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The Polaris (binocular) challenge – a learning exercise for new Splitters and a warm-up exercise for experienced ones

Polaris (Alpha [α] Ursa Minoris) (Σ 93)
MG: 2.0, 9.1  Sep: 18.2″  PA: 232°  (WDS 2009)

You can take this as a challenge, a lesson in splitting, or a warm-up exercise to use any time you feel like it  before moving on to something new.  For Northern Hemisphere observers Polaris is perfect in this respect because it is available every night, all night,  and stands virtually still. What’s more, it is not only a good challenge for binoculars, but also for any small scope.

Conclusions first

For me it has been all these things, but most importantly it has been a learning experience. I won’t keep you waiting – here’s what I have learned by taking the Polaris challenge:

  • I can easily split Polaris at 30X
  • I can intermittently split Polaris with 20X80 mounted binoculars in average conditions
  • All the numbers we use  – I use – are approximations, so don’t get anal retentive about them unless you have done a thorough, optical analysis of your equipment. (Something that is beyond my knowledge and patience.)
  • My binoculars say “20X80” on them. The effective objectives are really closer to 72mm, not 80mm –  that much I can measure.
  • I have  developed a simple, sure-fire way to always know where Polaris B should be
  • I have found that splitting nearby Mizar, with stars much more closely matched, but separated by 14 seconds instead of 18 seconds – is a great warm-up to splitting Polaris.
  • All the other general rules for splitting apply in spades –
    • be comfortable
    • be well rested
    • be fully dark adapted
    • focus with extreme care
    • position your head carefully in line with the eye lenses
    • spend time on target
      • look for at least a solid minute at a time

And now the details – how I got there from where I was

I start as a 70-year-old star splitter who has never sought the challenges John routinely takes on, but who has over half a century split more double stars more times than I care to remember and at the top of my list of stars frequently split is Polaris. I do a lot of outreach and I almost always include it in the show, usually using at least a six-inch scope so my  visitors have no trouble seeing that little faint dot next to the second magnitude primary and reminding them then that if our Sun were placed at the distance of Polaris it would be a bit fainter than the  secondary – Polaris B.

Recently I finally made a breakthrough splitting doubles with binoculars – something that was a real eye opener for me – and I was hooked on the kind of delicate images – and  elegant splits you get at low power – or any time your splitting takes you into the realm of the improbable. So I had to see if I could split Polaris with binoculars. And I thought I did on my first try with 25X100 Zhumells – only when I checked our own web site to remind myself  of the position angle  (pa) of Polaris I found that my supposed split was simply wrong – way wrong – 180 degrees wrong!  Now that is embarrassing to an experienced star splitter – but it came because I was really under estimating what I should see and because I didn’t have an easy way to know where to look for Polaris B.

Knowing where to look is a special case – and I have since seen writings where experienced observers screw it  up – because Polaris is rotating around the North Celestial Pole just like every other star – so Polaris B will, in the course of 24 hours, appear to rotate about Polaris. All this means is to use the Position Angle of 232 degrees for Polaris you have to apply the same two rules that you apply to any star:

  • west is the direction the stars appear to be moving
  • north is the direction from the primary towards Polaris

Whoops! Can’t use that second one here because we’re looking at Polaris. So which direction is true Celestiual North when you look at Polaris? To answer that find the crude asterism known as the “engagement ring” – those  stars will always be to the south of Polaris and the position angle of the brightest star in the ring is darned close to the position angle of Polaris B – all of which should become clear as you study this chart.

Low power finder chart for Polaris B showing field for 20X80 binoculars. Polaris B is at almost the same pa as the star marked 64 - brightest star in "Engagement Ring" asterism. Click for a larger version. Developed from Starry Nights Pro screen shot.

The preceding chart serves as a guide in two keys ways – first, use it to know where to look for Polaris B.  The secondary is, of course, much closer to Polaris than any of the stars shown, but with a pa of 232° it is for all practical purposes in the same direction as the brightest star in the Engagement Ring marked 64. Using Starry Nights Pro software I measured the PA of that star from Polaris as 229° – so Polaris B is just a tad  west of this line.

Not rocket science, but not easy – it pays to sneak up on this one

Knowing that Polaris B is magnitude 9.1, it helps to first find the other numbered stars on this chart. The numbers follow the standard convention for showing magnitudes on a chart in that the decimal point is left out – so the star marked “90” is of magnitude 9.0. Since Polaris B is impacted by the glare of Magnitude 2 Polaris, I used the 9.8 magnitude star as my rough guide to know what to expect to see.

The next question was, however, just how close should I expect Polaris B to be – and for this I took two approaches. First, I did a warm-up exercise with a Televue 85mm. I started at 85X with a 7mm Nagler. My eyes weren’t fully dark adapted and the night was far from perfect, so it took a few moments for Polaris B to pop into view even at that power – but it soon was shining steady and obvious right where it is supposed to be and you wonder why it wasn’t always so obvious.  I then dropped the power systematically, staying on each step until I had a solid, positive identification.  I went to 66X, 54X, 46X, and finally 30X with a 20mm TV Plossl.  That was my lowest solid, consistent view for that session.  I could see it from time to time with a 25mm Sirius Plossl (24X), but I could not see it with a 32mm Antares Plossl – a better 30 or 32mm Plossl should have produced it, though – at least intermittently.

Actually, before I did this with Polaris, I shifted to Mizar and got a clean split right down to 19X using the 32mm Plossl.

The Mizar connection

Why Mizar? Well, it’s nearby, so the sky conditions are pretty much the same.  The two stars, Mizar A and B are much more closely matched in brightness at magnitude 2.2 and 3.9. And finally, the split is similar to that of Polaris at 14.3 seconds vs 18.2 seconds.  While this was a piece of cake for the scope, it’s a good challenge star for the 20X80 binoculars. They do split it cleanly, but it gives me a real good idea of how close I expect Polaris B to be to Polaris A when seen in those same binoculars.

The PA of Mizar's companion is indicated by the short arrow - I think of it as always in the general direction of the 7.6 magnitude star that forms a triangle with Mizar and Alcor. Click chart for a larger version. Prepared from Starry Nights Pro screen shot.

So I split Mizar first with the 20X80 binoculars and once comfortable with that split, moved on to Polaris without changing the focus.

The numbers are precise, but the measurements aren’t

We can be such fools for numbers – see the book “Proofiness.”  But don’t let the numbers associated with binoculars fool you.  They sound precise, but they aren’t.

But first, should you even expect to be able to split Polaris with binoculars? In this case you need to forget about  the usual answer to that question which starts with checking the Dawes limit. The Dawes limit for an 80mm objective is 1.4″   ( Dawes limit = 4.56 arc seconds/Objective diameter in inches.) Ha! Good luck. As star splitters know this is seldom achieved and of course there are  many other variables – local conditions, observer, and the difference in magnitude in the two stars, for example. These all have a significant impact on the true limit of what you can split leaving you with a rough, but useful guide.  However, when you introduce binoculars into the equation all bets are off – the Dawes limit moves from being a reasonable guide to being virtually meaningless.

I like the formula the Sky and Telescope columnist Gary Seronik uses. Because binoculars are low powered instruments, he has a rule of thumb that says the practical splitting limit for a binocular is 300 divided by the power.  By that rule the 20X80 binoculars should be able to split stars separated by just 15 seconds. The Polaris pair is 18.2 seconds apart and should fit comfortably into that rule. And, of course, the Mizar pair are a little closer together than that – and a fairly easy split, which is why I say all of these numbers are just rough guidelines.

What’s more, the numbers on your astronomy equipment may simply be wrong. In fact, the more I read, the more I am convinced that the numbers most of us take on trust and use as if they were absolutes are really approximations. This is certainly the case with binoculars, as Ed Zarenski has frequently reported on Cloudy  Nights over the years.  And Seronik, in the December 2011 Sky and Telescope, pointed this out with one simple test anyone can perform. Point a flashlight into the eyepiece end of a binocular and measure the diameter of the projected circle of light – that’s your effective objective.

Rough and ready flashlight test of the 20X80 Zhummel binoculars. The bright circle of light in the inset shows the working objective diameter the system yields for my particular pair. So while the lens may be 80mm, the effective objective is something closer to 72mm. Click image for a larger view.

If the effective objective measure is off by that much, then what about magnification? I suspect it is wrong as well, not to mention field of view. In fact, it seems like every time someone put a stock binocular to the test they find that in most cases all the key numbers tend to be on the high side. Doesn’t mean there aren’t accurate ones out there – for me it just means take it all with several grains of salt.

That’s one reason why I am not suggesting we have a contest to see who can split Polaris with the least amount of power – but I do encourage you to give it a try and report your results here in the comments after this post being sure to mention exactly what you were using by make and model.  And if you’re new to this game, don’t get discouraged. A decade ago I was feeling very satisfied to split Polaris at high power with a 6-inch!

As for me, I’m going to keep trying. The “intermittent” split I got last night isn’t good enough. I could only hold it for about 10 seconds at a time. What’s more, I want to try the 25X100 Zhumells again on this, plus the 20X60 Pentax – and  I have some 16X70 Fujinons on the way. These last are, according to the reviews I’ve read, really, really good. Wonder if it will make a difference? Since you’re only using the center of the lens, I suspect the inexpensive Zhumells are pretty good – but the Fujinons may do better because of better contrast – and the individual focus eyepieces should do a better job of finding and holding the best focus.

I’ll see. Meanwhile, I expect to use Polaris as a frequent warm-up exercise for star splitting, helping me to fine tune focus, get used to the correct eye position, and telling me when I am well dark adapted – not to mention having something to say about transparency and seeing on any given night.