I photographed Uranus, but the colour of it was purple...

[WilliamAstro]

So last night I captured data of Uranus through my Celestron 127 SLT, and upon processing after stacking with Autostakkert!3 and sharpening wavelets with Registax V6 I was met with Uranus in a purplish colour... So I went over to Photoshop for further post processing and I went over to levels and turned the Red channel down to 0.50 by the middle slider. Should this be considered as a false colour image?

Before:

After:

Thanks and clear skies

[Pete Presland]

Congratulations on capturing Uranus. I guess it would be, not that it matters greatly. Most Uranus images seem to be false colour due to them being captured with mono cameras and IR pass filters. Imagers tend to be trying to show the lighter Polar region and maybe the Moons, so colour is a secondary consideration and added for aesthetic effect.

[vlaiv]

28 minutes ago, WilliamAstro said:

Should this be considered as a false colour image?

Yes.

If you want true color - you should do proper color handling / calibration.

This does not have to be very involved process - if you are using planetary camera and have access to small lens (they often come with small all sky lenses) - you can do simple "white balance" in controlled conditions to find best red and blue ratio (compared to green).

You can also do this while shooting the planets. Shoot an image of a star of known temperature (make sure you don't saturate / clip the recording). From star temperature you can get RGB ratios and then you can calibrate your recording.

There are also advanced ways of doing it by creating color correction matrix and so on, as well as doing correction for effects of atmosphere.

[WilliamAstro]

6 minutes ago, vlaiv said:

Yes.

If you want true color - you should do proper color handling / calibration.

This does not have to be very involved process - if you are using planetary camera and have access to small lens (they often come with small all sky lenses) - you can do simple "white balance" in controlled conditions to find best red and blue ratio (compared to green).

You can also do this while shooting the planets. Shoot an image of a star of known temperature (make sure you don't saturate / clip the recording). From star temperature you can get RGB ratios and then you can calibrate your recording.

There are also advanced ways of doing it by creating color correction matrix and so on, as well as doing correction for effects of atmosphere.

When I captured Uranus, it was not really that high on the horizon but enough to still be worthy for imaging. So the camera and whatever was not to blame.

[vlaiv]

8 minutes ago, WilliamAstro said:

When I captured Uranus, it was not really that high on the horizon but enough to still be worthy for imaging. So the camera and whatever was not to blame.

My planetary color camera (ASI178) gives distinct green cast because green QE is highest.

Reddish color with such camera can happen if I use inappropriate bayer matrix order. So that would be first thing to check.

Do have a look at this thread about color calibration and results:

 

[WilliamAstro]

1 minute ago, vlaiv said:

My planetary color camera (ASI178) gives distinct green cast because green QE is highest.

Reddish color with such camera can happen if I use inappropriate bayer matrix order. So that would be first thing to check.

Do have a look at this thread about color calibration and results:

 

Well, if it does not need to be an involved process. I guess I don't have to use colour calibration because many planetary images (like Uranus) tend to have false colour. But I could give this a go if there is ever an opportunity to do it.

[vlaiv]

1 minute ago, WilliamAstro said:

Well, if it does not need to be an involved process. I guess I don't have to use colour calibration because many planetary images (like Uranus) tend to have false colour. But I could give this a go if there is ever an opportunity to do it.

If you want simplest approach - then just shoot a star of known temperature.

Do it on the night of shooting your target and in same general direction and at same altitude (to get same effects of atmosphere).

Use this calculator to get RGB ratios:

http://www.brucelindbloom.com/index.html?ColorCalculator.html

Say you image F2 class star - it has temperature of 7020K.

Set calculator like this:

(reference white to D65 and Gamma to 1.0)

Put star color in CCT field and press CCT.

This will produce RGB triplet values that you can use to get scaling factors - in this case 1.004, 0.987, 1.112 (rounded).

You take recording of your star and measure RGB values from it and from that - you get R_scale and B_scale values that you'll apply to your planetary recording. Here is how to calculate those using simple proportion.

(scaled measured red to measured green has to be equal to above calculated red to calculated green)

R_scale * R_measured : G_measured = 1.004 : 0.987

R_scale = (G_measured * 1.004) / (0.987 * R_measured)

You do blue scale similarly and then apply R_scale and B_scale in channel mixer of your processing software. Add gamma of 2.2 at the end (set middle slider in levels to 2.2)