NGC 5286, and how to make colour-magnitude diagrams!

[SkyViking (Rolf)]

Hi All,

Here is yet another globular image I took during near-full Moon conditions recently. This time of NGC 5286. This globular forms a beautiful duo with the 4.6 magnitude star M Centauri. This star has a funny pale yellow-orange colour (G8) which is an interesting contrast to the cluster.

Hi-res image is here: http://www.pbase.com/rolfolsen/image/143299725/original
Image details:
LRGB 27:15:15:15m
02/05/2012
10" f/5 Serrurier truss Newtonian
QSI 683wsg with Lodestar guider


I have also been busy creating a couple of colour-magnitude diagrams for some of the globular clusters I have imaged recently. I was surprised with how successful it turned out and how simple the process was, so I'll briefly describe what I did here:

To create these diagrams I downloaded the demo version of AstroArt. The demo lets you export a list of stars from an image, so that is perfect for this purpose.
I used the stacked B and R raw FITS images to generate the data. First I loaded the B image into Astro Art. Then from the Tools menu I selected 'Find Stars' and entered desired number of stars and FWHM criteria. For Omega Centauri I chose for example 30,000 stars. For the other clusters I chose a smaller number since they are more compact and crowded and there aren't as many stars available for selection.
Then I went to View -> Stars and a table with all the selected stars is displayed. I sorted the rows according to X coordinate and saved the star data as a .sta file via the right-click menu.
I then repeated the proces for the R file, using the same star selection criteria.
(The .sta format is simply tab/space separated values and can be opened in any text editor)

The main challenge was to somehow match each star in the two B/R .sta files and create one new file with correct magnitudes for the matched stars. Not all stars will be matched since AstroArt will not necessarily select the same stars in each image. So for this purpose I wrote a small Gosu program that scans the two files and matches the entries by their coordinates. The program also calculates the magnitudes based on the ADU count. The result is a third file with the magnitudes for each star that was matched in the B and R file. This way I ended up with around 10,000 matched stars with magnitudes out of the 30,000 stars I selected for Omega Centauri.

Now I could open this new file in Excel and create a scatter plot for the data, and voila a textbook perfect colour-magnitude diagram emerged, complete with red giant branch and horizontal branch. Encouraged by this I repeated the process for some other globular clusters with equally effective results, allthough with fewer stars.

Click here to go to the gallery with colour-magnitude diagrams for the globulars Omega Centauri, NGC 6752 and NGC5286.

I had great fun with this little exercise and will probably continue making more diagrams of any globulars I image in the future. Hmm, how about a diagram of UKS-1...
Hope you enjoy the view. Comments and critique is welcome as usual.

Regards,
Rolf

[Mighty_oz (Marcus)]

Lovely shot, nice presentation of a cluster

[RickS (Rick)]

Very cool, Rolf. Nice colour in the image too.

I hadn't heard of Gosu. The language comparison page lists not being Lisp as a language feature which is a little unfair. Once you get past all the parentheses some Lisp variants aren't that bad

Cheers,
Rick.

[alistairsam]

Nice Analysis Rolf, is this commonly done?
would it indicate that there are a larger number of cooler stars?
what is your inference?

[jsmoraes (Jorge)]

Amazing idea. Congratulations for yours efforts. The graphics shows only magnitude or the actual color of stars too ?

[multiweb (Marc)]

Very cool Rolf and a superb shot to boot. Great stuff!

[Terry B]

Very clever and well done. Nice examples of HR diagrams. It would be worth doing the same thing for an open cluster as well as they are a different age to GCs.

[allan gould]

I read this and was just so impressed about what you have done. Absolutely brilliant and by the way, a great image.

[marco (Marco Lorenzi)]

Fantastic idea and extremely interesting result Rolf, I was not aware that such kind of analisys could be at the reach of amateurs.. I find the graphs really educative and interesting!

Clear skies
Marco

[Ross G]

A great photo Rolf.

Nice contrast and composition.

Ross.

[SkyViking (Rolf)]

Thanks for all your kind comments. I have added two more diagrams for M4 and M80 and changed the calculations slightly. First of all I now use the B and L files instead of B and R. This way the diagrams can be more easily compared to the standard B-V colour index used in astronomy.
I have also improved the Gosu program I mentioned, so it now takes the interstellar reddening into account. This is important in order to get the correct colour index rather than the observed one.
Hope that makes sense. I have updated the info on the site accordingly: http://www.pbase.com/rolfolsen/colourmagnitude_diagrams

Originally Posted by Mighty_oz

Lovely shot, nice presentation of a cluster

Thanks Marcus This one is quite photogenic right next to the bright star.

Originally Posted by RickS

Very cool, Rolf. Nice colour in the image too.

I hadn't heard of Gosu. The language comparison page lists not being Lisp as a language feature which is a little unfair. Once you get past all the parentheses some Lisp variants aren't that bad

Cheers,
Rick.

Thank you Rick, yeah Gosu is a fairly new language. Very 'clean' and not as verbose as Java but still with full access to all the solid Java API's. Very promising I think.

Originally Posted by alistairsam

Nice Analysis Rolf, is this commonly done?
would it indicate that there are a larger number of cooler stars?
what is your inference?

Thanks Alistair Yes these diagrams are standard astronomical tools but it's good fun to be able to create them from amateur data. There are quite a few interesting things visible in the diagrams, I'm thinking of making an annotated version.

Originally Posted by jsmoraes

Amazing idea. Congratulations for yours efforts. The graphics shows only magnitude or the actual color of stars too ?

Thank you Jorge, the horizontal axis is the colour index calculated as B magnitude minus L magnitude.

Originally Posted by multiweb

Very cool Rolf and a superb shot to boot. Great stuff!

Thanks very much Marc, glad you liked it

Originally Posted by Terry B

Very clever and well done. Nice examples of HR diagrams. It would be worth doing the same thing for an open cluster as well as they are a different age to GCs.

Thank you Terry Yes I'll try open clusters too, I just didn't have a good image of one where the stars are not overexposed (can't measure magnitudes correctly for saturated stars)

Originally Posted by allan gould

I read this and was just so impressed about what you have done. Absolutely brilliant and by the way, a great image.

Thank you very much Allan. It's a great little excercise, I'll probably add some more diagrams along the way.

Originally Posted by marco

Fantastic idea and extremely interesting result Rolf, I was not aware that such kind of analisys could be at the reach of amateurs.. I find the graphs really educative and interesting!

Clear skies
Marco

Thanks Marco Glad you liked it. I believe there are lots of things amateurs can do, we just haven't thought of it yet - too busy taking pretty pictures!

Originally Posted by Ross G

A great photo Rolf.

Nice contrast and composition.

Ross.

Thank you Ross, it's a pretty target I think.