Markarian's Chain is a group of galaxies in the Virgo cluster that forms a large sweeping arc through the sky.  The last time that I imaged this region was in 2015 using an Edge14 with a Hyperstar reducer and a Canon EOS 6D DSLR.  At F/1.9 with 6 micron pixels, I only used about 3 hours of total exposure time.  Of course that was back when I used to roll my scope out and manually run everything so 3 hours was just about at my nightly  limit.  This image was made at F/6.7 with my GTX130 refractor with a field flattener using 3 minute exposures spread over a total of a little more than 15 hours.  The full field of this system is 1.58 x 2.37 (d=2.85, r=1.42) degrees so this image shows roughly half of the full field.  The entire image shows most of the "chain" and is full of galaxies but I preferred to crop the field for a slightly more detailed look at the some of the bigger, brighter, and more interesting galaxies.  M84 is noteworthy because of its 1.5 billion solar mass central black hole.  It lies at a distance of 54.9 mly and has an apparent magnitude of 9.1.  The most interesting galaxy in this field is NGC 4438.  It shows a highly distorted disk along with tidal tails and dust clouds that are thought to be due to gravitational interactions with other galaxies in the cluster--in particular NGC 4435.

This region was already fairly high in the sky when I started this project so I could only gather data for around 3-4 hours each night.  Given the amount of noise that I found in the data, I think that my 3 minute exposures might have been a bit too short for the unity gain mode that I'm using with the QHY600M camera.  At only 3 minutes, I suspect that the level of read noise might have been a significant fraction of the photon noise, which might be why I saw a lot more noise than I expected for this much data.  Going forward, I'm going to increase my baseline broadband exposures to 5m and 10m for NB work.  I used to use 20m as my baseline with the 14" using a 16803 sensor and my stacked images were generally VERY clean.  Of course, I could also use a different gain mode in the camera to accomplish the same thing but then the sensor loses dynamic range, which is undesirable.  In general, I try to minimize the number of overexposed stars in any give field so that I can attempt to show a more colorful star field.

I used SPCC to color calibrate this image and as usual, it led to questions about the result.  In this case, SPCC produced a fair amount of green splotches and turned some stars violet/purple, which is never right.  On the other hand, the galaxies themselves came out a creamy yellow/orange, which are colors others have found as well.  I've posted the White Level Function plot so that you can what the fit looked like.  In the end, I fixed the green/purple/violet regions and went with the general SPCC tone.

After finishing my first run at this data, I was left feeling like the star field was too under-emphasized so I did something that I've never done before.  I used BXT to make the stars a little BIGGER!  Yes, it will do that.  The problem comes from the fact that the GTX is so perfectly sharp that the stars are extremely small.  The faint ones are even pixelated.  I love it that it's that sharp but for this image, the stars just looked too small for my eye.  I left a couple of the early versions posted so that you can take a look for yourself.  I didn't make the stars a lot bigger, but I think that the star field in the final version looks better to my eye.

As usual, C&C is more than welcome.

John


PS. As many of you know, I often incorporate a discussion of equipment problems into various image descriptions so you may wonder what's going on with this "new" refractor.  At the risk of cursing myself, I'll tell you:  Nothing!  So far, this GTX130 on my Mach2 has been a real workhorse.  It has been working perfectly it has been doing what I tell it to do with no drama.  Whew!  Hopefully it will keep up the good work--even after I've said that out loud.  : )))))