"Roll relative to north celestial pole" clarification

[bribrah]

I am a bit confused about what the roll results signify on the solve image results, it does not seem to be a standard astronomical measurement.

Thanks,

[gustavmpettersson]

Hi Brian,

This is the rotation of the image around the camera's optical axis (i.e. the centre of the image), so it does not signify anything regarding the pointing direction.

I haven't looked at this in a long time, but believe 0 rotation is when the "upwards" direction of the image points to the (celestial) North, with rotation measured clockwise from there.

Gustav

[bribrah]

When u say upwards direction of the image do you mean like upwards in the y direction of the 2d xy image? Or do you mean like up in 3d space, ie, if I have an image pointed where the middle was directly at the north celestial pole, the roll would be 0.

[gustavmpettersson]

Yes, upwards as in the -Y direction of the image. (Negative since the Y=0 is the first row of a 2D image). If you point perfectly at the North celestial pole the roll would be undefined.

Another way to explain it, when roll is zero, the X direction of the image points along the line of constant declination.