Mercator Projection: how to use the settings

Hi Mabula,

Version 1.075 introduces the Mercator Projection as one of the options to project data. This greatly improves big mosaics of (in my case) e.g. the Milky Way. In your release notes you have a picture with some examples of what the Center of Projection (CoP) settings do.

I tried to play around a bit with the settings in my 34 panel mosaic of the Milky Way from Scorpius to Perseus (about 150 degrees field of view), but viewing one individual frame in registered mode took already quite long to display and didn't give much insight in what the final mosaic would look like. I generated a couple of scaled-down test mosaics but that took even longer per setting.

There are 3 settings:

1. Rotation
2. Move CoP horizontally
3. Move CoP vertically

Playing around with the CoP settings rapidly made things worse than just leaving them at zero, and in combination with the rotation I was really getting confused. Things probably weren't helping that with the specific registration I used at that time was that the reference frame was the one at the extreme end of the long edge of the mosaic.

Could you please explain where the zero points for the projections are located, and in which order the rotation and moves are executed? Is there any coupling with the rotation angle in the Tools tab, I noticed that the angles didn't quite correspond?

This would really help me in getting the most out of this great new feature!

(And feel free to use equations, that only clarifies things more in my case.)

Thanks in advance