M 42 in HOS using HDR
December
2017
February 2018
The Orion Nebula is one of the most iconic deep sky objects. It one of
3 or 4 Messier objects naked eye visible from here. M42 has always
been on my to do list. Winters here are usually warm, but
cloudy. In past years I have used the limited time available for more
obscure objects. This year a small window of dark, clear sky presented
itself in February. I decided to use the test shots from December and do a
mad rush to complete this object in less than a week
M42 is a object that cannot be adequately imaged in a single
exposure. Hence the use of High Dynamic Range or HDR. With
this process images of several exposures ( each 16 bit integer) are
combined to form a single 64 bit floating point image. This image is
able to encode both the very bright and very dim. Originally I was
going to use just 60 second exposures for the Triangulum and 300 second
exposures for the nebula. The latter did well on M42, but did not
capture the surrounding H alpha clouds. Thus I had to add another
round of 600 second exposures.
I really love ACP Expert. Unfortunately ACP Expert adds a lot of
overhead. On my normal 900 sec images this extra time can be safely
ignored. If you are trying to shoot 60 60 sec images within 4 hours
then it cannot. Even with the 300 and 600 sec images I needed to
eliminate any useful, but unnecessary operations if I was going to
complete the project before the clouds returned. Thus I shot the
entire project directly with scripts in ACP. The scripts eliminated
plate solving and for the 60 second images eliminating RBI Flood (which is
itself 2 minutes). Doing these allowed me to capture > 15 images
at 60 seconds and 300 seconds in H, O, and S. I was only able to
capture Hydrogen at 600 seconds, but that was enough to add depth to the
gas clouds surrounding M42.
Annotated Image
The image is presented north up.
Close up of M42
This is a half size zoom into the heart of M42/M43. Clicking on
this image will bring up a higher resolution version.
Zoomable Image
The full size image is about 4096x4096. The following will allow the
reader to zoom into the image to explore it more closely. I like this
version since it places the nebula in the context of the surrounding
Hydrogen clouds.
Processing Details
Here are counts of the quality accepted frames that were used for the
various filters. Naturally more images were taken. These
represented the ones that made the cut.
|
60 |
300 |
600 |
Hydrogen
|
15
|
24 |
15 |
Oxygen
|
18
|
25
|
-
|
Sulfur
|
14
|
23
|
-
|
All images were processed by Pixinsight.
Since Hydrogen is stronger than any of the other signals I increased the
initial contribution of O and S by 25%. They were combined with
PixelMath
Red = Hydrogen
Green = Oxygen * 1.25
Blue = Sulfur * 1.25
The resulting image was
VERY noisy outside of the bright inner nebula. The final result is a
tribute to TGVDenoise.
Here are some other processing items of interest
- Getting the proper stretch required some tuning. I used the
manual settings of ScreenTransferFunction to find a proper balance of
background and stretch. I then transferred these setting into HT
to make the image non linear.
- I used a Median MMT
- The initial image was very noisy. It required multiple passes of
TGVDenoise with a variety of different masks to smooth the various
sections of the image.
- My first attempt at mid range and background masks each resulted in
very noisy masks. Since the goal was to use these in TGVDeNoise
that was not acceptable. To fix this I ended up using TGVDeNoise
with very strong settings to smooth the masks. I found this worked
much better than Convolution. Naturally after I smoothed the masks
I had to remove the stars by placing a starmask over the range mask and
then setting all of the locations with stars to 0 (fully masked).
- LocalHistogramEqualization brought out more detail in M42.
Copyrights For Photos
(c) 2018 Robert J Hawley.
Except as noted,all work on this site by Robert J. Hawley is copyrighted
under a Creative
Commons
Attribution-Noncommercial-Share Alike 3.0 United States License. This
permits the non commercial use of the material on this site, either in whole
or in part, in other works provided that I am credited for the work.
3/11/18