Helix Nebula, NGC7293, In Narrowband

[Mike Lewis]

I have wanted to image this amazing object for quite a while, and this last imaging session I was finally able to get some time on it.

The Helix Nebula is the closest planetary nebula to the Earth, at an estimated distance of about 650 light years. This then also makes it the largest planetary nebula in the night sky. Planetary nebula have nothing to do with planets, but are created when some stars, nearing the end of their stellar lifetime, begin to blow off layers of material. This object is low in the sky, even imaged from Southern AZ, but still makes for an interesting object, even with a 100mm refractor. The Helix Nebula has sometimes been referred to as the 'Eye of God', as well as the 'Eye of Sauron'.

This was imaged in the standard 3 NB filters (Ha, OIII, and SII) as well as a small amount of RGB data for the stars. The SII signal was very weak compared to the other 2, and in retrospect I might have been better off to put that integration time into the collection of more Ha and OII instead, although the SII is being used in the color mix.

Palette:
Red: (1.1 x Ha) + (0.5 x SII)
Green: (0.5 x ha) + (0.5 x OIII)
Blue: (1.2 x OIII)
RGB mixed and used for the star field

Rescaled to keep all channels inside of available dynamic range

The SII was blurred to mitigate the higher noise levels, as it was being used for a hint of color and not detail. Most of the detail is coming from the Ha channel, which is typical as it is almost always the strongest NB channel. In this case there is a pretty decent OIII signal as well, which accounts for the blue in the middle of the nebula. The small blue dot in the middle is the star responsible for the beautiful display. It is on its way to becoming a white dwarf, a star that has consumed its fuel but that will still shine due to residual heat for many billions of years. In fact, it is estimated to take ~15 billion years for a white dwarf star to cool to the point of not being visible - longer than the estimated current age of the Universe, at 13.8 billion years. So every star ever created in the Universe that is small enough not to go out as a supernova is still shining today...

Equipment:
ZWO ASI1600MM-C Camera @ -20C and
Gain:200 Offset:50
Software Bisque MyT Mount
Stellarvue SVQ100 Astrograph Refractor, 580mm @ f/5.8
Innovations Foresight ONAG

Software:
Pixinsight Commercial Version 1.8
Lightroom CC
Photoshop CC
Innovations Foresight SkyGuard

Light Frames:
Ha: 41 x 300 secs ( 3 hrs 25 mins)
OIII: 37 x 300 secs (3 hrs 5 mins)
SII: 34 x 300 secs (2 hrs 50 mins)
Red: 11 x 30 secs (5 mins 30 secs)
Green: 12 x 30 secs (6 mins)
Blue: 12 x 30 secs (6 mins)

9 hrs 37 mins 30 secs total

Dark Frames:
10 x 60 secs, RGB (30 mins)
10 x 360 secs, Ha,OIII,SII (3 hrs)

Bias Frames
100

Flat Frames
20 each filter

Comments and critiques welcome.

ML

 

[Jameel Hyder]

Amazing it is Mike. Enjoyed the details about it as much as the image. Nice work.

 

[AlanLichty]

Lovely result - looks printworthy. I like how you rendered the colors.

 

[Ben Egbert]

Mike this is really nice, looks like an eye. Great detail and very clean looking.

 

[JimFox]

This sure looks really sweet Mike! That's one super cool DSO to get to capture.

I noticed you used 100 Bias frames, 10 times the amount of darks and almost as many as the regular light frames. I typically do 20 dark, 20 flat and 20 Bias. Should I be using more Bias frames when I stack?

 

[JimFox]

Another question Mike. So your camera is a monochrome camera right? How do you get RGB stars from it then?

 

[Mike Lewis]

Jim,

2 answers:

1) I did 100 bias frames because they are so quick to do. But in reality, I am not convinced the difference of using 20 or 100 bias frames to make your master bias frame makes much real difference. So I would not worry about it. I think 20 dark frames is better than 10 too, but when you are taking 6 minute subframes like I was for a number of these images, that translates to 2 hours for 20 dark frames per filer. So that gets to be a bit tedious.

2) So I get colored RGB stars the same way I get colored NB images. I not only have separate filters for narrowband, but I have separate filters for colors, and also one filter that is for luminance (a fancy way of saying it lets in all the frequencies). So my RGB stars are the result of taking separate frames through a red, then a green, then a blue filter and combining the stacked master frames from each color to make a composite color image. You can see then from the Light Frames section for this image above that I used 6 separate filters for the image, Ha, OIII, SII, Red, Green and Blue, with the number of subframes taken for each filter listed. That means I made 6 separate master frames, one for each filter. The NB frames were stretched, then the stars were removed, then they were color combined (I am leaving out some steps, but you get the idea.) The RGB stacks were stretched and then color combined. And then the starless NB color composite frame was combined with the RGB frames, that were stretched only to show the stars. That can be done pretty easily in either Photoshop or Pixinsight - I have used both. I did not use the luminance filter for this image, ands I am finding I do not often use it for NB images.

Hope that is all clear.

ML

 

[JimFox]

Thanks for the answers, that sounds like a lot of work to get RGB stars!

One more newbie question, I hope it's not sounding too dumb. If there is a Narrowband, what's the opposite of it? Wideband? What makes it a Narrowband? Is it called that because you are using the Ha OIII and SII filters since they let in a Narrow wave length?

 

[Mike Lewis]

Thanks for the answers, that sounds like a lot of work to get RGB stars!

One more newbie question, I hope it's not sounding too dumb. If there is a Narrowband, what's the opposite of it? Wideband? What makes it a Narrowband? Is it called that because you are using the Ha OIII and SII filters since they let in a Narrow wave length?

Jim,

Not a silly question at all.

Basically yes, your response is correct. Narrowband filters come in different bandwidths (with the cost adjusted to match.) less expensive ones might have a passband as wide as 12 nanometers (nm), while high end ones get down to 3nm width. The visible light spectrum spans from about 380nm to almost 700nm, with human eye sensitivity falling off at the edges of this range, so even a 12 nm filter is a pretty narrow slice of spectrum. RGB filters are wide enough to let in all the spectrum for that 'color' - so Red might cover ~620 - 740 in an astro filter fopr instance. And a Luminance filter is made to collect the entire range of the visible spectrum. Because NB emiision filters are designed to pass the wavelengths of light for a particular energized element they have specific passbands. This aloows them to accentuate that energy while blocking other light sources, like light pollution, the moon, and airglow, to provide higher contrast images. That is why you can image with a Ha filter when the moon is out and not be affected too much by the moonlight in your captured frames.

ML

 

[Douglas Sherman]

Beautiful work, Mike.

 

[Tom Narwid]

As good as any I have seen.