combining moon images

[ian straton]

Hi,
I have a sequence of images showing the occultation of jupiter at the weekend, I want to combine them to show a "stationary" moon with the image of jupiter appearing at various points along the way.  Normally I would do this in photoshop using layers but unfortunately there is significant field rotation between images and the rotate tool in PS can't cope with the centre of rotation not being the centre of the frame  Is there a way I can use PI to combine these images?  Failing that can anyone recommend an alternative way of processing these images?

Thanks
Ian

[IanL]

You could probably do this in PS with a bit of extra legwork.  Depends on whether you have the whole moon in each frame or just a portion of it.

a) If the centre of rotation is inside all of the frames, you just need to crop (or pad) the images to move it to the centre of each frame.  Use the rotation tool, and you should then be able to mask/layer to get reasonably good results.

b) If the centre is outside the frame, but you have the whole moon in shot, cropping each frame so that the centre of the moon is at the centre of each cropped image and then rotating each frame until the moon has the same orientation.  Should be easy enough with a bit of trial and error, maybe draw in a cross-hair through the centre of each moon on another layer to help find the centre in each frame.

c) If it is a partial moon (i.e. the planet and just part of the limb), it would be more tricky.  You'd need to do the opposite and pad each frame around the edges so that the centre of the moon ends up in the centre of the frame, then do the rotation.  What you might try is drawing a triangle between three prominent points in the image and the same on another frame.  A bit of trial and error with padding and the rotation tool to get the triangles to exactly overlap.

From a geometric standpoint, you can certainly rotate around different centres of rotation and arrive at the same orientation.

With a sampled image like this though, whatever rotation you apply you might get noticeable aliasing artefacts between frames.  It still might be okay as I assume you will be using just one of the moon frames as the 'foreground' and only the planet images would be affected, probably depends on the scale of the original image and your final result as to how noticeable the effects are.

That said, I am a bit surprised that you cannot specify a centre of rotation in PS other than frame centre; seems like a blindingly obvious feature and one that I am sure I have used in numerous cheap/free photo/painting type packages down the years.  Sadly not PS though, since I can't justify the entry fee.  Maybe someone who has can point the way?

[pfile]

maybe try the FFTRegistration script? it worked great for my eclipse timelapse.

[Carlos Milovic]

Yeah, you may try the FFTRegistration script, although the rotation and scaling algorithm do not work very well. Fourier based registrations are not very robust, and may fail in a lot of cases.

[Don]

Hi Ian,

Actually, you can set the center of rotation anywhere you want in Photoshop if you use the Transform tool.  First select the entire image, then select Edit ... Transform .... Rotate.  The center of rotation is a movable mark that starts in the center of the image.  You can drag it with the mouse anywhere in the image.  If you select less than the entire image with one of the marquee tools, you can even move the center of rotation outside of the selection.  And if you use the Free Transform tool after making a selection (Edit ... Free Transform), you can rotate and translate in the same operation.

Don

[oldwexi]

Hi Ian!
Actually, you can set the center of rotation anywhere you want in PixInsight if you use the Dynamic Crop tool.  The center of rotation is a movable mark that starts in the center of the image.  You can drag it with the mouse anywhere in the image.  You can even move the center of rotation outside of the selection...

Aloha
Gerald

[ian straton]

Thanks don, took a while to work out the right technique for this.. was surprised to find that the rotation frame is independent of the image so as I moved the image in x/y the centre of rotation within the image moved relative to the image itself!  having noticed that I am now able to align each image by hand, its taking a while as there are 24 frames to go through but it will be done soon!