removing gradiant

[Erwin Kats]

Hey guys,

last wednesday i captured 9x1200 seconds of Ha from the california nebula, but i forgot to center the object and i went to bed.

afterall i did not make any flats, but because of the high humidity and the moon light i got some gradients in my photo.

here the file after stacking the 9x1200 seconds and some photoshop.

here a bit bigger file http://www.astro-land.net/wp-content/uploads/california-01.jpg

tips etc very welcome.

[Demonperformer]

In Making Every Photon Count, Steve suggests using the 'black point' eye dropper in the levels dialogue box as a quick fix to make the brightest part of the background black.

[roundycat]

using the black point eye dropper in either Levels or Curves will give a very poor result. You will be making a global change and all of the background will be made darker. The right hand side is already too dark...

Use a linear gradient and quick mask then you can adjust only the bit that wants adjusting. I could have made two iterations of this method to finish off the very edge at the left.

Dennis

[Euan]

Here is a quick run in PixInsight's Auto-Background Extraction

[Cloudwatcher]

Very nice but it looks a little pale......... so I applied the Cloudwatcher Palette.

[steppenwolf]

In Making Every Photon Count, Steve suggests using the 'black point' eye dropper in the levels dialogue box as a quick fix to make the brightest part of the background black.

As a quick fix the black point eye dropper works very effectively on normal light pollution that pervades the whole background but it doesn't work on gradients like this!

[Erwin Kats]

thanks for all the information !

it looks awesome cloudwatcher

a fellow dutch astrophotographer Arie Nagel ( http://www.astronomie.be/apn/index.htm )

He has been playing with curves in photoshop, and the result he managed is making me happy !

off-course its not a wonderfull image but it comes out way better!

[harry page]

Hi

I used dbe in pixinsight , A very hard gradient indeed

Harry

[Demonperformer]

As a quick fix the black point eye dropper works very effectively on normal light pollution that pervades the whole background but it doesn't work on gradients like this!

Sorry for misrepresenting you, Steve:o ... thanks for the clarification ... still so much to learn ...

[ollypenrice]

As soon as I saw that I knew Harry would be up and at in PixInsight! He is skilled in its use so it would not be dead easy to do as well first or second time but it clearly can be done. Gradient Xterminator is sometimes helpful but often eats up the object with the gradient, or so it does for me.

The bit that's left in Harry's treatment I would simply remove by hand. Select it in a well feathered marquee, put a pin on the curve at the general background level and drag the gradient residue down to match the pin.)

Olly

[steppenwolf]

Sorry for misrepresenting you, Steve:o ... thanks for the clarification ... still so much to learn ...

No problem at all - as you say, so much to learn and trust me, the learning process never stops, its all part of the fun!

Here is a re-worked version using 7 iterations of Gradient Xterminator, each one making a very small change. Not perfect but another way of tackling the issue:-