Gradient Removal
One of the clearly bad things about this data is there is an obvious gradient across it. Even worse, that gradient is a little tougher than average to remove because Messier 33 fills so much of the frame. I often will use automatic background extraction to get rid of gradients but it is hopeless here.
One of the clearly bad things about this data is there is an obvious gradient across it. Even worse, that gradient is a little tougher than average to remove because Messier 33 fills so much of the frame. I often will use automatic background extraction to get rid of gradients but it is hopeless here.
Here are the settings in ABE for that miserable failure:
As can be seen, the results are actually worse than what we started with. All kinds of dark artifacts have been introduced around M33, and the upper right and lower left corners are still bad. Various other settings can be tried. For example you might want to try reducing the Deviation setting on Global Rejection (often a handy thing to do), but nothing will work well here.
When automatic fails, it is time to bring out the big guns. In this case, that is DynamicBackgroundExtraction (DBE).
Unfortunately, the automatic settings for DBE will also fail miserably. Bring up DBE. Then click on the image. Open up sample generation, and the click on the generate button.
When automatic fails, it is time to bring out the big guns. In this case, that is DynamicBackgroundExtraction (DBE).
Unfortunately, the automatic settings for DBE will also fail miserably. Bring up DBE. Then click on the image. Open up sample generation, and the click on the generate button.
Your image will look something like this:
If you actually tried to apply this the results would be even more disastrous than ABE.
The trick is to actually place those little samples where we actually need them.
The trick is to actually place those little samples where we actually need them.
Then this actually needs to be applied to the image. Here is the settings box.
Note that I change the tolerance setting under model parameters so that all samples would be good (not red). I also had to change the correction setting under target image correction to subtraction. Otherwise, it won't actually do anything, just show you the background it would subtract.
Note that I change the tolerance setting under model parameters so that all samples would be good (not red). I also had to change the correction setting under target image correction to subtraction. Otherwise, it won't actually do anything, just show you the background it would subtract.
Here is the result. Those unsightly gradients are gone without the terrible artifacts introduced by ABE. By the way, to apply this, you drag the little triangle on the bottom left of the settings box over the image.