Stretched stars

[Sam Gowan]

When I make Milky Way panoramas, I often end up with elongated stars even though the original images have round stars. It just occurred to me that when I straighten the panorama, it will have to stretch parts of the image and cause this, but this is based upon the original stitching/control point. Is there a way to prevent this by somehow regenerating the control points after I’ve stretched/straightened out the panorama? This should then use more of the individual images and not require any image to be stretched/distorted. I always shoot with lots of overlap, thanks

Sam

[Erik Krause]

Am 30.05.2025 um 20:45 schrieb Sam Gowan:

> It just occurred to me that when I straighten the panorama, it will
> have to stretch parts of the image and cause this, but this is based
> upon the original stitching/control point.

The stretching happens because of the projection, not because of the
control points. The world around us, viewed from a single viewpoint,
resembles the inside of a sphere. Hence, PTGui stitches the images
internally on a spherical surface. For output, this has to be mapped to
a flat image. But this is not possible without distortion. The wider the
angle of view, the more.

There are different projections, that try to unwrap parts of the sphere
more or less without distortions. F.e. cylindrical, mercator and
equirectangular projections have no stretching along the horizon, but
are heavily stretched the further you look up or down. There are some
mixed projections, like stereographic or vedutismo, that try to minimize
stretching in other directions, too. But there is none that introduces
no stretching at all.

PTGui allows choosing between different output projections in the
Projection side menu in Panorama Editor. Just try which one suits you best.

--
Erik Krause
http://www.erik-krause.de

[Jesús Navas]

It is not only due to overlapping. I suggest using a longer focal length. The longer, the more images you need to cover the same field of view, the smaller the size of the stars, and the less distorted each image will be. Of course, this is not always possible or desirable for logistical, practical, or timing reasons. It is a way to consider, but be aware of the hard work needed later for processing, as PTGui is not optimized for mosaics of star scenes yet. I hope/wish it will be better sooner rather than later. :) 

Jesús

El vie, 30 may 2025 a las 23:16, Sam Gowan (<astroast...@gmail.com>) escribió:

When I make Milky Way panoramas, I often end up with elongated stars even though the original images have round stars. It just occurred to me that when I straighten the panorama, it will have to stretch parts of the image and cause this, but this is based upon the original stitching/control point. Is there a way to prevent this by somehow regenerating the control points after I’ve stretched/straightened out the panorama? This should then use more of the individual images and not require any image to be stretched/distorted. I always shoot with lots of overlap, thanks

Sam

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[Sam Gowan]

Thanks for the replies to my question. As a work around, rather than increasing my focal length, is there a way of leveling the horizon and selecting a new centre of image (this is where the image becomes stretched) (I use full frame fish eye projection) where by the image will split rather than distort where necessary and then I can just crop those parts? As I mentioned, I massively oversample the sky to ensure it is all covered. Or is this just the nature of flattening a semi spherical Image…

[Jesús Navas]

Yes, that distortion is unavoidable due to Maths reasons, something you cannot avoid when stretch the image whatever the overlay is.

But you can minimize the visible effect with longer focal length, asuming you will reduce the final image to the size you would have gzc4ot with the shorter focal length.

Moreover than choosing one  projection or another, I ignore if there is any specific way to use PTGui to minimize that effect with short focal length lenses from the beginning of the project. 

To view this discussion visit https://groups.google.com/d/msgid/ptgui/9a52e1e7-f269-445b-9ac5-2005761adeb0n%40googlegroups.com.

[Sam Gowan]

Thank you Jesus