural locations, especially deserts, the temperature can drop from 80°F to below freezing in a matter of hours. If the ambient temperature drops below the dew point temperature, water will condense causing dew on equipment, potentially fog, ice, and other issues.
Ideal conditions for astrophotography would be stable temperatures, little wind, arid, no cloud cover and little water vapor, a new moon or when it is below the horizon, high altitude above pollutants in thinner atmosphere, with no major light sources for 10 miles. This sounds almost impossible but it really isn't if you are willing to drive a bit. Wind shakes your camera and changes air density, distorting light as it passes through different gas densities at different speeds. Moisture in the atmosphere absorbs and refracts light. Condensation... Well that's obvious I hope. One thing that might not be obvious is "seeing". Seeing is an astronomy term which refers to distortions in the atmosphere caused by temperature differentials from the Earth radiating heat absorbed throughout the day. If the air temperature is very low, but the surrounding environment is radiating a lot of heat, then you will get subtle degradation of light, like a large scale version of the heat haze ripples which come off of hot asphalt during the day. The atmosphere itself interferes with light transmission as well, so higher altitudes generally yield better results. Even a few hundred meters can make a difference, so try to get to high ground. For these reasons I prefer to shoot in the high deserts of the southwest during winter, later at night or early morning. Also, different interesting objects are visible at different times of year, such as the Pleiades cluster or the Andromeda Galaxy.
Shooting from a tripod, you'll need a wide angle lens. In the span of 30 seconds, objects in the sky move a lot. Stars, planets, and other objects will leave streaks, or "star trails" at longer focal lengths. If you want a crisp image, use wide angle lenses with the widest aperture you can for shorter exposures. If you want star trails for your image, tighten up the shot a bit, stop down your lens, and go to a lower ISO setting. How long you expose for depends entirely on how much sky is in the frame, and how much light your setup can collect. If you're zoomed in at 250mm, just a few seconds will leave long trails, but at wide angles, you may need 20 minutes or more to get the desired effect. If you do not want star trails, this is why you need a wider lens. If trails are what you want, then also consider that the earth's axis is quite different from the celestial axis. Find Polaris and you'll know where the approximate celestial north pole is, and around what axis the stars will revolve. Stars will move more if they are closer to the celestial equator, so shooting away from the equator will yield less noticeable trailing if you are avoiding this.
If you want to capture images of faint deep sky objects like galaxies, you will need to look into other options for mounting your camera so it can follow the objects accurately during long exposures. Some common options include mounting the camera "piggyback" on a robotic guided telescope with a long telephoto lens, shooting through the telescope itself with a "T-adapter", shooting through the eyepiece of the scope, or using a simple "barn door" or "scotch" mount. This article will not attempt to explain these options in depth, but I have given you some keywords to search for more specialized articles.
Camera shake is the enemy of astrophotography, and it can come from many sources. Wind, as mentioned earlier, can cause movement of your camera which causes artifacts from shaking in the worst cases, and in minor cases can cause subtle vibrations which make the image appear fuzzy or out of focus. Simply walking on the ground nearby can cause this as well. It is imperative that you have a sturdy mount on a stable surface which absorbs impact. Sand is great, dirt is good, a porch or deck is probably less than ideal, and rock or concrete might be bad since they vibrate with footsteps. Use the sand spikes on your tripod if yours is equipped with them. Weight your tripod down with sandbags or something similar, but make sure the weight is touching the ground so wind doesn't cause it to swing. If you are using a less rigid telescopic tripod like the collapsible ones sold at Wal-Mart, then collapse the flimsy bottom portion of the legs and crank down the upper extension to make it a bit more sturdy. You could even use your camera bag if it's heavy enough. Place it on the ground between the legs, throw the strap over the central support braces, and tighten it down. If it's still kind of flimsy, try spreading the legs with your foot, allowing them to flex outward a bit. Investing in a good solid tripod with a ball head will save you a lot of grief if you can afford it. Use the "Mirror Lock-up" feature on your camera, if it has one, to avoid shake during shorter exposures. Also, use an infrared remote or cable release to control your shutter. This will allow you to shoot without causing vibrations from hitting the shutter button, which can take several seconds to die down.
Using a lens hood will help keep stray photons under control and may help keep moisture off the lens. With most DSLR cameras, light can also leak in from the eyepiece on the rear. On my Cannon 400D, the strap came with a cap attached to it that fits over the viewfinder's eyepiece and blocks out light from the rear. This is important if there are light sources behind the camera, such as laptops for tethered shooting, flashlights, or anything being used to light the scene's foreground. The moon, if it is in the wrong position, will cause lens flares the same way the sun does, a lens hood will help control this.
You should be using RAW images if possible. There are so many reasons for this that I won't attempt to explain them all here, but if your camera supports it, use it. RAW images can be fine tuned much more later, allow corrections for things like chromatic aberration (separation of light near the edges of the frame, causing a star to ghost, among other things), sensor noise, and they contain much more data about the light collected. This is critical if you choose to perform image stacking (which I will explain later) or if you want to filter out certain colors.
Noise is a major factor in low light photography. Noise comes from many sources, including electromagnetic interference in the camera, thermal noise from the sensor heating up, faulty "hot" pixels on the sensor, "dead" pixels which output black dots, and even background radiation. If you are lucky enough to own a low noise, high ISO camera with some fast lenses, this isn't as much of an issue. For us amateurs with less than ideal equipment, there are some things we can to to mitigate the noise problem. If you're shooting in RAW (as you should be... ahem) don't bother with in-camera "noise reduction" options, as they generally only affect JPEG output and will double the time necessary to shoot, while doubling thermal emissions from the sensor. Shoot at the lowest ISO you can. Higher sensitivity is achieved by amplifying the light signal through a gain circuit, which adds noise to the image. Instead, try to get the longest exposures you can without undesired artifacts at the widest aperture you can. We want to collect as many photons as possible in the shortest time possible. If you cannot get a bright enough image, don't worry. I'll discuss options for improving that later on. Noise is not only ugly, but destroys image data. We need all the data we can collect, so I will also show you a method for combining data from many images to remove noise and increase the light data in the image called "stacking".
Now that you're home and you have hundreds of bug bites, 3 full memory cards, and your leg is bleeding for no apparent reason, you load up your images in Adobe Lightroom and.... they look like garbage! Don't panic. There is still work to be done, unlike most terrestrial, normal light photos, you'll need to do a bit of work to get the image data to represent the scene you were shooting. First, if you've been out all night then go to sleep and work on them in the morning. Being exhausted and impatient will cause more harm to your images than taking bad ones to begin with. Work on them with a fresh pair of eyes and a fresh brain.
RAW image data needs to be processed into usable images. If you're not stacking them (more on this later) then you can go ahead and start processing them normally using Adobe Camera RAW, Lightroom, etc. You're going to change these images 500 times before you're happy, so make backups. Use your levels and curves tools to adjust contrast so that noise is minimized and the stars pop. Check the entire image at 100-200% zoom and eliminate any bright RGB dead pixels. Also watch for areas heavily noise infested, like one corner of the image being magenta. This is caused by interference from other components in the camera, like a hot image processor, or an improperly shielded RF source like the power supply. The more expensive the camera, the less likely this is to happen. My 400D has a hot spot in the bottom right corner which occasionally causes me grief, and some hot pixels, but I still manage. Here is an example of the noise pattern generated on my sensor over the course of several 9 second ISO 1600 exposures. It has been enhanced to show detail and converted to grayscale.
- see attached image 1 -
As you can see, that's a lot of noise! Here's where stacking images can help. If I want to capture an image of something faint like the Andromeda Galaxy, or the Orion Nebula in my image I will need long exposures, and I will generate a lot of noise. This noise, with the exception of hot or dead pixels, changes over time. We can take advantage of this with special software to increase the signal to noise ratio (SNR). Imagine you are recording the sound of a whistle, and there are many people talking in the background. You know the whistle emits a tone at 2600 Hz, but the frequencies of the talking are variable. By isolating the frequency that doesn't change, you can easily remove the ones which do, getting rid of the background noise. Image data is processed in a similar manner. When you have a single, low noise signal, you can simply reduce the amplitude (drop the low end of the dynamic range using the levels tool) and get rid of much of it. You can also use algorithmic tools like Noise Ninja to remove them, but you will lose some image clarity. If you take many similar exposures, and then combine the data mathematically, while simultaneously generating a map of the noise found in the images, you can remove the noise or increase the SNR. To do this, you will need a program like Deep Sky Stacker, which is free. The website has comprehensive information on how to use it, but I will give you an overview on how this process works for our purposes.
- see attached image 2 -
- see attached image 3 -
Now we're talking, right? Ok we can now save a 32 bit per pixel image for editing in Photoshop (Gimp if you're masochistic). In your editor of choice, adjust the Exposure and Gamma to get the image luminance right, or use an HDR workflow to get the image down to a normal 8 bpp. In Adobe Photoshop CS5 you can just switch modes to 8 bpp and use the Exposure/Gamma dialog which comes up to adjust the final output, then do further editing when you're in 8 bit mode. Adjust the image until you're satisfied with the color and contrast represented and show it off!
-see attached image 4 -
The core of our galaxy, with nebulae and dust lanes from our spiral arms.
This image has been processed to show maximum detail and much fainter objects.
- see attached image 5 -
The same image as above, processed to represent more of what you might see
with the naked eye. I simply changed the Exposure/Gamma settings
when converting to 8 bit, adjusted levels, and color balance.
This example was processed from the original images I took back when I was learning, and you can see I failed to control noise in the corner even with the dark frames, and the color is a little off. I'm showing you this example, rather than a perfect image, so you know what to expect when you start trying this. From here, you can do a lot more, such as compositing your stacked sky image with a static landscape exposure, and much more.
If you have minor trails burring your image, there's a little trick you can use to fix them.
Open the image in your editor, duplicate the background layer. Set the blending mode for the new layer to Darken. Use "offset" to shift the top layer back in the direction of the original objects before they trailed (usually just a couple of pixels). In Photoshop, it's under Filter > Other > Offset. Once everything looks sharp again, you're done.
- see attached image 1 - (left side untouched, right side offset processed)
Preservation of Night Vision
Your ability to see better in low light conditions is controlled by a few factors. First, of course, is pupil dilation which naturally occurs when light levels remain low for a short time. Second, your eyes respond to light which two structures, rods and cones. Rod cells react to as little as a single photon of light! These are your high sensitivity night vision cells. Cone cells need tens or hundreds of photons to trigger a reaction. Your rod cells are your friend during night observation and keeping them functioning is important because they take a while to return to maximum sensitivity. When rods are activated, they change the state of a molecule called rhodopsin. Rhodopsin (derived from vitamin A, a deficiency of which will cause night blindness) takes about 30 minutes to regenerate completely; most of the regeneration takes place in the first five to ten minutes however. Every time you get blasted in the face with a flashlight, your camera's LCD, a laptop, cigarette lighter, anything bright and broad spectrum, you reset that counter. To help prevent this, dim your camera's LCD by using its onboard settings or cutting a small piece of automotive window tint film to size and sticking it to the screen. Also, use red LED lights to see while you're out there. Red wavelength light depletes rhodopsin much more gradually, and is absorbed more by the cones than the rods in your eyes.
You may find it difficult to focus on stars in your viewfinder, and infinity focus markings on many lenses are inaccurate. Adding to this mess, if your vision isn't perfect and your viewfinder eyepiece is unable to perfectly correct for the discrepancy, what is in focus may appear out of focus in the eyepiece. I find it easiest to focus on a bright, clear object like a planet and then take a few shots, each time checking focus in the magnified LCD view until I get it spot on. Changing focal length on zoom lenses may change your focus, as will moving the camera if your lens isn't manufactured extremely well. Temperature changes over time as your warm camera from the car or house cools down will cause mechanical shrinkage of the optical assembly which changes your focus. Check it from time to time to make sure you didn't just take 40 exposures of blurry garbage.