Stitching A Lunar Panorama

Astrophotography today is becoming more and more about taking a series of individual images and assembling them into something else. This can be done in two sorts of ways. Very often, astophotographers will take repeated shots of the same area of the sky, and then use software to “stack” them, which has the effect of greatly reducing noise in the final image. As many of the objects in space are extremely faint, long, noise-free images are necessary to find them at all.

It’s also possible to make images of large areas of the sky by taking individual photos and joining them together, or stitching them. This can be a very effective way of using small cameras to produce large, high-resolution images. Despite the trend in some circles of seeking ever-bigger sensors dSLR cameras, stitching means that small cameras – like a web cam – can be used to great effect.

On the night of 16 August 2013 I was doing some observing and photograph form my west-facing balcony in Hamilton, and decided to try to make a large portrait of the moon using a small camera. I tried imaging the moon with several different cameras that night. My telescope was my Meade 125 Terabeam Maksutov Cassegrain catadioptric telescope mounted on an EQ6Pro mount with Synscan. Because of my balcony’s situation, it’s impossible to do an actual polar alignment; I can’t see Polaris. I can get to within a degree or so, which is good enough for taking a few images of bright objects like the moon.

The moon was lovely that night. After a few minutes of general moongazing, I mounted a Nikon D800 body on the telescope at prime focus, and was able to capture images of the whole moon’s face in one shot.

d800

A photo of the moon taken on the evening of 16 August 2013 from an apartment balcony in Hamilton, Ontario, using a Nikon D800 camera on a 125mm Maksutov Cassegrain catadipotric telescope.

I then changed the camera, replacing the big dSLR with a little imager sold specifically for lunar and planetary imaging, a Celestron NexImage 5. This little camera looks like a miniature hockey puck about 2″ across. Inside is a 5 megapixel colour image sensor, which attaches to a computer via a USB cable. In contrast to the very expensive Nikon camera, the NexImage 5 retails for under $200. Because the pixels are quite small compared to the dSLR, it is able to record images of much higher resolution on subjects like the moon, for a given magnification. It is able to do a variety of things, including recording AVI video files that can be used in the stacking process.

Lunar tile

One image captured by the NexImage 5 camera, including the famous craters Tycho and Clavius.

I decided to try making an image of the moon using single exposures with the NexImage 5. I set up the telescope to point at an area along the terminator of the moon, took a single image with the camera, and then manually moved the telescope so that it was pointing to an area that overlapped the first image by about 30 percent. In this fashion I worked my way across the entire surface of the moon, taking 41 individual images.

Having the pictures recorded was just the first step. The camera software had recorded the pictures as bitmap files (*.bmp), and so I used a free image processing software to convert them from bitmap to JPG files. I then used another free software package called Autostitch (downloaded free for personal use at: http://www.cs.bath.ac.uk/brown/autostitch/autostitch.html) to automatically bring all of the JPGs together, align them, and blend them together. I ran the program first on about a dozen overlapping tiles to see if it would work at all: Autostitch is designed for landscape photos – not moonscapes!

pano1 800px

A section of the lunar panorama made by using Autostitch to put together a dozen individual tiles.

Autostitch performed better than I could have hoped on my lunar frames! I re-ran the program using the whole set of 41 tiles, and in moments had my portrait of the moon in high resolution.

stitched

The moon, stitched together from 41 individual frames using Autostitch.

The final stage in assembling the image was to fill in the grey areas that were not actually captured by the imager.  I used another free software package, Paint.NET (not as capable as Photoshop but a whole lot less expensive!) to adjust contrast and brightness a bit, and paint in the grey areas with black. The results would be even better if each of the tiles had been prepared by the stacking process, which can result in dramatic improvements in such images. For now, I’m happy with yet another photo of the moon!

final

The final composite image of the moon, captured on 16 August 2013.

Copyright © 2013 David Allan Galbraith
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