Ever since I was a young kid and my parents bought me a copy of Carl Sagan’s Cosmos on DVD I have had an inescapable fascination with space and astronomy. This fascination has been central to developing my beliefs and philosophy on life, and has always provided me a grounding perspective when things become stressful or overwhelming in my work or personal life. With the advent of modern, high performance photographic equipment it has become possible for average joes like me to explore the universe and create images that just a decade ago, only astronomers with access to the most advanced equipment were privy to. This has resulted in magnificent images being created by hobbyists from all over the world.
Deep Sky Astrophotography is definitely one of the most difficult, frustrating and time consuming fields of photography. All the toiling, cold nights huddled in the dark have the effect of dissuading many novices from dedicating the time needed to accumulate the “data”, often hundreds of single pictures needed to compile a quality image of deep sky objects. While it may not be easy, and frustration is inherent, once one achieves a quality result, the gratification is enormous and instantly addicting.
I’d like to share with you a bit of the process involved with deep sky astrophotography, as well as my, so far, moderately successful results. This guide assumes a basic understanding of camera settings and image editing software. If you only use your camera in automatic mode, you may find this guide a bit too complex.
When first committing to taking these kinds of images, four major pieces of equipment are needed to achieve optimum results.
1: A quality DSLR style camera, with a remote shutter release. Canon, Nikon, Pentax, any brand will work. The camera will preferably be of the full frame variety, with a 35mm equivalent sensor, but these tend to be expensive, and many people (including me) cannot afford one. I use a Nikon D7000, which is a high end APS-C size sensor camera. Basically, the better the quality of the camera, the less “noise” or grain will be generated in your image, resulting in crisper, clearer final results.
2: A “fast”, or large aperture telephoto lens. For the lens, it is best to have an aperture of f2.8 or lower and a focal length of at least 100mm, although 200-400mm is ideal. I use an old Nikkor 105mm f2.5 lens I found at a garage sale, although I really wish I had a longer lens like a 300mm f2.8, but those are $6000 so I may have to hold off there for a while :)
3: A high quality tripod. I use this Slik Tripod, which is solid as a rock, and the best I have seen in the price range. The only downside is that this tripod is rather heavy, which may be an issue for some.
4: A computer with an image editing program such as Adobe Lightroom 5 or Adobe Photoshop CS6, and an image stacking program like Deep Sky Stacker. Both of these programs will be used to process the data from your camera.
The lens, while being an important piece of equipment, can be very costly. Do not shy away from trying these techniques if you only have a “kit lens” or a cheap telephoto. You can still get good results, and when you do get a fancy new lens in the future, you will already be ahead of the game and know what you are doing. With that being said, let’s get to shooting! In astrophotography, we are never at a loss for subjects, but let’s start with an “easy” subject:
M31 The Andromeda Galaxy
This image is the result of 90 separate 3 second exposures totaling 270 seconds of shutter open time. I put my camera onto a tripod. Located the Andromeda galaxy using the wonderful Google Sky Maps App on my smartphone and centered the camera on the subject. Putting my camera in manual mode, and making sure I am set to take RAW images, I set my ISO to 1200 (the highest I am comfortable going before the images become too “noisy” or grainy, this will depend on your particular camera model). I also put my camera in remote shutter mode and make sure my remote shutter release is handy.
Since the Earth rotates causing the stars move across the sky, you can only expose for a certain amount of time before you will see them streak in your image. How long you can expose depends on the focal length of the lens you are using. The more zoomed in you are, the shorter you can successfully expose before the stars begin to trail. A good rule of thumb is to take 400 and divide it by your focal length. 400/105= ~4 seconds. This rule is not set in stone, and your mileage may vary depending on where in the sky you are shooting and other factors.
With my settings ready, I now ensure my focus. I find the best way to do this is to put the camera into live view and digitally zoom into the image on the back of the screen. Find a bright star and gingerly adjust the focus ring on your lens until you think the focus is correct. This takes patience and practice, and is arguably the most important step in the process, so do not rush this step.
Once a good focus is achieved, it is time to take the first test image. Ensure your subject is centered, and your lens is at it’s widest aperture (you may want to step down one stop for sharper focus, this is up to you) and release the shutter with your remote. Once the exposure is done, review the image on the cameras LCD screen. Zoom in and make sure there is no trailing, and that your focus is sharp. Your image should look something like this:
Not too exciting, but we’re on the right track. If everything looks good, it’s time to take all of your exposures. Take pictures in succession, the more the better. Take as many as you can until the object noticeably changes position on the screen. If you have a long telephoto lens, the object will move rather quickly. Re adjust the camera so that the object is centered again, and continue shooting. Repeat this as many times as you desire, again, the more the better here.
Once you have taken a good number of exposures (100 is a good starting goal) it is time to take some funny pictures called darks, flats and biases. These are used by the stacking program to determine the unique performance and imperfections of your particular sensor. Basically they cancel the imperfections that are apparent in all camera sensors by allowing the program to recognize them and compensate for them.
Dark frames are taken using the same settings as your above exposures, but with the lens cap on, so no light reaches the sensor. take ten of these.
Bias frames are taken using the highest possible shutter speed (usually 1/8000th or 1/4000th of a second) and the lowest possible ISO (usually 100) with the lens cap on. Take ten of these as well.
Flat frames are uniformly exposed images. They can be taken many different ways. A good way to take these frames is to pull a white t shirt tightly over your lens and focus at infinity. Find a uniform light sourceand adjust the exposure time until you achieve a uniformly gray image. Take ten of these.
Whew! That was a lot of work! Well, the “hard” part is over. Now we move on to the computer end of the process. I will try in the future to do a write up on proper techniques and practices in the Deep Sky Stacker program. But for now, I will link you to this easy to follow tutorial by Doug German on youtube. DSS is a VERY complex program, and resembles the processing algorithms used by NASA for images from the Hubble Telescope, but mere mortals such as we can scrape by using many of the automatically generated settings. Doug German goes just deep enough to get good results without jumping into the deep end and being to too complex. You should get good results using his method.
Once you have an image generated by DSS and have adjusted the histogram as shown at the end of the video, it’s time to do the final touches. Don’t worry if DSS gives you a very dark picture on export. I nearly lost it and gave up when I saw my final image was barely visible! This is normal!
Open up Adobe Lightroom, or Photoshop. Adjust the exposure, contrast, and do some noise reduction. You can also do some selective color and white balance adjustments. This is the fun part! Mess around with applicable settings and see what looks good to YOU. You are allowed to go crazy. All of the amazing astro images you see on the net are various levels of edited. Some are totally overboard, while others are more reserved. This is where you can put your personal touch on the beauty of the universe and is the most rewarding and gratifying process I have ever encountered in photography.
I hope at least a few people out there find this little guide useful. Please leave a comment or ask any questions you may have in the comments. Thank you for reading!