Image Processing
Stitching together your panorama photos.
Most people think of astrophotography in terms of zooming in on tiny stars or galaxies. But I want to capture it all… literally. That’s why I create multi-pane panoramas. In this article we look at how to piece together those panes.
Once you’ve captured your images (see page 38) you need to merge them with as few errors and aberrations as possible. One of the most reliable pieces of software to do this is PTGui, which we’ll be using here.
First open your images in your preferred image editing software so that you can alter all the tones in every photograph to mid-range. Edit one photograph and then apply the same changes to all the other photographs. At this point we’re not worried about making the images pretty; we’re taking them to mid-tones so that PTGui can more easily match up their details. The principle is that you first create the panorama using the mid-tone images, then PTGui ‘records’ how it’s achieved the stitching together so that it can do exactly the same to the ‘real’ photos.
Export these mid-tone versions of the images with their meta data included so that PTGui can read the focal length used. Open PTGui and load in your mid-tone files using the Load Images button – the interface walks you through the process.
To the software stars all look pretty much the same, so it’s unlikely that PTGui’s automated stitching will be able to generate the final image without some help from you.
But press the Align Images button anyway and let the program have an attempt.
You have control
PTGui will probably be able to stitch together the foreground of the image but not the stars. In most cases it will report that there has been an error and invite you to review the ‘control points’. Control points are areas in different source images that coincide, and should overlap in the panorama. PTGui generates control points automatically but not always accurately. Stars are difficult for PTGui to recognise.
Instead you need to manually create your own control points. Click on one image and then click on the same star in the neighbouring images; repeat this process with three or four more stars to get a good spread of stars around the image.
If you were lucky enough to capture a meteor in one image and you want to use it in the final panorama you can mask it to ensure that it’s present in your final image. You do this by clicking on the Mask tab and selecting the Paint With Green option, then highlighting the detail you want to keep. You can also Paint With Red to hide details you don’t want appearing in the finished product. The image below left shows this technique being applied to car lights.
Once all this is done go to the Project Assistant and press the Align Images button. This time you should hopefully be presented with a completed image in a tab named Panorama Editor. Look for errors in the image. Sections of ‘double stars’ (normally on one axis) or distorted areas are common. To rectify this, open the Control Point Assistant and look at the Control Point Table. The larger numbers indicate control points that are least accurate; replace them with more accurate control points.
When you’re happy with the stitch of your image, it’s time to make the photo look pretty. Take your original source photos into your preferred image editing software and get them looking how you want them. Export them, then go back into PTGui and go to the Source Images tab; right click on each image in the left-hand panel and select Replace, then click on the corresponding ‘pretty’ image from wherever it’s saved on your computer. When you’ve replaced all the images you can click on Preview to see the stitched-together image.
At this point you can play around with different settings without fear of losing your work. At the bottom right-hand side of the window there are two sliders named Vertical and Horizontal Field of View. Leave the field of view wide and then crop the image later. Close the panorama editor.
Finally, export the image. It’s probably huge so it’s worth thinking about how you intend to use the image and how many pixels you really need, then scaling down your image to save your sanity. Finally select file type and press “Create Panorama”.