Misc image processing notes - how to combine multiple exposures of one planet on a single image. Combining images of a single planet on one wide field base image from multiple nights (in order to show a planet's positional change) presents some digital processing problems, which are discussed here. If the images are added, slight variations in the camera's position will result in two horizons overlapping at the far ends of the image. Also, the sky will progressively darken as each dark sky is added together. Another image processing method works around these problems, using GIMP or Adobe Photoshop: 1) Make your first wide-field planetary image the lower background. 2) Add the second image as a new layer; this layer is positioned as the top layer. Set the new layer to opacity 60%. 3) Store a second copy of this image below the background layer. 4) Select the new top layer (that contains the second night's image), and move and rotate the top layer so the horizon matches the bottom background layer as best as possible. 5) Add a layer mask set for ``full opacity'' to the top layer. ``Full opacity'' means any white on the layer mask will result in the top layer showing through. Black painting on the mask will make that portion of the top layer invisible. Experiment with your layer settings and paint colors until you get it right. 6) Now paint just the new planet's position so the planet will show through the layer mask. You are painting just a small circle around the planet in order to trigger opacity on the layer mask. 7) Select the planet on the top layer using a circle selection tool. Invert the selection of the planet on the top layer. Flood fill the remainder of the layer (everything but the planet) with the applicable foreground or background color so that the rest of the second image becomes invisible. 8) Apply the layer mask. 9) Reset the top layer's opacity to 100%. With the layer mask, only the second night's planet will be visible. Adjust the brightness of the top layer so the brightness of the planet on the second night reflects the observed relative naked-eye brightness. Examples: Venus on two nights processed by adding layers method http://www.slas.us/gallery2/main.php?g2_itemId=4229 (Two Venus's are faintly seen in the lower right-hand corner. Close inspection of the horizon line at the mountains reveals two horizons. Sky and the moon are disproportionately dark.) Venus on two nights processed using the layer mask method http://www.slas.us/gallery2/main.php?g2_itemId=4235 (Two Venus's are faintly, but more easily seen in the lower right-hand corner. The Venus's are better balanced in brightness. No errors in the horizon line or distortions in sky or lunar brightness.) Presumably, the same technique can be used to merge two telescopic exposures of bright and dim planet (Venus and Jupiter) in the same field of view into one image. - Clear Skies - Kurt