The colour images you’ve created so far using optical BVR filters capture the light that human eyes are sensitive to — but stars and nebulae emit light across a much broader range of wavelengths than the visible spectrum alone. Infrared light, for instance, penetrates interstellar dust more easily than optical light, revealing stars that would otherwise be hidden behind dense clouds. And certain narrowband filters isolate specific emission lines — such as Hα (hydrogen-alpha) and OIII (doubly ionized oxygen) — that trace the glowing gas in star-forming regions with extraordinary clarity. The first image on the opening page of this activity, IC 2948, was created using exactly this approach: BVR optical data combined with Hα and OIII narrowband data, which is what gives the nebula its rich, detailed structure and vivid red and blue tones. Without those additional layers, the image would show the stars clearly but the nebula would appear far more washed out.
In this optional example, you will learn how to supplement your own Skynet optical images with publicly available infrared data from NASA’s all-sky surveys — which are publicly available through NASA’s SkyView website — and blend them into your colour composite in Afterglow. The result is an image that reveals structure invisible to optical telescopes alone, and which can be genuinely striking for clusters embedded in or near regions of gas and dust. The steps below will show how you can add in WISE 12 and WISE 22 micron images, along with 2MASS K.
- Type your star cluster’s name into the Coordinates or Source search box. Then scroll down and select a survey, e.g. 2MASS K. Then scroll to Common Options.
- Next, you need to determine your field and pixel size so the FITS file you generate can be set to similar values as your Skynet images. Go to the Skynet Telescope Site page for the telescope that collected your images (e.g. PROMPT 2, 5, 6, etc are at CTIO, PROMPT-MO-1 is at Meckering Observatory, PROMPT-USASK is at Sleaford Observatory) and click the link to the telescope you used.
- Set the Image size to 150% the size of your image, to ensure you get the entire region regardless of telescope pointing offsets. For example, if imaging with a camera that has 1024×1024 pixels, set Image size (pixels) to 1536.
- Image sizes are listed on the Skynet page in arcminutes, while Skyview asks for degrees, so take the image size from the Skynet telescope page, multiply by 1.5 to increase to 150%, then divide by 60 to convert minutes to degrees. For example, if your camera’s field is listed as 10.1’x10.1′, you would enter Image Size (degrees) as 10.1*1.5/60 = 0.2525.
- Set the Projection to Rectangular (Car). Submit Request.
- When the image loads on the next page, scroll down to the FITS link below and download the image by clicking the link.
- Repeat for any other images you want, e.g. WISE 12 and WISE 22. Click Clear Survey Selections at the top of the page, then click the survey and filter you want to download, then Submit Request again.
- Save all files to someplace accessible on your computer, renaming them to include the cluster name and filter in each filename.
- Go back to Afterglow and click the File folder near the top-left, then navigate to the Workspace folder you created with your stacked cluster images, then Upload and select the files you downloaded. If any files fail to upload, you may need to delete some files from your Workspace first.
- When your survey images finish uploading, close any files in your Afterglow Workbench and open all files from your Workspace folder, including your stacked images.
- Select the Aligner tool, select all images, and set one of your stacked images as the Reference Image file. Mosaic Mode should be turned off, and all other options should be selected. Use WCS mode with 100 grid points, and Submit.
- For color image processing select the Display tool. Select all images in your Workbench and group the files. Click through each image to refresh the image display. Select the top-level combined image file. Click Color Composite Tools > Link All Layers (Pixel Values). Now click through all image layers and note that all images but one will say Brightness and Contrast Settings are linked to that one different file. That file should be one of your BVR images. If not, change the linked image so it is either B, V or R.
- Again, click Color Composite Tools > Photometric Calibration. Ensure your red, green and blue layers are set to your R (or iprime), V (or rprime) and B (or gprime) broadband filters. Measure zero points with field calibration, set extinction to the value you determined through isochrone modelling, and Calibrate Colors with Neutralize background checked.
- Click the top-level combined image file. Right-click the files below and set B (or gprime), V (or rprime) and R (or iprime) to blue, green and red color maps. Click the “eye” symbol next to every layer but your red, green and blue layers to hide them for now. Set stretch mode to Midtone and click Default Preset, then adjust levels to your liking.
- Drag each of the additional infrared images in the files list so they are all above your red, green and blue files. Right-click each of them and set the Blend Mode to Lighten. (Leave the red, green and blue layers set to the default “Screen” blend mode). Regarding color maps, you might start with setting I-band, zprime and WISE all to Heat, and set 2MASS to Cool. Sometimes setting WISE 22 to Gray looks nice.
- For each of these additional images, in turn:
- keep the top-level combined image selected in your files list
- click the eyeball button to make the file visible
- under Display Settings on the right, at the top select the image layer you made visible
- from the Brightness and Contrast Settings drop-down, select Manual
- select Percentile Mode, then Default Preset, then adjust levels to your liking. Your aim here should be a black background not including nebulosity, and relatively unaffected star colours from the original red, green, blue image.
- Each time you add in a layer and manually adjust its levels, use the eyeball to turn it on and off to compare how it affects the underlying image.
- Once you have all layers added in, lightening your base combined color image, use Export Image as JPG to save.
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