It seems like Thor's Helmet has been imaged by everyone; but, it has been one of my great white whales.  I've always thought that it looked like a pretty interesting target but every time that I've gone after it, it got away.  It was just too low in the sky when I was imaging from Oregon and when I moved my scope down to New Mexico, it wasn't much higher.  Then the big problem was that the airmass was just too deep to tolerate anything but nearly perfect seeing.  I had a short stretch of acceptable seeing over a year ago in New Mexico but I could never get enough data to feel like I could turn it into a good image.  When I went back to look through the data on all my disks, I found numerous folders with a few images here and there of Thor's Helmet but up until now, I never had enough to do anything meaningful with it.

Early in March I was setting up for a new object with the 20" and there...high in the Chilean sky, was Thor's Helmet just laughing at me!  The moon was out so I set up to go after a narrow band image.  I had it in the back of my head that I wanted to try to add some RGB data to have "real" star colors so that part waited for the moon to wane.   I was pretty busy at the time so I just let the scope run for a long stretch of data gathering.  By the time I finished, I was absorbed in getting ready for AIC  and that required a big chunk of my time available for processing so I'm finally getting around to processing this data set.  Frankly, I was stunned to find that I had accumulated over 44 hours of useable data!  Of course when it comes to narrowband, you can never have too much data!

Thor’s Helmet is an emission nebula located in the Canis Major at a distance of 11,960 ly and has a diameter of about 30 ly.  At an apparent magnitude of 11.45, this is a popular target for RGB imaging.  Ha and O3 emissions are very strong so this is a great target for narrowband imaging as well.  Like the Bubble Nebula, the helmet contains a Wolf-Rayet star, which is an extreme hot star producing intense radiation pressure on the surrounding molecular cloud.  This is what forms the filaments and tendrils in the structures of the helmet.  The expansion rate of different parts of the nebula have been measured in the range of 10-30 km/sec.  This nebula has been studied in both radio and X-ray wavelengths to try to determine it's composition, age, and origin, but many mysteries remain.

I started with quite a bit of NB data and this still turned out to be a moderately challenging processing job.  SII emissions are VERY weak so I added 20% of the Ha signal to the SII to form the red channel for this SHO palette combination.  That boosted the SII regions to display a nice deep orange color to highlight the surrounding detailed molecular clouds.  I took a little bit of raw RGB data to create a true-color star field.  I used Russell Croman's Star Xterminator within PI to work on a starless version of the nebula before recombining it with the RGB Star field.  This region of the sky is dense with stars, which can detract from the nebula image, so I dimmed and reduced the star sizes using MorphologicalTransform.  Be sure to zoom in to check out the detailed clouds of dust surrounding the nebula.  It is very reminiscent of the structures surrounding the Bubble Nebula.

As with any NB image, there is a lot of latitude available for determining the colors.  I tried a number of different variations but this one with a vibrant blue helmet surrounded by rusty-orange clouds appealed the most to my eye.  I have to mention that when I was at AIC, I was horrified by how my images looked on the monitor that they had setup in the lobby.  They were WAY over-saturated and very blue.  Seeing that monitor reminded me that virtually none of us see the same thing when we view these images.  I do all of my processing on a 2019 MaxBook Pro with a Retina display so that may be the best machine to get close to seeing what I see on my screen.  I've got a new MacBook Pro arriving shortly and I'm going to look into calibrating its display.  I may even break down, reorganize my office, get a bigger desk, and  buy a big monitor that I can calibrate.  In the meantime, let me know if it works on your monitor.  Hopefully, it's not too far out of whack.

John