ALHAMBRA Survey, field 7

[Joe DePasquale]

Thank you, Juan!  I am familiar with the CloneStamp tool, and although it's great for bad pixels and streak type artifacts, I find it to be fairly limited when it comes to repairing star blooms.  Perhaps there's something I'm missing with it.  I'd love to hear a new perspective on it. 

[Joe DePasquale]

Attempt number 2!

2 Major changes on this attempt:

[1] I finally gave Deconvolution a try...something I've been meaning to do for a long time.  I used this post from Juan as a guide:
http://pixinsight.com/forum/index.php?topic=2727.msg18512#msg18512
I think it produced pretty decent results, but it was a challenge to deal with the small scale noise brought up.  A few rounds of ATrousWaveletTransform helped with that.

[2] I had a major epiphany in using the CloneStamp tool for removing those star bloom artifacts!!  (Thanks Juan for leading me down this path).  I just had to adapt my Photoshop brain to PixInsight to make this work.  The trick is to make a preview of the star that you want to fix, pull it out into its own window, and then rotate it 90 deg and use the unbloomed diffraction spike as a replacement for the bloom in the original image using the clone tool.  It works surprisingly well! 

Here's the new version:

[Juan Conejero]

Hi all,

Here is my take on these wonderful ALHAMBRA data. I have been interested exclusively in finding new ways of combining the data, not in producing a finished image. I have explored several interesting possibilities and (inevitably!) have ended writing a small JavaScript script that performs the whole task of loading the images, computing the weights of each ALHAMBRA image for individual RGB filters, and synthesizing the final RGB image. The script is a bit long to include it inline (approx. 500 lines) so you'll find it attached to this post.

In order to evaluate the results some mouseover comparisons are very useful, which are not possible in a forum post. For this reason I have written a brief document in PIDoc format, where you'll find complete information on my approach to this task:

http://pixinsight.com/doc/docs/ALHAMBRA-1/ALHAMBRA-1.html

(you'll need an SVG enabled browser, which does not include Internet Explorer before version 9).

These are three crops with the best (IMO) filter set defined in the script (extended filter set):







I would like to draw your attention to an interesting fact: a visual representation (what is often called 'natural colors', whatever that means) is not appropriate to represent this ALHAMBRA dataset. In fact, the RGB filter set that I have called 'visual set' is much better than what visual perception actually is, and even with this concession the result is very poor. The best filter set is the extended set---I hope everybody will agree here---because it is the one that maximizes information representation. This is the documentary criterion that defines our deep-sky color calibration methodology.

[Joe DePasquale]

Hi Juan,

I have to say I'm fascinated with this approach!  Not only did I download and run your code, I went as far as to print it out and review it by hand, and run a few filter weights on my calculator to really understand the concept.  As a side note, I had no idea it was possible to create plots in PixInsight - how cool!

I just did a side-by-side comparison of your approach to my approach of giving all filters equal weight and just running them through pixel math (as referenced in my previous post).  Your approach gives you a very clean, color balanced image right off the bat, whereas my image requires a lot of color calibration before it even starts looking natural.  I totally agree that the 'extended filter' set is clearly the best option because it truly utilizes all of the available data while adhering to a rigorous approach to color. 

Just out of curiosity, how did you choose the central wavelength for the various filter sets (and specifically the extended set)?  I often find myself combining data from many different wavelengths (Radio, IR, optical, UV, X-ray, etc) often in the same image (I'm the Science Imager for NASA's Chandra X-ray Observatory).  How would you adapt your approach here to something like that?  Often times, when working in such disparate spectral regimes, you don't see the kind of overlap of morphological structures as you do here with the Alhambra dataset.  I wonder how that would affect the choice of spectra weights?

Many thanks for such an informative post!
-Joe

[Alejandro Tombolini]

Wonderful! not only the good final result, also the work you did explaining it. Thank you!

I have compared the result of the script (the extended image) with my approach and I have intense color in some galaxies due the lack of information of the other channels, and the big noise that I notice was considerable reduce due to the inclusion of more images.

Now is very noticeably the importance of having in consideration the weight of each filter but I would not have happened!
 
It was funny and very instructive, when the next challenge?

Saludos
Alejandro.

[Juan Conejero]

Hi Joe,

I am glad to know you like my script and the approach we have taken to render these data. Vicent Peris applied a similar concept when he processed the first ALHAMBRA fields back in 2006.

Overlapping filter response curves are crucial to this color methodology, since they provide rich and dense chroma components without any gaps. Typical RGB CCD filters usually cover almost disjoint regions of the spectrum with little overlapping, which leads to poor color renditions. For the same reason, DSLR cameras tend to provide richer color representations. See for example the spectral response curves for some popular digital cameras:

http://www.maxmax.com/spectral_response.htm

how did you choose the central wavelength for the various filter sets

For the visual set I chosen the approximate maxima of 'natural color' curves: 450, 550 and 650 nanometers. These central wavelengths are very similar to those implemented by most DSLR cameras (see the page linked above). For the other filter sets I simply distributed the central wavelengths equally across the main ALHAMBRA filter subset (from 370 nm to 950 nm, approximately), trying to gather as much data as possible with each set. The purpose of these sets is not to emulate human color  perception, but to use color as a means to maximize differentiation of image structures, and hence of the different objects represented in the image.

[Warhen]

Joe, it's great to see you here buddy! Hello to all.

[Lex]

Hi all,

I think I did not get something, I am really burning to try the process of such a wonderful professional data but.... I do really not know where to find it?
Could anyone help me out please?

Thanks

[astropixel]

Yes, me too!