*RGB color, Lab color and LRGB composit

*Drastic reduction of Exposure time by LRGB

RGB mode and Lab mode

1) Galaxies, Nebulae

Exposure time ; about 1/4 or more

2) Planets

One good monochrome + poor RGB = Best color image

Concept of LRGB composit

The most important point is:
The human's eye is very insensitive to the resolution in coloring data. The resolution of color image is determined by the resolution of the L-image only. This is true either for nebulae and planets.

Drastic Reduction of Exposure Time

In Most CCD camera, the 2x2 or 3x3 binning mode, where the sensitibity is improved in stead for deterioration in resolution, is available. By taking advantage of the binning mode in RGB image, the total exposure time can be reduced without any deterioration of the final LRGB image. This is the first merit of LRGB composite.

Example:

	RGB system without binning	LRGB system RGB with 3x3 binning L without binning
Red	9 min	1 min (1/9 of R)
Green	18 min	2 min (1/9 of G)
Blue	27 min	3 min (1/9 of B)
Luminosity		6 min (2/3 of R)
Total Exposure	54 min	12 min (1/4.5)

In order improve the final S/N and resolution, extending the L-exposure is most effective.

Example: M101 R=G=B=20 min, L=175 min

What is the 'Ideal RGB filters'?

1) Ideal RGB filters (broad band) Very good reproduction of the halftone colors But low transmission, usually 2) Ideal Dichroic RGB (no gap, sharp cut and high transmission rate) No hafltones for objects with line radiations (like planetary nebulae). This is a disadvantage of all dichroic types. However, if c-line is 500nm, H-alpha >> pure red OIII >> blue green -----(c-line) H-beta >> pure blue ---(b-line) This will be acceptable: My SBIG set is the case. 3) Worst case of dichroic RGB filters, with a large gap Some low cost dichroic filters may be this case.

1)L images of galaxies and clusters are taken without IR-B.
2)L-images of nebulae and planets are taken with IR-B
3)Thickness of Kenko R60, PO1 and Edmund-B is same (2mm)
L-image is taken with Kenko MC-protector (2mm)
4)IR-B is always used for the ST-7 + C8/C5 system

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