for GIMP - Australian Gemini Office
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1. The image will be much brighter now because the screen mode for each layer adds the intensity of the that layer to the overall image You can see the effect of each layer on the overall image by turning on and off the visibility of each layer This is done by clicking on the eyeball on the left side of each layer in the layer window When all of the layers are visible 1 e when there is an eyeball visible next to each layer the image should look similar to that shown below File Edit Select View Image Layer Colors Tools Filters Windows Help Rescaling and Colorizing Each Layer Now that a grayscale image for each filter is loaded as a layer into the main image file it is time to fine tune the intensity scaling for each layer so that we can best see the details in the faint and bright parts of each layer This is best done by working with only one layer visible at a time Rescale the intensity of the H alpha layer e Turn off the visibility of the OHI and SII layers by clicking on the eye ball next to each of these layers There should only be an eyeball visible on the H alpha layer e Select the H alpha layer by clicking it It should be the one highlighted e Choose Colors Levels A levels adjustment window like that shown to the right will appear Like in the FITS Liberator you can use 6 Image Tutorial X Layers The layers window in GIMP after the Ha OIll and SII images have been duplicate2. X ngc6751_demotrim psd 2 0 RGB 3 layers 1275x1650 GIMP File Edit Select View Image Layer Colors Tools Filters Windows Help 0 500 1000 px KH 33 3 Pism 125 9 MB The image is now nicely scaled to show the detail in the bright and faint portions of the nebula Note that your image may not look quite like what is shown above As always this is an excellent time to save the image Cleaning the Image The image is mostly done but there are still some minor steps that would be done to complete the image One issue is that there are cosmetic defects Zooming in will reveal that there are occasional dots and small streaks in the image The dots are caused by cosmic rays that hit the CCD detector during each exposure The streaks are artifacts from the stacking of multiple images They are therefore in different locations in each filter image and therefore have a color that matches that filter e g cosmic rays in the SII image look red These cosmic ray specs and scratches can be fixed in a variety of ways in GIMP A simple option is to use one of the Enhance filters e g the despeckle and destripe filters These can remove the cosmic rays but they also blur the image A more effective method is to use the healing brush tool to remove the cosmic rays one by one We will not do these steps here but if you are interested in learn ing how to do so the user s manual for GIMP explain
3. 50 This will make the image a dark deep blue Click OR Scale and colorize the SII layer Turn off the visibility of the OII layer Select the SII layer and turn on its visibility Choose Colors Levels Setthe black point and gray point in the same way you did for the H alpha and OIM layers For the provided TIFF file set the black point to 48 and the gray point to 0 93 Do not move the white point Choose Colors Colorize To make the SII layer red set the hue to 10 As before set the saturation to 100 and the lightness to 50 This will make the image a dark deep red Click OK Each image layer has now been rescaled and colorized To see how the overall image looks turn on the visibility of the all the layers This is a good time to save the image The image should look roughly as shown below X ngc6751_demo psd 2 0 RGB 3 layers 3108x2304 GIMP File Edit Select View Image Layer Colors Tools Filters Windows Help amp o 500 1000 1500 2000 For this image we have chosen to assign colors in the following manner Ha is yellow OMI is blue and SII is red Why these colors The color assign ments are in what is known as chromatic order where the shortest wavelenth filter OLI is assigned blue and the longest wavelength filter SII is red In chromatic order the middle wavelength filter is often assigned to green But Ha is very close in wavelength to SII so we chose to
4. at the bottom of the image window ee This selection will put the center of the nebula just above the center of Curve type the image Press Return to crop the image M Preview ten keset canca C o A Save the file with a different name Choose File Save As and save it as ngc6751_demotrim psd This way if you decide you would The curves window in GIMP before like to use a different cropping you can return to the original untrimmed any adjustment points have been image added Next we will fine tune the scaling of the image The image as it stands has a few minor problems First the sky background the area between the stars 1s too bright It should be closer to black Second the nebula at the center of the image is also too bright and it is hard to see detail We there fore need to rescale the image to bring down the brightness in these parts of the image But we do not want to darken the faint nebulosity in the top part of the image We can achieve this by using a curves adjustment layer Apply a curves adjustment layer to the entire image Xi Curves Adjust Color Curves Bee Select the SII layer SII 11 ngc6751_demotrim psd K reses C eo e Choose Colors Curves The window should look similar to Channel Value gi Reset chare E what is shown in the top right e To decrease the brightness in the background click on the diagonal line near the lower left corner This will add a
5. in detail below Note In this document an icon appears when a computer command is described 2 Image Tutorial Note Many of the steps described inthis process are subjective They are based upon the color calibration of the authors computer displays and their personal aesthetics If your final image does not look pleasing it is either because you did not follow astep correctly your computer s monitor is not properly calibrated many monitors are too dark or you do not share the authors excellent aesthetic palettes Rev 25 Jan 2012 Converting Datasets with FITS Liberator Before using GIMP to play with the images we will use another piece of software to convert the raw images to a format other than FITS This is because FITS is a data file format not an image format The FITS Liberator is a program that converts the pixels in a FITS file into pixels for an 8 bit or 16 bit grayscale image Ina FITS file the pixels may have any value they want including negative values In an 8 bit grayscale image the pixels may only be one of 256 possible values Thus a mathematical function often called a transfer or stretch function must be used to convert the data pixel values in the FITS files into grayscale image pixel values _ _ngc6751_ha fits ESA ESO NASA FITS Liberato E Merdara FITS Header Help Cuide Save amp Edit Preferences W E naass e Auto Scaling 3158 72 L Impo
6. make it yellow instead The actual color of OMI is actually green and Ho and SII are deep reds on the edge of human vision This image therefore does not represent what your eyes can see but what the telescope can see This object is too small and too faint for you to see with your eyes even with a very large telescope 8 Image Tutorial X Colorize Colorize the Image Ex H alpha 17 ngc6751_demo psd Presets O Select Color Hue o 60 6 Saturation _ O 100 Lightness 50 of Preview p Help Reset Cancel OK The colorize tool in GIMP for the H alpha layer Rev 25 Jan 2012 Fine Tuning the Image It looks pretty good But we can improve it First we need to trim the RL curves h n Adjust Color Curves a image down to only the portion we wish to keep Since most of the image siJ 11 ngc6751_demotrim psd contains only stars we will crop the image with a vertical portrait ori Presets pm e E Channel Reset Channel ia entation to include only the central nebula and the faint wisps above it Crop the image e Select the crop tool in Tools Transform Tools Select the region starting in the upper left corner at pixel location x 921 y 333 The pixel location is shown in the lower left corner of the image window Select a region with a width of 1275 and height of 1650 pixels The dimensions of the image are shown
7. I grayscale images are now loaded into the main image file each as a separate layer Finally import the SII image and duplicate it into the main image file Open the SII image in GIMP e Choose File Open As Layers andselectthe ngc6751_s2 tiff image In the layers window rename the layer to SII e Let s save the 3 layer image by choosing File Save All three grayscale image should now be in the main image file The layers window Rev 25 Jan 2012 Note You may also see an extra dialog box that asks you if you wish to maximize the compatibility of the file image If yousee this go ahead and select yes Image Tutorial 5 should look as shown on the right However you will only see the image in the top layer This is because the blending mode for each layer is set by default to Normal The blending mode menu is the one at the top of the layers window For the light from the images in each layer to combine the blending mode for each layer must be set to Screen Select the SII layer by clicking on it Change the blending mode from Normal to Screen Now select the OHNI layer and change the blend ing mode from Normal to Screen as well The opacity of each layer should be kept at 100 Note Technically you don t need to change the blending mode of the H alpha layer because it is the bottom layer but you can if you want just in case you change the order of the layers
8. NGC 6751 Tutorial Making Color Images Christopher A Onken Travis A Rector The Australian National University University of Alaska Anchorage Mount Stromlo Observatory Department of Physics and Astronomy Cotter Rd Weston Creek ACT Australia 3211 Providence Dr Anchorage AK USA email onken mso anu edu au email rector uaa alaska edu Introduction A Note from the Authors The purpose of this tutorial is to demonstrate how color images of astronomi cal objects can be generated from the original FITS data files The techniques described herein are used to create many of the astronomical images you see from professional observatories such as the Hubble Space Telescope Kitt Peak National Observatory and Gemini Observatory In this tutorial we will make an image of NGC 6751 a planetary nebula in the constellation of Aquila the eagle It is assumed that you are familiar with the prerequisites listed below If you have problems with the tutorial itself please feel free to contact the authors Prerequisites To participate in this tutorial you will need a basic understanding of the follow ing concepts and software e FITS datafiles GIMP The GNU Image Manipulation Program e The layering metaphor Description of the Data The datasets of NGC 6751 used in this tutorial were obtained with the Gemini South 8 1 meter telescope which is located at the summit of Cerro Pach n Chile The FITS files are 3108 x 2304 pixels and a
9. TIFF files set the black point to 42 Rev 25 Jan 2012 Nomenclature Contrast is a measure of how well differences in brightness can be seen In a high contrast image even small differences in bright ness can be seen Dynamic range is the ratio of the intensity in the brightest region to the darkest region Contrast and dynamic range are at odds with each other Overall an image with high dynamic range will have a low contrast and vice versa The levels and curves tools are used to maximize the contrast in the interesting parts of the image while maintaining a high dynamic range for the overall image Levels Adjust Color Levels H alpha 17 ngc6751_demo psd Presets H P O Channel Reset Channel Input Levels amp Sane 4 16 0 86 9 Output Levels 2 ess 16 i omg All Channels Edit these Settings as Curves Tl Preview 255 g ono 22 Help Reset Cancel OK The levels control window in GIMP that rescales the H alpha layer Note that the black point is nearthe left edge of the histogram s peak and the gray point is at the foot on the right side of the histogram Image Tutorial 7 and the gray point to 1 15 Do not move the white point Click OK To assign a color to the layer choose Colors Colorize To colorize the layer blue set the hue to 240 Set the saturation to 100 and the lightness to
10. are outside the range between the black and white points Pixels below the black point shown in blue will be set to a pure black when imported into GIMP And pixels above the white point shown in green will be set to a pure white These pixels are said to be clipped because you will not be able to see any detail in these clipped regions In brief make sure the black and white points are set so that there are very few blue or green pixels in the image Scaled Peak Level The scaled peak level adjusts the contrast in the image A small scaled peak level means there is high contrast better detail in the bright parts of the image but poorer contrast less detail in the faint parts of the image A large scaled peak level does just the opposite Thus the scaled peak level should be adjusted to best show the detail in the bright and faint parts of the image Rev 25 Jan 2012 Image Tutorial 3 pixels along the edge of the image Click the Auto Scaling button The image above should now appear nearly black To increase the contrast in the faint parts of the image change the Scaled Peak level from 10 to 10000 and hit return do not click on the Auto Scaling button again because it will reset the scaled peak level back to 10 The image should be brighter the Black Level should be 0 00 and the White Level should be 4 00 The window should look close to what is shown below Change the Channels bottom ce
11. bout 28 7 Mb in size each Datasets in the narrowband Ha OIM and SII filters are available for this object These data were obtained for the winning picture of the 2009 Gemini School Astronomy Contest in Australia About the Software This tutorial requires GIMP which is available for the Macintosh Windows and Unix Linux platforms The software is free and can be downloaded from http www gimp org In this tutorial GIMP v2 6 will be used This tutorial also requires the ESA ESO NASA FITS Liberator It is a program that coverts FITS data files into TIFF image files The current version is v3 0 The FITS Liberator is free and can be downloaded at http www spacetelescope org projects fits_liberator An instruction manual is also available at that URL Rev 25 Jan 2012 The Gemini South Telescope Nomenclature FITS stands for Flexible Image Transport System It is the stan dard format for storing astronomi cal data Nomenclature Ha is pronounced hydrogen alpha or H alpha Itis a specific color of red light produced by warm hydrogen atoms OIII is pronounced oxygen three or O three and is a specific color of green light produced by hot oxygen atoms And SIl is pro nounced sulfur two or S two It is a specific color of red light produced by hot sulfur atoms These are kinds of light commonly produced by planetary nebulae Decoding file names The FITS filena
12. d into the main image file Rev 25 Jan 2012 the sliders underneath the histogram to move the black and white points You ll also see a gray slider that adjusts the gamma correction The histogram window shows the distribution of pixel values The left side of the window corresponds to darker pixels and the right side cor responds to the brighter pixels Notice that most of the pixels are darker which is what you d expect from an astronomical image Move the black slider on the Input Levels a little to the right As you move the slider you ll notice the image gets darker Move it until the black slider is near the left edge of the histogram s peak as shown in the figure on the right If you are using the TIFF files provided with the tutorial the number below the black point should be 30 This is done so that the image will be almost completely black in the darkest portions of the image e g in the lower left corner Move the gray slider a little to the right As you move the slider you ll notice the image gets a little fainter Specifically the contrast in the faint regions is decreasing and the contrast in the dark regions is increasing If you re using the provided TIFF images the number below the gray point known as the gamma correction should be 0 86 This is done to improve the contrast in the faint parts of the nebula Do not move the white slider The white point should remain at 255 And don t change the Ou
13. handle that will allow you to change the shape of the line Clicking and dragging the handle down will make the image darker especially in the fainter regions If you are using the provided TIFF images move the handle down so the X value is 31 and the Y value is 17 The X and Y values are shown in the upper left of the curves window Preview Help Reset Cancel OK This will darken the sky background but it will also have the undesire able affect of making the faint nebulosity on the top too dark We must add another handle to the line Click on the line again this time near the The curves window in GIMP after center of the image Move the handle up so that it is at or slighly higher all three adjustment points have than the line This will have the effect of keeping the background darker been added but will make the faint nebulosity brighter For the provided images the handle should be set so the X value is 120 and the Y value is 124 e Finally we need to darken the bright regions in the nebula at the center of the image Click on the line near the upper right corner to produce a third handle Move the third handle down until you can see the detail in the central nebula more clearly For the provided images the handle should be set so the X value is 206 and the Y value is 186 The curves window should now look close to what is shown to the bottom right e Click OK Rev 25 Jan 2012 Image Tutorial 9
14. is which 4 Image Tutorial O Layers Ch ngc6751_hatiff 1 S D gt PEE a Opacity 100 0 8 Lock Bi oes Ouse He Bru shes Circle 11 13 x 13 The layers window in GIMP after the background layer has been renamed Rev 25 Jan 2012 e Currently GIMP considers this image to be grayscale and therefore disables the functions related to color To convert the image into a color image choose Image Mode RGB Note This won t change the appearance of the image It simply prepares the image so that it can be make into a color image later on e Now save the tutorial image as a separate file Choose File Save As to save the image as a Photoshop image In the dialog box choose the directory into which you want to save the image Name the file ngc6751_demo psd Make sure the format the setting on the bottom right is Photoshop image psd ece e X ngc6751_demo psd 2 0 RGB 1 layer 3108x2304 GIMP File Edit Select View Image Layer Colors Tools Filters Windows Help 500 1000 1500 2000 The image should look roughly as shown above We will scale and colorize the image later Opening the Other Datasets Now it is time to open the other datasets and copy them into the main image file Open the other TIFF images with GIMP e Choose File Open As Layers and select ngc6751_03 tiff e In the layers window rename the layer to OIII The Ho and ON
15. ity color images of astronomical objects in a purely digital form One particular advantage of image processing software such as GIMP is that they use a layering metaphor for assembling images In this system individual grayscale images which are stored as separate layers may be combined to form a single image Before this color images could only be assembled from only three grayscale images And each grayscale image could only be assigned to the channel colors of red green and blue With the layering metaphor it is possible to combine any number of images And each image may be assigned any color you like The layering technique has been used to produce astronomical images with as many as eight different images Each image is produced from a different dataset each of which shows different details in the astronomical object Such images are therefore richer in color and detail This tutorial explains how its done The steps followed to produce a color image can be summarized as e Convert each FITS dataset into a grayscale TIFF image using the FITS Liberator e Open each TIFF image with GIMP The image generated from each dataset will be stored as a separate layer in a single GIMP image e Rescale each layer to maximize the contrast and detail in the interesting parts of the image e Assign a different color to each layer e Fine tune the color balance of the image e Remove any cosmetic defects Each step is described
16. mes consist of the name of the object an underscore and then the filter name For example the ngc6751_ha FITS file is the data for NGC 6751 in the H alpha filter Image Tutorial 1 Description of the lmage Making Process When people see astronomical images they often ask Is that what it really looks like The answer is always no This is because our telescopes are look ing at objects that are usually too faint and often too small for our eyes to see Furthermore telescopes can also see kinds of light that our eyes cannot such as radio waves infrared ultraviolet and X rays In a sense telescopes give us super human vision Indeed that s why we build them It is important to note that telescopes work differently than a traditional camera The instruments on our telescopes are designed to detect and measure light We must then take the data that comes from the telescope and translate it into an image that our eyes can see and our minds can understand Astronomical images are made in a very different way than they were even just twenty years ago Instead of film or photographic plates we use electronic instru ments such as CCD charge coupled device cameras In addition to their better sensitivity these electronic devices also produce data that can be analyzed on a computer Finally thanks to the tremendous leaps in computing power and in image processing software it is now possible to produce high qual
17. nter to 8 bit Click the Save As button and save the file as ngc6751_ha tiff Jt SLATE SE I S ADA ELIS LIDETATOT S o aaarnas Se Metadata FITS Header Help Guide Image 1 Plane 1 ind e Repeat the above steps for the OINI and SII images Again when moving the black point and white point sliders choose settings so that there are few blue and green pixels in the image Loading the Images into GIMP You should now have three TIFF files one for each dataset If you want your final image to look exactly as shown in the tutorial you may use the TIFF files inclued with the tutorial Note when opening the pre packaged TIFF files you will see a warning that GIMP has converted the image to 8 bits per channel since GIMP cannot process 16 bit files These were generated using the above instructions We will now open the three TIFF images with GIMP Before we import the others we must prepare this image to become the main image file Prepare the main image file e Start GIMP Use File Open and select ngc6751_ha tiff e If it is not visible already choose Windows Dockable Dialogs Layers to make the layers window visible A single layer should be present titled background as shown on the right e In the layers window double click on the layer titled background You should now be able to edit the layer name Rename the layer to H alpha This will be useful later for keeping track of which layer
18. rt the Ha filter file of NGC 6751 with FITS Liberator e Launch FITS Liberator Choose File Open andselect ngc6751_ ha fits data file The window should look similar to shown above If you are unable to select the ngc6751_ha fits file or if the above window does not appear you may not have the FITS Liberator properly installed Note that there are gray edges on the left and right sides of the images These aren t important to us and we ll trim them off in GIMP later e If it is not set to it already change the stretch function to Log x e On the right side of the image make sure that the Undefined red White clipping green and Black clipping blue check boxes are all selected Make sure the flip image and freeze settings options are not selected e You should now see blue and green pixels on the image Move the black level slider under the histogram to the left until most of the blue pixels in the image are gone It is ok if you have a small sprinkling of blue pixels through the image or blue pixels along the edge of the image e Move the white level slider also under the histogram to the right until most of the green pixels in the image should now be gone It is ok if you have a small sprinkling of green pixels through the image or green 1 This is because 28 256 In a 16 bit image 65 536 2 values are available Nomenclature Clipping refers to the pixel values that
19. s how these tools are used That s it Our image is now essentially complete You can save the image into any of several image formats e g JPG TIFF This tutorial shows you nearly all of the steps that were used to produce the original image released by Gemini Observatory Final cosmetic cleaning and color balancing are the reason why this image does not look exactly like the released image 10 Image Tutorial The angles around the color wheel correspond to hue values Rev 25 Jan 2012
20. tput Levels If you re using the provided TIFF files the window should look as shown in the figure on the right Click OK Next we will assign a yellow color to the H alpha layer This is done with the Colorize tool Assign a yellow color to the H alpha layer Select the H alpha layer by clicking it It should be the one highlighted Choose Colors Colorize Awindow will appear that has three sliders for hue saturation and lightness The hue slider will determine what color they layer will be The numbers correspond to an angle on the color wheel between 0 and 360 see the figure on p 10 Set the hue to 60 The image will turn yellow Set saturation to 100 and lightness to 50 This will make the image dark yellow The adjustment window will look as shown to the lower right Click OK Now its time to rescale and colorize the OHI and SII layers by following the same steps as described above for the H alpha layer Scale and colorize the OII layer Turn off the visibility of the H alpha layer The eye next to the H alpha layer should disappear Select the OII layer and turn on its visibility An eye should now be visible next to the OII layer Choose Colors Levels Like you did for the H alpha layer set the black point to near the left edge of the peak in the histogram And set the gray point slider to near the foot of the histogram on the right side If you re using the provided
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