Thursday, November 15, 2007

IC 5217 PN, Lacerta

“...and we receive glances from the heavenly bodies, which call us to solitude and foretell the remotest future.”

I tried to spot this planetary with my 22-Inch Dob from the ASKC dark sky site when I was out with Rob Esson last weekend but couldn’t pick it out. The object is small, 6-7 arc seconds depending on the reference one checks. It looks stellar at low to moderate magnifcations and requires high magnification to differentiate from nearby stars. The seeing wasn’t terrific Friday night, either. In a large scope, stars appear swollen in less-than-ideal seeing conditions. This surely compounded the problem.

I decided to have another go from my backyard in midtown Kansas City using my XT-8 Dob, aided by several hyper-accurate star charts I made using MegaStar. The object’s visual magnitude is generally given as 11.5, easily within the reach of an 8-inch scope under urban skies. Here is one of the two finder charts, this is a wider view showing stars to 10.4 magnitude.

The eyepiece circles in this chart corespond to a 17mm Nagler Type 4 with a Paracorr coma corrector in an 8-inch f6 reflector. One of the excellent features of star-charting software is the ability to accurately depict the field of view seen with a specifc scope and eyepiece. The fields can even be mirror-reversed if necessary to show what is seen in an SCT or a refractor with a star diagonal (a critical navigation aid). The chart simplifies star hopping from an easily located star or object to a more challenging object like IC 5217.

Here is the second, more detailed chart. This shows the view in the same 17mm Type 4 eyepiece, but at a larger scale and with stars to 13 magnitude.

With these charts I was easily able to locate the planetary, even under urban skies. I used a 2-inch UHC filter to "blink" the planetary. This involves holding the filter in front of the eyepiece and then shifting it aside. The filter attenuates light from stars, making them seem fainter while the planetary is made to seem brighter. The technique is a valuable aid for confirming the identity of small planetary nebula. I also swapped in a 5mm Nagler Type 6 eyepiece, which provides a magnification of 280x with the XT-8 and Paracorr. At high magnification, in averted vision, the planetary could just be seen as non-stellar. That is, it had a tiny disk compared with stars in the field, which appeared as pinpoints.

Standard charts like the Sky Atlas 2000 and even the Uranometria don’t show faint enough stars to pinpoint objects like this, which is where custom charts really shine (if you’ll pardon the expression). Such observations are doubly fun because they can be made as easily from a backyard in the middle of the city as from a dark sky site in the country. And whereas a dark sky site trip requires hours of travel time, set up time, etc., making it all but impossible most nights, backyard observing can be easily done any clear night. It takes only minutes to set up an 8-inch Dob.

I went out around 9:30 PM, after Elly and I had a relaxed evening, fixing dinner together and watching a BBC mystery beforehand. I was back inside and going to bed by 10:30. In less than an hour of observing time, I saw not only this planetary, but another challenging planetary in Cassiopeia (IC 1747 near Epsilon Cassiopeia – more on that in my next astronomy post), comet 17P Holmes, Eta Persii (a fine double in Perseus), NGC 7789 and M52, open clusters in Cassiopeia, and the Dumbell Nebula, a planetary nebula in Perseus (M72), generally considered to be one of the more challenging Messier objects, but quite easy to observe compared with IC 5217.

IC 5217 is said to have a diameter of 15 arc-seconds in the Uranometria 2000 Deep Sky Field Guide by Cragin and Bonanno. This is obviously a mistake. The object I observed was much closer to the 6 or 7 arc seconds reported in other references.

No comments: