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Help with Ha in M27

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Below is 260 minutes of Ha shot with Meade 12 ACF at F8, 20 minute subs.  I cannot seem to get that outer ring to show up without the background overwhelming the image.  The image processing was identical to the LRGB as shown below, same use of appropriate flats, darks etc....

Is is just a case of not enough data?





drizzle_integration_DBE_DBE_ABE_ABE_HA New_Help.png



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Processing advice will depend on which software you use. Workflow is different in PS or PI.

Either way, you would start with removing the uneven background. In PixInsight, use dbe.

To reveal weak signal near the background level, do the stretching in steps, e.g. masked stretch in PI, followed by histogram transform and exponential transform, etc.

Hope this helps some

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Hi Mark,

You've done well to capture that outer stuff, it is very faint. If you can be bothered, there is a similar signal in Oiii too.

With such weak signal areas, I usually selectively combine the faint stuff using layers and masks in Photoshop. I think some of Ken Crawfords tutorials may help you out


Hope that helps.

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On mine I used separate layers for the starfield, the faint outer shell and the bright core.

DBE will certainly need to be well applied though, when you are going to stretch so hard for the faint shell. You have this showing well, for sure.


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Thanks guys for the help and input.


Wim.. I am new to this, so I am using both PI and PS, I started with PI and after a tough learning curve do most of the basic processing in PI, for layers etc I am finding it a bit hard to figure out, and the tutorials in PS seem to be easier, that said it is a slow learning curve, I could not get an old copy of PS in Canada, so I am using PS cc, the look and feel is much different than what I see in the tutorials, so it is taking me a bit of time to figure it out


Tim.. thanks for the heads up on the Oiii, right now M27 is only one hour away from meridian flip, so I am going to wait a bit before trying to capture Oiii, as it is a bit of a waste to get 3 subs, and then start the setup again.


Ollie,..  I did perform DBE, but perhaps I need a few more cycles before I do the stretch?



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You still have some stacking offsets visible in the mono image.

I always start by cropping out any of these stacking edges. As dbe will try to place samples here, edges will affect the gradient removal process.

I usually let DBE place samples to start with, but then I inspect every sample, to make sure it isn't positioned over a star or part of the nebula. Sometimes I use a copy of my image for placing the samples. This copy is stretched to the max, to reveal any faint nebulosity and gradient. I only use this copy for placing samples. By dragging the little triangle in the bottom left corner to the workspace, an instance of the tool is created. This can then be applied to the original image. I find that I get better results if the sample size in DBE is as large as the star field will allow, and the number of samples per row is large (20 or so) as well. Typical values for my dslr images are sample size of 15, 20 samples per row, and tolerance of about 2 - 2.5.

As for masked stretch: it can be used with default values most of the time. I usually set clipping factor to 0, to make sure no pixel values are clipped. Masked stretch is particularly efficient at keeping stars under control when you need to stretch aggressively.


Good luck

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