Pixinsight: synthetic luminance question

[konstantinos75]

Hi

Just a question about the synthetic Luminance frame in Pixinsight.

I have at my disposal these fit frames

- 20 red frames, bin 1x1

- 20 green frames, bin 1x1

- 20 blue frames, bin 1x1

These frames are already calibrated.

What are the steps to create the synthetic Luminance frame?

I want to use LRGBcombination module afterwards in order to add the synthetic Luminance to the RGB frame.

Some people argue that there's no need to take luminance frames if you have a synthetic Luminance.

Do you agree with this assumption?

Thank you

Konstantinos

[IanL]

A synthetic luminance is not a substitute for a real one. The reason for taking a luminance is that you can reach the same SNR in less acquisition time by binning the RGB frames 2x2 and taking the L at 1x1. The L is more important for resolution than the RGB due to the way human vision works.

Using a true L or creating a synthetic one can also be useful when processing using (say) Photoshop as you can apply different processes to the L and RGB frames before combining them. In PI it isn't generally necessary to do this as most of the important processes have the ability to apply different settings to the appropriate elements, so you can target (say) the luminance and colour data differently in one process without first separating them. That isn't to say that you can't or shouldn't create a synthetic L if that is what you need, but you shouldn't slavishly do so following Photoshop practices rather than understanding why you are doing it.

In short, I don't agree with your assumption. You are confusing two different issues; saving time during acquisition by taking an L and binned RGB frames vs. using an L to give you control over certain processing steps.

[konstantinos75]

Hi

I have found these three methods which describe how to make a synthetic L in Pixinsight.

I need to experiment now.

synthetic luminance method 1

===============================

R frames stack -> master R frame

G frames stack -> master G frame

B frames stack -> master B frame

Align R,G,B frames

ImageIntegration tool

- Combination = Average

- Normalization = additive with scaling

- Weights = Noise evaluation

- Scale estimator = iterative k-sigma

- Generate integrated image = enabled

- Evaluate noise = enabled

- Pixel rejection = No rejection

- Clip low range = disabled

- Clip high range = disabled

synthetic luminance method 2

===============================

R frames stack -> master R frame

G frames stack -> master G frame

B frames stack -> master B frame

Align R,G,B frames

pixel math: master R+master G+master B

synthetic luminance method 3

=============================

synL1 = ImageIntegration( red1 green1 blue1 ) 

synL2 = ImageIntegration( red2 green2 blue2 )

...

synLn = ImageIntegration( red(n) green(n) blue(n) )

finalLuminance = WinsorizedSigmaClipping ( L1, L2...Ln, synL1, synL2..., synLn)    // sigma =4, Normalization = scale zero offset

LRGB = LRGBCombinationTool ( finalLuminance/RGB)   // L=0,5, Sat=0,38-4,5

[Zakalwe]

If you have access to photoshop then it's dead easy.

Change the mode to LAB. Open the channels palette and select the L channel. That contains the luminance data.

[IanL]

Equally easy in PI, not sure where you are getting all these complex methods from!:

Calibrate, align and integrate your master image as usual (e.g. in your first method why would you turn off pixel rejection? You integrate using the appropriate rejection method for the number of frames as per tooltip and documentation).

Set the RGB working space for the integrated master image using the RGBWorkingSpace process. You want to give the RGB channels a weight of 1. (The default weights are sRGB and mean that your luminance will end up coming mainly from the green channel and only a small amount from R and B ). Drag the blue triangle on to the image to set its working space.

Use the ChannelExtraction process. Set the colour space to CIE XYZ make sure that only the Y channel is ticked and the drag the blue triangle on to your source image. The resulting new image will be a synthetic luminance.

[konstantinos75]

Hi

Thank you for the replies.

I have tested method 1 above and I am very happy with the result!