7 replies to this topic

[bill w]

curious why there's a dark area to the left of the comet which i didn't notice until after stacking:

shadow of the nucleus, shock wave, other phenomenon, artifact?

stack:

 

close up:

 

single frame:

 

7/21/20 approximately 9:20 PM pacific

sheep hole valley wilderness just north of eastern joshua tree

rented a nikon 850 D and 85 mm lens which pretty much blew away my D 60.

can be seen in this image on CN

 

any input appreciated

thanks

[Tom Glenn]

Looks like an artifact in the background to me.  I've been grappling with some comet shots recently, and I've found the processing to be somewhat tricky.  Gradients have been a major problem, and depending on how you remove them you can get uneven splotches in the background.  What do your raw subs look like?  I have recently reprocessed a few images, and don't see anything like that shadow you are showing.  See below. 

 

https://www.cloudyni...ail/?p=10392934

https://www.cloudyni...020/?p=10371989

[Tom Glenn]

OK I see you did include a single frame.  I'm not seeing the dark patch. Small variations in the background could have been amplified somehow during processing to yield that clearly defined darker patch.  

Edited by Tom Glenn, 04 August 2020 - 05:02 PM.

[Tom Glenn]

Also, what program did you use for stacking, and is this aligned on the stars, or did you process a separate star stack and comet stack, and then merge, or something else?  Fwiw I was using DSS initially, and found that it does a relatively poor job of combing the comet with the stars.  I'm testing out PixInsight now, and find it does a better job.  For a shot at small focal lengths, such as yours (and mine), I've found that just aligning on the stars and accepting some blur to the comet also yield a good result.  But the methods that were used to compose the stack, and then post process it could definitely introduce some artifacts around the comet itself.  

[bill w]

stacked in DSS comet and star aligned, worked like a charm for me. 

here's a single frame straight out of photoshop after import with no processing:

i think the dark patch is there to the left, but very subtle

 

here's the raw stack straight out of DSS, no processing:

[Tom Glenn]

Well, I'm just not seeing much (if any at all) of a dark patch in the raw frame, compared to the very distinct region in the processed stack.  Your raw frame has fairly uniform background and noise in that region, whereas the processed stack has a distinct patch that looks very irregular.  My feeling is that it's an artifact, but I'll let other comet imagers weigh in.  

[bill w]

gotcha

thanks for looking tom

[Tom Glenn]

gotcha

thanks for looking tom

I'm no expert at this type of imaging, but it seems plausible to me that the source of the dark area could be related to the recombination of the star image and the comet image.  DSS does this automatically, so I don't know the exact methods it uses, but having attempted a few of these in PixInsight, you can tackle the issue in multiple ways, but essentially the comet has to be subtracted from the image aligned on the stars, and then this is added to a starless comet aligned image.  For small comets this is pretty easy, but for a complex comet like NEOWISE, with such a large and bright tail, this is much trickier.  There can be remnants left behind, and I can easily envision a scenario in which the final result has some strange areas directly around the comet.  As it turns out, I was imaging on the same day as you, only about 25 minutes later, and only 100 miles away, at Mt. Laguna, CA.  I posted a raw sub in one of my posts, linked below, and again, there is no shadow near the nucleus.  

 

https://www.cloudyni...020/?p=10364729

 

But regardless of what the dark area is, nice images of the comet!

Edited by Tom Glenn, 04 August 2020 - 09:42 PM.