LAB #8

LRGB Tecnique: Capturing Colors Of Deep Sky Objects

M. Nicolini


The implementation of the LRGB technique in Astroart would allow you to capture the color information of an object from different sources: if you do not have a filter wheel but you wonder how would appear your image if colored, you can even avoid to grab the RGB frames and color your monochrome images with the splitted RGB channels frames coming from jpg images found (for example) on the WEB. I created this LAB because I was fascinated of the great skillness of Al Kelly in capturing true colors from deep sky objects: I grabbed some shots of M97 with a Starlight HX516 CCD camera at the Observatory of Cavezzo and tried to "paint" the resulting stacked image using the splitted RGB channels from Kelly's great image of M97 (see at http://www.ghg.net/akelly/m97wcmyl.jpg ). Don't forget to visit his web site, full of great images and documents to improve your CCD tecniques.

1. The LRGB Technique

This technique is based on the fact that the human's eye is very insensitive to the resolution of the colored data then the resolution of a color image is determined by the resolution of the L (luminance) image only: and this is true either for deep sky object and for solar sistem object like planets. This principle can allow the CCD imager to concentrate his efforts in producing one good monochrome with high S/N ratio image without wasting time in taking long and full resolution R (red), G (green), and B (blue) frames. Filtered frames can infact be taken with a lower resolution (i.e with binning 2x2 or 3x3) and with shorter exposure times (up to 1/9 of the exposure time in full resolution for each filtered frame).

Nevertheless, the LRGB and registering tools implemented in Astroart can help you to color your monochrome images even with single color images that come from third parties or that you can find or download as JPEG, BMP or TIFF (48 bit) from the WEB: and results are aestetically nice, like you can see in Fig. 1 below. How I did this? Follow the image processing steps described below.

Fig. 1 - On the left the original greyscale image of M 97 taken at the Observatory of Cavezzo and on the right the same image colored with Astroart using the Al Kelly's jpeg image downloaded from the WEB.

2. Sub-pixel Registration Of the Images

Now, you have found a wonderful and colored image of an object that you just captured the night before but ... with great disappointment (Fig. 2) the images have different scales and different field rotations ( a perfect match between two images from different observatories would be almost impossible ! ). Then the first step is to import the color image in Astroart and register its geometry with your monochrome image.

Fig. 2 - On the left greyscale image that we want to color and on the right the Al Kelly's jpeg image downloaded from the WEB. Both images were resampled to the 50% of the original for a better fitting and comparision in this page only.

NOTE

The monochrome image of M 97 was taken with the Starlight HX 516 CCD camera at the direct focus of the 400 mm. f/5.5 reflector of the Observatory of Cavezzo (MPC 107): it is the sum of 3 exposures of 300 sec. each. Note that the image is quite oversampled due to the smallness of the pixels (7.4x7.4 micron) and the focal lenght (2210 mm) of the telescope: this brings to a sampling of only 0.7 arcsec/pixel.

  1. With File | Import | JPEG color, open the color image in Astroart: the program will automatically split the image into the 3 RGB channels. For a color view of the imported image, open the Trichromy window with Tools | Trichromy, LRGB, LCMY, select the respective channel-image using the colored buttons on the bottom of the window and click on the Preview button (Fig. 3).

    Fig. 3 - The three channel-images with the Tricholor Window on the Astroart desktop .

    NOTE

    The colored images are splitted in three channel-image, one for each basic color: one for RED, one for GREEN and one for BLUE. You can easily take confidence with the Trichromy Window: every change or processing you are going to apply to each channel-image will take effect when you click on the preview button. Try for example to simply change the display thresholds of one of the channel-image using the cursors on the vertical gray-scale bar on the right of the desktop. Netherthless, remember to undo every change or simply reload the colored image when you want to keep the original color balance for your grayscale image.
  2. Now, we want to rotate and resize each of the three channel-image until they exactly fit the shape of the grayscale image. This is easily done with Astroart: select the grayscale image, click on the stars icon on the left of the desktop (or select View | Stars) to open the stars window and click on at least three stars on the image: these stars must be obviously present on the colored image too.

  3. Select one of the three channel-image, open its stars window and select the same stars in the same order of the greyscale image (Fig. 4).

    Fig. 4 - The four reference stars for each image selected for this LAB are outlined in yellow for clarity. On the left the grayscale image from Cavezzo Observatory, on the right the three channel-image from A. Kelly's JPEG and below the respective Stars Windows .

  4. Now, keeping selected the channel-image, click on Image | Coregister: at the prompt of the "Select an image" dialog window, select the image tha should be your reference, that is the grayscale image; a new image will be created with the same size and orientation of your grayscale image: save it and repeat the same last steps for the other two channel-images.

    NOTE

    It will often happen that Astroart will not automatically recognize the stars centroid in the channel-images: this is due to the distorsions of the JPEG compression. Remember that in this case you can always "force" the recognition holding down the Shift button and dragging a little square around the star.


3. The Final Touch

Now, if everything was well done, you shoud have on the desktop (and saved on your disk) the three channel-images "co-registered" with your grayscale image. Open the Trichromy window with Tools | Trichromy, LRGB, LCMY, select the respective channel-image using the colored buttons on the bottom of the window then select the Luminance image (your grayscale image) and click on the Preview button. After a while you should see the result of your work in the Trichromy window. Click on the image with the right mouse button: you can now save the result as a BMP or JPEG (compressed) 24 bit color image.

NOTE

You can set the compression factor of the JPEG files editing the value in the corresponding text box in Tools | Preferences | Program.

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