Globular Clusters and colours?

[iceman]

Hi all

Just wondering about the colours we see in Globular Clusters, particularly in photographs such as these two excellent shots of Omega Centauri taken by Scott Alder (http://www.iceinspace.com.au/forum/showthread.php?t=20162).

If Globular Clusters contain all older stars and there's no recent starbirth regions, then I'm wondering why we only see a scattering of the yellow/orange/red stars? I would've expected to see the majority of the cluster as orangey in colour due to the older redder stars.

Most images I see of globs are mostly blue in colour - but it doesn't make sense as the globular clusters don't contain the young, hot blue O/B type stars.

Is it simply a colour balance issue with how the images are taken and processed?

[iceman]

Ok no answer in Astronomy Science forum, so I'll try here in Deep Space :)

I'm sure Jase will know :whistle:

[Dennis]

Hi Mike

I had a quick peek at this yesterday, and managed to find the following information via a Google search.

The stars in globular clusters are generally older, redder and less massive than our Sun.

Globular clusters normally consist of Population II stars, which have a low metallic content compared to Population I stars such as the Sun. (To astronomers, metals includes all elements heavier than helium, such as lithium and carbon.)

In globular clusters a few stars known as blue stragglers are observed, apparently continuing the main sequence in the direction of brighter, bluer stars. The origins of these stars is still unclear, but most models suggest that these stars are the result of mass transfer in multiple star systems.

Cheers

Dennis

[iceman]

yeh exactly, thanks Dennis - that's what I understood as well, which is what prompted my question.

Most of the amateur images of globs appear bluish in colour - so I'm wondering why. Is it a colour balance issue?

[jase]

It’s usually the filter transmission characteristics that determines the outcome.

The additive combination of RGB filters can result in a natural looking image, however this does not necessarily depict a “true colour” image or the way we “think” the object should look based on scientific data. This topic of true colour imaging opens up a never ending debate.

For stellar objects, filters such as UVBRI are a better match than conventional RGB. In fact, V and B match a human’s green and blue sensitivity when correlating the wavelength spectral response. I is better than R as stars predominately radiate with continuum. In other words they don’t always fall in the visual emission lines. However, depending on the filter characteristics the wavelength of I can be vast, so mapping it to an R channel can result in weight problems if not managed correctly.

As the core of globulars are heavily populated/dense, there is typically a “haze”of radiating light. This is depicted with a subtle cool white tone through filters and can obscure/mask the stars past main sequence. Some filters will show this with a blue tone (which can be a weighting problem). I prefer to call it cool white (like what you get from the flourescent lighting - compared to warm).

Few astrophotographers use UVBRI filters for imaging “pretty pictures” but those that do obtain remarkable results. As an example – Stephane Guisard has produced an RVB image that displays a very appealing view of NGC5139 - http://astrosurf.com/sguisard/Pagim/Omega-FS128.html. You can clearly see plenty of the older stars, partly through the dense cool white haze. There are other RVB images on his site.

[ballaratdragons]

I have imaged a few orangey coloured Globs as well as the Blue ones.

[Dr Nick]

Interesting topic, I havn'y even given this a thought untill now... ;)

[little col]

i think my blue glob is due to overprocessing , but this a very interesting thread:)

[iceman]

Thanks for the info and explanation Jase, you're a legend :thumbsup: I knew I could count on you.

That image is incredible. I imagine the large bright blue stars in that image are scattered stars from the plane of the milky way i nthe same area of sky from our vantage point, rather than true members of the glob?

Even in that image, a lot of the fainter denser parts of the glob are whitey/blue in colour, so I guess you're right about the overall "haze" from the starlight, rather than a true indication of the type of age of the star.

When you talk about colour balance and calibrating with a G2 type star, how is that done?

[jase]

You raise a valid question considering our scientific understanding of the globular cluster stellar characteristics. Seriously, don’t take my word for it. I haven’t studied the structure of globular clusters, though have read a few research papers from ESO, but that’s about it.

Regarding G2V star calibration…

There are a few ways of calculating your colour balance for a specific imaging train. Ideally, you should perform the calibration through each optical system (telescopes, lenses etc) that you intended to use. Don’t expect the weighting for one optical system to be the same for others – even if you’re using the same CCD camera and filters!

There are many resources on the web that tell you how to do perform the calibration and there is more than one-way of doing it. Here’s how I do it. It may seem a little daunting at first, but once you’ve done it a couple of times it’s straight forward.
Firstly, look up a table of G2V stars and select one close to zenith. You want to minimise as much extinction factor as possible. Also choose a star that isn’t going to saturate easily. There is a great reference table here - http://www.gemini.edu/sciops/instruments/niri/standards/g2vstars.html
Once you’ve selected your target star, begin to focus the star as best you can through your optical system. Proceed to take 10 or more equal exposures through each individual filter (R, G, B). I generally go for 30 second exposures.
Register and average (combine) the frames.
In MaximDL or other astro imaging software, I then measure the intensity of the G2V star in each frame (R, G, B). Make sure you’ve selected the right star!!! This sounds stupid, but when you’ve got a huge FOV you may need to reference your astro planning software or star charts. You need to ensure the measurement encompass the entire star, not just the core.
Take note of what the intensity values are; As an example:
R = 600,000
G = 700,000
B = 500,000

So based on these (example – don’t use these!) figures, you can see the example optical system is most sensitive in the green wavelengths. What ever is the most sensitive channel, make that ratio value of 1. So now that we have established G = 1, we can compute the R and B weights. If you divide the green number (700,000) by the red number (600,000) you get 1.16. Divide the green number by the blue number you get 1.4.
So your calibrated RGB weightings are R:1.6, G:1,B:1.4 for that specific optical system/filter/camera combination.From this point on, you can take equal exposure lengths as you image certain objects and use the correct weightings for each channel to obtain the right balance.

You should perform this even if you’re using true-balance filters such as those made by Astrodon. You’ll find the spectral response of your entire optical train will not deliver a RGB of 1:1:1. There are likely to be subtle variations. My method above method can be also used for colour imaging with UVBRI filters – in particular BVR (not so much U and I as they are out of the visual spectrum).

[little col]

that is really interesting and technical jase does the same apply to dslr image processing?

[jase]

Modified DSLRs can benefit from a similar process as I detailed above. Typically, when the camera is modified by removing or replacing the filter and tweaking the cameras settings to improve sensitivity, factory setting are lost. The factory settings are tuned to work with the standard filter for optimal white balance. Hence, I don’t think there is a major benefit in using the above procedure for unmodified cameras. I’m not an astro-DSLR expert, but there are plenty of people on this forum that can assist you further regarding this.

[iceman]

Thanks again Jase - excellent info. :bowdown:

[avandonk]

Here is a good site Mike about colour
http://www.midnightkite.com/color.html
On the same site colour of stars in RGB values
http://www.vendian.org/mncharity/dir3/starcolor/
Black Body Colour
http://www.vendian.org/mncharity/dir3/blackbody/

and a program for generating a colour RGB for a given wavelength
http://www.miguelmoreno.net/sandbox/wavelengthtoRGB/default.aspx

Why colour is so difficult it does not exist! It is merely the sensation caused by the ratios of intensities detected by the three different types of cones in our eyes.

Bert