I’ve just run into a problem with the frame and mosaic tool, which resulted in a fair amount of work to diagnose and correct. I wanted to get the California Nebula, and since my setup (ASI 533, W-O ZS-61) has a very small FOV, I had to set up using the frame and mosaic wizard. I did this with a 2x2 mosaic, zero degree rotation, and the default 20% overlap. All seemed to work well, giving me a bit over 12 hours for each of the 4 panels. I put all the data into PixInsight to do the integration and mosaic processing, and that’s where I ran into trouble. The mosaic script in PI failed since the individual frames did not overlap, so I spent most of the day digging into this further.
The frame data from PixInsight (which corresponds to the SGP parameters within a few arc seconds) is as follows:
Frame 1:
Image center … RA: 3 58 02.142 Dec: +37 03 55.54
Image bounds:
top-left … RA: 3 55 36.897 Dec: +38 13 00.80
top-right … RA: 3 52 17.568 Dec: +36 34 52.96
bottom-left … RA: 4 03 51.062 Dec: +37 31 55.04
bottom-right … RA: 4 00 23.038 Dec: +35 54 39.55
Frame 2:
Image center … RA: 3 58 09.074 Dec: +35 38 00.48
Image bounds:
top-left … RA: 3 55 43.229 Dec: +36 46 48.97
top-right … RA: 3 52 32.047 Dec: +35 08 18.92
bottom-left … RA: 4 03 50.229 Dec: +36 06 42.65
bottom-right … RA: 4 00 30.787 Dec: +34 29 01.31
Frame 3:
Image center … RA: 4 05 14.894 Dec: +37 03 18.77
Image bounds:
top-left … RA: 4 02 46.088 Dec: +38 12 06.36
top-right … RA: 3 59 31.837 Dec: +36 33 33.72
bottom-left … RA: 4 11 02.384 Dec: +37 32 01.28
bottom-right … RA: 4 07 39.264 Dec: +35 54 19.91
Frame 4:
Image center … RA: 4 05 17.230 Dec: +35 38 09.78
Image bounds:
top-left … RA: 4 02 51.352 Dec: +36 46 58.31
top-right … RA: 3 59 40.175 Dec: +35 08 27.94
bottom-left … RA: 4 10 58.414 Dec: +36 06 52.21
bottom-right … RA: 4 07 38.975 Dec: +34 29 10.56
It’s probably not obvious from the above data, but there’s a “hole” in the middle of all these frames where there’s no data at all. It took me a while for plot the individual frame coordinates on graph paper, but sure enough, there are some overlap issues. Frames 1 and 4 do not overlap, nor do frames 2 and 3. The other combinations (1 to 2, 1 to 3, 2 to 4, and 3 to 4) are fine, but the two “failure to overlap” frames resulted on no data in the center, which killed the mosaic. This is both surprising and troubling to me, since I assumed that the mosaic wizard would be able to create a sequence with overlapping frames, especially since it looks that way in the preview and also has a 20% overlap specified.
Luckily I discovered this right after the last image capture so I’m setting up tonight (last clear night for a while) and shooting centered on the “hole” produced by the mosaic wizard. That should take care of my current mosaic, but I do worry about the possibility of this happening in the future. Is this a known issue, or is there something I need to do beforehand to assure that this won’t happen for future mosaics? I don’t do mosaics often, but with the small FOV that I have with my equipment, it is necessary from time to time on the larger targets.
Thanks in advance for any advice,
- Ken