Comets in 3D

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Ever since Dr Brian May’s presentation at Astrofest in February 2015, I have been wrestling with the idea of genuine 3D imaging of comets for viewing with one of Dr May’s stereoscopes.  Having presented my “proof of concept” to the Comet Section of the BAA in June (2017), it is now time to share this more widely and encourage others to join in.

For the purposes of this project, there are three types of stereoscopic image:

Type 1: Simultaneous imaging

This is surely the holy grail of 3D comet imaging, taking two images simultaneously from different locations.  It is also the only technique which renders a true 3D image.

The “simultaneous” constraint could be relaxed, as long as there is full disclosure of the time interval, but if relaxed too far then the image becomes effectively a Type 2(b) as described below.

Click to display full size…

These two images of 41/P were assembled from simultaneous LRGB stacks of 22 subframes of 120-seconds each taken from New Mexico and Spain.  The first 120-sec subframe of the right image started 29 seconds after the first 120-sec subframe of the left image.  That’s a 75% overlap – which I count as simultaneous!

Type 2: Delayed imaging

2(a) Two images taken from the same location, but with an appropriate interval between them. For example, a comet rising in the east at dusk will be setting in the west at dawn – if that’s eight hours later then the Earth will have rotated by one third, and – more importantly – will also have moved one third of one day in its orbit around the Sun.

2(b) Two images from different locations and taken at different times. A brief search among the images posted online suggests that there might already be many candidate images available.

These render as exciting stereoscopic images, but the 3D effect is not genuine.  The geometry of the Earth’s orbit and the Comet’s orbit are artificially realigned in the final presentation to produce a convincing impression of a 3D image, but these should be viewed with caution as the result is not a reliable representation of the comet and its tail or tails.

Type 2 images also carry the risk that a fast-moving comet will not occupy the same field of view in the left and right frames, so they cannot be presented with the same star background.

Finally, the tail could also change substantially, reducing the impact of the 3D effect.

Type 3: Synthetic 3D images

These are achieved by simply moving the comet from one image to the next in post-processing, creating an entirely synthetic 3D effect. The comet will appear as a 2D image in front of the stars, and the tail will have no depth. I’m not interested in these images.

 

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