Example 7 (Optional): Create Stacked images of NGC 3766

The previous examples guided you through the analysis of a representative set of star clusters. You learned how to make effective use of Skynet’s Afterglow software to photometer astronomical image data and to create tri-colour images. And you learned how to use Skynet’s Clustermancer tool to analyse star cluster data to determine physical properties like a cluster’s age and metallicity. Along the way, you also learned about the basic physical principles astronomers use to explain the various features and trends we observe in cluster HR diagrams.

In short, you learned nearly everything you need to know about stellar evolution, cluster evolution, and the various aspects of astronomical data analysis that form the foundation astronomers need to be able to study star clusters.

But with regard to practical data analysis there is one aspect we skipped over at the beginning that is essential when using real telescope images to extract photometric data and create colour images: aligning and stacking images.

These are straightforward procedural steps, so Example 1 began by giving you a set of images that were already aligned and stacked. However, if you plan to observe your own star cluster with Skynet and combine multiple images to reduce noise and improve data quality, you will need to know how to do these steps as well.

In this example, we will therefore begin with the raw image data used for the NGC 3766 examples above. By following the steps below, you will be able to create your own BVRI images that will be identical to the ones you began with in Example 1.

1. Open Afterglow Access in a separate window and login. Close any images you currently have open in your Afterglow Workbench. Open all images (40 files) in the “Sample > MWU > Module 5 – HR Diagrams > NGC3766 > individual” directory.
2. Select the Cosmetic Correction tool (the “magic wand”). Select all images and click Submit. This will automatically remove artefacts, such as bad columns, due to imperfections in the CCD.
3. Select the WCS Calibration tool. Select All images and use Platesolve mode. Click on any image in the set and click Extract values from current viewer, then Submit. It will take a minute or so for your WCS calibration job to complete, updating the astrometric solution for each of your images.
4. While WCS processing will successfully complete for this sample data set, if with your own data set you find at this point that one or more of your images fails to solve you should identify those images and close them, as they will not be able to be stacked with the rest.
5. Select the Aligner tool from the right-hand menu. Select all images to align, and choose a good looking image (such as the top one in the data set) to serve as your reference image file.
6. Do not select Mosaic mode, but leave the remaining settings selected as they are by default. Click Submit.
7. Finally, to create stacked images with noise significantly reduced, select the Stacker tool from the right-hand menu. Here, you want to stack only the images taken with the same filter. In this case, images were taken with B, V, R, and I, so we will make four stacked images, identical to those you used in Example 1.1 above. To start with the B filter, type “B” in the filter list above the list of images, then select all images to stack.
8. The simplest method is to create median stacks, which calculate median values for each pixel from the corresponding pixels in the individual images. Therefore, change the mode to “percentile” and leave the value at 50.
9. Under Smart Stacking, choose “SNR”. This will ensure only images that increase the signal-to-noise ratio of the resulting image stack are used. Leave the remaining settings with default values and click Submit.
10. Repeat steps 7-9 for V, R and I, but when filtering the I-band images note that you should type “I_” since the letter “i” appears in “fits” in each image’s filename.
11. Once you’ve created all four stacked images, in Filter list type “stack”, then select all files and Save Selected Files twice. Open your Workspace, click New Directory and name the folder “NGC 3766”. Double-click the new folder and click Save, then Auto-save to same location.
12. You are now right back at the start, with four stacked images of NGC 3766 open and ready for analysis. Except at this point you’ve already done all that analysis, so you should probably close these files and follow the steps above to create stacks of your own cluster images. Then you can go back and follow the analysis steps you learned in Examples 1-6 above to analyse your cluster images!