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Project 8: "Advanced Imaging Techniques"
by Blake Bartosh
Level: Advanced
Requirements: SkyPro Exposure Time, Resolution, Background & Range, Unsharp Mask
Objective
The sources of noise, signal to noise ratio, the importance of exposure time, dark frame, flat field, resolution, background and range on the quality of images is learned. Images taken with different exposure characteristics are compared. The Student/Amateur is introduced to the advanced image processing capability of SkyPro, including "Track and Accumulate" and "Unsharp Mask."
Background
CCDs record images by converting photons into electrical signals and then to digital words, as described in the Introduction to CCD Astronomy, found earlier in this User's Guide. One question that is always asked is "...what is the best exposure time for this object we're imaging?" Certainly, virtually any exposure will yield some results. But there's a big difference between an image that just records an object and one that brings the full potential of the CCD to bear, showing intense images of stars and smooth-looking nebulosity.
The operator will provide an exposure time that has been proven, through experience, to yield the best image. But why is there a "best" exposure time? The answer lies in the fundamental physics of the CCD--in short, we are talking about "Signal to Noise Ratio." The signal to noise ratio, S/N, is a measure of the strength of the signal, the wanted information, relative to the strength of the noise, the unwanted information.
As was described earlier, the CCD's job is to convert photons, which is the wanted signal, ultimately to a digital word so the image can be saved as a computer file. Counting photons, which is essentially what the CCD camera does when the number of photons collected by a pixel is converted to a digital word, includes an element of randomness. Even if a pixel were illuminated by a constant light source, the exact number of photons recorded for a given exposure time would vary unpredictably. This unpredictable variation is the noise.
So what causes noise in a CCD image? Some of it arises from the fundamental properties of light itself. This starts getting into the quantum efficiency of the detector (the CCD array construction) and is a very advanced subject, and can be researched independently through the references below. Easier sources of noise to understand are those contributed by the CCD itself, i.e. readout noise and dark count. When the signal generated by light falling on a CCD array is collected, amplified, and converted to a digital word, noise is introduced every step of the way. This noise is called readout noise. There is some basic level of noise that exists even in a digital image that has a zero exposure time.
The dark count of a CCD occurs because even in the absence of light, electrons accumulate in the CCD pixels, as if photons were indeed falling onto the array. This noise is indistinguishable from the wanted signal.
How do we solve this dilemma? First, the dark count can be reduced by cooling the CCD. Our SBIG ST-6 uses a thermoelectric cooler to ensure a minimum dark count. Second, the readout noise and remaining dark count noise is somewhat repeatable. Therefore, by taking a "dark frame" image prior to opening the shutter and taking the actual image, a record of how much noise creeps into each pixel during the exposure time is made. Once the image is taken, the dark frame information is literally subtracted, pixel by pixel, from the image data to remove this repeatable dark current phenomenon. SkyPro takes a dark frame prior to the image when Automatic Dark Frame mode is selected in the Take Exposure window. When this feature is selected, SkyPro automatically removes the dark count noise from the image.
So now we have effectively reduced the unwanted noise inherent in the CCD camera itself, and thus increased our S/N for our image. There is another source of noise which we can mitigate with SkyPro's exposure control settings and image processing procedures--the background noise of the sky itself.
Background noise from the sky includes natural sky glow, moonlight, light pollution and atmospheric light scattering. Fortunately, the tools provided by SkyPro help to effectively reduce these sources of background noise. These tools include:
Exposure time
Resolution (High, Medium and Low)
Background & Range
The exposure time is the first line of defense against background noise. The longer the exposure time, the more photons will get to the array and provide a stronger signal. The goal of selecting an exposure time is to nearly "fill up" the pixels without filling them up entirely. Then we are assured we are getting an image that makes use of nearly the full capability of the CCD, and therefore maximizing the S/N. The result is an image which contains as much information as possible, and such an image typically looks beautiful!
Further, once we have achieved an image with maximum information, we can do image manipulation to really bring out details without loosing its dynamic impact.
When the Resolution is chosen, SkyPro uses more or less pixels to capture the image. In general, this is always set to High. When set to Medium, four adjacent pixels on the CCD array are "binned" into a single image pixel and displayed that way. Similarly, when set to Low, a three square pixel region is binned into a single pixel and displayed that way.
Background & Range settings independently adjust the "black level" and the allowable dynamic range of the pixels. The capability to adjust the black level, or background, is especially useful when there is a lot of sky glow, moonlight or light pollution. By raising the background level to just equal sky glow, for example, the CCD image will have a black background, and the star images will then appear crisp, not washed out as would be if the background were left at "0." SkyPro can do this automatically when Background & Range is set to Automatic.
Of course, when the background level is raised, that amount of raising means that we have reduced the dynamic range of the pixels in the array by that amount. So there is a limit to which the background should be raised.
The Range setting puts limits on the range of values any pixel can have. The range should be kept at maximum for best S/N, but there are occasions when the image contains an object that may be so bright that pixels are saturated, preventing the investigation of some faint object in the same field of view. Therefore, the range may be adjusted so that the fainter object detail is more emphasized.
Once these settings are understood, we can take an image with maximum S/N and do some interesting image manipulation with other SkyPro tools.
Unsharp Masking is a technique originally used for photographic images and printing to bring out fine detail in an otherwise washed out print. For CCD images, the Unsharp Mask provides a means of studying fine detail in galaxy images, for example, which have a highly luminous central core that washes out detail in the delicate spiral arms. The way an unsharp mask works for, say a galaxy image is a "mask" is created which is an unfocused or blurred negative image of the luminous central core. This unsharp mask is overlaid onto the original image and the two summed together. The resulting image contains only the small scale detail of the original image because the highly luminous areas of the unsharp mask cancel with the highly luminous areas of the original image.
Finally, to truly gain the most S/N for an image, many images can be stacked together to form a single image, with increasing amounts of information being added by each image. Since information is systematic, and noise is random, adding images that contain systematic information is reinforcing of that information, and destructive of the noise because the noise can cancel itself out. Therefore, SkyPro has incorporated a feature called Track & Accumulate, patented by SBIG and licensed to Software Bisque.
Track & Accumulate is a unique method of taking an exposure. It reduces the effects of poor telescope tracking and exposing, aligning and summing many short length exposures (with minimal tracking errors). The resulting image has an effective exposure time equal to the sum of the individual exposures, yet is free of saturated pixels and is virtually free of tracking errors.
Discussion of Work
Choose an object of varying detail, such as a galaxy or globular cluster. Take several exposures, perhaps two that are less than the "best" exposure time suggested by the operator, and two exposures that are more than the best exposure time. Be sure Resolution is set to High and Background & Range is set to Automatic. After each image is downloaded, use the Histogram function, found under the Image menu, to determine the maximum pixel value of the image. The goal is to find an exposure time that maximizes the full range of the pixels, without saturation. Saturation would be indicated in the histogram as a large percentage, over 10%, of pixels reading 65535, the maximum value of the pixel.
It is instructive to determine the sensitivity of the saturation with respect to exposure times. The user may wish to plot the percentage of pixels with, say half maximum value (32767) versus the exposure time. Is it a straight line? An exponential? Can you use this plot to predict what the exposure time would have to be in order to fill up pixels to 2/3 of maximum? Try the predicted exposure time to see if this hypothesis is correct.
Experiment with the Resolution setting on one image, keeping the exposure time constant. Keep an album of the results to chart the effect of the Resolution setting on the images taken.
Experiment with the Background & Range settings on one image, perhaps a galaxy or globular cluster. Again, keep an album of the results to chart the effect of the Background & Range settings on the images taken.
Using a candidate galaxy image, utilize the SkyPro Image menu, Unsharp Mask function to create an Unsharp Mask and apply it to the image. Keep a record of the resulting images to get familiar with the action of the Unsharp Mask function for different Mask Radii.
Choose a candidate object for testing Track & Accumulate. Before starting this exercise, be sure there is enough time in your session to allow you to complete the imaging run, as once the run is started interruption leads to loss of the image. Because Track & Accumulate images the same object several times, the telescope should be left to track that object throughout, and each image must be downloaded to the user's computer.
To take an image using Track & Accumulate, choose Take Exposure from the Camera menu, and select Track & Accumulate. Set the following parameters:
Snapshot
Time (exposure time)
Number of Snapshots (4 - 8 is a good number for best effect)
Resolution (same as above, High is a good setting)
Background & Range (same as above, Automatic is a good setting)
Frame Size (select the portion of the CCD you wish to expose; typically the
whole frame size is used)
Number of Exposures Before New Dark Frame (taking a dark frame before each snapshot greatly increases the amount of time required to obtain the final image). It is recommended that there be at least one dark frame taken every 4 exposures.
Choose OK, and the Track & Accumulate process starts. The first snapshot is taken, and you are asked to select a guide star. Use the mouse to select the guide star and click OK. The guide star is used to ensure that successive images are properly aligned. Thereafter, the creation of the final image is automatic.
References
CCD
Astronomy, Vol. 3 No. 3, "What Can You Really Get From Your CCD Camera,
P. Rybski, pp17-19, Sky Publishing, Cambridge, MA
CCD Astronomy, all issues, Sky Publishing, Cambridge, MA
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