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Figure C. 10 Your telescope mounting may not track perfectly for an hour, but by summing short exposures in AIP4Win software, you can have perfect tracking with an imperfect telescope. In this tutorial, you will stack eight deep Ha images to make one super-deep image of a section of the Veil Nebula.
AIP4Win provides a set of multiple-image processing features that allow you to perform repetitive operations on a specified set of images. These processes provide the ability to:
• Calibrate an entire observing session's worth of images at one time automatically.
• Align and enhance a set of images in preparation for creating a movie.
• Align and combine a group of images to create a single, "deeper" image.
• Enhance a set of planetary images.
This tutorial demonstrates creation of images with Track & Stack, one of the most useful of the multiple-image operations.
Track and stack is a technique by which multiple shorter-exposure images are aligned with each other and added together to achieve the effect of a single, deeper image. This method is useful when short exposures are desired to prevent bright stars in the field from blooming. It is also frequently used with CCD cameras attached to telescopes that don't track by themselves long enough to avoid trailing on longer exposures. Some people even use this method to avoid guiding images altogether.
Nothing comes for free, however. While the image on a CCD adds directly, making the final image value equal to a single exposure with the same total integration time, the quantization and readout noise adds with the square root of the number of images combined. So a stack of images is noisier than the equivalent single exposure.
In this tutorial we will walk through the steps used to combine a stack of eight exposures into a single image. The relevant files are found on the CD-ROM in the Tutorials\Multiple Images directory and comprise a series of images of the Veil Nebula along with their corresponding calibration frames.
Step 1: Set up the Image Calibration. You need to have the calibration set up before you begin the AutoProcessing. Click on the Calibrate]Setup... menu item to open the Calibration setup window. Select Advanced for the Calibration Protocol.
Select the Bias tab and click the Use Bias Frame radio button. Click the Select Bias Frame(s) button and when the file selection dialog pops up, navigate to the "TutorialsYMultiple Images" directory on the CDROM and select the file named "lxlSXVMasterBias.fts" and click Open. Now that the bias frame has been loaded, click the Process Bias Frame(s) button; you will see that the Subtract Bias checkbox is now checked.
Select the Dark tab and click the Select Dark Frame(s) button. Use the file "lxlSXVMasterDark480.fts" for the dark frame. Click the Automatic Dark Matching radio button and then click the Process Dark Frame(s) button; a status bar will pop up as the dark frame is processed. When it finishes, you will see that the Subtract Dark Frame box is now checked.
Select the Flat tab and click the Select Flat Frame(s) button. Use the file "lxlSXVMasterFlat.fts" for the flat frame. Click the Process Flat Frame(s) button; you will see that the Apply Flatfield Correction box is now checked.
Select the Defect tab and click the Select Defect Map button. Use the file "lxlSXVDefectMap040714.fts" for the defect map; you will see that the Correct Defects box is now checked.
Now dismiss the Calibration setup window by clicking Close.
Step 2: Invoke the Deep Sky AutoProcess Tool. Click on the Multi-Image IAuto-Process I Deep Sky... menu item, and the AutoProcess Multiple Images window will appear. This is definitely one of the most complex windows in the entire program, but for this tutorial we will only be using a few of its features.
The Deep Sky AutoProcess window is arranged in a series of tabs, in the order in which each is used. Rather than describe each one here, we urge you to read the help file associated with this window. You can access the context-sensitive help screen for this window by clicking its titlebar and then pressing the F1 key. You don't need to read it now, but be aware that this tool is completely described in the on-line help, as are all the others.
Until a set of files is selected, the tabs are disabled, in order to prevent unintended user actions.
Step 3: Select the Files to be Stacked. Click the Select Files button and select the files "NGC6992Ha_001.fit" through "NGC6992Ha_008.fit" by clicking the first of the files, holding down the shift key, and clicking the last of the files and click Open.
Step 4: Setup the AutoProcess Parameters. For this tutorial you will need to select the following options:
• Process Type. Select Average Stack if it is not already selected. It will cause the images to be combined in a stack and averaged together. Other possibilities are Sum Stack, Median Stack, k-sigma Stack and Individual Files.
• Calibrate Image. Check this box on the Pre-Process tab to use the calibration we just set up.
• 2X Resample. Check this to preserve as much of the image resolution as possible when the image is shifted. Leave it unchecked for this tutorial.
• Prescale. Check this box to cause pixel values in each image to be multiplied by the Scale Factor before the image is added to the stack. Leave it unchecked for this tutorial.
• Scale Factor. Leave this set to the default of 10.
• Noise Filter. Check this box and set the Deviation to its maximum value of 3. This will remove any remaining hot pixels or cosmic ray hits in the images as they are stacked.
Step 5: Select the Master Frame. On the Alignment tab, use this drop-down list to select one of the images for use as a master when doing an alignment. Generally, you will have fewer difficulties tracking if you select the first image in the series; in this case you would select "NGC6992Ha_001.fit." The image will now be calibrated, noise filtered and displayed. You can see that it is a bit noisy. By lowering the White value on the Image Display Control, you will be able to more easily visualize the noise. This is a large image. To make room on your screen, you can set the Zoom on the Image Display Control to some convenient value less than 100%. Now when each subsequent image is displayed, it will be zoomed to this size.
Step 6: Select the Alignment Mode. For this image stack you will be using the 2 Star alignment mode. This gives results superior to 1-star alignment, but it involves a little bit of additional work, since two stars need to be selected. It has the advantage of correcting field rotation between images.
Click on the 2 Star alignment radio button, and you will see a set of alignment star selection buttons appear. There is also a Track Radius control, which is used to adjust the size of the region in which the program will attempt to find a stellar centroid. In dense areas it helps to make this small enough so that only a single star fits inside. In sparser starfields it can be set larger to make it easier to capture a centroid. For this tutorial you should set it to a value of 12.
Step 7: Choose the Alignment Stars. The alignment stars in the image consist of a pair of stars as widely separated as possible, which exhibit nice round images. The only serious requirement for selecting each star is that it be present in all of the images you are trying to stack. To this end you will want to keep well away from the edges of the image. Generally, selecting stars in opposite corners works the best.
There is an excellent candidate at X = 286,Y = 138. If the image covers the Display Control window, you can move it over a bit until the window is visible, or reduce the image size by zooming to a value less than 100%. Click on this star, and a circle will appear around it. Now click the Star 1 button, and a yellow circle with the number 1 below it will appear around the star.
There is another good candidate at X = 1136,Y = 916. Click this star, and then click the Star 2 button. A yellow circle will now appear around this star with the number 2 below it.
If for some reason you decide to try a different set of stars, the Clear button allows you to clear your selection and start over. Go ahead and click the Clear button and select the same stars again.
Don't worry about selecting the exact center of the star; AIP4Win has an excellent centroiding routine that mathematically determines the exact center of the star. It can, however, be fooled if the track radius is set so large that multiple stars fall within inside its boundaries. In this case, it will find the "center of mass" of the group of stars and center on that.
Notice that as you select each star, a message displaying the star's elongation is displayed in the Status box. Knowing this value can be used to help you select a value for the Elongation Limit when the Skip frames with elongated stars box is checked. This feature of A!P4Win will automatically reject frames with tracking errors. We will not be using this feature in this tutorial, as the set of images we are using is quite well tracked.
Notice also that the Track Radius control is disabled once you select the first alignment star. If you need to adjust this value, you will need to click the Clear button in order make this control active again.
If you wish to manually inspect your star images as you select them, use the Magnifying Glass tool to zoom in on a selected region of the image without changing how it is displayed.
Step 8: Start the Track & Stack Operation. Now we are ready to start the Track & Stack operation. For this tutorial we will use the Manual Slave Alignment Star Selection option. Make sure the Manual button is clicked. In manual mode, the user must select the alignment stars in each subsequent image. A guide is provided to show you where A\P4Wm thinks the guide star is. If the images are well-tracked, and the Track Radius is set large enough, you can use Automatic mode and AIP4Win will find the stars for you.
Now click the OK button, and you will be presented with each of the remaining images in turn. A Manual Alignment panel will also appear as an extension at the bottom of the AutoProcess Multiple Images tool window. The stars you selected on the master image are displayed with numbered yellow circles.
If the yellow circles contain your alignment star selections, all you need to do is to click OK. If they don't line up, just click the correct stars and the Star 1 or Star 2 button, and then click OK. You can correct the position of either star.
If an image shows signs of obvious tracking errors, satellite trails, or has other defects, you can click Skip to skip it. Skipped images are not added to the stack.
This image set was nicely guided, and A!P4Win will follow the alignment stars as best it can as they drift across the frame from image to image, so you will most likely be able to click OK for each image without needing to correct the alignment star position.
When it is finished you will see a nice, deep image of the Waterfall portion of the Veil Nebula that has much less noise than any of the individual images in the set. Right-click on the image to bring up the Image Status window and select the History tab. A description of the "Track and Stack" operation is stored in the FITS header when you save the file. This record keeping is performed for each tool in AIP4Win and allows you to see how each image was processed.
Click the Close button to dismiss the Auto-Process window. Restore the Auto-Process Log and scroll down through it. You will see the coordinates of the alignment stars in each image and see how the images were translated, rotated and scaled to bring them into alignment.
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