Tuesday, November 10, 2015

The quest for the deep...hunting Stephan's Quintet and the Deer Lick Galaxy's fleas in Pegasus

"Annotated deep field of the Deer Lick and Stephan's Quintet Galaxy Groups"


One of the things I set out to do as a visual astronomer was to strive to go deeper whenever possible. I made a pledge to do this when I got my 10" GS dobsonian back in 2007, and go deep I did, seeking out fainter fare, detail in fainter objects, as well as pushing the magnifications as high as I could to eek out as much detail in the brighter planetary nebulae and globular clusters. One thing that always eluded me though was galaxy clusters. The Abell's and the Hickson's were still out of reach, and my best guess is that the smallest sized scope for such an endeavour would be at least a 12.5" scope. I am partially right though, some of these are doable in a 10". That said there is no detail visible, and under most circumstances all you see is one extremely faint blob with no resolution of its individual members.

Abell and Hickson Galaxy Clusters

So what are the Abell and Hickson's? The Abell catalog of galaxy clusters is an all sky catalog containing 4,073 rich galaxy clusters. The original list, published in 1958 by George O. Abell only contained 2,712 galaxy clusters, the "Northern Survey". This catalog was revised and 1,361 clusters from the "Southern Survey" was added by co-authors Harold G. Corwin and Ronal P. Olowin in 1989.

The Hickson Compact Group, also known as HCG is another collection of faint and compact galaxy groups, numbered at 100 and published by Paul Hickson in 1982. The most famous of this is of course one of my quarry's, HCG92, Stephan's Quintet. Both the Abell and Hickson catalog of objects are regarded as observational targets for advanced visual astronomers with big telescopes from the darkest skies!

The most famous and probably most observed of these clusters is Stephan's Quintet and NGC 7331, the Deer Lick galaxy group (and its smaller companions, the fleas). These two objects are only just above the threshold of a 10" scope. While NGC7331 is easy, the fleas are not visible. As for Stephan's Quintet, well lest just say all I though I saw was a faint haze that was barely there with averted vision. There was no galaxy cluster per se. That makes what I just accomplished all the more impressive, and this with a scope a fraction of the size of my 10". Least to say I am a convert to CCD imaging. Presented below is an image I took of section of the cosmos that showcases the above-mentioned galaxy clusters, in the northern reaches of Pegasus the winged horse. Not only does the Quintet make an appearance, I can actually make out its individual members (see asteriks denoting the members).

Also in the same frame, we have NGC 7331, the Deer Lick Galaxy and its companion galaxies, the fleas. This image was a composite of 10x42sec exposures through a 70mm Celestron Travelscope optical tube assembly riding on a NexStar GOTO alt-az mount, no tracking! The CCD camera of choice is my new-old Meade DSI II OSC. Since the focal length of the OTA is only 400mm, my guess is that my 102mm OTA at 1000mm would frame it better...and frame it way better it did. Below is the result of last weekends dark sky imaging trip to Saratoga Gap. All the major members are now fully resolved, and I have annotated my images. See figure legends for details of the subs and exposure times. I can only imagine how much more can be captured if I had a EQ GOTO mount for longer sub exposures. But I am happy with what I captured with my cheap and simple imaging gear!!!!

Wide Field Imaging of both clusters in the 70TS at 400mm focal length on NexStar GOTO

"Stephan's Quintet and Deer Lick Galaxy clusters: 70TS: 400mm focal length: 10x42sec"

A Closer Look: Imaging with the 102GT at full focal length 1000mm on NexStar GOTO

"Deer Lick Galaxy: NGC7331, NGC 7735, NGC7336, NGC7337, NGC7340: 102GT: 1000mm focal length 20x21sec"

"Stephan's Quintet: NGC7317, NGC7318, NGC7319, NGC7320, NGC7320C: 102GT: 1000mm focal length 20x21sec"

The results I obtained for this "deeper sky" imaging session was indeed very encouraging. I might try my hand with imaging some of the brighter Abell and Hickson clusters the next time the new moon comes around. Stay tuned for more......

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