Author Topic: what is going on?  (Read 2604 times)

Offline georgian82

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what is going on?
« on: 2016 December 06 21:04:09 »
Hi everybody!

I am trying to understand why the image that was processed with flat frames comes out red and the one that was processed without flat frames comes out green (what I am used to).

The problem is that it is difficult to apply DynamicBackgroundExtraction tool on the image that is red because it seems to also take out the red from the California Nebula. Is there a way to subtract this red color but leaving the California Nebula intact? I would prefer to use the image that has the flat frames applied to it so that I can get rid of the vignetting.

For reference...I used a Nikon 180mm f/2.8 ED lens at f/2.8 and an Orion Skyglow imaging filter in a Baader modified camera.

Thank you all!

Offline Greg Schwimer

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Re: what is going on?
« Reply #1 on: 2016 December 06 21:16:36 »
If you haven't done this already, open the STF process and uncheck the "link channels" button and re-apply STF. This should give you a more or less natural color image in most cases.

For seeing faint regions in your image, you can try a boosted stretch (nuclear icon with the up arrow on the toolbar). Another approach is to run an initial DBE on the image and then adjust your samples. Save DBE to an icon, undo the first DBE on the image, then open it again and apply it again.
- Greg
Scottsdale, Arizona, USA

Offline jkmorse

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Re: what is going on?
« Reply #2 on: 2016 December 07 08:30:58 »
Also, there is no reason you can't do background neutralization and color balance first, before you do DBE.  That should clean up the color problem as well.  Note also that if the flats do their job, you may not need DBE at all.  I rarely use it any longer since the flats flatten my images already.

Best,

Jim
Really, are clear skies, low wind and no moon that much to ask for? 

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Offline cdavid

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Re: what is going on?
« Reply #3 on: 2016 December 07 08:46:16 »
....just to point out, unless you are imaging from a mountain top or you have pristine skies from an absolute dark site....you will have gradients.  DBE is not used just to flatten an image.  It is primarily geared towards removing gradients and in my personal opinion, I can not imagine any images taken from earth that would not benefit from it applied properly.

As Greg points out, unlink your channels in STF so you can see what is going on.  Then use just a few samples per row...6-8 or place them manually on areas of clear sky.  Make sure your radius is not too small, keep it above 10 or you will mainly be sampling noise.  If some of your samples are rejected (they appear as red), gradually increase the tolerance and hit resize until they are green. 

At this point save an instance of the settings for reapplication if needed.  Run the DBE process without subtraction and look at the model.  Superimpose the model on your image and see how well it matches up.  If you are happy, then run it with subtraction, normalization checked, and discard the model. 

Sometimes running it a second time is helpful if you have complex gradients.  After DBE, run background neutralization with a preview that again is not too small.  A few stars are ok because the process uses the median for sky background representation and the stars will not be considered in the calculations.  Finally run your color calibration.

After you have completed the DBE and BN steps, you can re-link the STF channels and should no longer have the heavy color bias.

Hope this helps
Carlos


Offline jkmorse

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Re: what is going on?
« Reply #4 on: 2016 December 07 08:57:15 »
Carlos,

I must just be lucky, imaging at 7300 ft from some of the darkest skies in the US, but I take your point.

Jim
Really, are clear skies, low wind and no moon that much to ask for? 

New Mexico Skies Observatory
FLI Microline 16803
Planewave CDK17 - Paramount MEII
Planewave IFR90 - Astrodon LRGB & NB filters
SkyX - MaximDL - ACP

http://www.jimmorse-astronomy.com
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Offline cdavid

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Re: what is going on?
« Reply #5 on: 2016 December 07 09:09:16 »
Ha! I envy you!
....imaging just west of the Boston light dome is a totally different story.
Carlos

Offline chris.bailey

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Re: what is going on?
« Reply #6 on: 2016 December 07 11:53:14 »
Imaging from sea level in the UK, DBE is a must :D

Offline georgian82

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Re: what is going on?
« Reply #7 on: 2016 December 07 12:09:26 »
If you haven't done this already, open the STF process and uncheck the "link channels" button and re-apply STF. This should give you a more or less natural color image in most cases.

For seeing faint regions in your image, you can try a boosted stretch (nuclear icon with the up arrow on the toolbar). Another approach is to run an initial DBE on the image and then adjust your samples. Save DBE to an icon, undo the first DBE on the image, then open it again and apply it again.

Hi Greg,

Thank you for the response. I am not exactly sure what you mean by "uncheck the link channels". I usually just do a boosted stretch using the nuclear icon to reveal more faint regions before proceeding to do a DBE. But perhaps I am doing it all wrong???

Thanks for the input!

Sebastian

Offline georgian82

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Re: what is going on?
« Reply #8 on: 2016 December 07 12:13:30 »
Also, there is no reason you can't do background neutralization and color balance first, before you do DBE.  That should clean up the color problem as well.  Note also that if the flats do their job, you may not need DBE at all.  I rarely use it any longer since the flats flatten my images already.

Best,

Jim

Hi Jim, thank you for the response. In all honesty, I got in the habit of using DBE for removing the color gradient and have not used background neutralization tool yet. Is it as good as DBE? In which instances would I use it over DBE?

Also, even though I am using flats, I use DBE to get rid of the color, so to me it is still a must.

Thank you for the input!

Sebastian

Offline Greg Schwimer

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Re: what is going on?
« Reply #9 on: 2016 December 07 17:13:45 »
Also, there is no reason you can't do background neutralization and color balance first, before you do DBE.  That should clean up the color problem as well.  Note also that if the flats do their job, you may not need DBE at all.  I rarely use it any longer since the flats flatten my images already.

Best,

Jim

Jim,

I'm interested in this. Theoretically it makes sense, but I had a thought about it. Doesn't DBE affect color balance somewhat, even if slightly? Also, if you run BN using a background sample that has gradient, have you really neutralized the background correctly? Or are you suggesting to run BN and CC to help expose the nebula, then build your samples with DBE and go back to the initial image and apply that saved DBE process?

Greg

PS - we're certainly not suffering a shortage of acronyms in this hobby. TMA (Too Many Acronyms). :)
- Greg
Scottsdale, Arizona, USA

Offline jkmorse

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Re: what is going on?
« Reply #10 on: 2016 December 07 18:52:34 »
Greg,

Hadn't thought of it that way, but I really like your idea of using BN and CC to identify the background, then apply it to the uncorrected version.  That is the best of both world's clever  ;)

Best,

Jim
Really, are clear skies, low wind and no moon that much to ask for? 

New Mexico Skies Observatory
FLI Microline 16803
Planewave CDK17 - Paramount MEII
Planewave IFR90 - Astrodon LRGB & NB filters
SkyX - MaximDL - ACP

http://www.jimmorse-astronomy.com
http://www.astrobin.com/users/JimMorse

Offline Greg Schwimer

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Re: what is going on?
« Reply #11 on: 2016 December 07 19:23:29 »
Hah! Well, I thought that was what *you* were suggesting, so who gets the credit for it? :)
- Greg
Scottsdale, Arizona, USA