C11 Deep Sky Images

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Up to this point, the tutorials have been focussed on specific features of AIP4Win. This and the following tutorial demonstrate using a group of AIP4Win functions to solve typical image processing challenges. This tutorial focuses on enhancing deep-sky images.

The task to be performed in this tutorial is the calibration and enhancement of a typical track-and-stacked deep-sky image. Images of this type share a number of common characteristics.

• Since they are images of faint objects, they are usually fairly long exposures (as CCD exposures go).

• They typically have a wide dynamic range (especially nebulous objects).

• The pixels in the image are dominated by the sky background (i.e., most of the pixels are black).

The common characteristics of deep-sky images lend themselves to a standard processing flow which consists of:

1. Calibration

2. Evaluation

3. Enhancement (assuming that the images are not being acquired for photometric or astrometric purposes).

The type of enhancement that will be performed depends on:

• the subject of the image (nebulosity vs. stars),

• the amount of visible noise, and

• the dynamic range of the image and how it is distributed across the range of available pixel values.

Due to the length of the exposure, and the desire to remove system-related artifacts from the image, the diligent CCD user will have taken the requisite calibration frames, including:

• dark frames of duration at least equal to the image frames,

Trying to enhance a raw CCD image can be a futile endeavor without calibration frames. For most CCD cameras, the dark current pattern tends to overwhelm the low-level detail which we are trying to bring out, and you end up with a noisy-looking image.

The image set which we are going to work on here consists of four 4-minute exposures of the Helix Nebula taken with a Cookbook 245 CCD camera w/LDC, attached to an 1 l-inch/76.3 telescope. These files are found in the Deep Sky Images subdirectory of the Tutorial directory on the CD-ROM. This image set is the white-light portion of a much larger set of color-component frames which are found in the Images\McMickle\ColorHelix directory on the CD-ROM. Included with the four exposures are darks, flats and flat-darks. These CB245 files are saved with a fixed bias value of 100, so bias frames are not needed.

Step 1: Examine the Raw Images. Click the Load an Image File button on the toolbar, open the files "Helwl.fts" through "Helw4.fts," and briefly examine each frame. Make sure that the Auto Low/High Stretch is the default display mode and that the image is stretched sufficiently to make its structure visible. What you will see is a faint image of the Helix Nebula sprinkled liberally with hot pixels. This is a pretty typical raw image frame. Notice that the stars are well formed and show no signs of trailing over the duration of their four-minute exposures. This is an indication of good guiding. Notice the absence of any pattern noise, such as horizontal banding, which would indicate electrical interference. Notice the darkening in the lower corners due to vignetting in the optical system.

It is good to give the raw frames at least a cursory glance before they are combined into an image, to avoid using frames with obvious flaws. It is also worthwhile to check them from time to time just to see how well the camera is functioning. Close the files when you are finished.

Step 2: Examine the Calibration Frames. In the same manner as described above, open the files "1240drk.fts" through "5240drk.fts." Compare them to each other. They should all appear consistent. They should appear as a field of white dots on a dark gray or black background, with some slight electroluminescence from the output amplifier visible on the left edge. When you are finished examining these files, close them.

Open the files "FlatOOl.fts" through "FlatOlO.fts." You will see a series of typical flat frames, with the same signs of vignetting that were visible in the image frames. These flats are only 11 seconds long, and the camera was well-chilled and stable, so they show very little dark current noise. When you are finished examining these files, close them.

Now open "DrfltOOl.fts" through "DrfltOlO.fts," and you will see a "salt and pepper" noise pattern typical of a short dark frame for the CB245 camera with the Low Dark Current feature turned off. If you run the cursor around the image, you will see that the variation from pixel to pixel is only a few ADUs. If you fire up the Pixel Tool and sample groups of pixels randomly, you will see a standard deviation of less than one. This shows that the frame is pretty consistent. When you are finished examining these files, close any open images.

Examining your calibration frames is a good practice to make sure that no pathological problems exist. Any problems that appear in your calibration frames will appear in any images you process with them. Weed out the defective frames now, before they cause you any trouble.

Fortunately, this is a high-quality image set, and you will not need to discard any frames.

Step 3: Set Up Advanced Calibration. The advanced calibration protocol allows us to use the automatic dark matching feature of AIP4Win, and it is the preferred method of calibrating images. It only requires that the bias value of the camera be known, or that a bias frame be available. It allows a set of dark frames to be used that may be of a different (preferably longer) exposure time or taken at a slightly different temperature.

Click the Setup Calibration button on the toolbar, or use the menu item to open the Calibration Setup window. When the window opens, select the Advanced calibration protocol and activate the Bias tab, and select the Use Constant bias button and leave the value at its default of 100. Activate the Dark tab and click the Select Dark frame(s) button; then select the files "1240drk.fts" through "5240drk.fts." After you click Open in the file dialog box, the green light next to the Select Dark frames(s) button should go on. Click the Median Combine and Automatic Dark Matching radio buttons; then click the Process Dark Frame(s) button. The dark frames you selected will now be median-combined, and a table of all the hot pixels will be created for use in the dark matching function. Click the Process Dark Frame(s) button, and you will see that the Subtract Dark Frame box is checked.

Next set up the flat-frame correction. Select the Flat tab and then click the Select Flat Frame(s) button to invoke the Select Flat frames dialog. Select the files "FlatOOl.fts" through "FlatOlO.fts" and click Open; the dialog box will close and the green light next to the Select Flat Frame(s) button will turn on. Click the Median Combine radio button in the Flatframe Correction area to set this mode for combining the flat frames. Check the Subtract Flat-Dark box, and the Select Fiat-Darks button will become enabled. Click it and select the files "DrfltOOl.fts" through "DrfltOlO.fts" for use as flat-darks and click Open. Select the Median Combine radio button in the Flat-Darks area as well. Finally, click the Process Flat Frame(s) button, and the master flat frame will be created, finishing your calibration setup.

Since we are not performing defect correction for this exercise, this completes the setup for calibration. Dismiss the Calibration Setup window by clicking the Close button.

Step 4: Set Up and Run the Deep Sky Auto-Process Tool. To bring up the Deep Sky Auto-Process Tool, click on Multi-Image I Auto-Process I Deep Sky Select the files "Helwl.fts" through "Helw4.fts," and set the Process Type to Average Stack. Initialize the following controls to their indicated settings:

Calibrate Image should be checked

2X Resample should be checked

Prescale should be left unchecked

Scale Factor is not used

Noise Filter should be left unchecked. Now activate the Alignment tab and select the file "Helwl.fts" on the Select Master Frame drop-down list. Select 2 Star for the alignment mode and set the Track Radius to 12.

The Master Image window should now be visible; click on the star at X = 58, Y = 67, and click the Star 1 button to choose it as your first alignment star. Pick the star at X = 680, Y = 407 as Star 2. If it is not already selected, select Automatic as the Slave Alignment Star Selection mode. These images were guided well, so we will let AIP4Win take care of stacking these images for us. Click OK.

Watch as AIP4Win flashes each image on the screen, calibrates it, and produces a stacked image. The process is finished when the image, now two times larger, is displayed as an image labelled "Track & Stack: Average of 4 images." You can now close the AutoProcess Multiple Images window; it has completed the job you asked it to do.

Step 5: Square Up the Image Pixels. Since this image has been resampled by 2x during processing, it no longer conforms to any known CCD camera chip size. If this image is ever read back into this program, or any other image processor, it will be displayed with an incorrect aspect ratio, because the program will not know the pixel size. To prevent this, we need to resample the image to square up the pixels.

Click on the Transform\ Re sample... menu item to bring up the Resample Tool. Make sure the By Percentage tab is active and set the Resample to: value to 100%, if it not already set. Check the Make Pixels Square box and click Resample Image. A new image will be created in which the pixels are square. Because A!P4Win automatically displays images with the correct aspect ratio, where possible, the image will not look any different; but if you bring up the Image Status tool by clicking the Measure\Statistics\Image... menu item and look on the Specs tab, you will see that the original image was 756 x 484, and the resampled image is 756 x 562.

Step 6: Save the Stacked Image. Save the image you just created in your C:\Windows\Temp directory, or another of your choosing. Click the Save Image in FITS Format button on the toolbar. (This is the picture of the floppy disk with a red "F" on it. A Tool Tip will appear if you float the cursor over the button.) The Save Image as FITS File window will appear, giving you a choice of FITS formats, as well as the option to edit the various FITS header fields and save the file.

If you click on the Examine/Edit FITS Header button, you will see the contents of the FITS header for the file you are saving. Included in the FITS header you will see a complete processing history of the file. This is exceptionally useful information to have when you are trying to reproduce a processing sequence later and need to know how you got a certain result. You can click on the Edit WITS Header... menu item to bring up the FITS header with the processing history of any image on the screen, whether or not it has been saved as a FITS file.

Step 7: Evaluate the Image. Now that the image has been calibrated and stacked, you can evaluate it. Upon visual examination you see that it is a bit noisy, but not objectionably so. Some of this is due to the stacking process, in which the noise adds with the square root of the number of images stacked. This is unavoidable unless you just want to make a single, longer exposure. You will notice a blooming trail in the upper right corner of the image. This trail imposes limits on any single exposure, and will get objectionably longer and brighter as the exposure time increases. So this exposure is about as long as you want for an image that has that bright star in the frame.

Another thing you will notice about the image is the artifacts along the top and left edges due to the stacking process. The scope drifted about 10 pixels to the right and about 10 pixels down between the first and last exposure. This left an area along these two edges that does not contain contributions from all of the images. The pixel values encountered in these regions throw off the statistics for the image and can often cause the Auto-Stretch function to display it incorrectly. It will also affect image enhancement tools that rely on image statistics to do their jobs. You will usually want to crop these edges.

Step 8: Crop Off the Edges. When a series of images is stacked, there is usually a band of pixels along one or two adjacent edges that has values inconsistent with those in the rest of the image. This results from areas where the source images did not completely overlap due to drifting between the exposures. It helps to crop these edges off so that they do not affect measurements made on the image. It also results in a more cosmetically pleasing image.

Invoke the Crop Tool by clicking the Transform I Crop... menu item. You may want to move this tool to the left edge of the screen to give yourself some room. Make sure that the Select With Mouse radio button is active, press the left mouse button and scroll the mouse across a region of the image, excluding the edge pixels. You can repeatedly select the region until you are satisfied with it. Fine adjustments of its location can be made using the arrow keys on your keyboard. When you have selected the area, just click the Apply button and a new, cropped image will be created.

Step 9: Measure the Image. Let's make some measurements on this image. Invoke the Statistics Tool by right-clicking the image button. This tool will show you some things about the image that you cannot easily visualize. First notice the Min and Max Pixel Values. They show a reasonably wide range, over 2,400 ADUs wide, depending on the region that you cropped. This range will give us a good deal of data to work with. You can better visualize this if you invoke the Histogram Tool to display the image histogram.

Step 10: Histogram Shaping. You can use the Histogram Shaping Tool to bring out more detail in the image by redistributing the histogram to create more contrast between the available brightness levels. Invoke this tool and try some of the available histogram shapes to see which ones provide the most pleasing results.

Step 11: Brightness Scaling. This is another useful technique for bringing out image detail. Invoke this tool and experiment with the different scaling types

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Auto Min/'Max r Display Image as Negative

File Edit View Utilities Calibrate Measure Enhance Transform Color Multi-Image Preferences Window Help

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