Introduction to Astrophotography - Part IV

Final image, Great Orion Nebula (R) with Running Man Nebula (L)

If you've been following the previous three posts in this series on astrophotography (Part I, Part II, and Part III), you're appetite for astrophotography has been whetted and you're itching to get out there and start creating some images of your own. In the first three parts of this series, I've touched on some astrophotography basics but haven't talked much about things you can do to help your images look their very best while you're shooting. Before continuing with actual image acquisition beyond what I've covered so far in Parts I, II, and III, this is a good time to digress for a moment and discuss some things that will minimize noise, increase contrast, eliminate vignetting, and otherwise make it possible for you to create the best possible final image.

Anything you can do with your camera before importing an image into Photoshop serves as the foundation for any image. But it's especially critical in astrophotography because with astrophotography you will be dealing with very dimly lit subjects that will push your imaging equipment to its limits (and beyond). That is why you must do anything and everything you can to maximize the signal to noise ratio in your images.

Signal to noise ratio is not a concern for me when I'm shooting portraits, daytime landscapes, or even sporting events under the lights/indoors. Today's professional digital cameras and the technology they incorporate can adequately deal with varied lighting conditions and minimize noise to an acceptable level. For the most part, signal overpowers noise making the images look "grainless" or at least not so grainy that the grain is objectionable. Even when I'm shooting an indoor basketball game at ISO 6400, as long as I make sure to use a correct exposure setting, the image will be relatively clean from a noise perspective.

Unfortunately, astrophotographic images are almost always underexposed. Underexposed images are a nightmare to correct in post processing as any efforts made to correct exposure will reveal a lot of undesirable, objectionable noise. The solution? When you're shooting a celestial target, take as many images as you reasonably can and also include a set of flats, darks, and bias frames. Import all of these images into "stacking" software, generate a "stacked" image, and then complete the image processing in Photoshop.

Before getting into the nitty gritty, I will assume that you will be shooting your images in RAW as opposed to a JPEG format. Never, ever shoot in JPEG. Doing so will drastically limit your ability to post process images. Now, let's start with a discussion on shooting flats, darks, bias frames, and why they're important.

DEFINING OUR TERMS - FLATS. DARKS AND BIAS FRAMES

A "flat" frame

Flats
“Flats” are images taken to eliminate as much dirt, dust, reflections, and other undesirable things in astro images. "Flats" are also used by the stacking software to correct any difference in brightness in the main image. They are created by covering the end of the lens with a white T-shirt and shining a flashlight on it, or taking an image of an evenly lit flat surface such as a computer monitor, a light box, or the sky at twilight."Flats" must be taken with the same ISO, f stop, and shutter speed as the astro image you intend to shoot. Ten to twelve "flats" usually suffice.

Darks
“Darks” are images taken after covering the end of the lens or telescope with the lens cap. Darks correct the dark signal flaws in image sensors, which is essentially a form of noise reduction. Ten to twelve "darks' will do with half taken at the beginning of the image session and the other half at the end with the same f stop, ISO, and shutter speed as the main image.

As an aside, you may not know it but DSLR's can internally create "darks" when you use the camera's high-ISO or Long Exposure noise reduction features.  These settings do essentially what you are doing when shooting "darks" manually - the camera takes a "dark" frame immediately after taking the main image and subtracts it (the noise generated) from the main image for you. If you are shooting with a DSLR, you have the option of using the camera's long exposure noise reduction or high ISO noise reduction features if you would rather let your camera take the place of shooting darks. When I'm shooting with my CCD cameras (a whole different animal than a DSLR, to be discussed in a later post), I don't have the option of in camera noise reduction features so shooting darks are a part of my imaging process.

But even when I use a DSLR, I've always resisted the temptation to use in camera noise reduction as I believe that manually taking my "darks" will always give me more flexibility and generate better results than any image processing performed internally in my DSLR's. As an analogy, I don't program my camera bodies to sharpen my images in camera as I can do a much better job of sharpening post-process. I would rather create the image and have the flexibility to decide how much or how little I want to sharpen the image, but that's impossible if I set the camera to sharpen the image for me. The same goes for noise reduction,

Bias
“Bias” frames are images taken with the fastest possible shutter speed the camera can shoot and the lens cap on. They contain only the noise generated by the camera’s electronics on the sensor which is subtracted from the data in the darks to identify the true sensor noise. Ten or so bias frames will suffice. 

These extra images are time consuming but they will allow you to create the best possible final image. All of these images are used in the pre-Photoshop processing of images in software commonly known as "stacking". "Stacking" images in software specifically created for this purpose is an essential step in the image creation process.  Images are "stacked" in the software, generating an image that will then be opened in Photoshop as the final step in the image creation process.

WHAT IS "STACKING?"?

One image of the Great Orion Nebula without any stacking.

When you shoot multiple images of the same scene, a camera basically takes a number of samples of the scene. The more samples you record of the scene, the more uniform it can ultimately become when software uses the best portion(s) of each image to create an image that uses the best of the best from each image - exposure, noise, detail, contrast, etc.

Several images stacked of the Great Orion Nebula. Detail, exposure and color saturation are improving.

This is similar to the concept of HDR images where you shoot various frames of the same scene with a range of exposure differences.  HDR software then takes the best exposure for any given part of the scene from each image and merges all of this information into one image that improves on the overall exposure by having both shadows and highlights exposed correctly.

All images of the target are now stacked. Detail, saturation and exposure are much better with room for improvement. Noise will be eliminated once darks, flats and bias frames are introduced into the mix and the overall exposure will be improved as well. Afterwards, the image will be ready for importing into Photoshop.

Like HDR software, stacking software takes information from all the images, averages the brightness, darkness, noise, etc. and from every image it grabs detail and contrast from all this additional information and adds it to the final product. But that's not all. Stacking software allows you to remove any images that you may not want to include, such as images where an aircraft travels across the night sky, or images that include cloud cover that passed through the area you were imaging. It then performs another critical step, aligning all the images so they are superimposed precisely one on top of the other.

Darks, flats, bias frames have been stacked with the other images. Image was then finalized in Photoshop.

After image alignment, stacking software uses the flats, darks, and bias frames that were taken. It uses the flats to eliminate any brightness variances and uses the darks and bias frames to subtract noise.

STACKING SOFTWARE

Deep Sky Stacker and RegiStax are two free stacking programs you can download and use. Deep Sky Stacker would be my pick from the two but experiment with each and make your own decision. If you don't mind the $99 cost, Nebulosity (v. 4) is a great choice and is the stacking software I prefer to use.

That wraps up the discussion on pre-Photoshop image enhancement techniques. In Part V we'll get into more sophisticated imaging with telescopes and CCD cameras.