CHANGES IN THE SKY - DIGITALLY

Transcription

1 CHANGES IN THE SKY - DIGITALLY Sakari Ekko EAAE Summer School Working Group (Finland) Abstract Various changes in the sky are monitored using photographs taken with a common digital camera. Participants learn how to make star trail photographs and multiple exposures to visualize and measure slow motions in the sky: the path of the Sun or Moon, the motion of a comet or planet among the stars. A method to photograph the analemma during the year is shown. In digital photography, this is easier and not so prone to errors as the old method of fastening your camera in certain position for the whole year and making all the exposures on only one frame. A comparison method to monitor a variable star (Algol) by dragging the variable and comparison stars together is practiced. Comparing the magnitudes is easier this way. INTRODUCTION Digital imaging is a very useful tool in astronomy teaching. The immediate checking or use of the image and the possibility to process the images by the students or yourself are the main reasons for this, but with the limited resources in school the low cost is significant too: you do not have to buy, develop and print a whole roll of film to photograph a single phenomenon. You need a digital camera (not necessarily a digital single-lens reflex (DSLR)), a tripod, a computer and image processing software. A piece of Baader Astrosolar ND5 filter foil is needed for Solar photography. I have done the processing with Adobe Photoshop Elements, but many other types of processing software will do the same, with some differences in commands and logic. LAYER IS THE WORD Clyde Tombaugh discovered Pluto by comparing countless pairs of photographs to find an object that had moved during the interval between the exposures. He used the blinking method, but we can do the same with layers in our computer. It is done like this: You have two digital images, 1 and 2, of the same part of the sky. Open them, copy Image 2 using commands Select>All and Edit>Copy and paste it over Image 1 by Edit>Paste. Now you have Image 2 as a layer on Image 1, but it is opaque, and you can see only Image 2. Think of the situation like having two almost similar transparencies stacked, but you can alter the transparency of the upper one, and more. Select Layers>Layer 2 (click it in the small window under Layers). In the Layers window you find the slider for Opacity and a drop window with the text Normal. Try altering the 1

2 opacity. The smaller the percentage, the better you can see Layer 1 (background layer) through Layer 2. Move the slider back to 100% opacity. Click the arrow at the drop window, and then Difference in the drop menu. Now all that is similar in the two layers is black, and you see only the differences. Click Move in the toolbar and use the arrow keys to position Layer 2 over the background Image 1 as accurately as possible. The combined image is black, where the two layers are similar, but any difference will be more or less light, depending on the amount of the difference. If any of the objects has moved between the exposures, you can clearly see the two images of it. There is your moving object! Next, click Lighten in the Layers window drop menu. Now the lighter pixel of each image is selected, so you get two images of your moving object and only one of the fixed objects. This way you can get the path of a comet or other celestial body using several images taken at suitable intervals. When satisfied with the result, click Layers>Flatten image and you have the layers combined to one background image to be saved. Beware: after the Flatten image command, you cannot move or otherwise process the different layers, because you do not have layers anymore. You can always go back using History windows, ctrl-z or Edit>Step backwards, of course. If you want to file the image with all the layers, use the Photoshop format, not JPG or TIFF. You can create a layer from one picture: select all or a part of it (with a lasso or some marquee tool), then copy and paste it again on the original. Thus you can drag the layer with the mouse (hold the left key down when dragging) where you want it to compare the brightness or size. Again, the Difference or Lighten command is useful. In a perfect world, the images would align totally except for the moving object, but in practice the different distortions in even the best camera lens usually cause uneven alignment of the two images, if the two views are not exactly the same. The solution is to use the nearest objects to the moving object for alignment, and erase the other parts of the layer before using Lighten command. THE PATH OF THE SUN To photograph the rising / setting Sun (or Moon) you need a good tripod, a fresh battery for your camera and a piece of Baader Astrosolar filter foil, if you photograph the Sun. Use manual exposure and landscape (infinity) focusing. With a DSLR, use preferably a fixed lens, and with a compact camera make sure that the camera retains the focal length of the zoom between exposures. Usually cameras do this, if they are left on, despite going to standby mode in about a minute. Point the camera so that the estimated sunset point in the horizon is near the right edge of the frame (sunrise point in the left edge, of course), attach the filter over the lens (take care not to damage the lens) and wait until the Sun is seen in the left upper corner in the viewfinder or trial exposure. Make the first exposure at, say, 1/1000 s f:8, ISO , and check the result on the camera monitor. You will see a small light disk on a black background. If it is too bright, shorten the exposure time to 1/1000 s or 1/2000 s; if too dark, make a longer exposure. After you have found the right exposure, check the time and make a new exposure every 10 or 15 minutes until the Sun sets. Always check the image and correct the exposure, if necessary. Bracket (vary) the exposure every time anyway. You can select the best image in the computer afterwards. Maybe you have to take the filter away, when the 2

3 Sun is near the horizon, but take care not to move the camera! After sunset make some normal images of the landscape, the camera still on the tripod. At the computer, select the best landscape image and then add the first right-exposed image of the Sun as a layer, as described above. You can process the layer, if necessary, then use the Lighten command and the first of the Sun images is in the sky. Go on in the same way with other Sun images until you have your final image. If you want to save on computer memory you can flatten the image after every Lighten-command, but you lose the flexibility of the layers. When satisfied with the result, flatten the image and save it. In Figure 1, made with this method, I have calculated and plotted the real path of the Sun to show the effect of atmospheric refraction. You can photograph the Moon by the same method, but you do not need a filter. Expose the disk of the Moon at 1/500 s f:8 ISO 100 and check the result. Correct the exposure, if necessary. The Moon is a sun-illuminated landscape, so the normal daylight exposure is probably OK. Note: At sunset, the filter is not always needed, if haze filters enough of the sunlight and you can set the aperture of your camera small enough, like f:22 or f:32 (not possible in compact cameras). If you have a cord release for your camera, use it. The camera can be released gently with the self-timer, too. And remember to shade the camera from the sun between exposures the camera can be damaged by the Sunlight collected by the lens. Figure 1. Sunset in Finland , 62 o N. The broken line shows the real path of the Sun, corrected for refraction. 3

4 THE ANALEMMA To photograph the analemma you need a whole year, and usually it is not possible to attach your camera permanently as in sunset photography. Instead, you need some good reference points in the image to align your images in the computer. Find a suitable location, attach your camera on tripod and estimate the height of the analemma. If you begin around summer solstice, the Sun is highest. Select a suitable framing and focal length (shortest in a compact camera, preferably a fixed lens for a DSLR), point your camera accordingly, put the filter over the lens and take a photo of the Sun at the time you have chosen. Shading the lens from direct sunshine, take the filter away and, without moving the camera or lens, make a normal exposure of the landscape. This is for alignment only, so you can shade the lens with your hand as the direct sunlight can damage the camera CCD. Make sure, that your reference points are seen in the image, and combine the two images with Lighten command in your computer. Make new exposures with the same focal length at intervals of 10 to 30 days at exactly at the same time of the day (but neglect the daylight saving time!) during one year and then combine the images using your reference points to align them. Use the Lighten command and erase unwanted parts of the reference images before flattening the image, only leaving the sun images and one landscape image taken without shading the lens, the Sun out of frame as in the after-sunset image above. With patience, care and some luck, you will have an image with a nice analemma figure in the sky. Sorry, I have not done this yet I still have to find the patience and the location. OTHER MOVING OBJECTS The planets and comets move against the background stars. If you photograph the field with the moving object in on successive nights, you can again combine the images as layers, using the stars to align them (Figure 2.). The retrograde loop of Mars makes a good project. Figure 2. The movement of Venus and Jupiter

5 Figure 3. Comparing the brightness of Algol. The star field is used as a background. Circled are two comparison groupings, maximum and minimum of Algol. LUNAR AND SOLAR ECLIPSES The technique described above can be used to photograph lunar or solar eclipses. The eclipsed Moon is difficult to expose correctly, and the possibility to bracket the exposures is a great asset, compared with multiple exposure on film, where every exposure must be spot on, or the whole image is spoilt. If you have a telescope with good tracking (use sidereal rate and/or some guide star), you can combine 3 images of the eclipsed Moon, one with the Moon halfway in the umbra, second in mid-eclipse and third with the Moon halfway out of the umbra. The Moon moves to the left against the stars, and the round shadow of Earth is seen on the Moon images. If you can get some stars in the images, you can align different frames using them as guides, and do not need accurate tracking. One application of image processing is to find the subtle brightness differences in the Moon, when it is entering or leaving the penumbra. After combining a photograph of the Moon before or after the eclipse with another taken in penumbral phase, you can use the Difference command in the Layers menu to find the difference in brightness. Thus, you can find such subtle darkening, that it is impossible to see in a photograph or in the sky. It is important to use the same camera settings for both images. STAR TRAILS The problem with star trail photography is the brightness of the background sky, usually mostly caused by light pollution. If you expose short enough to have the sky tolerably dark and the stars as bright as possible, you can combine as many successive exposures as you want with the Lighten command to get star trails with dark sky. The brightness of the sky in the final image is not more than in the brightest individual frame. A good tripod is essential, of course. THE ROTATION OF THE SUN Photograph the Sun through a telescope equipped with Astrosolar filter at intervals of 2-3 days. Make two exposures every time: first with the Sun in the middle of the frame 5

6 and then another after minutes without moving the telescope between the exposures (tracking off). Combine the images by the Lighten command. The celestial north is in the direction of the line through the intersecting points of the disc images. Find the tilt and inclination of the Sun for the date from tables or a planetarium program and super-impose a coordination grid with the right tilt as a layer on the image. If you have to alter the size of the grid, use the Move tool and hold the shift key down when altering the size with the mouse (hold the left ear of the mouse down). This way you retain the shape of the layer. Turn the grid to the right inclination angle with the help of the celestial N-S line. Use the Darken command and flatten image. Now you can read the coordinates of the sunspots and find the movement during the intervals. From this, it is easy to calculate the rotation period of the Sun (or better, for the latitude of the sunspot used for this the Sun does not rotate as a solid body). See Proceedings of the 6 th EAAE Summer School, pp BRIGHTNESS COMPARISON Algol is a good object to practise the estimation of the brightness of variable stars. Photograph the field with the variable star and comparison stars using suitable zoom setting. The illumination on the chip is not uniform, because even the best lenses tend to give more or less darker image towards the edges and corners of the frame (vignetting), so try to place the important stars symmetrically around the centre of the frame. Bracket the exposures 2 s 4 s 8 s at f:2.8, ISO 400, for example. Find the suitable bracketing by experimentation before the session, and check the result every time, because the transparency of the sky can change, and you have to get the most useful exposure of every phase. Expose at minute intervals around the predicted minimum. For Algol, this takes the whole night let us hope you have clear weather all night. At your computer, select one comparison star with the circular Marquee tool, copy and paste it as a layer on the original (background layer) image. Now you have the image of the comparison star as a new layer. Click the Move tool and drag the layer with the comparison star near Algol. Click Lighten in the Layers palette, and you have Algol and the comparison star near each other. Do the same with other comparison stars. You can label the stars using the Type tool. Flatten the image and select the dragged stars with Algol using the rectangular Marquee tool, copy the selection and save it as a new file (File>New, Edit>Paste and File>Save As). Give the date and time as a file name. Having done this for all your material, you can begin to estimate the brightness of Algol. It is easier this way than having the comparison stars widely separated on the frame, and you can enlarge the images using the Navigator windows zoom slide. Estimating is best done on your monitor screen. The Argelander method for brightness comparison and other information is found in the Proceedings of the 8 th EAAE Summer School, pp References Ros, Rosa M. Proceedings of the 8 th EAAE Summer School, Barcelona Ros, Rosa M. Proceedings of the 6 th EAAE Summer School, Barcelona Ros, Rosa M., Vinuales, Ederlinda and Saurina, Carmen: La Fotografía, una herramienta para hacer Astronomía, Mira Editores, Zaragoza