Globular Clusters M3 and M13

[steppenwolf]

Globular Clusters M3 and M13

Springtime is a great time for galaxy hunting but if you’re prepared to stay up late and into the wee small hours it is also a great time for an early look at some globular clusters. Two of my favourites are M3 and M13 and the recent series of clear nights had me ditching my narrowband filters and rotating in my LRGB set – except that I then remembered that I had been doing some comparison tests between my Baader 8.5nm OIII filter and my Astrodon 3nm OIII filters earlier in the season and had removed the green filter to make room for the Baader OIII- Doh! Not to be done out of my images, I decided to capture just the Red and Blue channel data and synthesise the less important green to produce my RGB images.

M3 (NGC 5272)

Introduction

This beautiful globular cluster in the constellation of Canes Venatici was the first Messier object to actually be discovered by Charles Messier himself who came across it on 3rd May, 1764 although he originally mistook it for a starless nebula, a mistake that was corrected 20 years later by William Herschel who was able to resolve the cluster’s individual stars. It is now believed to contain over half a million stars, some of which I have managed to capture here!!

Image Stats

Mount: Mesu 200
Telescope: Sky-Watcher Esprit 150
Flattener: Sky-Watcher Esprit specific
Camera: QSI 683 WSG-8
Filters: Astronomik LRB (no G!)
Subframes: 36 x 300 sec L, 23 x 300 sec R, 25 x 300 sec B
Total Integration: 7 hours
Control: CCD Commander
Capture: MaxIm DL
Calibration, Stacking and Deconvolution: PixInsight
Post-Processing: PhotoShop PS3

Location

Constellation	Canes Venatici
RA	13° 43' 04.0"
DEC	+28° 17' 26.0"
Distance	~34000ly

 

 

M13 (NGC 6205)

Introduction

The second of my two recent captures, M13, often known by its common names of ‘The Hercules Globular Cluster’ or ‘The Great Globular Cluster in Hercules’ (!) is another beautiful object which out-shadows its nearby cousin, M92, in the constellation of - yes, you guessed it – Hercules! This is probably the best known globular cluster in the heavens and was discovered in 1714 by the English astronomer Edmond Halley who like Messier and his M3 was convinced that it was starless! It is now known to contain over 300,000 stars. Although I captured 22 x 300 sec L exposures and they are of excellent quality, I felt that they added nothing to the image so I didn’t include them!

Image Stats

Mount: Mesu 200
Telescope: Sky-Watcher Esprit 150
Flattener: Sky-Watcher Esprit specific
Camera: QSI 683 WSG-8
Filters: Astronomik LRB (no G!)
Subframes: (22 x 300 sec L – not used), 23 x 300 sec R, 12 x 300 sec B
Total Integration: 2.9 hours
Control: CCD Commander
Capture: MaxIm DL
Calibration, Stacking and Deconvolution: PixInsight
Post-Processing: PhotoShop PS3

Location

Constellation	Hercules
RA	16° 42' 21.0"
DEC	+36° 26' 17.0"
Distance	~23000ly

Location Chart for both clusters

 

[simmo39]

V nice and clear, I will show my poor effort of M3 later.

[City9Town0]

Very nice Steppenwolf. Funnily enough I imaged M3 for the first time last night (total 16 minutes)... I'll put my image up as well...

[Tommohawk]

Very nice images - I really ought to have a go at some globs. Somehow the clusters seem to generate a 3D appearance which isnt apparent from sparse star fields

[vlaiv]

@steppenwolf

I just carefully read your capture details and saw that you used no G approach to capturing.

I wonder why you decided to go without G instead on other color - like B for example?

I presume that you used simple math to calculate missing color component prior to color calibration G = L - (R+B).

In these kind of images G carries a lot of signal so G channel gets good SNR. Usually due to sensor response and atmospheric scattering, B is the weakest of the three - so it needs quite a bit more exposure to reach same SNR.

Maybe B would be better candidate to calculate rather than G because of that?

[Laurin Dave]

Very nice Steve, particularly impressive result given that you had no green subs.

Dave

[reddoss]

Globular clusters are quite hard to image without over exposing the core. Well done Steve.

 

[steppenwolf]

4 hours ago, simmo39 said:

V nice and clear, I will show my poor effort of M3 later.

Thank you, I look forward to seeing your version.

4 hours ago, City9Town0 said:

Very nice Steppenwolf. Funnily enough I imaged M3 for the first time last night (total 16 minutes)... I'll put my image up as well...

Thank you, yes, definitely post up yours.

4 hours ago, Tommohawk said:

Very nice images - I really ought to have a go at some globs. Somehow the clusters seem to generate a 3D appearance which isnt apparent from sparse star fields

Yes they do produce a 3D effect naturally - beautiful objects.

4 hours ago, vlaiv said:

I wonder why you decided to go without G instead on other color - like B for example?

I presume that you used simple math to calculate missing color component prior to color calibration G = L - (R+B).

The slot that normally holds my Green filter in my filter wheel was occupied by another filter and I wasn't prepared to take the system apart to replace the green filter for these images and as I already use Noel Carboni's excellent PhotoShop 'Actions', I knew that he had a 'Synthesise Green' from Red and Blue channels action. It is, of course, quite true that the blue channel is the worst effected channel with regard to atmospheric conditions but my choice was one of necessity (no green filter available) rather than a deliberate choice for imaging purposes! The PhotoShop action performs the maths for me and to be honest, it does a pretty good job as I hope the two images demonstrate.

3 hours ago, Laurin Dave said:

Very nice Steve, particularly impressive result given that you had no green subs.

Thank you, it is amazing what you can get away with and I am used to bi-colour narrowband imaging so was quite comfortable with this approach.

1 hour ago, reddoss said:

Globular clusters are quite hard to image without over exposing the core. Well done Steve.

Yes, indeed they are and this is why I felt that the Luminance channel data that I got didn't really contribute anything to the M13 image so I junked the subs. I could have used them with a lot of faffing around but didn't feel that was worth the additional effort when I already like the R(g)B image I had produced.

[geeklee]

Lovely images. It's the colours and especially the depth that really pull you in.

[kirkster501]

Both superb. 

Interesting you didn't use the luminance and I likewise don't use luminance for globular or open clusters.  RGB at 1x1 is enough in my opinion.  I cannot give a scientific reason for that, just a "feel" that a separate luminance just doesn't add anything to the final result. 

If you don't mind, here is my version of M13 by way of comparison and I prefer yours very much; your rendition looks much more "natural" than mine.  You've inspired me to go back at this data set and have another go.

[steppenwolf]

36 minutes ago, kirkster501 said:

Interesting you didn't use the luminance and I likewise don't use luminance for globular or open clusters.  RGB at 1x1 is enough in my opinion.  I cannot give a scientific reason for that, just a "feel" that a separate luminance just doesn't add anything to the final result. 

Yes, Stephen, it is an interesting observation as we are so used to producing a luminance set to give 'body' and detail to deep sky images but globular clusters don't seem to need it! I suspect that it is simply the stellar nature of these objects so there isn't any detail as such, just point sources and the presence of the Luminance data becomes a little overwhelming and risks spoiling the subtlety of the individual stars. Our images of M13 are very different but I really like the incredible number of stars in your image!

 

[Knight of Clear Skies]

14 hours ago, geeklee said:

Lovely images. It's the colours and especially the depth that really pull you in.

Yes, it's the sense of depth that makes these images really stand out.