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D'OH! Decent Helix Nebula, But Could've Been Better

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#1 Phillip Creed

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Posted 01 October 2020 - 02:58 PM

I did some imaging on the night of September 18, and was intent on getting NGC 7293 (Helix Nebula).

My problem was it was too low after dusk, so I got close to an hour on The Dumbbell Nebula in my AT115EDT using the 0.8X FF/FR.  I was a bit worried about only having an hour, but it turned out alright.

Once the Helix got 20° up, I decided to start taking images.  I had re-checked focus (it had drifted slightly), centered up NGC 7293, taking some test subs, and then let it go.  The plan was to image a good 2.5 to 3 hours.

Emphasis on the word, "plan".

You know that thing called a, "dew shield"?  It acts as a, um, SHIELD.  Against DEW.  Especially if the shield is USED.  'Cuz a shield only works if it's used.

Guess what happens when it's not?

Yep.  I forgot to extend the dew shield.  The result was that I was only able to salvage 78 minutes' worth of data.  But 1.3 hours is still 1.3 hours more than I started the night.

Both were shot from Wilmot, OH (~20 miles SW of Canton) with a Nikon D5300 using unguided 30-sec subs @ 800 ISO on a Sirius EQ-G mount.

Clear Skies,

Phil

Attached Thumbnails

  • NGC 7293.jpg
  • M27 9-20.jpg

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#2 sg6

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Posted 01 October 2020 - 03:10 PM

What I like is that it wet wrong. Sorry for putting it that way.

You have 2 good images, of effectively it reads just over an hours total and at 30 seconds each.

What I like is that if you read so very many posts you have to get several days worth of images, and if each is not a minimum of 600 seconds then you are doing it wrong.

 

You have good images at what "breaks the rules" of around 80 minutes total and each of 30 seconds.



#3 Jschroedl

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Posted 01 October 2020 - 03:10 PM

Amazing shots!



#4 Jim Waters

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Posted 01 October 2020 - 03:24 PM

These are very nice images Phil.  Nice colors and sharp stars. How were they processed?



#5 Phillip Creed

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Posted 01 October 2020 - 03:26 PM

What I like is that it wet wrong. Sorry for putting it that way.

You have 2 good images, of effectively it reads just over an hours total and at 30 seconds each.

What I like is that if you read so very many posts you have to get several days worth of images, and if each is not a minimum of 600 seconds then you are doing it wrong.

 

You have good images at what "breaks the rules" of around 80 minutes total and each of 30 seconds.

I can make up some of the difference through processing.  PixInsight is a godsend, and the Nikon D5300 DSLR I was using has a pretty good dynamic range if you keep the ISO low.  I typically shoot at 800, though I understand I could squeeze out a bit more dynamic range at 200/400 ISO.

If you're shooting 10 minute subs here in Ohio, you're either (a) doing narrowband, or (b) showing off.  Most sites I've been to in Ohio won't even allow for a 10-minute sub @ f/5.6 on a OSC or DSLR.   Mono's a different story.

One reason I haven't done guiding (though, admittedly, I SHOULD...) is that I pretty much have to burn a night to make sure I did everything right, and hope I don't have any software/hardware issues.  Oh, and a lot more power.  Maybe spending a night getting the process nailed down isn't a problem in some parts of the country/world.  But this state's weather is abysmal and clear, moonless nights are precious commodities here.  There's also family and work commitments that eat away into the time I can devote to imaging.

Unguided is NOT recommended long term.  I have to toss 20% of my subs (it's unguided at 644mm focal length on a Sirius EQ-G, after all), but I get enough to make it work.  What I need for the time being as a beginner is simplicity, and a mount, scope, DSLR and a $70 17-Ah 12V supply from Harbor Freight is about as simple as it gets.

Clear Skies,

Phil


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#6 Phillip Creed

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Posted 01 October 2020 - 03:38 PM

These are very nice images Phil.  Nice colors and sharp stars. How were they processed?

Stacked in Deep Sky Stacker and processed in PixInsight.  My main PXI workflow is generally:

 

Linear state:

Crop extreme edges to eliminate stacking artifacts
ACDNR Noise reduction
Automatic Background Extraction (typically division)
Color Calibration (and noting estimated background brightness)

Non-linear (various steps done in multiple iterations):
First histogram stretch to get peak ~15% over.
Use ABE/DBE if needed again.
Star mask
Morphological transformation (erosion) on masked stars (x2 if needed; I try not to do it full strength each iteration, though)
Range mask on nebula
Curves transformation on masked nebula (stretch a bit, but not too much)
Arcsinh stretch (preview to find estimated zero point, go to about 70% of the way there, then stretch by a factor of 2 or so.  Repeat if needed.
HDR Multiscale Transform with lightmask on
Deconvolution to sharpen it up.

It's a quick-and-dirty approach.  I don't have time to watch 17-part tutorials that are each 45 minutes long.  Okay; they're not that long, but you get the point...

Clear Skies,

Phil




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