LRGB VS RGB imaging of deep sky objects

[5ash (Philip)]

what is the best way to take deep sky images.I find that with LRGB i get a change of focus as i change to no filter for the L frames. This when using a focal reducer seems to result in a slight change in field size when refocussed . I will try less focal reduction next time .However what are the pros and cons of lrgb and rgb imaging.
regards philip

[iceman (Mike)]

With what camera and filters?

[[1ponders] (Paul)]

When taking an L frame I use a clear filter (IR I believe). This basically eiliminates the need to refocus between the RGB and L captures.

[jase (Jason)]

There are many benefits that LRGB provides over conventional RGB. Possibly too many to list conclusively here, but here are a few;

• Reduction of total exposure time - through chrominance (RGB) binning
• Human eye does not detect great detail in colour information, thus using the luminance "greyscale" for detail makes obvious sense.
• Ability to separate and image process the luminance data from the chrominance data is a major advantage. I think many underestimate this.
  
• Typically, the luminance will pick up much greater detail in the target object as it's permitting a broader wavelength window.
• Provides improved flexibility - Luminance can also be combined with Hydrogen Alpha to bring out more details.

Of course, there are also problems with LRGB. Most commonly, problems occur when processing an LRGB image as the luminance data is much stronger than the RGB. This results in the typical "salmon pink" tones. Imagers avoid this as much as possible. You can get around it by blending channels and/or luminance layering.

Its worth acknowledging the founders of the LRGB Process - Kunihiko Okano and Robert Dalby. You can see some information of the process from Dr. Okano's web site - http://www.asahi-net.or.jp/~rt6k-okn/its98/lrgb.htm

"The basic premise of luminance layering is that by combining an unfiltered high resolution and high S/N greyscale image with the weaker color data we can in essense "buy back" the signal and detail lost in our filtered RGB exposures. The end result potentially should be a more aesthetically pleasing high contrast color image." - Rob Gendler

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Re: Your problem. Its sounds like your filters are not parfocal. Always use the Luminance filter instead of unfiltered for LRGB images. Unless of course you intend to refocus.

[5ash (Philip)]

Hi Mike ,
Im using the g star ex .
regards philip

[5ash (Philip)]

thanks for the info Jase , i hadnt heard of a luminance filter .Is that just a glass ? Where do you get them?
regards philip

[jase (Jason)]

Philip, the luminance filter typically comes in a set with the RGB filters. You can still buy them separately though. Its important that you match what you are using for your RGB filters. I.e don't use different manufacturers as the filters will not be parfocal!

The luminance filter is usually multicoated/anti-reflective and permits the entire visual spectrum (wavelengths). They normally block IR and UV, though you can get ones that also permit these. I personally don't see much value in this, but depends on your astronomy goals. What's important is that the RGB filters are IR/UV blocked. This will ensure no colour contamination occurs and will permit the channels to be balanced.

http://www.astronomik.com/english/eng_ir.html
http://www.customscientific.com/astronomy.html
There are a few other manufactures that produce them as well. Of course, as I previously mentioned, if you don't mind refocusing you can use any filter you choose.