Table of Contents Image Acquisition Types 2 Image Acquisition Exposure 3 Image Acquisition Some Extra Notes 4 Stacking Setup 5 Stacking 7 Preparing for Post Processing 8 Preparing your Photoshop File 9 Post Processing your Stacked Image 11 Other DeepSkyStacker Options 12 1
Image Acquisition Types One cannot create astrophotographs without acquiring a set of images. The number of images varies depending on the circumstances. Generally speaking, these images fall into one of the following four categories: Light Frames As the name suggests, light frames refer to gathering photons; in other words, these are the photos of the object(s) you are trying to photograph. Taking multiple photos of the same object increases the signal to noise ratio (STNR). Dark Frames Dark frames are photographs taken at the same camera settings as the light frames, but with a cap over the lens or telescope. The stacking software will remove the dark frame data from each light frame, resulting in a cleaner overall image in the end. Flat Frames If your lens or telescope causes vignetting in the images, you may want to take a flat frame. This frame is taken while covering the lens or telescope with a white cloth and lighting it evenly with white light. The stacking software uses this frame to remove the vignetting. If using a telescope, this frame must be taken with the camera oriented in exactly the same way as when the light frames were acquired. Bias Frames Bias frames are taken with the same camera settings, but at the shortest possible shutter speed setting in the camera. I currently only use light and dark frames in my processing. At present, I don t consider vignetting to be an issue. Additionally, through research, I have found that bias information is included in each dark frame and, thus, is subtracted along with the dark information. Not only do I cut down on the image acquisition time by following these steps, but I also save my camera s shutter system from additional frames and, therefore, undue wear and tear. 2
Image Acquisition Exposure Now that we know about the types of frames, I generally use the following steps in order to get my exposures. Please note that I use BackyardEOS to program my exposures; if you use a different image acquisition program (or none at all) your process will be slightly different than mine. Step 1 Determine Proper Exposure Take test shots with your camera to determine the proper exposure time for your image. ISO 800 tends to be a good balance between low noise and fast shutter times, but ISO 1600 can be used if necessary. If using a lens, it is recommended that it be stopped down one stop from fully open to increase clarity and quality. Preview each test photo; a properly exposed photo needs to have the histogram peak between 25% and 50%, and you need to make sure the lightest parts of the object are not overexposed. Step 2 Determine Light Frame Count and Capture Light Frames Proper light frame count is less a function of number of exposures as it is the total exposure time (total photons collected). A total exposure time of 1 hour is considered to be good for very bright objects; average objects will look better with 1 2 hours total, and very dim objects may require 4 hours total or even more. Knowing your exposure time from step 1, determine the number of frames needed to give the desired total exposure time. Program this into your capture program and begin capturing. Set a timer so you will be back when the exposures are completed. Step 3 Determine Dark Frame Count and Capture Dark Frames The dark frame count can be substantially less than light frames. I usually recommend at least 20 30 minutes of total dark frame exposure time. Put the cap onto the lens or scope, and expose with the same settings as the light frames. Step 4 Rename Files (If Necessary) When BackyardEOS downloads frames from the camera to the computer, it automatically renames files based on the object name and exposure type. Alternatively, you can manually organize the files. Whichever you choose, I highly recommend that you sort your image files by object name and exposure type to make the stacking process setup easier. 3
Image Acquisition Some Extra Notes Since I use a Digital SLR, I also have several tips for making your images better and the process smoother: Make sure your camera format is set to include RAW files. You can have JPEG files enabled if you wish, but these will not be used and take up extra space on your camera s memory card. Cameras that have swivel screens, such as the Canon 60D, are much more friendly to use for this purpose because of the various angles the camera may be at while looking at the screen. Cameras that have live view are an absolute must. Not only can the live view be used for focusing, but it also allows BackyardEOS to be used for focusing, which can be even more precise. If your camera supports it, turn your camera s LCD brightness down to the minimum setting. This avoids destroying your night vision. If you are using BackyardEOS to capture your photos, turn image review off in your camera settings. You will save a bit of battery power by not having the LCD screen come on after each image, and it also avoids destroying your night vision. 4
Stacking Setup Now that we have our light and dark frames, it s time to stack the images so that we can get closer to our final product. I use DeepSkyStacker for this. The latest beta version (necessary for camera raw file support) is available at the following site: http://tech.groups.yahoo.com/group/deepskystacker/ Before importing any photos, I recommend that you set your settings to match mine. These have worked very well for me, and are a good starting point for anybody that is trying to stack photos for the first time. Refer to the screen shots and the associated text for instructions. Options > Settings > Register Settings If not already set, set the star detection threshold to 10%. Leave the Reduce the noise by using a Median Filter checkbox checked. Options > Settings > Stacking Settings > Result Tab Set the program to Intersection Mode. Make sure 2X and 3X Drizzle are turned off. Make sure Align RGB Channels in final image is turned on. (The 2X and 3X drizzle options are fantastic for enhancing image resolution and will be covered later in this document) 5
Options > Settings > Stacking Settings > Light Tab Set the stacking mode to Kappa Sigma clipping. Make sure Kappa is 2.00 and Number of iterations is set to 5. Below Auto Adaptive Weighted Average, click the text and change to Per Channel Background Calibration if it doesn t already state that. Options > Settings > Stacking Settings > Dark Tab Set the stacking mode to Median. Make sure that Hot Pixels detection and removal is checked. 6
Stacking It s time to stack your images! There are multiple ways to import your images into DeepSkyStacker for stacking. This is just one of them. Import Light and Dark Frames In the left menu, click Open picture files, select all of your light frames, and click Open to import the files. Then click on dark files, select all of your dark frames, and click Open to import the files. Once all of the files are imported, click on the Check All link to select every file you have imported. When ready to begin the stacking process, click on Register checked pictures to proceed. Register Settings All three checkboxes should be checked, and I recommend setting your percentage to 90% unless you are sure all of your images are of good quality. I personally use 95% so that I have as much light data as possible. Click OK. Stacking Steps This just shows you a summary of what the program is about to do to stack your frames. Click OK and the stacking process will begin. Depending on your camera s resolution, number of frames, and computing power, this may take a few minutes or several hours. 7
Preparing for Post Processing When the stacking process finishes, you will be shown a window that looks something like this: While DeepSkyStacker does have rudimentary adjustment controls, I recommend doing all post processing in Adobe Photoshop. Therefore, on the left menu, I choose the Save picture to file setting, and save the final picture. Make sure you save the image as a TIFF Image (32 bit/ch rational) file. Choose None for compression, and choose the Embed adjustments in the saved image but do not apply them option. Type your desired file name and save the file in a location of your choosing. You may now close DeepSkyStacker, and open Adobe Photoshop for post processing of your image. 8
Preparing Your Photoshop File Before working with your image, you will need to do several things in Adobe Photoshop first. Please note that the screen images come from Adobe Photoshop CS 5.5. Under the Edit menu, go to Color Settings. Under Working Spaces, make sure that the RGB option is srgb IEC61966 2.1 or something similar. This only needs to be done once. Open the file you saved in DeepSkyStacker, and you will be presented with the following dialog box: Choose the Assign Profile option, choose srgb, and then make sure the convert option directly below is checked. Press OK and your file will be opened. Convert your picture to 16 Bit color. Under the Image menu, choose Mode, and then 16 Bits/Channel Change the Method to Exposure and Gamma, and leave both sliders at their default levels. Click OK to convert your file. 9
Now, create three adjustment layers for your photo: Layer > New Adjustment Layer > Curves Layer > New Adjustment Layer > Levels Layer > New Adjustment Layer > Hue/Saturation Your layers palette in Photoshop should now look something like this: Finally, save your file as a Photoshop (.PSD) file. From the File menu, choose Save As Be sure to change the format to Photoshop. Also, be sure that Layers is checked, as well as ICC Profile: srgb. Name the file as you desire, and click the Save button. Your stacked image is now ready for post processing in Photoshop. 10
Post Processing Your Stacked Image Up until this point, everything has been extremely technical. Beyond this point, however, we now will be going into the realm of artistic expression. That means that there is no one right way to process images. Generally, however, I process in the following order: Curves Levels I use the Curves adjustment layer to increase the contrast of the image and bring out the midtones some. Please note that I don t use it to make the blacks totally black, nor the whites totally white. On the histogram window, I drag the darks slider to the right, then the lights slider to the left, and the midtone slider until the midtones are satisfactory to me. To the left is an example from one image I edited. The Levels adjustment is similar; I use the dark and light sliders to increase the contrast and bring the blacks darker, and the whites whiter. Then I adjust the midtones to achieve a good balance. If contrast is too high, stars will look unnatural; be careful when making these adjustments. Hue/Saturation If you want to try to bring out the colors more, use the Hue/Saturation sliders. First I change the master saturation, and then I look at each color individually from the dropdown menu, and adjust until I get something that I like. Make sure you save your Photoshop file after each adjustment. The best thing about using Adjustment Layers is that nothing is done to the image itself; you can always reset the adjustment layer if you want to start over. 11
Once you achieve a result that you like, there are just two steps left. Sharpening Almost all digital photos need some level of sharpening, and astrophotographs are no exception. First, save your layered file. Then create a duplicate of it: Image > Duplicate Name it whatever you like, and then choose Duplicate Merged Layers Only. Then, use whatever sharpening filter you like. I normally stick with Unsharp Mask as it gives me the most flexibility. Once the sharpening is complete, crop and resize your image as you desire. Save your final photo as a different file name, and you re done! It s always good to keep your layered.psd file separate you can easily go back to it at any time. 12
Color Correction Other DeepSkyStacker Options After stacking, some images may be difficult to process due to the background color. If that is the case, you may try re stacking with a different background calibration option. Options > Settings > Stacking Settings Open the Stacking Settings dialog box, and go to the Light tab. Below Auto Adaptive Weighted Average, click the text and change RGB Channels Background Calibration to Per Channel Background Calibration instead. Then go through the entire stacking routine again. Custom Rectangle and Drizzle Options You may want to focus on just one area of your image, and possibly magnify it. First, create a custom rectangle. After all of your frames are imported into DeepSkyStacker, click on one of the Light Frame files. After several moments, your image will be shown on the main screen. Drag a rectangle around the area you wish to process. NOTE: If you wish to use the 2X drizzle option, your square must be equal to or smaller than the 2x rectangle when you are creating your custom rectangle. If you wish to use the 3X drizzle option, it must be equal to or smaller than the 3x rectangle. To clear your rectangle, click anywhere outside your selection. Go to Options > Settings > Stacking Settings. Under the Result tab, the Custom Rectangle option should now be checked. If you wish to enlarge the selection, choose the 2X Drizzle or 3X Drizzle option. Click OK, and then go through the entire stacking routine again. 13