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A wonderful observing session April 15, 2007

Posted by dorigo in astronomy, personal, science, travel.
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62 non-stellar objects identified and described, among which 3 planetary nebulas, 2 globular clusters, and 57 galaxies ( many of the latter contained within 17 ARP catalog objects) are the booty of a rich observing session I had yesterday night with my friend Mauro in a deserted mountain pass on the Alps, a locality called “Casera Razzo” located at 5300 feet above sea level between Veneto and Friuli.

We arrived on the site at 9PM and were amazed to see how good the site was. The strongest source of light pollution was bright Venus and later Jupiter, while no detectable light was seen anywhere in the mountainous landscape. The night was perfectly clear, with above freezing temperature, no wind, and no dampness to wet our optics. The limiting visual magnitude reached close to +7.0 towards the end of the observations.

We used my 16″ dobsonian telescope, with a large collection of eyepieces; mostly used were Mauro’s Pentax series, particularly the 14mm (yielding 145x) and the 10.5 (for 190x). The seeing was not perfect, so we seldom pushed the magnification to 400x with a 5mm Hyperion.

I was amazed to see a few details on well-known objects that I had never thought possible to detect with my instrument. First and foremost, a galaxy of visual magnitude +15.8 (!!), PGC35618, was clearly visible in ARP 322 – and Mauro saw hints of the arc-shaped structure which contained PGC35615, which is at the unreachably faint magnitude of +16.8. You can see a group of five tiny galaxies near the bottom edge of this b/w plate centered on the intriguing NGC3718, and the arc I am talking about is made up of the three central galaxies.

(UPDATE: for a much better picture of this cluster of tiny galaxies, which lie 425 million light years away from us, please visit this site: http://www.starimager.com/Image%20Gallery%20Pages/Galaxies/ngc%203718.htm ).

What impressed me most, however, was detecting five individual, long arm segments in M101, a galaxy which is well-known for its troublesome visual appearance, and which more than a few amateurs with large Schmidt-Cassegrain instruments has never even seen. The galaxy is in fact glorious to stare at, with lots of knots of H-alpha regions, dark bands amidst the arms, and a rich stellar field. But to do that, you need a fast instrument and a perfectly black background sky.

I saw many interacting pairs of faint galaxies for the first time. A pair which was especially nice to look at was ARP 269, “the cocoon” (also labeled NGC4485-4490), where the bridge of material connecting the two objects was clearly visible with averted vision.

Another pair which took a lot of our time was of course M51-NGC5195, which stayed close to the zenith during the best hours and provided us with lots of fun by following its arms through entire 360 degree loops. A bridge between the two galaxies was also seen there.

In six hours of observations, we did not do all the objects of the Arp catalog which I had earmarked for observation, despite the large number of objects we studied. The reason is that I could locate only a few of the fainter ones, and had to lose my adaptation to darkness in order to consult my laptop – it just was not worth it to then have to wait for 20 more minutes to get back to full night vision…

Next month we plan to go back to the site, if the weather allows it. I hope we will be able to bring there more visual observers – many had obligations today and had to give up at the last minute.

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