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  #1  
Old 26-05-2017, 06:34 AM
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Weltevreden SA (Dana)
Dana in SA

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Join Date: Apr 2012
Location: Nieu Bethesda, Karoo, South Africa
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First Light, Intes-Alter MN84, #3

Pleasey: Palomar 6

OK, the easy of my MN84’s first light was nicely taken care of, let’s go duck hunting.

Pal 6 (ESO 520-21 on NED & Simbad) should be a lark. Its Mv is 11.55, its core radius is 1.1 arcmin, and its half-light radius 1.2 arcmin. It is a Class XI cluster, which should show in the eyepiece of an 8-inch scope as an evanescent tremour that resolves into stellar scintillations more easily seen than its surface glow. Whet your knife by spending half an hour with nearby NGC 6256 in Sco, which has the same low star density and weak glow, but whose surface brightness is 2.4 mags brighter.

Now for the bad news:WikiSky clearly shows that Pal 6 lies in a crowded field of stars of nearly the same brightness as Pal 6’s. The ever-useful Where is M13? website shows Pal 6 cuddled right next to the Milky Way bulge only 2° to the right of dead centre and 1.7° above the Galactic plane. Also according to WikiSky the surrounding 10 arcmin circle is swiss-cheesed with stellar aggregates merging into extinction blobs, many of them about Pal 6’s own diameter. The Harris catalog lists Pal 6’s surface brightness as 21.5 mag arcsec2 (see Part III, Col. 10, mu_V). BUT: this is in turn reddened by 1.46 mag. The term “reddened” refers to the value “E(B–V)”, which is a number astronomers use to calculate several values, one of them photographic surface brightness which is different from eyeball surface brightness.*

*The most exotic E(B–V) calculation tells astronomers the number of hydrogen atoms lying between a circle the diameter of our eyes’ exit pupil and a surface exactly the same diameter in the middle of the object we are looking at. See here and have some fun with your pocket calculator.

When using Harris to estimate seeability,, we have to multiply a value for E(B–V) by 3.1 to learn the actual dimming we can expect at the eyepiece. For Pal 6, E(B–V) 1.46 x 3.1 = 4.5. Hence we must add 4.5 mag/sq arcsec to the Harris value of 21.5 and . . . Uhh-ohhh . . . it comes out to a surface brightness of 26 mag arcsec2.

Lads, technically we can’t see this thing. Yet Uwe Glahn has (see below). What gives?

Here, note two things:

(a) We are not looking for a dim glow. This is a Class XI globular. We are looking for an overdensity of nearly sub-visible scintillations popping in and out of view in a field 2.5 arcmins in diameter floating in a field of stars with a much greater spread of brightnesses. Put another way, we are looking for a patch of scintillations of exactly the same brightness in a region where average brightnesses differ a lot. Recall that the Horsehead in Orion is said to be like looking for dark on slightly darker. With Pal 6 we are looking for even lying beneath uneven. Sort of like spotting fireflies during the full moon. When we prepped for Pal 6 by examining nearby N6256 in Sco we were looking for the same inverse density disparity, only brighter.

Now (b): WikiSky throws us another curveball. In the dense soup of over- and underdensities of the Milky Way bulge, WikiSky is prone to giving overoptimistic impressions of GC brightness compared with surroundings. Globulars are basically red objects due to their great stellar ages and low masses. WikiSky’s SDSS database reaches mag 22 in the V band, which puts a lot more luminosity density into crowded regions than we see visually with our mag 14–16 backyarder’s eyes. Also, our human-being eyes can’t stack up photons like pixels on a CCD chip. Add to this that WikiSky’s B-band colour brightness and R-band dimness give the impression of objects being bluer than our visual impressions. Y'know, if I didn’t r-e-e-e-l-l-y want to see Pal 6, I’d be taking my chips off the green beize table right now.

Offsetting these, WikiSky’s filamentary rendering is crisp enough that we can trace differential reddening adjacent to and across an extended object which we otherwise wouldn’t notice in the eyepiece. Pal 6 location is a fairly easy find because it makes a lazy isosceles with mag 8.4 HD 160972 and mag 9.9 TYC-6831-169-1. Notice that between the latter and Pal 6 there is a bird-on-the-wing looking extinction patch with 4 arcmin “wings” and a head and beak point directly at Pal 6. The bird-on-the-wing is not at all as self-evident in the eyepiece, but the overall patch in which the bird blob resides clearly reaches directly from TYC-6831-169-1 to the cluster.

All that just to get us to the right spot to look. Now we wait. And wait. And . . . in a half hour under ideal skies, the even-on-uneven scintillation pattern build up just enough glimpses to look like a very very diaphanous underdensity within an uneven overdensity next to a nondensity. It’s about 2 arcmin around. Three nights in a row of this and you can call it a definite maybe.

For this we spend thousands of dollars and half freeze to death night after night cursing the dew and clouds and mozzies and bats dive-bombing our scalps. No wonder our partners think we’re nuts. They console themselves with the thought that at least we are safely out there in the back yard and not out on some dance floor with a floozie.

What do other observers say? As far as I know, only Uwe Glahn has reported observing Pal 6 with an 8-inch. He used 89x in 7th mag skies, reporting, Very faint glow, can be hold for seconds, two other observers also see the glow, a little bit concentrated to its middle. I used 89x using a Lunt 9mm 100° and 145x in an Explore Scientific 5.5mm 100°. [Reports by observers with much larger instruments are here.] My luck was about the same as Uwe Glahn’s, in good part because the Sco-Sagg region is near zenith during the midnight to 3 a.m. hours this time of the year. Conditions were seeing 8/10 transparency 9/10.

Happy ending: I'm so delighted with my not-so-brand-new Intes MN84 that I will part with it only when they put me six feet under.

People say, “You can’t take it with you.”

Pity.
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  #2  
Old 26-05-2017, 11:38 AM
Wavytone
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I think I should take the floor with a floozy more often after that lot
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  #3  
Old 31-05-2017, 01:25 PM
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Tinderboxsky (Steve)
I can see clearly now ...

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Join Date: Feb 2012
Location: Tinderbox TAS
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Another top read Dana.
Steve.
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