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.
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.
Processing Details
Data was collected in November and December 2016 mostly at
-30C. The low temperature helped reduce the number of images
required
Filter
Exposure
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
Nitrogen
1.7
Hydrogen
1.0
Sulfur
4.2
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.
