Is DBE correct?

[pvelez]

I have a quandary.

I've combined about 12 hours of RGB data for LDN673 - a dusty region of the Milky Way. I've used Image Integration, applied a Linear Fit based on G to the B and R masters and then applied both Background Neutralisation and Colour Calibration. The result has a significant region with a greenish hue. It doesn't look right to me.

So I ran DBE over the combined image and extracted the background - it replicates the greenish hue that I saw in the RGB image. My question is this - How do I know if this is a genuine feature of the image (ie it should be green) or if its a function of uneven illumination of the G Master?

I ran DBE over the R, G and B Masters and found that the background produced for all 3 was roughly the same except for the anomalous region.

Here are some screenshots - happy to post the individual master backgrounds if that helps.

Pete

[RickS]

Hi Pete,

Did you blink through your subs and check if there might be some that are cloud affected?

It's a difficult region to process.  Finding an area of actual background is tricky (using the dark neb is not a good idea since it isn't necessarily a neutral colour) and with all the dust around using the stars as a white ref is also suspect.  Looking back at notes from when I processed an image of this area I tried a couple of different colour calibration methods and then blended the results.  I also used SCNR so I had some green that I wanted to get rid of but I don't remember having a region that looked like yours.

Cheers,
Rick.

[pvelez]

Thanks Rick

Yes, I used the dark neb for background.

And yes, I used the target image for the white reference.

How could you predict so accurately what I was trying to do?

I'll have a look at the subs - doubtless there was some cloud about. Data is hard won and its a challenge to throw any of it out.

My question though is how does one tell what is true gradient and what is not? My thinking was that if its a true gradient, then it should not be filter specific - so if I look at the background extracted with DBE from the Masters using the same settings, I should end up with a similar background - am I being simplistic here? I do accept that light pollution is not colour neutral (though my sky is pretty dark and I image quite high anyway)

Pete

[pvelez]

Checked them - yep, some subs have high cloud. The way I checked was to blink through them while keeping an eye on the dark structure in the offending area. There were quite a few where the structure disappeared - presumably as cloud crept in.

I'll integrate without those and see what happens

Pete

[msmythers]

Pete

As something to try you might do a linear fit using green as your reference, combine the channels and run ABE. With ABE check  the 'Draw sample boxes' and 'Just try samples'. Adjust the Deviation until you have a good coverage of samples. Then uncheck those boxes and use Subtraction for the correction and the Normalize box is checked. This might give you a good starting point with a tricky nebula. You might still have some green but SCNR with an amount between 60 and 70 should take care of that. 

Like Rick was saying with all the dust finding a true black or gray background might be difficult and the stars can suffer the same problem.

Like I said, something to try.


Mike


Mike

[pvelez]

Cheers Mike - I'll give it a go.

Its a tricky area of sky that's for sure!

Pete

[RickS]

How could you predict so accurately what I was trying to do?

I've been there before, Pete 

My question though is how does one tell what is true gradient and what is not? My thinking was that if its a true gradient, then it should not be filter specific - so if I look at the background extracted with DBE from the Masters using the same settings, I should end up with a similar background - am I being simplistic here? I do accept that light pollution is not colour neutral (though my sky is pretty dark and I image quite high anyway)

I see a lot of variation in subs/backgrounds even at the darkest locations.  Even removing man-made light pollution as a major factor there's a lot of stuff going on in the atmosphere over a period of hours (or days) that's significant to the tiny signals that we capture.  Not sure that helps but I think it's complicated and unpredictable.

Interested to see how you go trying Mike's suggestions...

Cheers,
Rick.