Author Topic: Trouble with gradients  (Read 3080 times)

Offline mcgillca

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Trouble with gradients
« on: 2012 December 16 09:15:59 »
Hi - I am still very new to PixInsight and am having trouble with colour gradients in my image.

The enclosed image is about 2.5 hours each for RGB, taken at a dark site in Spain, and has just been colour combined, cropped, and put through DBE, then stretched.

As you will see, there is still a clear gradient in the image. When I take the image processing further, I'm still left with this at the end - further applications of DBE do not seem to help. If I try to remove the gradient through moving the black point, much of my image disappears too!

I've looked at the raw data - there is probably not more than a one photon difference between the top left and bottom right per 5 minute sub, so the gradient is very small.

Any help much appreciated.

Thank you,

Colin

Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/

Offline Harry page

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Re: Trouble with gradients
« Reply #1 on: 2012 December 16 10:16:22 »
Hi

This is should be a easy target for dbe  :D as there is plenty of free background !

When you place the samples do you make sure that they are not red ( so excluded from background model )

If they are red raise the tollerance a bit and don't forget to subtract the model  :surprised:

Regards Harry

Harry Page

Offline mcgillca

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Re: Trouble with gradients
« Reply #2 on: 2012 December 17 01:42:06 »
Thanks, Harry. Since I'd tried this, I wondered why you were getting a better result.

I suppose it is obvious in hindsight, but I was merely re-running the same DBE as I'd carried out previously. What you did was to run it again with different points selected for the DBE (since you had no idea which points I'd used).

Since the original points removed whatever gradient they could find, they didn't see the colour cast of the image I posted - otherwise they would have removed it in the first pass.

Enclosed the original image, with three successive runs of the DBE tool using different points. The first shows the same gradient I'd shown previously. The second is definitely better, but with a slight gradient remaining. The third with almost all the gradient removed.

So, I've learned that iterative use of DBE can give better results, but ONLY if you select different points to use for successive iterations.

Thank you very much, Harry.

Colin

Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/

Offline mcgillca

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Re: Trouble with gradients
« Reply #3 on: 2012 December 17 01:44:16 »
Oops - didn't post second iteration.


Colin
Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/

Offline mcgillca

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Re: Trouble with gradients
« Reply #4 on: 2012 December 17 02:32:47 »
And here are a couple of images fully processed. There are still remnants of the gradient in the background, so I moved the black point to hide them, but it looks a little artificially black to my eyes. Any other suggestions?

Thank you,

Colin
Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/

Offline Harry page

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Re: Trouble with gradients
« Reply #5 on: 2012 December 17 11:32:46 »
Hi

May I suggest you do not have the samples in the correct place in the first place as a simple one go should sort this image out ( nothing wrong with multiple goes though)

Can you post a fits file to look at ?

Harry
Harry Page

Offline mcgillca

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Re: Trouble with gradients
« Reply #6 on: 2012 December 20 08:34:57 »
Dear Harry,

Hi - I suspect you are right. Taken me a couple of days to work out that I should just use Dropbox to share the fits file (duh).

Enclosed link to the fits file:

https://www.dropbox.com/s/n2s3v48w2tvkj0q/M74%20RBG%20Combined.fit

I've also added a snapshot of the DBE locations and settings I've used to process this:

https://www.dropbox.com/s/u8jga73p42rqwpu/DBE%20Settings.jpg

Any thoughts much appreciated.

Colin
Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/

Offline chris.bailey

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Re: Trouble with gradients
« Reply #7 on: 2012 December 20 09:06:07 »
Hi Colin

I dont think the distribution of your samples does it justice.

Try auto-generating the samples with a Tolerance of 0.8 and a Minimum Sample Weight of 0.45 with a Radius of 12 and 25 rows. Delete a few that are within the galaxy itself and subtract. A single pass does a very good job leaving a bluish border down each side which can be cropped off.

Chris

Offline Harry page

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Re: Trouble with gradients
« Reply #8 on: 2012 December 20 10:00:26 »
Hi

First thing I would do ( and did ) is crop the image tp remove all the misaligned frames and the I placed a few samples around the image making sure they were
right into the corner and close to the edge , :-*

Please notice I did not need many samples  :angel: I also raised the tollerance to 1 to remove the gradient in one go  :D

Regards

Harry
Harry Page

Offline mcgillca

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Re: Trouble with gradients
« Reply #9 on: 2012 December 21 10:04:12 »
Thanks, Harry - between the two, that sorted it.

I think part of the problem is that I did not bin the colour data, so there was a lot of variability, hence the need to increase the tolerance.

Thank you (and Chris) very much,

Colin
Paramount MX
Ikharos 8" Carbon Truss RC
Atik 460ex
Atik EFW2 with Baader LRGB and HA SII OIII
ONAG with Atik 314L+
http://www.astrobin.com/users/mcgillca/