Questions Star Color Calibration

Can anybody help on this? Thanx, Stephan

Sorry for not being able to answer at the moment, very busy weekend. I'll tag Mabula to see if he's able to (@mabula-admin).

Thanx

Any news here?

I had exactly the same questions in mind....  😉 

Nope, I've asked Bula in a PM to try to get to the tagged questions. 🙂 Sorry for the delays lately! (@mabula-admin)

Hi Mabula,

Good to have you back in the forum. Happy new year to you!

If you time, could you please reply to this question. I have many raw data in the pipeline and I want to process them in the right way.

Thanx, Stephan

I think he missed this one. Maybe another tag works.. @mabula-admin If not, you can also mail him about it.

Hi Stefan @schurig, @betelgeuze and @vincent-mod,

Please accept my apologies for the long delay in answering this.

Let me respond to your questions:

I watched your latest video tutorial about star color calibration. As I understood, star color calibration takes place in 3 steps:

1. adjust Magenta/Green cast for both constant sliders equally in percentage
2. adjust the slope for both slope sliders equally in percentage, so that the black body model line (green) goes right through the middle of the blue star cloud in the diagramm
3. adjust the Blue-Red slider that the 0,0-point of the diagram of the blue star cloud is shifted in order to represent more red stars (a bigger part of the blue star cloud lies to top/right than to bottom/left)

I have a couple of questions:

• When adjusting slope slider and constant sliders equally in percentage is recommended, why is there two sliders? Or at least wouldn't it be comfortable to connect the sliders so moving one results in an equal move of the other?

There are 2 sliders, because it really depends on the filters used if the sliders need to be adjusted a similar percentage. If you use a light pollution filter for instance, the amount of adjustment of the sliders are not related as such. If you have a DSLR and have not used any filter, then the percentage approach is a good approach. I could make a selectbox to fix the sliders but I will need to think about this.

• Step 1 is a totally subjective process, right? It obviously strongly depends on a very good calibrated screen and user experience. Or is there a way to see in the diagramm how much I have to move the slider?

Well not entirely so I would argue, the Color-Color diagrams show you quite well what is happening with colors. Check how the star population is spread realtive to the origin of the plots. If there are stars to the bottom left of the origin, these would be magenta, if there are stars to the top-right of the origin, these would be green.

A calibrated screen does help a lot in being able to visually see acurate color off course and get better/good experience. If your monitor is not calibrated, then your stuck sort of speak, because you can't really build up experience because there is never a reference ;-). Any monitor over time will start to show the colors differently, because of age and this is significant. The monitors will also become less bright over time, which affects color as well... And the ambient color in your work/data processing environment (daylight or desk light) will affect things as well.

The best that I can recommend in this regard is to invest in a screen calibration sensor like the datacolor spyder. Even the cheaper and non-IPS monitors can be calibrated very well with such a sensor. The problem most of the times, is that cheaper monitors come from the factory not or badly calibrated. And even if they come well calibrated from the factory, after several months of use, the calibration is gone and thus not reliable, really 😉

• Is there a fixed ratio of red to blue stars in the universe? So that I can set the 0,0 point to divide the star cloud e.g. 1/3 to 2/3? Or is this also a subjective process?

No, there is not for the whole universe as a whole. It does depends on the age of the star population that you are looking at for instance.

The Blue-Red adjustment basically is the white star calibration. by shifiting the slider, you are telling APP where in the star population the white star is. So it depends directly on the star population selected. You should take into account that the blue stars are hotter and brighter than red stars, so there is also a strong practical bias here introduced by your data and your optical setup. The blue stars are much more likely to be detected and analysed then the colder and fainter red stars in our images.

In our images, most stars our obviously from our own MilkyWay and they are most likely Main Sequence stars in hydrostatic equilibrium fusing Hydrogen to Helium in their cores. And the far majority of these stars will be red. They simply live much longer than blue stars and therefor have the majority.

Perhaps I can add to the tool with some quantive output with regard to this, like indication a % of green to magenta pixels for the entire image, to make this more controllable. And perhaps indicate in the image and Color Color diagram what a white star would be with the current Blue-Red slider setting. I will put this on my list to think about and possibly improve.

Kind regards,

Mabula