How to do mosaic on comets?

[Savio Fong]

hi there, this is my first time posting here.

I have questions on how to do mosaic. Here is my mosaic of 3 images of the comet C2014 Q2, and also the mask generated from Star Alignment process. I align the left one with the middle one first, generated a 2-panel tail mosaic, then align the right panel (the comet head) with the tail mosaic,

- not sure if it is the best approach? seems this will cause least distortion to the final image.

- as the images are not fully aligned with each other at the edge, so what is the best approach to modify the mask, to make the edge transition more smoother, as the transition at the edge of a image, could be to another image, or to the "black" area.

- not sure why the comet head background seems to be a little darker, any method to align the 3 images background more even?

- is it better to combine in linear stage, or no linear stage, e.g., stretched first?

Thanks for your advice.

[georg.viehoever]

Did you try GradientMergeMosaic?

[Savio Fong]

Thank you Georg, I tried it and much better now.

However, it's not perfect, may I know how to further improve it?

I tried mosaic the 5 panel at linear (1st image below), but the texture between panels look much different, than I tried at non-linear stage (2nd image below), it's even worst, and I can still see seams between.

See my 5 panels below, all taken with the same exposure, just one minutes apart from each other, stable sky condition, so the images should show such big difference?

[Savio Fong]

other 3 panels,

I tried stack from head to tails, or start from center and then to head and to end of tail, slight difference, but not much, is there a better way to handle this?

[Savio Fong]

and then this is the image yesterday, see the dark artifact (also lighter) along the edge, the original single panels don't have that, only appear after GradientMergeMosaic, per the tutorial, I tried to increase the shrink radius but only getting worse, now I am using 1 so it's smallest.

Not sure how to handle this?

Thanks you for all your advice.

[georg.viehoever]

To analyze this I would need the individual FITS files. Plus I currently dont have the time to do it. Anybody out there who can help Savio?

Georg

[lucchett]

I confess i don't know The tool, but what about linear fitting the panels first and may be dbe?
Just a try if you run out of ideas
Andrea

[Savio Fong]

personally I want to avoid DBE on these, I took this via my robotic observatory in Tibet, at 4300m altitude and need zenith, so there was no sky gradient to such extend and all images are taken within few minutes, so I believe there is something I don't process it right, so want to fix it.

See the image taken tonight, it's much better, so I don't know how to fix this problem when it appear.....