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Posts Tagged ‘o-iii’

The Astrophotography Sky Atlas

November 22, 2015 Leave a comment

TASA Cover 500px

The Astrophotography Sky Atlas is now available at Amazon!

I spent two years coding, researching, and writing this book with a simple goal:  to create a compact, reasonably-priced atlas designed for the imager. Over 2000 deep-sky objects are plotted in their correct size and shape, including many faint nebulae not shown in other atlases. Stars are shown down to 9th magnitude.  The entire sky is covered in 70 full-color charts.

A tabular index contains important details on each object, including a description, the best time of year to capture it, and the required field of view.

What’s shown:

  • 416 emission nebulae and supernova remnants, including the complete Sharpless (Sh2) and RCW catalogs.
  • 171 reflection nebulae, including the complete van den Bergh (vdB) catalog.
  • 146 planetary nebulae, including the complete Abell catalog
  • 52 dark nebulae and molecular clouds
  • 792 galaxies (larger than 3 arcminutes)
  • 38 galaxy groups from the Abell and Hickson catalogs
  • 108 globular clusters (larger than 5 arcminutes)
  • 309 open clusters (larger than 5 arcminutes)

Keeping a focus on what is important to imaging, sparse open clusters and galaxies smaller than 3 arcminutes (unless part of a group) were left off the maps.

With information on nearly every possible photographic object in the night sky, The Astrophotography Sky Atlas will help you choose your targets and plan your imaging.

The Cave Nebula (Sh2-155)

November 22, 2014 Leave a comment

The Cave Nebula is probably better suited for 600 mm of focal length, but I thought I’d try a wide field view of the area.  The whole nebula is fainter than I expected, especially the diffuse OIII spread about the region.  20- or 30-minute sub-exposures would do a much better job of revealing it, but because it was so windy, I wanted to keep the subs at 10 minutes, so I really had to fight a lot of noise in the blue channel.

The Cave Nebula (Sh2-155) in narrowband SHO palette

The Cave Nebula (Sh2-155) in narrowband SHO palette

Image data:

  • Exposures: 55 x 10 min Ha, 28 x 10 min OIII, 28 x 10 min SII
  • Telescopes: Two William Optics Star 71s (360mm f/5)
  • Cameras: SBIG ST-8300M and QSI 583wsg, 2×2 binned
  • Mount: CGEM
  • Guiding: QHY 5L-II mono, guided using PHD2
  • Conditions:  gusting winds
  • Processing: DeepSkyStacker -> PixInsight -> Photoshop
  • Date: Nov 9-10, 2014

NGC7000

November 18, 2014 Leave a comment

This is not exactly a shining moment as an astroimager, because I completely messed up the polar alignment routine, resulting in smeared stars.  (When the CGEM tells you not to pick polar alignment stars too close to the pole, it means it!)  But NGC7000, more commonly known as the North America Nebula, is such a compelling object, I’m posting the image anyway.  It also gives a sense of how wide the field of view on the Star 71 is.

NGC7000 "The North America Nebula" in Narrowband

NGC7000 “The North America Nebula” in Narrowband

Image data:

  • Exposures: 34 x 10 min Ha, 14 x 10 min OIII, 17 x 10 min SII
  • Telescopes: Two William Optics Star 71s (360mm f/5)
  • Cameras: SBIG ST-8300M and QSI 583wsg, 2×2 binned
  • Mount: CGEM
  • Guiding: QHY 5L-II mono, guided using PHD2
  • Conditions:  wind gusts over 20 mph, gibbous moon
  • Processing: DeepSkyStacker, PixInsight, Photoshop
  • Date: Nov 2, 2014

IC1396 (most of it) and the Elephant’s Trunk

October 23, 2012 Leave a comment

This is most of the nebula IC1396. IC1396 is huge, about the width of 10 full moons, and it lies in the constellation Cepheus. In astronomical terms, it’s pretty close at only 2000 light years away. The structure in the bottom right is the famous “Elephant’s Trunk,” (vdB 142) which is thought to be an active star-forming region. The largest black dust lane in the middle is Barnard 161.

[ASIDE:  There is a tiny blue blotch directly below Mu Cephei, the big yellow star on the left.  It sticks out like a sore thumb in the O-III images, is visible in the H-alpha images, but it’s absent from the S-II, hence its blue color here.  After a good bit of research, it appears that this semi-anonymous planetery nebula is known as PN G100.4+04.6.  I can find very little else about it other than the basic stats in SIMBAD.  If anyone knows more, let me know.]

As with most of my recent images, this one combines images taken through narrowband filters into a mapped color image where the Sulfur-II emission line is mapped to red, Hydrogen-alpha (+Nitrogen-II) to green, and Oxygen-III to blue.

Image data:

Exposures:  13 x 1200s Ha , 22 x 1200s O-III, 12 x 1200s S-II (15h 40m total)

Software:  guiding by PHD, stacking in DeepSkyStacker

Processing:  Photoshop CS3, modified Hubble palette

Telescope:  Televue NP101 with 0.8x reducer (at about  f/4.3)

Camera:  SBIG ST-8300M with Baader standard narrowband filters, 2×2 binned

Mount:  CGEM

October 17, 20, and 21, 2012

In Defense of Mapped Color Imaging

October 21, 2012 Leave a comment

Bob Berman recently wrote a column in Astronomy (October 2012) where he says of mapped color imaging, “It’s Disneyland meets Sagittarius.  But is it ethical?”  He admits that “Emission nebulae are always the same repetitious shade of red,” but describes (accurately, I’d allow) Hubble palette images as “fake colors” and “lovely but unreal.”  All fair points.  And it seems that the Astronomy editorial board generally agrees–in the same issue, they show 100 of the “greatest” pictures of the universe.  Less than 20% are mapped color images by my estimate, and nearly all of those are from either ESO or NASA.  Images of emission nebulae from amateurs, with only a couple of exceptions, are limited to true-ish colors.  I take the message to be, “we like our nebulae red, unless you are a professional.  (It’s still okay to turn the saturation knob to 11 for galaxy image, though.)”

The question of ethics in image processing goes back to the beginning of photography, though electronic imaging certainly brings the issue to the fore.  Should we expect an image to exactly represent reality as we would perceive it?  The question comes up most frequently with fashion photography:  when a model’s skin is digitally airbrushed, is that an acceptable improvement or has a line been crossed?  But images have been enhanced for nearly as long as they have existed.  Ansel Adams extensively burned and dodged his prints, not only to increase the perceived dynamic range, but also for purely aesthetic reasons.  These were subjective alterations designed to emphasize the more interesting objects by creating artificial contrast.

The problem with such questions is that they are by definition subjective:  an image is never an accurate rendition of reality.  Cameras and lenses are nothing like our visual system, so photography’s relationship with reality can only be described on a spectrum (pardon the pun) that starts with “slightly inaccurate” and goes to “completely fabricated.”  If we truly limited ourselves to staying close to the way our eye-brain system “sees” things, then nearly everything would be out of bounds.  No wide-angle or telephoto lenses. No exposures longer than a second, so no  images where motion is blurred.  The very fact that we use long exposures in astronomical imaging should disqualify every image, since the human eye can integrate less than a second of photons in total.  Certainly any radio frequency, x-ray, or ultraviolet spectrum images could not be allowed, and at best you could argue that they should be monochromatic.

The point is that images are not reality, but aesthetic representations of reality.

It seems that Mr. Berman’s primary issue is with mapped colors in narrowband images.  I see the point.  Not only are we assigning light from specific emission lines to different colors in the final image, but we also equalize these spectral lines so they are approximately equally bright, when the reality is that one of them (H-alpha) is typically far brighter than the others.  H-alpha, N-II, and S-II are all red.  O-III is blue-green.  That is the extent of our palette for the vast majority of emission objects in the sky, which when rendered as the eye would see it amounts to red.  And visually, they are all too dim to trigger cone cells anyway, so they are all grey unless you can put an eyepiece in the Keck telescope (which would have too narrow of a field, even if you could!).  The diagram below shows how the Hubble palette maps the narrowband emission lines to RGB color.

For me, mapped color narrowband images reflect reality even better than a “true-color” image.  Nearly all of the visual spectrum energy from these objects is emitted in these three or four narrow wavelength slices, each one emitted by a different type of ionized gas.  Emission nebulae are not broad spectrum objects, so to render them the same way we would a galaxy, or for that matter a family photo, seems to fall short of their potential.  We have readily available filters to separate these emission lines, so why would we choose to render them in a way that doesn’t distinguish them in the final image?

I avoid any arbitrary adjustments, additions, or deletions to my images, other than fixing optical defects like dust motes.  (I think Apple’s deletion of M110 from an image of the Andromeda Galaxy for the OS X Lion wallpaper was clearly beyond the pale, but then again, commercial images have different goals.)  I don’t believe that mapping emission lines from ionized gases in a nebula is augmenting reality; it reveals structure that is quite real, but would otherwise be unresolved.  I think it is perfectly acceptable to create contrast in luminance or color that reveals and emphasizes what is actually there.  Just because it’s not visible at the eyepiece doesn’t make it a fake.

Ced 214 (Sh2-171) and NGC 7822

September 23, 2012 Leave a comment

Cygnus is not the only summer narrowband delight in the northern hemisphere.  Cepheus also hosts some beautiful emission nebulae, two of which are shown here. The object on the left has the lovely name Cederblad 214, also known as Sharpless 171.  The object on the right (that I can’t help but see as a seahorse) is NGC 7822.

Ced 214 is fairly commonly imaged at longer focal lengths to capture its “elephant trunk” pillars of expanding gas, but here I’ve captured a little bit wider of a field at about 430 mm (on the KAF-8300 chip).  If you go even wider, the whole region is covered in nebulosity that when framed properly is known as the Cosmic Question Mark.

This image took five evenings in September to capture.  This object has a low surface brightness, especially in O-III.  This image represents just shy of 20 hours of total exposure time.

Ced 214 (Sh2-171) and NGC 7822

Ced 214 (Sh2-171) and NGC 7822

 

Image data:

Exposures:  16 x 1200s Ha , 16 x 1200s O-III, 27 x 1200s S-II (19h 40m total)

Software:  guiding by PHD, stacking in DeepSkyStacker

Processing:  Photoshop CS3, modified Hubble palette

Telescope:  Televue NP101 with 0.8x reducer (at about  f/4.3)

Camera:  SBIG ST-8300M with Baader standard narrowband filters, 2×2 binned

Mount:  CGEM

September 9, 10, 11, 12, and 19, 2012

The Western Veil Nebula and Pickering’s Triangle

September 19, 2012 Leave a comment

If Cygnus would stay in the sky all year, I don’t know if I could exhaust all of its imaging possibilities.  This is the Western Veil Nebula, NGC 6960 and Pickering’s Triangle (bottom left). Both are remnants of a supernova from about 5,000 years ago that also includes the beautiful NGC 6888 (not shown) among others.  This nebula is surprisingly bright in all three major narrowband lines, H-alpha, S-II, and O-III, which meant that I could capture a reasonable amount of data in a single night.

NGC 6960 in narrowband

NGC 6960 in narrowband

Image data:

Exposures:  9 x 600s Ha , 10 x 600s O-III, 9 x 600s S-II (6h 40m total), binned 2×2

Software:  guiding by PHD, stacking in DeepSkyStacker

Processing:  Photoshop CS3, tone mapping using modified Hubble palette

Telescope:  Televue NP101 with 0.8x reducer (at about  f/4.3)

Camera:  SBIG ST-8300M with Baader standard narrowband filters

Mount:  CGEM

September 14, 2012

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