Some more info:
I tested this with the simulator creating a 2x2 mosaic.
The problem also is evident here too. I've also had the same issue in real world use with bigger mosaics like this.

Mosaic tool settings:


Simulating CCD view in Stellarium using the RA/DEC from the mosaic jobs, there is a 30% overlap on horizontal panels when there should be no overlap. Vertical panels are correct.

I calculated the same 2Wx1H mosaic of NGC5139 layout using Telescopius. Interestingly it generated :
Pane, RA, DEC, Position Angle (East)
PANE 1, 13hr 29' 07", -47º 28' 46", 0
PANE 2, 13hr 24' 27", -47º 28' 46", 0

These are 4' 40" apart in RA. Compared to 3' 10.7" apart from the Ekos tool. The Telescopius panels line up correctly when checked in Stellarium.

As the DEC calculations are OK, I wonder if the issue relates to the difference in units used between RA and DEC - maybe the code behind the tool is treating these the same where they should be different?

Thank you for the detail investigation. This indeed appears to be a bug but I didn't have enough time to investigate it. Maybe you can spot the issue here ? This is where it's calculating the center points for each mosaic panel.

Cheers knro.

My maths or coding isn't good enough for this sorry. I did manage to recreate the calculations in Excel using the method from mosaic.cpp and found I got the wrong RA value as well.
I can say adding the width of the sensor, minus overlap (in degrees) to the RA position (degrees) gives the wrong position, though this method works for DEC.

I had a look at how Telescopius do their mosaic tool (( pastbin ) and they do some spherical and cartesian coordinate conversion - perhaps that's a hint for you.

I'll keep giving it some thought, and hopefully someone can assist.

I do not understand or maybe I am missing something but why is the RA divided by 15 and the DEC not?


tile->skyCenter.setRA0( (center.ra0().Degrees() + (diffFromSkyMapCenter.x() / (pixelsPerArcmin * 60.0))) / 15.0);
tile->skyCenter.setDec0( center.dec0().Degrees() + (diffFromSkyMapCenter.y() / (pixelsPerArcmin * 60.0)));

The only other reference in code when I looked for diffFromSkyMapCenter was in the comments:

" QPointF diffFromSkyMapCenter = skymapCenterPoint - mosaicCenterPoint "

I understand how its calculated, this is why I don't understand why one is divided by 15 and the other is not. My math for RA and DEC along with rotation isn't allowing me to 'get' the divided by 15.

If it is beyond me disregard this post as my math skills being weak, just curious at this point

Cheers
Jim

RA is in hours, but yeah it's more likely to do with spherical trig. The calculation in Mosaic assumes a flat plane which is only approximate for very small angles and breaks down over large areas. Will have to look at it more in the future unless someone wants to step it to fix that.

I haven't yet used the mosaic tool (but soon will). Big Noel, I'm curious whether you've tried to mosaic at the celestial equator (Dec=0). Is the answer correct at Dec=0? If so, could this be a simple issue with not correctly factoring cos(dec) in the RA calc? The error for NGC5139 is suspicious in this regard.... Just a thought.

Okay that makes sense. In a flat plane I would assume that each quadrant would have a value in relation to the one above, below, left, and right. On a four panel mosaic this would work out:

top left: Position lower, move to right.
top right: Position lower, move left.
bottom left: Position higher, move to right.
bottom right: Position higher, move to left.

Anything beyond 4 panels is way over my head...

Now to try and wrap my head around this with RA and DEC numbers. Thank you Jasem for the explanation, I now see that to change hours to degrees the calculation is one hour equals 15 degrees.

Cheers
Jim

Good test dmsummers and you might be on to something.
I'll test Kstars more tomorrow night, but knowing the EKOS tool gives a 3' 10" difference in RA using the simulator when using NGC5139 as a test (DEC -47 28 36). I compared that to the Telescopius results, at RA 0, DEC 0, the difference in RA between centers of a 2x1 mosaic using the SimulatorCCD specs is... 3' 10" !
I entered a different target in Telescopius; RA 0, DEC 45. The difference in centers is 4' 28" . Clearly the Telescopius algorithm is factoring in something relating to the DEC. This might be enough for someone to solve the issue.

When it works, the mosaic tool and scheduler is magic, I really hope to see this issue fixed so I can use it more! AstroPixelProcessor does a good job at compiling the mosaic too.

Big Noel and dmsummers

It appears that within Telscopius they use +- 90, +- 180, +- 270 to achieve the overlap. The math is beyond my understanding when 'cos' and 'sin' is involved.

Gonna remove myself from this conversation while I still have a single brain cell left...

Cheers
Jim

I'll just leave this here as it may be insightful.
people.sc.fsu.edu/~jburkardt/m_src/spher...sphere_llq_grid.html
people.sc.fsu.edu/~jburkardt/m_src/sphere_grid/sphere_grid.html

Sorry, I don't understand either of the last two posts. What seems to make sense (although it could also be totally wrong) is that the total RA field must need a mosaic panel RA position offset adjustment proportional to (1-cos(dec))/N RA panels (or similar). At DEC=45, and for a 2 panel RA mosaic, this would be a 30% total RA panel field size position adjustment divided by the 2 panels (left panel left, right panel right). The devil's always in the details (e.g. likely need to map camera rotation to N/S, E/W coordinate vectors), so a deeper code eval with a few good sim tests is needed to sort and fix the bug. BigNoel, since you're already good at setting up the test(s), it might be nice to know what the sim says about a 2W x 1H mosaic on Polaris.....