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1. M33 Tutorial Making Color Images Travis A Rector University of Alaska Anchorage and NOAO Department of Physics and Astronomy 3211 Providence Dr Anchorage AK USA email rector uaa alaska edu Introduction A Note from the Author The purpose of this tutorial is to demonstrate how color images of astronomical 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 profes sional observatories such as the Hubble Space Telescope Kitt Peak Gemini and Spitzer In this tutorial we will make an image of M33 the Triangulum Galaxy M33 is a spectacular face on spiral galaxy that is relatively nearby 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 author at the email address above Prerequisites To participate in this tutorial you will need a basic understanding of the follow ing concepts and software e FITS datafiles e Adobe Photoshop or Photoshop Elements e The layering metaphor Description of the Data The datasets of M33 used in this tutorial were obtained with the WIYN 0 9 meter telescope on Kitt Peak which is located about 40 miles west of Tucson Arizona The FITS files are 2048 x 2048 pixels and about 16 Mb in size each Datasets in the broadband UBVRI and narrowband Ha filters are available For this tutorial we will2. back to 10 The image should be brighter the Black Level should be 0 00 and the White Level should be 3 00 The window should look like what is shown below e If you are using Photoshop 7 0 or Photoshop Elements change the Chan nels bottom center to 8 bit Otherwise leave it as 16 bit e Click OK and the image will be imported into Photoshop Layers X Channels Paths The ESA ESO NASA Photoshop FITS Liberator m33_b fits 2 Ey aio l HH Opacity D Processing _ Metadata FITS Header ox Normal P j 100 cy Image 1 Plane 1 He Jp Ji shil al Fill F R Cancel Lock E j a 100 X Image size L L oe Width 2047 px Height 2046 px Reset F 7 E log 1000 d Image data aa v Preview _ Freeze settings Bg 0 px Input 0 00 A Pg Y O px Stretched 0 00 Mark pixels in preview Image statistics Undefined red Input Stretched aA White clipping Min 51564 70 1 90 green Black clipping Max 59353 09 3 00 A lie Mean 318 03 0 36 Median 300 46 0 34 Show image information for STDEV 418 30 0 15 Stretched Scaling and stretch Advanced f4 Scaled 17 74 F Stretch function Log x B 1 90 3 00 Background level 225 29 a l Peak level 59353 09 Scaled Peak level 1000 00 d Auto Scaling Channels _ Undefined values O 8 bit Black z 16 bit Transparent
3. Caution If you double click on the layer mask thumbnail the white square to the right of the layer thumbnail a Layer Mask Display Options window will appear If this occurs click Cancel and try again SOx E Doc 24 0M 71 9M gt Rev 4 1 08 Image Tutorial 11 The colors are now more in balance The image may also be improved by adjusting the black and gray points gamma correction of the level adjustment layers Because this image uses only three datasets the image can be vastly improved by using the additional M33 datasets These should be imported rescaled and colorized in the same manner Finally there are many additional tools that are available in Photoshop e g the curves tool that enable the layers to be manipulated more precisely For simplicity these methods and tools are not discussed here As always this is an excellent time to save the image Fine Tuning the Image The image is mostly done but there are still several steps necessary to complete the image One issue that has not yet been addressed is the alignment of the image layers Because these images were taken with the same telescope and camera and on the same night one after another the images are already very closely aligned However if you zoom in on the image say to 500 as shown on the right you will notice that the stars are not perfectly aligned To align the images use the move tool to shift each image layer so that t
4. layer by clicking on it Choose Layer Create Clipping Mask The adjustment layer will indent and an arrow will point down to the B band image layer Finally we will assign a color to the layer This is done with the Hue Satura tion adjustment layer Assign a blue color to the B band layer e Select the Levels layer above the B band layer by clicking it It should be the one highlighted e Choose Layer New Adjustment Layer Hue Saturation A New Layer dialog box will appear Click OK and a new window will appear A window will appear that has three sliders for hue satura tion and lightness Rev 4 1 08 Note In Photoshop Elements the imput levels will be above the histogram in the dialog box to the left Note In Photoshop Elements the menu option is called Layer Create Clipping Mask You can also select this option in the levels dialog box Image Tutorial 8 Before moving any of the sliders click on the Colorize checkbox in the lower right corner of the window The image will go red Once the colorize checkbox is selected 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 to the right Since this is the blue filter set the hue to 240 The image will turn blue Set the saturation to 100 and the lightness to 50 This will make the image a dark deep blue The window should look as sho
5. to save the image Name the file m33_demo psd Make sure the format is set to Photoshop and the Layers box is checked Don t worry about the other check boxes Rev 4 1 08 Image Tutorial 4 e 08a BB m33_demo psd 50 B log 1000 RGB 16 The image should look as shown above We will fine tune the intensity scaling later Importing the Other Datasets Now it is time to import the other datasets and copy them into the main image file We will start with the V band dataset Import the V filter file of M33 with FITS Liberator e Choose File 0pen and select m33_v fits data file The FITS Liberator window should appear Make sure that the Undefined red White clipping green and Black clipping blue check boxes are all selected e Change the stretch function to Log x e Move the black level slider so that the Black Level is 2 54 Most of the blue pixels in the image should now be gone e Move the white level slider so that the White Level is 4 79 Most of the green pixels in the image should now be gone e Click the Auto Scaling button The image above should now appear nearly black To get the same contrast as the B layer change the Scaled Peak level to 1000 and hit return remember do not click the Auto Scal ing button again e If you are using Photoshop 7 0 or Photoshop Elements change the Chan nels bottom center to 8 bit Otherwise leave it as 16 bit
6. use only the B V and Ha datasets But you are encouraged to play with the other datasets afterwards These datasets are provided to be used strictly for educational purposes They may not be redistributed without permission If you wish to use these files for other purposes please contact the author About the Software This tutorial requires Adobe Photoshop version 7 0 and later or Adobe Pho toshop Elements version 2 0 and later Both run on the Macintosh OS 9 and OS X and PC Microsoft Windows platforms In this tutorial Photoshop CS3 will be used but it is very similar for Photoshop Elements Differences between the two programs are occasionally noted in the sidebar Rev 4 1 08 The WIYN 0 9 meter Telescope Nomenclature FITS stands for Flexible Image Transport System It is the stan dard format for storing astronomi cal data Decoding file names The FITS filenames consist of the name of the object an underscore and then the filter name For example the m33_ha FITS file is M33 in the H alpha filter Image Tutorial 1 This tutorial also requires the ESA ESO NASA FITS Liberator It is a plug in for Photoshop and Photoshop Elements that enables these programs to import FITS data files The current version is v2 1 The FITS Liberator is free and can be downloaded at http www spacetelescope org projects fits_liberator Amore in depth instruction manual on all of the capabilities of the FITS Liberator is al
7. 5 arr 32 bit Black Level 0 00 o White Level 3 00 Preparing the Main Image File You should now see a grayscale image of M33 This is only one of the three images we will import into Photoshop Before we import the others we must prepare this image to become the main image file A l l ol alal Prepare the main image file Ifitis not visible already choose Window Layers to make the layers me leyete new nora a i after the background layer has window visible A single layer should be present titled background eem eared e In the layers window double click on the background layer A dialog box titled New Layer will appear Rename the layer to B log 1000 This will be useful later for keeping track of which layer is which Currently Photoshop considers this image to be grayscale and therefore disables the functions related to color Choose Image Mode RGB Color to convert the image into a color image 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 Note You may also see an extra dialog box that asks you if you wish to a er maximize the compatibility of the e Now itis time to save the tutorial image as a separate file Choose File file image If yousee this go ahead ave AS to save the image as a Photoshop document In the dialog and select yes box choose the directory into which you want
8. 51564 70 Max 59353 09 59353 09 Mean 318 03 318 03 Median 300 46 300 46 STDEV 418 30 418 30 White clipping 4 gree Black clipping blue information for Stretched Scaled Scaling and stretch Advanced 17 74 P Stretch function Linear i is 51564 70 59353 09 Background level 0 00 a Peak level 4371 14 A Scaled Peak level 4371 14 i amp Auto Scaling SS gt a Channels Undefined values 8 bit Black Cesa 7 16 bit Transparent l gt 32 bit ES Black Level 43 38 White Level 4371 14 5 oO D Launch Adobe Photoshop Import the B filter file of M33 with FITS Liberator Choose File Open and select m33_b fits data file A window should appear that looks similar to the one shown above If you are unable to select the m33_b fits file or if the above window does not appear you may not have the FITS Liberator properly installed Change the stretch function to Log x 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 You should now see blue and green pixels on the image Move the black level slider under the histogram so that the Black Level shown in the lower left corner is 2 35 Most of the blue pixels in the image should now be gone Move the white level slider also under the histogram all the way to the right so that the Whit
9. e Click OK and the image will be imported into Photoshop e In the layers window rename the layer to V log 1000 Rev 4 1 08 Image Tutorial 5 The V band grayscale image now needs to be copied into the main image file Choose Layer Duplicate Layer and a dialog box will appear Change the destination to m33_demo psd and click OK Now the V band image is copied into the main image file so we can get rid of this copy Choose File Close and Photoshop will ask you if you wish to save the image Click on Don t Save The B band and V band grayscale images are now loaded into the main image file each as a separate layer Finally import the Ha image and duplicate it into the main image file Import the Ha filter file of M33 with FITS Liberator e Choose File Open and select m33_ha fits data file The FITS Liberator window should appear Make sure that the Undefined red White clipping green and Black clipping blue check boxes are all selected e Change the stretch function to Log x e Move the black level slider so that the Black Level is 1 47 Most of the blue pixels in the image should now be gone e Move the white level slider so that the White Level is 3 70 Most of the green pixels in the image should now be gone Click the Auto Scaling button Change the Scaled Peak level to 1000 and hit return do not click the Auto Scaling button again e If yo
10. e Level shown at the bottom is 4 77 Most of the green pixels in the image should now be gone 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 to 1000 and hit return do not click on This is because 28 256 In a 16 bit image 65 536 21 values are available Rev 4 1 08 Nomenclature Clipping refers to the pixel values that 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 Photo shop 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 Image Tutorial 3 the Auto Scaling button again because it will reset the scaled peak level
11. egions is decreas ing The number below the gray point known as the gamma correction should be about 1 32 This is done to improve the contrast in the center of the galaxy e Donot move the white slider The white point should remain at 255 And don t change the Output Levels The window should look as shown Levels Channel i RGE HH fo 9 Input Levels a Load Save Aute Y E j o A e B 1 32 255 Options Output Levels Fj Fd S a b s S v Preview o 255 e Click OK and a new layer will appear above the B band layer The new layer is called an adjustment layer It does not consist of an image rather it adjusts the appearance of the layers below it You can see the effect of the adjustment layer by toggling the visibility of the layer 1 e clicking on and off the eyeball for the adjustment layer The effect of the adjustment layer is that it improves the contrast in the center of the galaxy while keeping the outer edges of the galaxy from getting too bright An adjustment layer will affect all of the layers below it The Levels 1 adjust ment layer must therefore be grouped to the B band layer so it only adjusts it Note For the B band image layer this is not a problem because there are no other layers below it but it will be an issue for the other image layers Select the Levels 1 adjustment
12. f the steps described inthis process are subjective They are based upon the color calibration of the author s computer display and his 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 author s excellent aesthetic palette Rev 4 1 08 Importing Datasets with FITS Liberator Adobe Photoshop by itself cannot open FITS files This is because FITS is a data file format not an image format The FITS Liberator is a plug in that con verts the pixels in a FITS file into pixels for an 8 or 16 bit grayscale image In a 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 The ESA ESO NASA Photoshop FITS Liberator m33_b fits Processing _ Metadata FITS Header Cancel Reset v Preview __ Freeze settings Image 1 Plane 1 Hy Image size Width 2047 px Height 2046 px Image data X 0 px Input 0 00 Y 0 px Stretched 0 00 Mark pixels in preview Image statistics Mi Undefined red Input Stretched Min 51564 70
13. he color wheel again shown on the right To start we ll try by adjusting the color assigments to each layer Adjust the color assignments to the B V and Ha layers ar e Leave the visibility of all of the layers on as you do this Double click on the layer thumbnail for the B band adjustment layer to bring up the hue saturation settings for this layer e To increase the red in this layer the hue value needs to be moved more to the red This is done by increasing the hue value Change the hue value from 240 to 260 Click OK The angles around the color wheel e Toincrease the red in the V band layer the hue also needs to be moved more correspond to hue values to the red In this case this is done by decreasing the hue value Double click on the layer thumbnail for the V band adjustment layer Change the hue value of the V band layer from 120 to 100 Click OK e The center of the galaxy now looks a little too red It should be more yellow Double click on the layer thumbnail for the Ha adjustment a layer To add more yellow to the image change the hue value of the Ha Norma fag Opacity 100x ja layer from 0 to 15 Click OK The image should now look as shown a Slalels Gone ck Fill 100 below _ eed p Ar A lag Hue Satur L I OOA m33_demo psd 50 Hue Saturation 3 Layer Mask 16 Layer thumbnail Layer mask thumbnail
14. hey are aligned with one another Note Shifting the adjustment layers will have no effect Select the Ha layer by clicking on it Choose the move tool and use the arrow keys to shift the layer until it is well aligned with the B band layer Now select the V band layer and shift it as well so that it is aligned with the other two Zooming will also reveal that there are numerous dots and small streaks in the image These are caused by cosmic rays that hit the CCD detector during each exposure 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 V band image look green In the case of this image nearly all of the cosmic rays are in the V band image These cosmic ray specs and scratches can be fixed in a variety of ways in Photoshop A simple option is to use one of the noise filters e g the dust and scratches filter These can remove the cosmic rays but they also blur the image A more effective but time consuming method is to use the cloning stamp and healing brush tools to remove the cosmic rays one by one The user s manual for Photoshop explains how these tools are used Rev 4 1 08 Zooming into the image to 500 shows that the stars are not aligned Use the move tool in Photoshop to align the image layers Image Tutorial 12
15. 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 Rev 4 1 08 Layers Channels Paths Lock otla i Eo ha log 1000 V logi1000 B logil00d The layers window in Photoshop after the V band and Ha images has been duplicated into the main image file Image Tutorial 6 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 as shown on the next page e208 E m33_demo psd 50 ha log 1000 RGB 16 If you have not done so recently this is an excellent time to save the image 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 B band layer e Turn off the visibility of the V and Ha layers There should only be an eyeball next to the B layer e Select the B layer by clicking it It should be the one highlighted e Choose Layer New Adjustment Layer Level
16. r Click OK and the hue satura tion window will appear Click on the Colorize checkbox Since the V band filter is green set the hue to 120 Set the saturation to 100 and the lightness to 50 This will make the image a dark deep green Click OK and then group the hue saturation adjustment layer by choosing Layer Create Clipping Mask Rev 4 1 08 The angles around the color wheel correspond to hue values 5 Layers Channels inl Paths a ha log 1000 log 1000 Hue Saburat_ gt ae r et iaki The layers window in Photoshop after the B band layer has been rescaled and colorized The Ha and V layers have not yet been rescaled and colorized Image Tutorial 9 Rescale and colorize the Ha layer e Turn off the visibility of the V band layer Select the Ho layer and turn on its visibility ee 7 e Choose Layer New Adjustment Layer Levels A New _ fae Opacity 100 Layer dialog box will appear Click OK and the levels window will Lock 5 ia Fill 100 appear Set the black point to 61 and the gray point to 1 05 Do not A move the white point Click OK and then group the levels adjustment layer by choosing Layer Create Clipping Mask Choose Layer New Adjustment Layer Huer Saturation A New Layer dialog box will appear Click OK and the hue satura tion window will appear Click on the Colorize checkbox Since the Ha fil
17. rticular advantage of image processing software such as Photoshop is that they use a layering metaphor for assembling images In this system indi vidual 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 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 Import each dataset to be used into Photoshop using the FITS Liberator The image from each dataset will be stored as a separate layer 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 Align the image layers and remove any cosmetic defects Each step is described in detail below Note In this document an H icon appears when a computer command is described 2 Image Tutorial Note Many o
18. s A New Layer dialog box will appear Click OK and a new window will appear A histogram will be shown in the Input Levels area of the window Like in the FITS Liberator you can use the sliders underneath the histogram to move the black and white points You ll also see a gray slider that adjusts the gamma correction e Move the black slider a little to the right As you move the slider you Il Rev 4 1 08 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 Image Tutorial 7 notice the image gets darker Move it until the number below the black point is 8 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 e Move the gray slider a little to the left As you move the slider you ll notice the image gets a little brighter Specifically the contrast in the faint regions is increasing and the contrast in the dark r
19. so available at that URL Be sure that this plug in is properly installed before starting the tutorial 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 looking at objects that are usually too faint and often too small for our eyes to see Fur thermore our 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 very 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 quality color images of astronomical objects in a purely digital form One pa
20. ter is red set the hue to 0 As before set the saturation to 100 and the lightness to 50 This will make the image a dark deep red Click OK and then group the hue saturation adjustment layer by choosing Layer Create Clipping Mask Hue Satur Each image has now been rescaled and colorized Make sure that each adjustment layer is grouped to the image layer below it To see how the overall image looks turn on the visibility of the B band and V band layers The layers window should look as shown on the right and the image should look as shown below eA a E m33_demo psd 50 Hue Saturation 3 Layer Mask 16 a A alal The layers window in Photoshop after all three layers have been rescaled and colorized Its not bad but the galaxy has too much cyan This is because the B band and V band filters primarily show the stars in the galaxy whereas the Ha filter pri marily shows the warm hydrogen gas in the galaxy where stars are forming Thus the stars will look cyan which is a mix of blue and green Cyan is on the Rev 4 1 08 Image Tutorial 10 opposite side of the color wheel as red Thus to bring the image better into balance we need to either decrease the cyan or increase the red This can be done by changing the setting in the levels and hue saturation adjustment layers There are so many options that can be changed it is confusing How to do this is more easily understood if you think in terms of t
21. u are using Photoshop 7 0 or Photoshop Elements change the Chan nels bottom center to 8 bit Otherwise leave it as 16 bit e Click OK and the image will be imported into Photoshop e In the layers window rename the layer to ha log 1000 Choose Layer Duplicate Layer and a dialog box will appear Change the destination to m33_demo psd and click OK Now that the Ha band image is copied into the main image file we can get rid of it Choose File Close and Photoshop will ask you if you wish to save the image Click on Don t Save All three grayscale image should now be in the main image file The layers window 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 ha log 1000 layer by clicking on it Change the blending mode from Normal to Screen Change the modes for the other layers as well by selecting each layer and changing its blending mode The opacity of each layer should be kept at 100 Note Technically you don t need to change the blending mode of the bottom layer but you might as well do it anyway in case you change the order of the layers The image will be
22. wn below Hue Saturation Edit Master Hue 240 l Cancel Load Saturation 100 l o Saven Lightness 50 zl Colorize ar a PIa Fi Br Preview Click OK Anew adjustment layer titled Hue Saturation 1 will appear above the Levels 1 layer Select the Hue Saturation 1 adjustment layer by clicking on it Choose Layer Create Clipping Mask The adjustment layer will indent and an arrow will point down to the Levels 1 image layer Since the Levels 1 and Hue Saturation 1 adjustment layers are grouped to B band layer they will only adjust that layer The layers window should look as shown to the right Now its time to rescale and colorize the V band and Ha layers by following the same steps as described above for the B band layer Rescale and colorize the V band layer Turn off the visibility of the B band layer Select the V band layer and turn on its visibility Choose Layer New Adjustment Layer Levels A New Layer dialog box will appear Click OK and the levels window will appear To darken the background and increase the contrast in the faint parts of the galaxy set the black point to 35 and the gray point to 1 23 Do not move the white point Click OK and then group the levels adjustment layer by choosing Layer Create Clipping Mask Choose Layer New Adjustment Layer Huer Saturation A New Layer dialog box will appea
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