Capturing Details Digitally
Upon first attempting digital astrophotography through a telescope, we're elated to simply capture a recognizable image. Then we strive to capture more details, always looking for the next image to be better.
Today there are several ways to capture highly detailed images with digital technology: dedicated CCD camera, video, webcam, or digital camera. The only sure way to determine which methods work best is to take lots of images with your equipment. Here are some random thoughts related to the various digital formats.
How Long an Exposure...?
No one can say exactly how long an exposure you'll need for a particular object. Your equipment is just one of the factors. For example, the light grasp of your telescope influences the length of an exposure. It also depends on any eyepiece you're using. A higher power eyepiece spreads the light out more. Target objects vary greatly. Deep sky objects are inherently faint. Then there's the moon - here there are factors such as the phase, whether you're imaging the entire moon or just a small piece of the terminator or somewhere out toward the limb. If possible, be guided by how your target looks on the camera's LCD screen or on a monitor. The final image may be darker or lighter, but the screen view is a valid starting point. Then bracket the exposures like crazy. It's best to err toward under-exposure because it's impossible to recover detail from a burned-in image.
Stacking uses computer software to overlay one image on top of another to create a denser and sharper final image, and bring out detail you wouldn't otherwise see. Stacking of images is so easy and the results so good that it has become a common practice. Users of video and webcams may stack hundreds of images to create a final picture with amazing detail. The resolution of each video frame is much lower than a single digital camera image, but the combined effect of all the stacking compensates for that. With digital still cameras and dedicated CCDs, fewer images are required in order to get good results.
I'm not aware of any guideline that says just how many images are needed for stacking, but it's a good practice to use as many as possible. On planetary photography with a digicam I take anywhere from 10 to 15 images, which is on the low side compared to what many others do. Begin by deleting the poorer images. Stacking a lot of images that include some marginal ones will not give as good a final result as culling through the images and picking just those that are sharpest. For example, I got a far better final image of Mars from three carefully chosen raw images than I did from a dozen that included inferior ones. As you select each image to stack, ask yourself, "What will this image contribute?" A raw image that's blurrier than the rest will only smear the final result.
If you take a lot of images, it's very time-consuming sorting through all of them to find the best ones to stack. But the freeware programs Registax and AstroStack do this for you. Using a good single frame as a guide, you can tell the programs how far you are willing to deviate in quality from that standard in selecting the rest of the images to stack. This feature is of special value to those who use video cameras or webcams. To download Registax, go to http://registax.astronomy.net/ and for AstroStack it's http://www.astrostack.com/. For still camera imaging, I find Registax easier to work with. Since both programs are freebies, try each to see which you prefer.
Stacking also is widely used by professionals. For instance, in March 2003 European astronomers tracked down Comet Halley at a distance of 28.06 astronomical units from the Sun, about as far out as Neptune. They simultaneously used three of the 8.2-meter Very Large Telescope reflectors in Chile to image the comet's predicted field. The nucleus was invisible on any single exposure, but when they stacked all 81 exposures (totaling 9 hours), they were able to see Halley.
Video/Webcams vs Still Imaging
Video cameras and webcams are increasingly being used for astronomical imaging. They have an advantage when seeing conditions are not the best because among all those individual frames, they can capture the few moments when the image stabilizes. After a "movie" of the object is taken, individual frames are chosen to be stacked and processed to produce the final still image. When it comes to choosing between a video and a still camera, it's hard to decide. Both have produced great results. However, the still camera process is easier because you don't have to work with a frame grabber and there are far fewer individual images. And with a webcam you must be connected to a computer while taking the image, so you also need a laptop.
Seizing the Moment
Another reason for taking multiple images is the constantly changing nature of the atmosphere. If you take ten images on a night of normal seeing, you'll generally find that some will be sub-par for no reason other than at the moment the image was taken, the seeing turned gunky. Image processing software has been referred to as the "amateur's adaptive optics". While it's true that to some extent you can rescue an image from less than perfect seeing conditions, you still can't make a silk purse out of a sow's ear. Herculean efforts to improve the image may only result in overcooking it with software, giving a completely unnatural-looking result. You'll get far better final images when atmospheric conditions are good than when they're not. Your eyes somewhat compensate for atmospheric instability, but the camera doesn't.
The atmosphere is especially unstable during the day, so if you're shooting the sun you'll generally end up getting a sharper image by taking multiple exposures in hopes that at least one will capture a moment of better seeing. The best time to take solar images is either early or late in the day, at which point the atmosphere is more stable. To stack images of the sun, they have to be taken in rapid succession, since solar features change quickly. This is also the case with Jupiter, which rotates very fast. If you dawdle for 10 minutes taking multiple exposures, Jupiter will have rotated about 6o in that time, pretty much negating the advantage of stacking.
Astrophotographers generally agree that the most critical issue is to attain the best possible focus. A long focal ratio telescope is more forgiving of focus imperfections, but it's not the best choice for wide fieldwork. With the small LCD displays on cameras, it's difficult to see whether you've reached the point of best focus. There are three basic methods that can help. " A small external monitor connected to your camera will enlarge the image enough to allow a better view. Of course, CCD imagers use computer monitors. " A Hartmann mask is a covering in front of the scope with two or three holes in it. These are available commercially; Kendrick Astro Instruments calls theirs the "Kwik Focus". They're also easy to make yourself. Aiming at a bright object shows two or three images on the camera's LCD screen, and then you adjust the focus so the multiple images merge into a single image. Remove the mask and you should be well focused. " A workable but less precise method is to use digital zoom to enlarge the image as much as possible on the camera's display, and then use that as a guide to adjust the focus. Before taking the image, be sure to back off to the optical zoom range.
Deep Sky Objects
Deep sky objects are faint and generally diffuse, which makes them more difficult subjects. Add to that the fact that most can't be seen on a camera LCD screen or even on a monitor. So we start with easier targets such as star clusters and bright nebulas. (Who hasn't done M42!?) And since DSOs don't change appearance, you can stack a set of images taken over the span of several nights.
Narrow band filters, which isolate specific emission lines (H?, OIII), can be used to accentuate nebulosity. They don't really make the nebulae brighter, though they seem brighter to our eyes because the filters block extraneous light that competes with the nebula. As with visual observing through these filters, stars will be dimmed and colors will be tinted by the filter. You might opt in this case to convert the image to gray scale or use it as a luminance image when stacking others.
Some of the same criteria apply in the digital world as in emulsion photography. (You remember film, don't you?) There are mainly three ways to pick up more detail: " A telescope with greater aperture " Longer exposure " Stacking multiple images The longer exposures required for DSOs are limited by the electronic noise ("dark current") that results, though some cameras have a built-in noise reduction feature. If your digital camera has noise reduction, give it a try. In my camera, the NR feature saves the images in .TIFF format, which really eats up memory and battery power. I've found that taking multiple exposures and stacking is more effective in getting rid of noise. It also helps to turn off the LCD display and take images on a cold night.
Nebulas are particularly colorful, but your image may not show the actual tones of the nebula, depending on the color sensitivities of the CCD device in your camera. Thus you may need a longer exposure to pick up red hues, for example. It's an analog to what is called the temperature of film - a "warm" film is more red-sensitive, whereas a "cool" film is more blue-sensitive. Most dedicated CCD cameras take monochrome images of varying duration through red, green, and blue ("RGB") filters, then combine the images with software to produce a naturally-colored result.
Knowing how to capture more detail is one thing, doing it is another. Some day I hope to look back on my best current astrophotos and be struck by how crude they are.Published in the February 2005 issue of the NightTimes