Splitting the Double-Double

A couple of nights ago I got the chance to set my Vixen A80MF telescope up for a session under the stars. It’s been a while since I last used it, I’ve been using a newtonian and binoculars recently, but the it’s reminded me why I like refractors so much.

This isn’t an instrument for wide field views normally, it’s my double star scope, but with a 16mm Skywatcher Nirvana eye piece (82 degree AFOV) the view of the double cluster was fantastic.

It’s so easy to get the focus just right on these ‘slow’ scopes, and it tends to stay right whatever the sky condition… almost. Lovely sharp stars with a hint of the circular diffraction pattern.

This is a great scope for general use – in the same mold as the Skywatcher Evostar range – but I’m not entirely satisfied with it’s performance at high magnification on double stars. So decided to try it out on the double-double (epsilon Lyrae) which has become a standard for testing optics and handy at the moment near the zenith.

Finder map of the double-double (epsilon Lyrae)

There’s a very widely spaced pair – eps01 and eps02 – that can be seen with binoculars and even the naked eye, but each of these is a double pair too, and that’s the test.

With my 7mm SW Nirvana in the diagonal, giving angular magnification of 130 times and about 38 arcmins TFOV, I could cleanly split both pairs.

What I found interesting is that the southerly eps02, comprising the C and D components of the double-double, was much easier to split even though it’s the closer of the two.

True, it’s components are separated by 2.3 arcseconds whilst the components of eps01 (A and B) are 2.4 arcseconds apart, but the visible difference was huge. I was using a 2x barlow on the 7mm eye piece to get a clean split between A and B with an angular magnification of 260 times.

The difference was clearly down to the apparent magnitudes of the component stars. The CD pair are very even in magnitude at about 5.5, whilst the AB pair have a difference of about a magnitude (A at 5.2 and B at 6.1).

It’s impressive how much difference that makes to splitting a double star, and what it says about the limiting performance of telescopes given the traditional methods of calculating angular resolution. These are based on even pairs.

Sissy Haas has a project to empirically derive a correlation for telescope performance on uneven double stars. It’s open to, reliant in fact on, contributions from the amateur public, so why not have a go.

Stephenson 1

I’ve finally made a start on the open cluster observing programme I talked about in… February this year! It seemed sensible to start with an easy one because I was determined not just to observe and classify, but to produce a sketch too.

The chosen target was Stephenson 1, which is also known as the Delta Lyra Cluster. As you’ve probably guessed, the delta Lyrae visual double – and many would say that is enough reason to take a look – plays a key role in the structure of this open cluster.

The delta02 Lyrae component lies right at its heart, and makes finding this cluster pretty straight forward with nothing more than a red dot finder.

The instrument I chose was my Skywatcher 150PDS (a recent addition) furnished with a 16mm Skywatcher Nirvana eye piece (also a new addition). This combination provided a field of view of about 1.75 degrees, or 105 arcminutes, at an angular magnification of 47x.

My observation

And so, for your enjoyment, here’s the sketch drawn at the eye piece and later scanned from my logbook.

My original sketch of Stephenson 1 open cluster

Ok, there are two sketches really, one with the field stop, and another drawn larger with more detail. It might be a little messy, but I think it’s a good representation of the cluster.

I’ve also tried using digital means to clean it up a bit. The image below was produced by tracing the original scan using a paintbrush tool to make round stars. I wouldn’t need to do this if I’d been more careful. A lesson for next time.

Digitised version of the original sketch

Using the field stop as a guide, I’d estimate that the cluster spans about a third of the field which gives it an angular width of about 35 arcminutes.

Classification

Now to the matter of classification. I’m supposed to classify each of the open clusters I observe using the Trumpler system (which is explained in this nice article.

It’s clearly not rich with stars, I could count no more than 20 actually in the cluster. I suspect larger apertures would find more. So the rating for the number of stars in the cluster would have to be p (poor).

By virtue of fourth magnitude del02 Lyrae and many stars I’d estimate at magnitude 8 to 12, it’s got both bright and faint stars alright. This yields a range of brightness rating of 3.

Finally there’s the question of concentration, and I have to admit to struggling with this one. I’m not clear on what constitutes a cluster being deattached, or not.

Stephenson 1 certainly lacks any central concentration. This leaves me with a rating of III or IV, but which one? I guess I’d have to say that it’s more of an increase in the density of stars than a clear concentration, so perhaps IV, but it could have gone either way for my money. I guess I’ll get better at this with practice.

Piecing together my work, I’d classify Stephenson 1 as a IV3p open cluster of about 35 arcminutes angular diameter.

According to the professional data del02 Lyrae isn’t likely to be part of the cluster as it’s nearly 100 parsecs closer to us. It’s still a pretty yellow coloured star though.

Digitally inverted detailed sketch of Stephenson 1

I really enjoyed doing this, and I’m looking forward to the next one. But which will it be?

Charles Messier’s objects

Charles Messier

Charles Messier was born in 1730, in France, and grew up to be a comet obsessed astronomer working in Paris. To be fair to Messier, most 18th century astronomers were obsessed with comets since discovery could bring fame and riches.

Whilst hunting for these bringers of wealth, Messier kept finding himself confounded by other objects that appear non-stellar, but lacked one of a comet’s defining features: they weren’t moving. Whatever they were, they weren’t what Charles was devoted to.

It’s curious then that Charles’s lasting claim to fame is the Messier catalogue in which he listed 103 ‘nebulae’ – a definition that included almost anything that wasn’t a comet or clearly stellar – with 7 more being added after his death.

The binocular observer.

The thing about these 110 objects that might look like comets, but aren’t comets, is that they were objects not to confuse with a comet and as such, in binoculars they’re generally not impressive. The main exceptions tend to open clusters due to their looser stellar nature. Even so, many fail to break the fuzzy blob mould even with the benefit of modern optics.

Perhaps binocular astronomers are getting a good impression of what Charles Messier saw when we’re looking at globular clusters in the 15x70s. They’re little more than faint fuzzy object with a brighter core… perhaps.

It’s noticeable that many of the best binocular astronomy targets were ignored by Messier completely, but perhaps it’s not a surprise.

The Double Cluster

With his goals in mind, he was hardly going to mistake the double cluster (NGC 869 and NGC 884) above for a comet. They neither display background nebulosity, nor appear as a nebulous object as so many globular clusters have a tendency to do in antique telescopes and binoculars.

So what’s the interest in Messier’s catalogue?

Observing the Messier catalogue with binoculars is mostly an exercise in finding things. Most of these objects stand out well from the stellar background with a reasonably dark sky; that’s why they’re on the list in the first place.

Finally, a pair of binoculars is very compatible with this activity. Many of the Messier objects are near the horizon from the UK as Messier was working further south. Additionally, dark skies can be hard to come by to finish off those faint galaxies. So to locate the lot you’ll probably need to move around, which is home turf for binoculars.

I might sound disparaging about Messier’s objects. He certainly had no interest in them, however some of them are really beautiful, even in binoculars, take M45 Pleiades for example. But to see many of them at their best you need a telescope.

Which is your favourite binocular Messier?

Comet C/2014 E2 Jacques

I managed to find this comet quite easily as a large fuzzy dot with my 10×50 binoculars on the 3rd August 2014 when it was south of eta Aurigae. Visually I couldn’t see a tail, but I’ve been informed that one appears on images. It was around magnitude 6–7, so quite bright, and it’s moving towards Perseus, fading as it goes. Here’s a chart (Carte du Ciel again) to help you find it (click on it for a larger view).

Finder chart for Comet C2014 E2 Jacques

Each point on the path is at 23:00 on the day indicated, when it should be clear of the eastern horizon. Auriga and Perseus are distinctive constellations that show up even in light polluted skies.

I’d recommend making the effort to see this comet as we don’t get many in the northern hemisphere within range of binocular astronomers.

Beautiful binocular clusters

I’m chasing Messier objects with binoculars at the moment. To be honest most aren’t all that impressive, but whilst searching out these objects in Ophiuchus I encountered something fabulous.

I found a huge cluster of bright stars with an obvious profusion of faint stars forming the background field. It immediately struck me that after a lean time on Messiers this object was what binoculars were made for.

But I wasn’t done!

As usual I let the field wander around only to catch sight of an even more impressive cluster. This one wasn’t as big, that much was immediately evident, but it sported a bright band of stars from north to south through the middle of a rich cluster.

A quick look at my copy of Sky & Telescope’s Pocket Sky Atlas told me that what I had originally found was the open cluster IC 4756, and that I was now gazing upon the splendour of NGC 6633. Here’s an idea of what they look like provided by Carte du Ciel, but it’s nothing like the real thing.

Detailed view of IC4756 and ngc6633

The red circle around NGC 6633 shows a 4 degree field of view which is pretty modest for moderate power binoculars. My 15×70 binoculars have about this field, and my 10x50s give about 5 degrees. So you can see why these are such good binocular objects. First they’re a bright starry magnitude 4 to 5, and secondly they’re really big, I would guess at least a degree across.

They’re not particularly hard to find either. There are plenty of bright stars around to help and binoculars are great for star hopping. Here’s a sky map courtesy of Stellarium to help.

Finding IC4756 and NGC6633

You’re missing out if you haven’t seen these two, so if you’ve got a pair of binocular take a look before they disappear into the morning glare until next year.