Quote:
Originally Posted by Placidus
That's just gorgeous. The HII regions look brilliant. Well done!
Really hoping that you can complete this project and that you'll show us the result.
We agree that as far as possible one wants to do each panel of a mosaic under the same conditions. Close to new moon at a really dark site with no high altitude cloud is really helpful. Close to the meridian is really helpful. Really bad is bright moon in one frame but not another, light pollution or moonlight bouncing off high altitude cloud in one panel but not another is bad. Low down in the murk in one frame and at the zenith in another is really bad.
We calibrate to set the black point of the histogram (the leftmost point of the histogram where the frequency is nonzero) to be identical in each frame. As others have mentioned, a big fat overlap really helps.
The absolute killer is if you have different, incompatible gradients in different frames, e.g. moon on the east in one panel, moon on the west in another, or horizon glow to the south of one panel but not in another. If you do have different gradients in each panel, no amount of overlap, no practicable amount of feathering, will get rid of the visible joins. It is far better to take the panels under similar conditions. If that isn't possible, then in an object like this, where you don't have continuous nebulosity throughout the object, you can reduce gradients on each panel before trying to combine the panels.
One other tip: if you're running out of memory, it might be necessary to do the mosaic in say three sub-mosaics. Fully and seamlessly assemble each sub-mosaic. Then combine the three sub-mosaics. This technique will only work if there are no incompatible gradients.
So far, your image looks magnificent!
Best,
MnT
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Thanks for the encouragement MnT. I particularly like the reference to setting consistent black points in each frame. A good suggestion. At least with the SMC it is conveniently placed to track the object for a few hours each session so there should be good opportunities to collect data.