This project was a lot of fun!  First, these two galaxies are pretty bright, very detailed, and really thought provoking.  Second, the faint tidal tails cover a pretty big region and it's interesting to figure out how to capture the details in the core and still pull out the faint tails.  I had in mind trying to mix in some Ha signal to amplify what I thought might be some interesting detail in Ha so I invested 25 hours into taking 150 10 minute subs in Ha.  When I went to process the pure Ha signal, I discovered that when you subtract the scaled red signal from the Ha signal, there is virtually no Ha signal remaining!  I though that for sure, I'd find some interesting features emitting Ha; but, no dice.  If it's there, it's at a VERY low level--below what I could find in 25 hours of exposure.

These two galaxies appear in the constellation Corvus at a visual magnitude of 11.2.  They are thought to be at a distance of about 45 Mly.   Around 1.2 billion years ago, the Antennae were two separate galaxies.  It is thought that NGC 4038 started out as a barred spiral and NGC 4039 was a spiral galaxy.  So, how do we know?  It turns out that these galaxies were used to develop computer models that could fairly accurately predict how galaxies interact in a collision.  The result of this modeling suggests that as these two galaxies passed through each other, stars and gas were released forming the long tidal streams known as the Antennae.  Modeling suggests that within another 400 million years, the nuclei of both galaxies will collide to eventually become a single core.  Models show that the overall shape will ultimately stabilize to become an elliptical galaxy.  It is fascinating to consider the scale and magnitude of this kind of collision.  Indeed, what would the sky look like from inside?

I started shooting this target while I was still in Chile and that provided an opportunity to compare data from my 20" with data from the 24" that  @Matt Dieterich kindly shared.  We were both imaging the same object on the same night under the same sky from the same location with the same camera model at nearly the same focal ratio (F/6.5 vs F/6.8 ) so it was an apples to apples comparison. We were also very lucky that the seeing was quite good that night--maybe just a little better than 1". With a difference of only 4" in aperture, it was hard to see a huge difference in image quality between this two scopes.  Some regions were a bit sharper with the 24" and some were sharper with the 20".  You can see for yourself in rev B.  These were two single raw, uncalibrated subs from each scope.  We both used 600s exposures but Matt was binning 2x2 with different gain/mode settings. I was binning 1x1 with Mode 3, gain 25.  Either way, I love comparing results from different scopes.  Take a look and see if you can tell which one is which.  I'll put the answer at the end.

I set the upper bound on FWHM to be at 1.8" so the average might be around 1.6".  When I first sat down to process this data, I had plenty through the Lum filter but G and B data was thin.  So, I waited through a lunar cycle and grabbed more--actually a LOT more.  Things were running well and I was busy so I didn't bother to change to a new object until I had like 150 subs for each channel!  Even after selecting the best, I still wound up with over 38 hours of useable data.  That's a lot for such a bright object.

The biggest challenge is getting the color calibration right and I struggled a bit with it.  My first "quickie" test run using only the RGB channels showed pretty good balance but with too much green.  I quickly discovered that using SCRN to minimize the green fixed the problem but it left too much noise in the orange regions.  Hmm...I guess that I have seen that happen before; but, what to do about it?

Next I went back to try PCC.  Lately, I've had a number of conversations with folks around here about it and I've never had much success with it but the tool has had a number of updates so I wanted to give it one more shot.  Once again, it produced a balance that was WAY too warm with very few blue stars and mostly white stars in the background.  One thing I wanted to experiment with was to try different star catalogs to see how that might affect the results.  No dice.  I could only get PI to connect to a single catalog--and that one would only match around 120 stars in this field and most of them were pretty bright.

Finally, I started just fooling around with the PCC corrected data and defined a white region around the core and a dark background area.  I used that for calibration and it came out "sort of" right.  The blues were still very diffuse--mostly bright baby blue.  So I set up a preview on the core to look at the colors and for some reason, I re-stretched the preview and BINGO, the colors looked perfect!  There were deep blues, bright oranges and yellows.  Hmm, I've never had that happen before.  So I took the parameters from the preview, copied them to the HistogramTransfer tool and stretched the whole image--and that did the trick.  I suspect that regions outside of the galaxy cores were screwing up the computation of the global stretch values.  I don't think that it was the over-scan regions but at the time I didn't stop to do the right experiment to test that idea.  Maybe that was the problem.  Either way, I liked the result so I rolled with it.  I did have to go back to fiddle with the yellow/blue balance just a hair using the AssistedColorCalibration tool and that moved the most of the yellows to become more orange.  I still liked the result so I went with it.

I went through 18 versions before I settled on this one.  Normally with that many versions, I might go back for another round from the beginning but those 18 changes were all pretty small to fix one tiny thing after another.  I just couldn't spot them all at once.  Hopefully it works.  As usual, C&C is always welcome so fire a way.

John



PS. The 24" data is on the left and the 20" data is on the right.  If you looked carefully, there are a couple of good clues indicating which I which so I hope you spotted them...