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Thread: Question about LRGB imaging... Can I use regular colored filters?

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    Default Question about LRGB imaging... Can I use regular colored filters?



    So, I have a bunch of standard colored filters in various colors, of which I have red, green and blue. So like the title says... Could I just use these filters to do LRGB imaging? Or... is there something specific about actual LRGB filters that would be lacking in the regular filters?
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    You probably can, though your colour balance may end up slightly different from "official" filters. It will be close, though.

    One thing a matched set of filters gives you that standard filters won't is consistent focus. Any filter will shift the focus point. With ordinary filters, it doesn't matter, so there is no requirement to have them be consistent. They might be consistent or they might not. With LRGB sets, not only are the R, G, and B filters all parfocal, there is also a parfocal clear glass (L "filter"), which allows you to record the white light luminance images without having to refocus.

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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    If you're shooting with your Canon it doesn't make sense. Your Canon already has an RGGB Bayer matrix, so every photosite (pixel) is covered by a tiny filter of red, green, or blue.
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    To be sure, LRGB imaging is done with a monochrome detector, and LRGB filters - as indicated, a DSLR has a Bayer filter layer already in place.

    In the monochrome detector case, "good" LRGB filters will be par focal, and have well defined transmission bands, combined with specific transmission properties to allow what's called 1:1:1:1 imaging. This does two things; it allows integration times in each filter to be identical while achieving equal 'signal', and good color balance in each wavelength region, and allows for use of a single dark frame set with all four filters.

    Low-end LRGB filters won't necessarily be par focal, and may not allow 1:1:1:1 integration times. These (deficits) issues add complexity to the process, because they require different exposure times, manual color balancing in post-processing, and require multiple dark frame sets to match the different integration times.
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    Thanks for all of your replies. That makes a lot of sense now. I guess I just don't really know a lot about LRGB, but as of lately I kind of do want to get a CCD camera and move down this route as opposed to my DLSR... I just have to figure out which one would be best for me first.
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    Here are a couple of examples, using "non-standard" filters as a proxy for a balanced, par focal LRGB set:

    NGC 281 - The Pacman Nebula, 0.8m Telescope, PFC/LF1 @ McDonald Obs.

    NCG 1499, The California Nebula in R-band, 0.8m Telescope, PFC/LF1 @ McDonald

    In these examples, the filters are Johnson-Bessel photometric (broadband) filters, which are U (ultraviolet), B (blue), V (green), R (red), and I (near infrared) filters.

    These are not par focal, and require re-focusing the telescope for each filter as it is used.

    They are not color-balanced, nor do they have identical transmission characteristics, and this means each filter requires a different exposure duration (integration time) to collect the same signal level from a target.

    This in turn leads to the requirement to use a dark frame set that matches the integration time used in each filter/light frame combination. In this particular case, I avoid this specific part of the issue, because the camera is cryogenically cooled. In all other cases, the separate dark frame durations would be required.

    The final set of issues is in post-processing - color balancing must be done to combine the images in the appropriate 'ratios' and to match the data from the non-standard wavelength bands to a standard color palette.

    All of that could be avoided, for an equal or perhaps slightly better end result by using a high quality set of standard LRGB filters.
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    Quote Originally Posted by AustinPSD View Post
    Here are a couple of examples, using "non-standard" filters as a proxy for a balanced, par focal LRGB set:

    NGC 281 - The Pacman Nebula, 0.8m Telescope, PFC/LF1 @ McDonald Obs.

    NCG 1499, The California Nebula in R-band, 0.8m Telescope, PFC/LF1 @ McDonald

    In these examples, the filters are Johnson-Bessel photometric (broadband) filters, which are U (ultraviolet), B (blue), V (green), R (red), and I (near infrared) filters.

    These are not par focal, and require re-focusing the telescope for each filter as it is used.

    They are not color-balanced, nor do they have identical transmission characteristics, and this means each filter requires a different exposure duration (integration time) to collect the same signal level from a target.

    This in turn leads to the requirement to use a dark frame set that matches the integration time used in each filter/light frame combination. In this particular case, I avoid this specific part of the issue, because the camera is cryogenically cooled. In all other cases, the separate dark frame durations would be required.

    The final set of issues is in post-processing - color balancing must be done to combine the images in the appropriate 'ratios' and to match the data from the non-standard wavelength bands to a standard color palette.

    All of that could be avoided, for an equal or perhaps slightly better end result by using a high quality set of standard LRGB filters.

    Interesting, that all makes even more sense now. I didn't even think about how if you needed different exposures to balance the variations between filters, you would need to take extra darks. The cooling thing seems like a good solution, but between the exposure differences and the focusing... I can see the benefits now to purchasing a set.
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    Default Re: Question about LRGB imaging... Can I use regular colored filters?

    I'm a monochrome guy but I will say that the 60D is a really good, low noise camera. I'd stick with this for awhile if I were you. As an aside, I'm not a fan of narrowband filters in front of DSLRs. For Ha, for instance, which is deep in the red, only 1/4 of your photosites will be sensitive to it.
    Stuart Forman


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    Default

    Quote Originally Posted by s24man View Post
    I'm a monochrome guy but I will say that the 60D is a really good, low noise camera. I'd stick with this for awhile if I were you. As an aside, I'm not a fan of narrowband filters in front of DSLRs. For Ha, for instance, which is deep in the red, only 1/4 of your photosites will be sensitive to it.
    I will say it is a great camera, so maybe once I get my auto guider I'll be able to get more out of it.

 

 

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