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Could someone help me with these artifacts?

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#1 moxican


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Posted 12 January 2018 - 09:16 PM

This image was taken a couple of weeks ago with Equipment ASI1600mm-C + Baader 7nm H-alpha filter + Nikon Nikkor 180mm F2.8 Ai-s. -15C; Gain:200; offset:50; 80x30s exposures, Bin 2x2. 

Darks and Bias were subtracted from this version, but I've tried not subtracting them only to come to the same issues. I did image from white area that is though. I do not know what are those stripes on it. They look like some sort of artifacts. They show up on other targets too. This target was about 50 degrees above horizon when I shot it.

I'd appreciate any help or opinion on the subject



#2 WadeH237



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Posted 13 January 2018 - 12:16 PM

Those streaks are likely an artifact from alignment and stacking.


Your calibration is not perfect, and it is leaving some stuff behind.  No calibration is ever perfect, so this is somewhat to be expected.  You are then registering the images.  When you do this, it shifts the subexposures to align the stars.  This also shifts the stuff that was left behind by calibration.  When you stack the images, some of the artifacts remain.


So why are they organized into straight lines?


I am going to guess that you are either unguided, or using a guide scope.  As a result, you are either getting drift or flexure.  The effect of this is that all of your images are shifted by a small amount in the same direction for each subexposure.  This direction causes the remaining artifacts to be streaks.  To see the effect dramatically, just blink through your stack of uncalibrated and unregistered subexposures.  You will probably see your target slowly march in one direction over time.


There are three things that you can do to address it.


The most important is to dither.  By dithering, you destroy the organization of the straight lines.  This has two positive effects.  First, it makes the sigma rejection more effective during the stacking.  And second, any remaining artifacts lack organization and don't appear nearly so obvious.  This will take care of most of the problem by itself.


Next, you should consider off axis guiding (or on axis guiding, depending on your setup).  This will nearly eliminate flexure, and (without dithering), your images would need near zero shift to be aligned.  You still want to dither, though.  If you don't, then any artifacts remaining after calibration would be reinforced during stacking.


Finally, you should look critically at your images before and after calibration.  Most of us start out by just using the default settings in whatever package that we are using.  In reality, there are a few options for proper calibration.  You want to learn exactly what's happening to the pixels and understand the way to get the most out of it.  You also want to learn to evaluate your calibration masters.  Nothing that you can do with the software settings will correct problems if the calibration masters are not appropriate for your images.  Oh, and the same thing applies to the stacking step.  The noise rejection settings during stacking make a difference - and different images may have different needs for this, so you might choose different settings for each image that you do.


I've listed them in order of importance to your images.  As it happens, they are also listed in ease of implementing.  Dithering is probably just a checkbox in your guiding or image capture software.  Off axis guiding is a hardware change that has some learning curve to setting up, buy once done is just as easy as a guide scope.  Calibration and stacking is a topic that has generated many threads on this forum, and there are probably whole books dedicated to it.


Getting all of these exactly right will really improve your images - and make them easier to process as well.

#3 moxican


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Posted 13 January 2018 - 01:06 PM

Well, thanks for the wealth of information. You are right, I was guiding, but not dithering. Drift I was getting for sure sine my polar alignment was not accurate, I am not positive about flexure though. In any ways it makes sense. Thanks again. 

I'll look into dithering and off axis guiding as well.

#4 Dan Crowson

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Posted 13 January 2018 - 01:22 PM



definitely do the dithering as that should help. You probably can't do off-axis guiding using a lens.



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