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.


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?

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.

Finding your target

Navigating to open clusters with a red dot finder is difficult. Many are some distance from a visible bright star – bright is magnitude 4 or more around here – and an Alt-Az (Altitude–Azimuth) mount is next to useless without an optical finder under these conditions.

When double star hunting I usually prefer my Equatorial mount, and I think it’d be the best tool for the open clusters program too. Heavy though it is, once aligned it’s much easier to find your way around using the slow motion controls for RA (Right Accession) or Dec (Declination) whilst looking through the eye piece.

A reliable way to find NGC1662

For example, lets assume I’m trying to find NGC1662.

Finding NGC1662 from Meissa

There’s not much nearby that’s brighter than magnitude 4, but I could find it using Meissa (lambda Ori), which is magnitude 3.5, and my RA control.

First, use the lowest power eye piece in the box. Trying to find things at high power is usually a bad idea.

Meissa is bright enough to use my red dot finder, and once in the view I use the RA control to move the mount towards the celestial West in the direction of the blue arrow.

Remember the eye piece? Mine has a field of view of about 2.4 degrees in my reflector, and Meissa is about 1 degree south of NGC1662, so it should be visible in the eye piece as it passes by… as long as I know what it looks like.

I could even stop when that bright star at the top of Orion’s bow appears and centre the scope on that before continuing. That’d move me towards my target and make it easier to spot. That’s how you begin star-hopping around the sky.

Cluster confusion

You may remember that I announced my intention to complete the open cluster program this year. Well we’re a couple of months in and a few things are clearer to me now. And that year’s seeming short than I’d expected.

When is a group of stars a cluster?

As a relatively new observer I’m used to open clusters like M37 which, courtesy of Stellarium, looks something like this.

Open Cluster M37 in Stellarium

They’re pretty obviously a cluster of stars aren’t they, so no problem there, even I can find that as long as I’m looking at the right one.

Unfortunately they’re not all like that. I was looking for NGC1662 last night in Orion, and it turns out to look like this.

Open Cluster NGC1662 in Stellarium

Now I found it, and it looked very much like that, but it wasn’t what I’d expected. I’ve been trying not to cheat by looking up the clusters ahead of time in case that distorts my observation. But you do need to know what you’re looking for.

This gives me a headache: when do a few nearby stars actually become a cluster?

In order to complete the program, I not only need to draw some of these clusters, but I need to classify all of them. To do that I’m going to need to answer my own question. Practice anyone?

Becoming a better observer

This is my third year as an astronomical observer, and I’ve decided the time has come to kick off a few projects aimed at improving my practical skills.

Observing open clusters.

The Astronomical League have some fantastic material and programs that are available online, even to non-members. I wish British astronomical organisations were more like their American counterparts (a discussion for another day).

Anyway, the program that’s caught my imagination is the Open Cluster Observing Program which provides a list of 125 objects from various catalogues, most of which I’ve never encountered. I’ve decided the best, and most obvious, place to start is the Basic program which states the following requirements.

  • Observe any 100 of the 125 open clusters on the provided list.

  • Sketch any 25 of the 100 open clusters that you observe.

  • Classify all 100 observed clusters under the Trumpler classification system.

I don’t believe there’s a deadline. I’m only doing this for my education and entertainment, but I’d like to try and complete the program this year. So the plan is to start with some of the open clusters listed for Taurus, Orion and Monoceros before they’re gone in Spring.

Why open clusters? Simple really, they’re one of the few features of the deep sky that are reasonably visible in the suburban night sky, and some of them are quite spectacular.

Choosing my equipment.

It’s suggested that using the same telescope with a limited range of magnifications gives the observer a better appreciation of the differences between these clusters.

I’d estimate that there are about 30 clusters on the list the I’m going to struggle with because of their low altitude. That’s not good since I need to view 100 out of 125 to complete the basic program. I’m going to have to move about to bag a few of the tricky ones.

A small telescope like my SW Explorer 130P on an Alt-Az mount is relatively portable, and I don’t have anything with more light grasp, so that’s the one I’m going to use. I’m going try and stick to 50 and 100 times magnification depending on the actual object.

For the drawings I’ll try and stick to high altitude objects and switch to my heavier equatorial mount. It’ll be much easier to track them whilst I draw.

What am I hoping to gain?

I’m not a member of the Astronomical League (yet), so I’m not eligible for any awards, so that’s not the focus here. If you’re anything like me you tend to find your target, give it a minute or so, and move on to the next. For me getting past this behaviour is the main point of the project.

You remember they said observe? Well that means making notes with descriptions as well as the classification. It’s very hard to describe or draw an object accurately without really studying it in the eye piece.

I think programs like this one will make me a much better observer, and possibly one day an astronomer.