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A selection of open clusters using live RGB imaging

Martin Meredith

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In the brief window after darkness and before the the moon came up too far last night I decided to do some 'live' RGB compositions of open clusters using the multispectral feature of StarlightLive. Apart from a couple of old favourites, this was largely unplanned, based on browsing the charts for interesting-looking possibilities.

Equipment: SW Quattro 8" f/4 + Lodestar X2 mono + Baader RGB filters. For clusters I typically use short subs (5-10s) to prevent saturation and loss of colour/incorrect colour balance; also, it isn't necessary to use many subs.

Here's M6, the Butterfly Cluster in Scorpius, which was rather low at the time (13.5 degrees above horizon). This is 4x5s in each of RGB (the number shown is the total). The bright reddish-orange star is the variable BM Sco. Most of the cluster fits in the FOV but some parts are missing. This would benefit from a larger FOV to show the context and boundary of the cluster.


Even nearer the horizon at 11.5 degrees is M7 (Ptolemy's Cluster). This is actually the 3rd brightest Messier OC but is also the most southerly Messier object and not as well appreciated as it would otherwise be by many northern hemisphere observers. The whole area is packed with stars which meld into a browny-red mily way background. 


Next, a lesser-known cluster, Dolizde 37 in Cygnus, whcih sits about a third-way along the line from Sadr to Albireo. Since the members stars are fainter, I used 10s subs. Although the cluster is not much to look at, it sits at one end of a linear dark nebula, Barnard 145, which can be seen running diagonally in this image. At the other end the dark nebula is classified as LDN 865 but seems to be part of the same structure. But what I really like about this is the many-coloured but understated 'gritty' starfield.


About a degree away is the first member of the Dolidze catalogue. I was drawn to this mainly by the very close multiple (14 components!) star SEI 928 at the edge of the cluster.


Close by, and in similar vein is the compact cluster IC 4996. The 'dogleg' grouping of bright stars is the multiple system Burnham 442 which has no fewer than 31 components listed (presumably not all physically-related though!).


Moving to Vulpecula, this is NGC 6830, a couple of degrees away from the Dumbbell Nebula. I imagine this would be hard to pick out from the surrounding stars.


Not so for the last cluster though, which along with Dolidze 37 was for me the surprise of the evening. The reddened hazy mass of faint stars constitutes NGC 6791 in Lyra, not far from the border with Cygnus, and along with NGC 6743 one of only two NGC OCs in that constellation.  This could easily pass as a loose globular, it seems to me. I can't help thinking this object should be better known but I've found very little mention of it in my observing guides. I've also marked the 11-component multiple star system ES 2490 which lies outside the cluster.


As an aside, I've been using the manual filter wheel for nearly a year now (on and off, mainly off) and though I'm happy with the results it produces in terms of colour rendition using the Lodestar X2 mono, I wish I could justify the expense of a motorised wheel! As you can see, it is possible to observe a great many OCs in a short time due to the short exposures, but each filter requires me to get up from my observing position and ever so carefully shift the wheel, sit down, wait 10 seconds for any vibration to stop, then continue the exposure. A motorised wheel with control directly from StarlightLive would cut this time in half I'm sure. However, the benefit of my manual wheel is that it holds 9 filters so acts as convenient storage ...

Thanks for looking (and hopefully ignoring the dreadful collimation...)




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Great captures Martin! Your and others 'mark up' of your images are fantastic and are a real inspiration that got me me digging deeper into my VA/EAA images, and your very helpful Pretty Deep Maps are invaluable.

If you noticed my threads over on the other site, I have started working through the Astronomical League observing programs that allow imaging. The AL Open Cluster program is one that just opened up for imaging, so am going to start working on these myself.

I lost my CGEM DX a few weeks ago to a close lightning strike and the replacement Atlas Pro EZ/EQ-G arrives from Orion tomorrow, albeit with a long stretch of poor weather forecasts. Can't wait to get imaging again!

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NGC6791 is a great find.  There are a number of OC in the standard Herschel 400 that are very nice through a big scope (clearly very large assemblages of stars viewed at a great distance) and I think would be similarly impressive EAA subjects.  I'll try to remember to look at my notes and post some.

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Fantastic captures Martin, NGC6791 is particularly interesting. On you capture of  Dolidze 37, which is a fascinating area due to the dark nebula alone, there is a very red star at the edge of the frame at about 2 o'clock - is this a genuine red star or an artefact of some sort? I am assuming the latter as it is very VERY red!

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Thanks Brandon. I feel your pain over the mount. Mine (the Berlebach, not the AZEq6) was nicked from the car a few weeks back; that's after it got lost in the Air France luggage system and spent 2 weeks at Charles de Gaulle. I replaced it with the same which I suppose is a kind of recommendation!

Thanks Alex and Astrojedi. I'm certainly tempted to look at some of these through the eyepiece in a larger scope.

Rob, you've inspired me to check out that and it is quite interesting. There is a Mira-type variable (V1301 Cyg; also known as VV 249) at that location, according to this DSS image. I've not found much on it except this paper where it is described as a long period variable with magnitude ranging from 16.5 to 21+.

Screen Shot 2016-07-26 at 14.41.20.png

here's a close up (DSS left, mine right; you'll need to mentally rotate a little)

Screen Shot 2016-07-26 at 14.57.29.png


So it looks like I have my colour saturation up a little too high.... but it does appear to be a rather red star. I'd love to find some colour data on it.





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Just in from the shortest session yet, racing against clouds to get this capture. Fortunately that part of Cygnus was clear but within seconds of the shot it clouded over. So here's the same shot (just 3x10s in each of RGB this time) with the red star just SW of centre near the warm yellow sun. I've turned the saturation down to about 25% here.


Here's a zoom showing the low saturation (left) compared to a slightly higher saturation (about 40% on the scale). Probably the left panel (low sat, matching the main shot) is more 'realistic'.

Screen Shot 2016-07-26 at 23.22.14.png

But either way, yes, it is pretty red! The double star at the top reminds me of a visual view of Albireo.

I think the shot I got the other day may have been a combination of slightly too high saturation and coma at the edge causing the RGB misalignment (note the effect on the double in the earlier shot which is not separated due to the coma). But at least it wasn't some crazy red pixel track which was my initial suspicion when you mentioned it.

So thanks for pointing this star out Rob because having dug out the 1969 paper I link to above I'm determined to take a look at the 9 red stars he mentions in Cygnus to see how they compare to this one. The brightest is mag 13.7 at maximum and mostly they're like this one, around 16 at max, so I'm not surprised they're neglected (yet well within the range of short exposures).



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Well done for getting out again so quickly! Nicely captured and researched. The paper doesn't say much about spectral classes and I am guessing that even if it did, spectral classifications do not go down to the granularity required to compare shades/intensities of red? The paper does not say much about VV249 (your target) but it does mention that VV250 is 'extremely red' which sounds promising. 

It's interesting to see the subtle colour differences between all the stars in your shot.

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Remarkably, this shot also contains VV 250 (V1303 Cyg in new money) and VV 248 (V1512 Cyg) although the latter is not visible (it varies from mag 17 to 19).  I've marked their locations below along with the original object of the discussion, VV 249. 

VV 250 doesn't look as red as suggested, but perhaps it has something to do with what part of the cycle it is on (it varies from mag 15 to 18.5). These are B mags; actually, Simbad lists its V mag is 9.8, giving it a B-V of 5.2, making it very red indeed!



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