Abell 2218 – A Monster of a Cluster!
Roll back nearly ten years ago and you’d find the most distant galaxy ever detected by humans in the constellation Draco, Abell 2218. This huge cluster of over 10,000 galaxies is heavily shifted into the red by it’s recessional velocity. A galaxy behind it is nearly 13 billion years back (note this cluster is actually around 2 billion years away.) This compared with my previous distance record of 7.8 billion years with the Double Quasar, QSO 0957+561. Aside from it’s massive distance, Abell 2218 holds another treasure of nature: gravitational lensing. As in the double quasar, gravitational lensing occurs when space bends and light from distance objects can be magnified – as if space itself were creating a giant lens. This allows us to see things even more distant than we might otherwise be able to see and also creating some interesting observational illusions.
Many amateurs know the “best sky” the sky right above you: you’re looking through the least amount of atmosphere and therefore less gunk can mes up your seeing. Abell 2218 is pretty well near zenith so I thought now would bea fun time to try. I’d previously tried imaging this cluster on three separate occasions and never had enough data to process. This time however I really cranked it up and raced against the full moon coming above the horizon. Over the course of two days, I took nearly six hours of imaging time with the AT10RCF – with each exposure was set at 20 minutes.
I also altered by typical processing routine a little to try to eek out the most detail I could. Here my dark came from MaximDL (which I find builds better darks than Nebulosity — why?) and applied some flats from a twilight sky. These were then applied to each sub, converted from raw to color, and the historgrams were matched against the best single image I had. Then I used software to bin 2×2 and applied a drizzle stack. These last few steps may seem a little weird, but I was trying to improve my signal to noise and allow drizzle to improve the undersampling of this cluster that would result. With features in the cluster in the sub arc-second range, there is very little hope of gathering real detail. But it was worth a try!
This cluster is easily the most challenging DSO I have tried. While I can’t see any lensing popping out like the hubble space pictures, I’m still pretty happy to be able to register some of these galaxies which start around magnitude 17 and go past magnitude 20 pretty quickly. Zooming in on the core provides a little more detail — and noise.
If you’d like to try your hand at processing this area better, a crop of the stacked file is available for download: abell-2218-driz5-crop.fit.gz