Celestial hemisphere:  Southern  ·  Constellation: Virgo (Vir)
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SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm

SH2 Catalogue Highlights (3 Posters - SHO, HOO & H)

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SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm

SH2 Catalogue Highlights (3 Posters - SHO, HOO & H)

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Description

Note – last month I posted a similar set of images of my favorite Sharpless objects.  Several of you commented and asked for more guidance about which specific narrowband filters were used and about where the objects are located.  This post is a complete rework in response to that request. 

Dr. Stewart Sharpless, born in my hometown of Milwaukee, published his first “Catalogue of HII regions” in 1953. His second (Sh2) and final list was published in 1959.  The list includes 313 objects north of declination −40°.

The Sharpless Catalogue contains some of the strongest emission nebulae in the night sky.  These objects make for great images.  For some of these nebulae, their emission strength is so strong that normal RGB (or LRGB) imaging of them can provide good results without the use of narrowband filters.  In my experience, the Sharpless objects which are most suited for RGB-only imaging are Sh2-9 (Rho Ophiuchi), Sh2-279 (Running Man Nebula), and Sh2-281 (Orion Nebula). 

However, in many cases, narrowband filters are used to bring out the faint details of the emission nebulae.  These filters are typically hydrogen (HII), oxygen (OIII), and sulfur (SII).  The resulting narrowband subs can be assigned to the RGB color channels in many different ways.  For full narrowband, I prefer the conventional SHO palette which assigns sulfur to red, hydrogen to green, and oxygen to blue.  For bi-color work (when the SII signal is minimal), I prefer the HOO palette which assigns hydrogen to red and oxygen to both green and blue.

The narrowband results of the attached 3 posters consist of the following 3 narrowband combinations.  The use of a combination for a specific object is dependent upon the relative strength of the 3 narrowband channels for that object:
  1. SHO Palette - The first poster shows my 25 favorite nebulae when narrowband is captured in all 3 channels (SII, HII, and OIII) and combined using the full narrowband SHO palette.  For these nebulae, the subs show a decent signal level in each channel. Although the level of signal typically is greatest in HII and least in SII, there is enough SII to make it worthwhile to add it as a third channel. These nebulae are distinguished by their deep orange and blue colors throughout, often supplemented by hints of other colors such as yellow, purple and red.
  2. HOO Palette – The second poster shows my 25 favorite nebulae when narrowband is captured in 2 channels (HII and OIII) and combined using the bi-color HOO narrowband palette.  These nebulae are distinguished by their red and cyan colors throughout.  The SII signal for these nebulae is weak.  If used to develop SHO images for these nebulae, the results will look washed out with less detail than their HOO counterparts.  I sometimes generate results using both palettes and then pick the one which provides the best combination of color, detail, and contrast, although my preference is to make the decision to use HOO up front so that I can allocate the SII time to obtain more HII and OIII subs.
  3. H Palette - The third poster shows my 25 favorite nebulae when narrowband is captured in only 1 channel (HII) and combined with the red channel of the RGB result.  These nebulae are distinguished by their deep red colors throughout.  Both the OIII and SII signals are weak for these nebulae.   Because only 1 narrowband channel is required, I am able to obtain more HII subs for the same total integration time and therefore capture significant detail for the HII channel.

The location, object and filter data for the nebulae on these 3 posters is shown in Revisions D, E and F.  My upcoming eBook and spreadsheet will include hyperlinks, mouseover images, and more technical data for these objects.

Please note that the above categorizations are simply my recommendations - any of these objects can be imaged in all 4 methods (RGB, SHO, HOO, and H) given enough integration time and processing.

In Revison G (and the mouseover above), I have included a skymap of the object locations.  The map shows the entire sky, with RA (right ascension) shown on the horizontal axis and Dec (declination) shown on the vertical axis.   Note that Dr. Sharpless numbered his objects in the order of right ascension, starting at 17 hrs. The Milky Way is depicted in blue. The transit date is superimposed on the horizontal axis.  This is the date when the object transits (crosses the meridian) at midnight.  During daylight savings time, transit occurs at 1 a.m.    If you want image an object at transit earlier in the evening, simply use the fact that objects move across the sky at a rate of 2 hours per month.  For example, if it is December 21 and you want to image an object which has an 8 p.m. transit time (4 hours before midnight), look for objects which have an October 21 transit date (2 months ahead) on the poster above.  

A final note - although narrowband imaging is great for capturing the nebulae details, the resulting star colors will not be “true color”.  This is not a significant factor for some people, but I prefer having true star colors in my images.  Consequently, for narrowband imaging, I always take a set of broadband (RGB) subs in addition to the narrowband subs.  I then combine the narrowband and broadband subs together for the final image.

Comments

Revisions

  • Final
    SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    Original
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    B
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    C
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    D
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    E
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    F
  • SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm
    G

B

Description: HOO Highlights

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C

Description: H Highlights

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D

Description: SHO Technical Data

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E

Description: HOO Technical Data

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F

Description: H Technical Data

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G

Description: SH2 Object Locations

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Sky plot

Sky plot

Histogram

SH2 Catalogue Highlights (3 Posters - SHO, HOO & H), Gary Imm