Video Here we will take another detour from processing our single original JPEG image of Orion to discuss the advantages of shooting multiple light frames to improve the signal-to-noise ratio in your image. Along with using dark frames at temperatures that produce a significant amount of thermal signal, shooting multiple light frames and averaging them is one of the most beneficial things you can do to improve your images. Image quality is all about signal-to-noise ratio, and there is no other way to increase the signal except by gathering more photons. If you have shot multiple light frames, this is when you should combine them into a master light frame. You should do this after calibrating the light frames with dark frames. Even if you have not shot any dark frames, your image will still benefit greatly from combining more than one original light frame. The more light frames you can combine, the better the signal-to-noise ratio will be in the final image. It's pretty easy to shoot multiple light frames of the same object. You have already set up, focused, and framed the object, so all you have to do is trip the shutter again.
In the mouse-over example above, we can see how the noise in the sky background is improved when nine calibrated light frames are averaged together. Averaging multiple light frames increases the signal-to-noise ratio in the image. Because noise only goes up with the square root of the total number of images combined, but the signal goes up directly, you will gain much by averaging many light frames. For example, if you shoot 9 light frames, say each one with an exposure of 5 minutes, then you will have 9 times the signal of a single frame, but only the square root of 9 (3) the amount of noise. So the signal-to-noise ratio will be 9 to 3 or 3 times greater than a single frame. Having a higher signal-to-noise ratio has the benefit of providing a much smoother sky background. When the image is stretched later to increase the contrast the result will be better because of this. Combining Light Frames can be done with many different mathematical operations, but for basic imaging, using a simple average is probably the best and easiest way to accomplish this. Averaging can be done in a general image processing program such as Photoshop as well as in astronomical image processing programs such as Image Plus. The method is pretty much the same as when we averaged the dark frames in the previous step.
Open the first light frame that you have shot. This will be your base image. Open the second light frame. Copy and paste the second image as a layer on top of the first image. Change the blending percentage to 50 percent in the Layer's palette. Close the second image. Open the third light frame. Copy and paste it into the first image. Change the blending percentage of this third layer to 33 percent. Repeat for each additional light frame that you have. The following list gives the blending percentage for each layer as they are seen in Photoshop's Layers Palette. The background layer will always be on the bottom, and each additional layer is added on top.
You don't have to have the total number of layers listed here, you may have more or less, although more than 10 are probably more easily combined in groups or in a dedicated astronomical image processing program. After you have all of the individual light frames pasted as layers, and the blending percentages set correctly, then flatten the image. Go to Layer > Flatten Image. Then save the flattened image as an uncompressed TIFF File with the name "MasterLight.TIF".
An Easier Way If you are going to go to the trouble of shooting multiple light frames, and averaging them together, it is much easier to do this in a dedicated astronomical image processing program like DeepSky Stacker (which is free), or Images Plus. These programs will take the complexity out of the process and do it automatically. |
||||||
|
Back | Up | Next |
