To photograph the moon using a telescope and to study various features found on its surface.

Transcription

1 0

2 5 LUNAR IMAGING 34 5 LUNAR IMAGING OBJECTIVE To photograph the moon using a telescope and to study various features found on its surface. PROCEDURE We will be using the 0.5 m telescope of the Climenhaga Observatory to get a number of CCD (charge-couple device) images of the moon. Take the covers off the telescope and the finder scopes. Make sure the drive is on, camera cooler and controller is on, printer is on, open the dome, find the moon. Take a few initial frames to position the moon on the CCD. Exposure Time: approximately = 0-1 seconds, depending on the phase of the moon and the brightness of the sky. Our CCD has a maximum brightness of 4096 Digital Units so it is necessary to make sure that all the pixels on the frame are less than this. Filters: Lately we have been using the Hα filter, but 3 I (3rd magnitude neutral density and I-band filter) or 5I also works. Focal Reducer Lens is to be used. This will increase the field of view from 5 x 8 (minutes of arc) to around 11 x 18. Since the moon subtends about 30 on the sky, only 6-8 frames should be needed to cover the whole moon. Use the RA and Dec displays on the telescope control console to move the telescope half a frame between students. (Remember that 1 min of time is equivalent to 15 of arc.) Phase of Moon: The best time to photograph the moon is when there are shadows to show the relief of the lunar features. At full moon the sun shines down into every nook and cranny and the light becomes so flat that many of the features disappear. Therefore we want to take our pictures near when the moon is half lit. This point is when the moon is one quarter of the way around its orbit so we call it a first quarter moon even though it is half lit. The moon takes days to go from new moon to new moon and we call this the synodic period. Notice that days is approximately the number of days in a moonth, so if we have a new moon on the 1 January we will have a new moon close to the 1 February etc. Therefore if we know the age of he moon on the thirty-first of December (the epact), we can find the

3 5 LUNAR IMAGING 35 phase of the moon for any date of the year from: Lunar Phase = Modulo(Epact+Month+Day,30) The Epact will increase by about 11 days each year and is 9 for 2007 and 21 for 2008, and 2 for 2009 and 12 for 2010 where the phase starts at 0 for new moon and 14 for a full moon. DATA GATHERING We will be logged on to the computers in a directory set up especially for this exercise. The image frames must be moved from /dol2/uvccd to our working directory and changed from *.igh files to *.imh files. We will use the imrd program to do this: Start up IRAF in the IRAF directory by typing: /astro/a200 >cd iraf to change directory to the IRAF directory. Notice how the prompt changed. Then type: /astro/a200/iraf >cl to start IRAF command language. Change to the moon directory by using: cl >cd../moon Check to see that the moon pictures are there by typing: cl >ls If the files are there skip to the DISPLAY section below. You can make the frames into IRAF format with the following steps. cl >task imrd = home$imrd.cl will define a new iraf task which will read in camera files and change them into iraf format. cl >imrd /dol2/uvccd/363o.igh will use the new task imrd to change the file 363o.igh into iraf format. You can get back the last command you typed in by typing an e. You can edit the command using the arrow key and the delete key. This way you can imrd all your files without retyping all those characters.

4 5 LUNAR IMAGING 36 DISPLAY To see the image on the screen you will need to start the program SAOimage Deep Space nine version with: cl >!ds9 & A new window will appear which you can paste on the right side of the screen with the left mouse button. Figure 1. Deep Space Nine image viewer with a lunar image. To load SAOimage with a frame use the task display by typing: cl >epar displ where an * marks the parameters that may need to be changed. PACKAGE = tv TASK = display I R A F Image Reduction and Analysis Facility image = 365o image to be displayed * frame = 1 frame to be written into * (bpmask = BPM) bad pixel mask (bpdispl= none) bad pixel display(none overlay interpolate) (bpcolor= red) bad pixel colours (overlay= ) overlay mask

5 5 LUNAR IMAGING 37 (ocolors= green) overlay colors (erase = yes) erase frame (border_= no) erase unfilled area of window (select_= yes) display frame being loaded (repeat = no) repeat previous display parameters (fill = no) scale image to fit display window (zscale = yes) display range of greylevels near median * (contras= 0.25) contrast adjustment for zscale algorithm (zrange = yes) display full image intensity range * (zmask = ) sample mask (nsample= 1000) maximum number of sample pixels to use (xcenter= 0.5) display window horizontal center (ycenter= 0.5) display window vertical center (xsize = 1.) display window horizontal size (ysize = 1.) display window vertical size (xmag = 1.) display window horizontal magnification (ymag = 1.) display window vertical magnification (order = 0) spatial interpolator order (0=replicate, 1=linea (z1 = ) minimum greylevel to be displayed * (z2 =.) maximum greylevel to be displayed * (ztrans = linear) greylevel transformation(linear log none user) (lutfile= ) file containing user defined look up table (mode = ql) Execute the task by typing :go. The contrast and brightness of the image can be changed by clicking the left hand mouse button on Color and then dragging the right mouse button across the image. CONVOLUTION Because the image is stored in the computer as a series of numbers representing the brightness of each pixel, we can enhance the image. One method is to convolve the image with a kernel or matrix. If the kernel is the 3X3 matrix (1 1 1; 1 1 1; 1 1 1) then each pixel is replaced by the sum of itself and its 8 neighbors. This is useful to smooth a noisy image. The image can be convolved with a sharpening kernel to enhance the small details and to suppress the slowly changing background. This is sometimes called unsharp

6 5 LUNAR IMAGING 38 masking. One sharpening kernel we can use is the 3X3 matrix ( ; ; ) which replaces each pixel in the original image with 9 times itself minus each of the surrounding pixels. We can use the IRAF task Convolve with: cl >epar convol PACKAGE = imfilter TASK = convolve I R A F Image Reduction and Analysis Facility input = 365o.imh Input images to be fit * output = c365o.imh Output images * kernel = ; ; Kernel file * xkernel = X dimension kernel file bilinear kernels ykernel = Y dimension kernel file bilinear kernels (bilinea= no) Is the kernel bilinear? (radsym = yes) Is the kernel radially symmetric? (boundar= nearest) Boundary (constant,nearest,reflect,wrap) (constan= 0.) Constant for boundary extension (row_del= ;) Kernel row delimiter (mode = ql) The task can be executed by typing :go. The new image c365o.imh will be created and you can look at it with your saoimage by typing: cl >epar displ where you need only change the image parameter to c365o.imh. LABEL DS9 will let us label the picture. Click on [Region] and [Shape] and [Text] to allow text labels. Click on [Region] [Color] [Black] and [Region] [Font] [12] to make the text legible. To delete a label click on it and type the delete key. Add lines with [Region] [Shape] [Line]. For exercise 1. we want to label five craters and five Maria and five landing sites.

7 5 LUNAR IMAGING 39 PRINT You will want to make a hardcopy of your image once you have labeled the things in Exercise 1 and maybe 2. To print a copy of your image click on the [file] button with the left hand mouse button and then click on the [print] button. EXERCISES 1. Identify five craters, five Maria, and five landing sites of spacecraft. Do this by working with different images, if necessary. A website with the crater names, diameters, longitude and latitude is found at 2. Craters follow the rule of superposition. Can you find an example of a crater which was formed after another; i.e., overlies it? We know that the dark Maria are about 3.5 Billion years old. Can you find a crater which formed after this? How about before the Maria? Explain. The sun produces a particle wind which blows continuously against the moon and turns the lunar rock very black. When new craters form, the explosion blasts the underlying lighter colored rock across the surface. Can you find some lighter colored craters? Can you find some rays of lighter colored material emanating from a crater? Identify the craters, if possible by marking them on your print. 3. What is the diameters of a big crater and a little crater and convert them to kilometers. We know the field of the CCD is 11 by 18 and each pixel is arc seconds per pixel. The average distance to the moon is km. The diameter of the moon is 3476 km. Compare these diameters to the Barringer Crater in Arizona, 1.2 km. and the Manicouagan Crater in Quebec, 100 km. The size of the craters are about 25 times the size of the meteoroid that originally hit the Earth. The rocks in space are called meteoroids, in the air they are meteors and on the ground they are meteorites. How big were the meteoroids which made these lunar craters? 4. About 65 million years ago at the Cretaceous-Tertiary boundary, the dinosaurs became extinct most likely due to a terrestrial impact by a

8 5 LUNAR IMAGING 40 comet or an asteroid. From the iridium found world wide at this strata, it has been estimated that the asteroid was 10 km in diameter. A crater which is 180 kilometers in diameter and 65 million years old has now been identified in the Yucatan. Is this crater about the right size? This impact is roughly the equivalent of an explosion of 100 million megatons of TNT or 10,000 times the combined arsenals of the U.S. and Russia. It has been estimated that a much smaller impact (1 km. rock) would be sufficient to kill the human population of the earth. How often this happens can be estimated by examining the moon. There are 29 ± 5 craters 25 km in diameter and bigger on the lunar Maria. The maria are 3.5 billion years old, so how often do craters of this size form on the maria? If the area of the maria is about 6 million sq. km., how often would you expect a crater of this size to be formed on the Earth if the area of the Earth is 500 million square kilometers? 5. Starting on July 16, 1994, the 21-odd pieces of Comet Shoemaker-Levy 9 slammed into the atmosphere of Jupiter. Their velocity upon entry was 60 km/s. Although there was wide disagreement among physicists as to how deep the fragments would go and how much energy would actually be converted directly into light and heat, there was no doubt about how much energy would actually be delivered. We will assume the comet was mostly water with a density of 1 gm, and that the pieces cm 3 were more or less spherical. What was the kinetic energy of a 1 km diameter piece? If there are Joules in a 1 megaton TNT blast, how many megatons is equivalent to this impact? The Jupiter show was spectacular beyond the wildest hopes. How spectacular would an impact with Earth have been? Use the more precise relation for the diameter D in km of a crater and the energy E in Joules of an explosion: log D = 0.29 log E 4.9 (6) and find the size of the crater from this relation if a piece 1 km in diameter hit one of the continents on Earth. An outer-planet specialist, Heidi Hammel of MIT, quipped, I feel sorry for Jupiter. It s really getting pummelled.

9 5 LUNAR IMAGING Every year on the 12 August the Perseid Meteor shower occurs. This meteor shower is caused by bits of gravel lost from comet Swift-Tuttle, but still traveling in almost the same orbit. The Earth s orbit intersects the comet s orbit at the place that the Earth is on 12 August. If the date of perihelion of the periodic comet Swift-Tuttle changes by +15 days (it changed by several years in its last orbit), it will hit the earth on August 14, It has a diameter of about 2km, and would hit the earth with a speed of about 60 km/s. How big a crater would it make? There would be a 75% chance that it would land in an ocean and make a tsunami. Find its height from the equation: log H = log R log E 6.70 (7) where H is height in meters, R is distance from impact in km, E is the energy in Joules. How large would the tsunami be 300km from the impact site? Would I be safe on Mt. Doug? For more information see: