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NGC 1499 California Nebula in NHS

November - December 2016

The California Nebula is an emission nebula in Perseus.  Test shots showed that it had little Oxygen. So like the North American Nebula I decided to shoot with Nitrogen instead of Oxygen. 

The processing of these images was greatly influenced by the 2014 Katonah NY workshop.  I tried a couple different methods of mixing the colors, but the method we were taught in Katonah of picking a bright section and setting the white point their produced the dramatic colors seen in the image.

This was also one of the fastest times to complete a project. With the 1100AE I am no longer throwing out 50% of my images.  I collected the Hydrogen and Sulfur and then started on my Nitrogen.  I only had 9 good Nitrogen images when the weather turned nasty.  Their quality was good so I proceeded with test processing which produced results good enough to publish.

Map of NGC 1499

NGC 1499 in NHS

click for a 1/2 size image

PixInsight processing tends to emphasize edges thus this looks different than some of the other narrowband images I looked at.  Still I like a display that shows how much is happening in the nebula

Choice of Palette

I tried a couple different combinations for this image.  The most striking was NHS.  Again I did not use Oxygen since I found their was little in the image.

Nitrogen=red Hydrogen=Green Sulfur=Blue

Processing Details

Data was collected in November and December 2016 mostly at -30C.  The low temperature helped reduce the number of images required

Hydrogen 15x900
Nitrogen 9x900
Sulfur 15x900

Color Calibration

As I learned in Katonah instead of trying to preserve the relative levels of the filters we used color calibration on the brightest parts of the nebula as a "white" reference.   This resulted in a correction of


I tried an alternative method where I stretched each color separately and then combined the non linear images together.  That produced much more muted colors than just doing as I was taught.

Managing star shapes and size

One of the most important things we learned at Katonah was how to build good star masks. PixInsight provides a tool for doing this directly, but it is difficult to fine tune the results of the tool.  At the workshop we learned to use MLT to perform an initial extract.  We then used Curves, MT, and convolution in whatever combination gave us a satisfactory mask. Once I had the masks I protected the stars particularly from the sharpening operations. Different operations required different masks.  Sharping operations used relatively broad masks while color operations used narrow masks.

Many of these techniques are also covered in IP4P section 3 PI-11 and PI-12.

Managing star colors

Since the palettes used in narrowband are arbitary the result of combining the three colors is nothing like real star color.  Add to that that the red and blue channels are stretched much more than the green and you end up with really ugly rings.  I addressed this in several ways.  First using the masks described above I desaturated the stars. That sets them to white (although technically grey). I made several passes to insure both the large and small stars were fixed.  Ultimately some were so badly damaged that I ended up replacing all three channels of the stars with the original un-colorcorrected H channel that received a simple HT.

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