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Messages - astroedo

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The Astro Imaging Channel: Challenge nr 1

The Astroimaging Channel on Google+ proposed an image processing challenge (read here) based on a set of RGB images of the Markarian chain.

This was the challenge:

Gradient Removal while preserving star color.
A few months back, I wanted to try imaging my blue channel high up in the sky.  It didn't work out as planned and I have a terrible gradient through the image.  I've been able to get rid of the gradient, but it leaves a bad color cast through my stars.  The challenge is to remove the gradient and preserve the star color.

Here my solution in PixInsight.

Hi, Thank YOU for reading!

This Tutorial is thought to work mainly with object with mixed continuum and narrowband emission like galaxies.
Nebulas are mainly narrowband line emission.
It can work, but it is not suited for it.

Much better is the Vicent Peris article here

I don't know if this method can work with OIII emission: OIII is a mix of green and blue, so, maybe we can try to apply the method to both the channels and try to enhance the green and the blue component...

but I've never done, it would be very interesting  ;)

Hi All here my latest tutorial: as from the subject it's about using an H alpha image to enhance HII regions in spiral galaxies.

The method is directly derived from Vicent Peris formula for composition of Narrow band data with broad band ones.

I how you will enjoy

Anche in italiano

I suppose it's completely correct.

I always export TIFF in 16 bit format.

Very Strange the behavior with FIT in 32bit, maybe the software you are using does not supports this format  ???

Hi Gunny it's perfectly normal that you find tiny values in PI:
It works on normalized images so all your values lay between 0 and 1 (where 1 means the maximum value allowed by your dynamic range, 65536 in your 16 bit images).

Less normal is that you find different aspect opening the image with different software.

Which is your work flow in PI?
Which processes are you using?
Could you share an image on dropbox just to understand better the problem?


Simply GREAT!

It can be an Idea for a script or module in the next version of PI  ;)

General / Re: Need Help on Processing Comet
« on: 2015 January 05 10:23:16 »
I'm trying right now with the same comet  ;D :laugh: :cheesy:

General / Re: Need Help on Processing Comet
« on: 2015 January 05 10:05:42 »
Hi boys. I cannot reproduce Juan's tutorial. Images of comet and comet+stars are so different, that after substraction I got near black image. Could someone help me with this problem? I uploaded set of images to
Many thanks for any suggest.

Hi! I have the same problem because the background in Full and Comet images is the same so many pixels are clipped to 0 when I perform
Full-Comet operation to get the stars image.

I've solved changing Juan expressions as follows

for the stars image:

Full-Comet+med(Comet) this is the new stars image

Star removal from full image

x = k*med( stars );
$T - iif( stars > x, stars, stars*(1 - (x - stars)/x) )+med( stars)

I really don't know if it is the right solution, but it works!

bye Edoardo

Hi, I sometimes use InterChannelCurves Module using Green (or another color channel) as Both reference and target channel.

InterChannelCurves is an optional module that you can downolad for free from Carlos Milovic website

you can also find information here.

I hope this helps  ;)

General / Re: using previews in pixelmath expression
« on: 2014 December 18 09:06:02 »

Open an image, change its Id to "A". Create a preview in A. Apply the PixelMath expression "A" with "Create new image" to A's preview. PixelMath generates no image nor an error message (i.e. size mismatch?). Bug?


In my experience create new image never works with previews.

This is not a BUG in my opinion because previews are intended to preview ( :D ;D ) the result of the expression before applying to the whole image.

General / Re: LRGB combination
« on: 2014 November 02 16:16:03 »
On a side note, combining linear L and RGB is possible in PixInsight. However you must perform a YRGB combination instead of LRGB.

In this case, instead of the CIE L*a*b* space, the CIE XYZ space must be used. CIE XYZ is a linear color space; however, the underlying RGB working space must also be linear in order to preserve linearity in the combined image. The basic procedure would be as follows:

- Define a linear RGB working space. With the RGBWorkingSpace tool, set gamma = 1.0 (disable sRGB gamma function). In addition, I would define a uniform RGB working space in the same operation. So the steps with RGBWorkingSpace are:

* Set the three luminance coefficients = 1.
* Define a uniform energy illuminant by setting the six chromaticity coordinates = 0.333333.
* Disable the sRGB gamma function and set gamma = 1.
* Apply to both the RGB and L images.

- Use the ChannelCombination tool to perform a YRGB combination. Select the CIE XYZ space, disable the X and Z components, and select the L image for the Y slot. Apply to the RGB image and you're done.

- To process the linear luminance of the combined YRGB image, you must select "Linear luminance" in the applied tools, when applicable (for example, Deconvolution and ATrousWaveletTransform allow you to select it).

- After the initial nonlinear stretch with HistogramTransformation, you may want to apply the default RGBWorkingSpace parameters, as a linear uniform space no longer will describe the image appropriately.

This should work :)

Hi Juan,
I've tried this method just for curiosity, but when I try to use RGBWorkingSpace with all six chromaticity set to 0.333333 I get the error:

Instance Validation Failed
Reason: Singular Matrix in RGB working color space inizialization

any Idea?

Its goal is to keep the background with a neutral color.
without this correction you would have a strongly colored background because its value would be boosted.

Thank you Geoff!

Yes the star mask is crucial, sometime I use two different settings to get all the stars, and I use a range mask enlarged with morphological transformations for the big saturated stars.

Then I merge the different masks using the max() operator in PixelMath.

I've Just downloaded Tone Mapping V2 by Metsavainio where it explains how to recover stars colors and I'm working to integrate it in the normal workflow in PI.

Unfortunately I don't have so much time as I would like to make tests and write tutorials...  :-[

Great! I can't wait for it

I also have an Idea for unconventional starmask building:

I'm experimenting with PixelMath to create a "clipped gradient mask" to use as a Deconvolution support.

I will post as soon as possible

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