M103 in Cassiopeia

It’s time for another open cluster, and in truth I’ve built myself a bit of a backlog. The issue is that I’m too lazy to scan my drawings in a timely fashion. That’s my excuse and I’m sticking to it.

This time I’ve chosen to turn my telescope, with pencil in hand, on M103 (one of Charles Messier’s objects) in the constellation of Cassiopeia for the second time this season. The first was with my ST-80, but I felt that 44x magnification didn’t to it justice. So on the night of 4/5 October I returned with the Vixen A80MF and 7mm Nirvana eye piece for a higher powered attempt (130x).

If you know what you’re looking for M103 isn’t hard to find being about 1 degree from the 2.5 magnitude star, delta Cassiopeiae. It’s not one of the “obvious” clusters, in fact I’m not sure why Messier found this one and not the nearby NGC 663, perhaps it looks more “cometary”.

My Observation

So once again, here’s a scan from my logbook. I’ve tidied up the notes on the computer, but the sketch has just had the contrast increased as the scanner leaves it a bit faint. My field of view is about 38 arcminutes with this telescope and eye piece combination.

Sketch of M103 in Cassiopeia

You’ll get to see the scan with handmade notes if you click on the sketch. And this is what the Deep-Sky Survey makes of M103.

DSS view of M103 in Cassiopeia

I don’t think I’ve done too badly.


To my eye, and telescope, M103 isn’t rich with stars. I’ve drawn pretty much all I could see, and some of those weren’t constantly visible. Yet there weren’t many more than a dozen stars in the cluster. So I classed this cluster as p (poor) using the Trumpler system.

There aren’t any really bright stars in M103, but there are obviously very faint ones (I estimate that I could see down to about 11.3 magnitude). This led me to settle on a brightness range of 2.

This cluster is clearly detached from the Milky Way background – quite a feat that was contested for some time after it’s discovery – and there’s concentration of the brightest stars, but its not highly condensed to my inexperienced eye.

So to reach my final Trumpler classification for M103 I threaded these components together estimating M103 to be a II2p open cluster of about 10 arcminutes angular diameter.

How does this compare with the official data? My estimate of the angular diameter is a little above the 6 arcminutes in the literature which also thinks that M103 is a bit richer giving it a rating of m rather than p.

I can live with that comparing my sketch with the DSS image.

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?