Physical Description

NGC 6951, sometimes cataloged as NGC 6952, is a barred spiral galaxy in Cepheus, situated between Cepheus and Draco as shown in the Sky Plot. It is about 75 Mly distant, 100 kly across, apparent magnitude of 11.0, apparent size 3.9' x 3.2', redshift 0.004750. (Ref: Wikipedia)

Lewis Swift discovered NGC 6951 in 1885. (Ref: Jeff Kanipe and Dennis Webb, Annals of the Deep Sky, vol. 5) Heber Curtis noted NGC 6951 for its symmetrical spiral shape in 1918. (His name rang a bell - see the next section below if you're interested). Following this, the galaxy has been classified mostly as a barred spiral with inner S-shaped ring and bright nucleus. This nucleus is now regarded as having a massive black hole with mass on the order of 200 million suns. There is a measured in-flow of gas onto its accretion disk that accounts for the brightness and activity of the nucleus.

A distinctive feature is a circumnuclear ring, with major axis of 9 arcsec, comprised of knots about its circumference. The main inflow of gas is thought to occur at two contact points at either side of this ring, with star and star cluster formation moving away from these points along the ring. Interestingly, I am just able to discern a ring-like structure in my image with notable knots, which I noticed prior to reading about this structure. (My usual first thought was that it was a consequence of image saturation and that I should ignore it. Turns out I actually did not saturate the core here, although my image is heavily stretched to emphasize IFN, not the nucleus.) I measure it has an angular size commensurate with what Kanipe and Webb cite in their book. My resolution is not impressive, certainly not pretty, but it's neat to have spotted a feature on my own and then later learn that it was a notable part of this galaxy's active nuclear region.

Another feature of this image is quite a bit of Integrated Flux Nebula (IFN) . This is dust and gas in the outer regions of our galaxy that is illuminated by the general emission of all the stars (hence the name "integrated flux"). I've not emphasized this before in my images - see notes below.

The Great Debate

Mention of Heber Curtis rings a bell from some reading I've been doing recently. Pardon the diversion. Feel free to skip this part if you're not interested ... (Actually, you can skip any or all of this if you want as I'll never know anyway ...)

I've recently been reading about one of the big cosmological arguments of the early 20th Century, the question of whether or not ours was the only galaxy in the universe or if the cosmos consisted of many galaxies. I find it surprising to remember that it was actually not quite a century ago that scientists had not yet settled this argument! In 1920 Harlow Shapely and Heber Curtis held the Great Debate, held by the Smithsonian, to argue this very question. Shapely held the position of the universe being comprised only of the Milky Way, and the small fuzzy "nebulae" being clouds of stars contained within it, albeit far away at its outskirts. Curtis held the position, being increasingly supported through observations and early distance measurements and deductions, that these small, faint fuzzies were themselves other, much more distant galaxies, or "island universes" of their own. The debate was not immediately settled at this event, and there was actually contemporary evidence and scientific findings supporting both positions.

Of course, Curtis' position was proven to be correct, particularly by Edwin Hubble just a few years later, when he trained his 100in telescope on those "nebulae" and could not just determine that there were many of these galaxies or "island universes", but also determined their distances, red shifts, and that the universe was expanding. The 1920's and 30's were a profound and prolific time for astronomy. I for one had not appreciated just how much advancement there was in our cosmological understanding during a relatively short time!

Acquisition Details

I acquired data on NGC 6951 over five nights, the first several unfortunately around the Full Moon, but the latter two closer to the next New Moon. Despite the moon, I was pleased to have captured some integrated flux. Even with a total of 12 hours integration time, however, I appreciate that more time would be needed to do the IFN more justice and balance out the background.

My FLI ML16200 camera underwent quite a bit of service and repair through two trips back to FLI. It is now essentially a brand new camera with new sensor and circuit boards, and this was technically its "first light". I'm pleased with its performance, and the fact I could calibrate my frames well enough to emphasize the IFN is a testament to its performance.

I chose to use 180s frames for all channels, binning the RGB at 2x2. This was a good choice in that I have only a few truly saturated stars, and the galaxy core is not blown out. (It may be hard to evaluate that claim with this image since I wished to emphasize the IFN.)

One annoying issue I have been contending with for quite some time is a vibration that is coupled between the FLI camera's fan and something inside my C11 scope, most likely the primary mirror mounting. This vibration has been partially mitigated by now using a set of rings to mount the scope to the AP 1100. This is a more rigid attachment than the rail I used before, however I can still discern the vibration. I will need to tear into the scope and figure this out, but the vibration is mitigated well enough for now that I don't wish to mess with it during my short, active imaging window of late summer.

Processing Details

As I mentioned before, this was my first time to emphasize IFN in one of my images. The challenge here is that IFN is really just barely above background noise. The more integration time (the "deeper") the image, the better. I had just enough data here to give it a try.

Calibration of the frames was therefore even more critical than usual given the small flux from the IFN. I have recently purchased a new flat panel that I have measured to produce a much more uniform illumination than my old panels. This was critical to not have the master flat itself add its own appreciable nonuniformity to the image. Unbeknownst to me I was doing just that with my old panels, and they have now been retired.

Second to that, I wanted to make sure that I was removing background sky gradient without obliterating IFN. I think I did a fair job of that by using PixInsight's ABE process, but only up to order 2. Looking at the image, however, I think that I could have applied perhaps an additional DBE process to smooth out more of the background. However, here again, comparing with others' images, I may indeed be capturing even more of the IFN than just the more obvious, denser regions. I therefore erred on the side of caution by applying background removal conservatively.

Summary

I hope you enjoy the image and have found my write-up interesting. The main result of this endeavor was checking out the new, post-serviced camera and finding it to be performing well. Capturing IFN and emphasizing it for the first time in my processing was a fun challenge.