L2RGB combine LRGB and drizzling techniques. Compared to the traditional LRGB, I image in the HSI space is here replaced by the drizzled combination of many subsampled images.

We test the algorithm for situation where CCD greatly undersample the image.This the case then the goal is large field of view for survey program (new comets detection program, novae, variable stars). My test images were taken through photographic optics having short focal lenght, between 55 and 80 millimeters.

In the following example (M20 region) I used a 80 mm objective with a f-number of 4 for R, G and B frames (Audine CCD camera + KAF-0401E used in 2x2 binning). The exposure time is of 10 x 60 seconds in of R, 10 x 60 seconds in green and 10 x 90 seconds in blue.

For the I image, which is required finest, the f-number is 5.6 for the 80 mm objective. The CCD camera is the same and I use an R filter. The FWHM in this situation is about 1.2 pixels, i.e. the image is severely undersampled. The I  image in the HSI space (or L image in the L2RGB designation) is the drizzling combination of 15 diphered 60 seconds exposure time frames.

Figure 17. Left, shift and add combining (note the severe chromatism of the objective). Right, one used L2RGB technique.

Figure 18. Click on the image for examine the full frame L2RGB result. 80 mm objective lens and KAF-0401E CCD.

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Figure 19. Another example of application of L2RGB procedure. We observe here the M31 galaxy. Top, standard combining of the RGB components. Bottom, the L image of the L2RGB processing (or I image in the HSI space) is the result of the optimal composite of 15 diphered frames. 80 mm photographic lens and  Audine camera with KAF-0401E. The blue diffusion around brigth stars is caused by the strong chromatic aberration of the objective. Click on the image to look the final format. Images processed with Iris software.

For other techniques about tricolour clicks here (RGB versus CMY discussion).