Sunday, January 30, 2011

Sword of Orion on the ST80

The Sword of Orion is a group of open clusters and nebulae located south of Orion's Belt. The most prominent object in the Sword is the Great Orion Nebula, cataloged as Messier 42. The Great Orion Nebula is one of the few nebulae that are visible to the naked eye--the Lagoon Nebula is another.

I once imaged the Sword of Orion with the Epsilon-200, and the results were OK in my opinion. I intend to redo it on the Epsilon-200 when I get a chance. A while back I also imaged the Sword with my ST80 on the EQ-1 mount. Most of the subs were useless, and I only took 30-second exposures. The results were promising, though.

Last week I had the opportunity to re-image the Sword of Orion with the ST80. I used the Vixen GP mount and the Canon EOS Rebel XS camera. I had to take multiple images at different exposure times because the contrast among different parts of the nebulae is so great.  Here is the breakdown of sub-frames:

  • 10x1 second (that is, 10 one-second images)
  • 10x5 seconds
  • 10x10 seconds
  • 10x20 seconds
  • 23x60 seconds
  • 17x120 seconds
I stacked each group of exposures separately, then loaded the six final images as layers in GIMP. I brought out the detail in the brighter parts of the nebulae by using layer masks to cause the shorter-exposure images on lower layers to show through. (If anyone is curious how to do this in GIMP, send me a message and I'll explain in greater detail in another post.)

After the layer masks were complete, I flattened the image and then applied my usual fringe mitigation process.  (Again, e-mail me if you're curious how I remove the purple fringes from the stars.)

Here is the final product:

Sword of Orion, ST80 on Vixen SP, multiple exposures
There are several separately cataloged objects in the Sword. The group of stars on the left edge of the image is an open cluster that is cataloged as NGC 1981. To the right of that is the Running Man Nebula, which is a combination of diffuse nebulae NGC 1973 and NGC 1975, and open cluster NGC 1977. Near the middle of the image is De Mairan's Nebula, which is cataloged as Messier 43. To the right of that is Messier 42, The Great Orion Nebula. (To me, M43 looks like a bird's head, an M42 looks like the bird's body and wings.) Near the right edge of the image is diffuse nebula NGC 1980, which is illuminated by a 2.75 magnitude class B star called Na'ir al Saif (Arabic for "the Bright One in the Sword").


  1. Very impressed with what you are able to do with the ST-80, mate. Really good stuff.

  2. Congratulation! This is a really good picture, far better than any of my tries. Yes, it would be most appreciated if you could post about how this picture was processed step by step using GIMP. I think a lot of us could learn so much from it.

  3. Will do! I'll post something within the next couple of weeks.

  4. I wrote up instructions on how I processed the Sword of Orion. Check out the following post:

  5. Would really like to hear how you deal with the purple fringing. I've played with some GIMP scripts but they aren't up to much - a semi-automated way would be very welcome!

    1. I've been meaning to write something up on this for some time. I'll try to post something soon!

    2. It took me a long time to get to it, but I finally wrote up some articles on how I deal with purple fringing. Here is the link to the first article:

      I've also dabbled with the Darla Purple Fringe Fix in GIMP, but the results are not always consistent. Here is the link to the plug-in if you haven't already tried it:

  6. Great! Much appreciated. I stumbled across this blog since I also have an ST80 on the tabletop (non-motorised) EQ1. I prefer to use it visually on a camera tripod as a convenient travel scope, but I think I might invest in the cheap RA motor for the EQ1 so at the very least I can attach my DSLR to it for some widefield (and limited DSO) testing using a lens up to 200mm in length. Do you happen to have any idea how long an exposure the ST80 could sustain on this cheap mount/motor? Is it even worth bothering with?

    1. I used a full-size EQ-1 with one of the "simple type" drive motors early on with the ST80 for astroimaging. It didn't work very well. The drive had an adjustable speed, so it was really difficult to find the right setting for good tracking. This may not be an issue with the EQ-1M motor that Orion sells now. The other problem was getting decent polar alignment. Without a polar alignment scope I had to memorize the positions of some of the stars around the NCP and visually estimate its location in an eyepiece. Alignment was rough, at best. Still, with some patience I was able to get a few decent 30-second exposures on the ST80 now and then. It worked pretty well for wide-angle shots with my zoom lens set at 18mm.

      Here are some images on the blog that I acquired with the ST80 on the EQ1:

      The images of Messier 44 and Messier 93:

      The wide-angle images of the Orion constellation and the image of the Sword of Orion taken with the ST80:

    2. Also, the image of Messier 78 in the second link was taken with the ST80 on the EQ-1.