Blue stars?

[groberts]

Attached is my LRGB processed image from early May, which in general I think isn't too bad but wonder why there are so many what appear to be blue stars? I've gone back and reprocessed the blue channel (less aggressively) but with limited improvement.  I've subsequently imaged and processed M13 globular cluster without any such problem. Any ideas please?

Thanks, Graham

WO GT81 + guided , 24 x 180' Lum + 10x180' RGB + Calibration Darks/Bias/Flats. Processing DSS and Photoshop 

 

[JohnSadlerAstro]

Hi,

My image of M3 came out pretty much the same when processing, I found that there were several very red and then lots of very blue stars that messed up the colour balance constantly. Just the object, perhaps?

John

[Some Dude With A Mak- Cass]

I don't know much about imaging, but there are a lot of blue stars in that cluster. What cluster did you say it was? It could just be that it has lots of O and B class stars, because the image looks fine to me.

[groberts]

Sorry, it is M3 and there are apparently a high number of so-called 'blue stragglers' - younger main-sequence stars derived from stellar interaction of the older cluster stars. Just thought there were too many + didn't look right somehow, 

[Some Dude With A Mak- Cass]

2 minutes ago, groberts said:

Sorry, it is M3 and there are apparently a high number of so-called 'blue stragglers' - younger main-sequence stars derived from stellar interaction of the older cluster stars. Just thought there were too many + didn't look right somehow, 

I know the feeling. When I first saw a star cluster (I don't remember which one it was now) I thought it looked too red, but now I know it just had a weird number of red stars. Before photographing an object, it's a good idea to look it up so you know what to expect from the object.

[JohnSadlerAstro]

1 hour ago, groberts said:

Sorry, it is M3

No problem, it's fun to do some detective work! That tiny galaxy just above the cluster gives it away though!

John

[ollypenrice]

My planetarium (SkyMap Pro) has the B-V values for many stars and have a B-V colour chart saved in my imaging folder. Here's one: https://stackoverflow.com/questions/21977786/star-b-v-color-index-to-apparent-rgb-color

This means I can check some stars to make sure my colour balance is credible.

Olly

[groberts]

Thanks all.

I'll have to spend more time to understand the B-V values concept but at first glance it looks useful Olly.  Is there an equivalent for other DSO features?

Graham

 

[celestron8g8]

I think your pretty close to good to go on your colors . Take a look at Robert Gendlers image of M13 . Robert is a pro at his imaging . Your not far off his colors . 

http://www.robgendlerastropics.com/M13NM.html

[groberts]

Thanks Ron that's encouraging, though his additional integration time shows too. 

[celestron8g8]

2 minutes ago, groberts said:

Thanks Ron that's encouraging, though his additional integration time shows too. 

But remember , Robert is a pro so I don’t expect things to be better . Robert is a surgeon doctor but he has always had time for astro work . I love looking at all his work . I’ve emailed him a few times in the last 15 yrs . He is quite a gentleman . Takes a little time but i always got a reply sooner or later . 

[ollypenrice]

21 hours ago, groberts said:

Thanks all.

I'll have to spend more time to understand the B-V values concept but at first glance it looks useful Olly.  Is there an equivalent for other DSO features?

Graham

 

You can actually use it without understanding it. Just find the B-V value for a given star and read off its colour from the chart. As the others have said, your colour looks about right with maybe a tad too much green? But really a tad.

In PS you can use the colour sampler tool on the background sky. Set the sampler to 3x3 average, not point sample, and it will give you readings in R,G and B. Personally I go for parity in all three. This is a very good test of general colour balance. If the background is balanced the rest shouldn't be too far out.

You can't really have an equivalent for nebulae, though if an Ha-dominated target doesn't look red then something's gone wrong! 

Olly

[groberts]

2 hours ago, ollypenrice said:

You can't really have an equivalent for nebulae, though if an Ha-dominated target doesn't look red then something's gone wrong! 

Thanks Olly. 

Yes I figured that but I'm always concerned what is the 'right' colour for objects such as nebulae, other than aesthetically and wondered if there was some way of benchmarking this objectively?

Graham  

[ollypenrice]

40 minutes ago, groberts said:

Thanks Olly. 

Yes I figured that but I'm always concerned what is the 'right' colour for objects such as nebulae, other than aesthetically and wondered if there was some way of benchmarking this objectively?

Graham  

I think that the best any of can do is perform a careful colour calibration and trust it. When I enhance natural colour images in RGB or LRGB with Ha and/or OIII I have the unenhanced image open on the screens for comparison. Sometimes this gives a result which is 'controversial' in the sense of being unconventional. Sometimes I end up in the same place as most other people. If the stars agree with the astrophysics and the background is neutral then I stick with it.

Olly