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  1. #1
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    Default Uses and the pros/cons of Hα/OIII/SII vs RGB



    Hello all!

    I was looking into some astrophotography items and I came across two types of filters, RGB, and Hα/OIII/SII. Stop me if I am wrong, but it is my understanding that RGB pass a band of the spectrum in red green and blue (What about common light polluted wavelengths like sodium vapor?). And then later in photoshop or whatever, you can combine them into an image.

    But Hα/OIII/SII capture only an extremely narrow band of wavelengths, excluding common pollution wavelengths. But looking on Orions filter set, these three wavelengths compose a turquoise color, and two reddish colors. So would these have to be combined in Photoshop in a false color way?

    Also, what are the main uses (i.e. the main subjects you would be trying to capture) with each, or are they the same, just two similar means to an end? And what are the pros/cons to each? Thanks a bunch!

  2. #2
    Phil Leigh's Avatar
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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    The RGB filter set comes in two flavours; one that has no gaps between the red and green and so can create "full spectrum" colour and one that has a gap between red and green that can help to reduce some light pollution effects. RGB is used to allow mono cameras to behave the same as OSC cameras. This is called "broadband imaging".

    Ha/Hb/Si/Oiii is "narrowband" imaging. The end result is a completely false colour image. It is used to image certain types of object that have strong emmissions at certain frequencies. It can eliminate pretty much all light pollution effects and allow you to image objects that are otherwise very dim/invisible under normal RGB imaging - but the downside is that much longer exposures are required.

    The two techniques are used on different types of object. Emission nebulae respond well to narrowband imaging, whereas globular clusters respond well to broadband imaging - I'm barely scratching the surface of the subject here.

    They are different means to different ends.

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  4. #3
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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    To expand a bit:
    There are three basic types of Cameras used for Astrophotography:
    1) Webcams - cheap and easy way to capture Video (vs Still images) through a telescope - useful for Planets and Moon where "Stacking" allows one to really improve the captured details of "Near Space Objects".
    2) DSLRs (and most consumer Point-n-Shoot and Smartphone Cameras) - slightly adapted consumer products used for "regular" photography as well as Astrophotography. These are "One-Shot-Color" (OSC) High Resolution cameras (because general consumer expects Hi-Res Vibrant Color shots) which do quite well when mated with most telescopes and controlled via settings and remote-shutter to generate long exposures of 30sec to 4-5min.
    3) Astronomy CCD Cameras - specialized cameras with a type of image sensor that is especially well adapted to taking very long exposures of very low-light targets. The packaging and features designed into these CCDs are very specific to Astrophotography, making these cameras "single-purpose" as well as relatively expensive. Most of these CCD are designed as monochromatic cameras - they don't differentiate amongst the colors of the incoming light - a tradeoff that allows them to be more sensitive to low-light and wavelengths that the human eye doesn't see well.

    It is these CCD Mono cameras for which the RGB Filter Sets are targeted. Taking images of the same target with each of the Red Blue and Green filters allows the user to combine the results into a final Full-Color image (like the DSLRs and P&S and Cameraphones to automatically) - but usually with more image data captured than DSLRs due to the extra sensitivity of the CCD sensor.

    Quote Originally Posted by Phil Leigh View Post
    The RGB filter set comes in two flavours; one that has no gaps between the red and green and so can create "full spectrum" colour and one that has a gap between red and green that can help to reduce some light pollution effects. RGB is used to allow mono cameras to behave the same as OSC cameras. This is called "broadband imaging".
    Quote Originally Posted by Phil Leigh View Post
    Ha/Hb/Si/Oiii is "narrowband" imaging. The end result is a completely false colour image. It is used to image certain types of object that have strong emmissions at certain frequencies. It can eliminate pretty much all light pollution effects and allow you to image objects that are otherwise very dim/invisible under normal RGB imaging - but the downside is that much longer exposures are required.
    The main purpose of Narrowband filters (Ha, Oiii, etc) are to allow Mono CCD Astronomy cameras to capture images isolated to just a specific wavelength of light that is generated by a specific type of DSO, and thereby emphasize the features of that DSO as well as isolate it from all other incoming light (whether from the same DSO or simply Light Pollution or stray Moonlight). "Ha" Hydrogen-Alpha light is the prime example, as it is generated by most nebulae when energized Hydrogen emits its energy as light that is deep into the Infrared beyond what human eyes or consumer cameras can see.

    Quote Originally Posted by Phil Leigh View Post
    The two techniques are used on different types of object. Emission nebulae respond well to narrowband imaging, whereas globular clusters respond well to broadband imaging - I'm barely scratching the surface of the subject here.
    They are different means to different ends.
    There is a 3rd type of Astrophotography Filter - Light Pollution / Contrast Enhancement - which should also be mentioned (and possibly is what you really were interested in when your search found RGB and Narrowband Filters). These "LP" and "UHC" (Ultra High Contrast) filters work by attempting to block portions of the light spectrum that are typically generated by Light Pollution sources such as Streetlights as well as Glare from Moonlight scattered through the atmosphere. Depending on your location and your gear, LP filters may enable you to "see" (usually for imaging more than visual use) DSO which otherwise would be hidden in the glare and glow of the usual light polluted nighttime sky.

    There is also a 4th type of filter - Moon Filter - that is useful for Visual use (as well as Imaging). These are either Neutral Density or Polarizing filters which work to reduce the overall brightness of the Moon through the telescope eyepiece, allowing the human eye a better chance to see the features of the Moon rather than simply struggle with the glare of its brightness in the nighttime sky (after all it is mainly a giant rocky mirror for the Sun's full light).

    (I won't go into Solar Filters - we're talking nighttime sky here - as I don't want to go into the "obligatory" harsh warnings about the safety concerns necessary to not permanently harm your vision.)

    Unless you are rather sure that you want to explore the intricacies of Narrowband or CCD RGB imaging, the LP / UHC filter is the place to start for improved DSO Astrophotography (of course so is travelling out to a Dark Site).
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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    Great responses, thank you guys.

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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    Right now, I've got a Canon 10D DSLR camera. Would there be any benefit to getting RGB or Ha/Sii/Oiii filters? Or is the point and shoot capability enough to capture good DSOs?

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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    Quote Originally Posted by SpaceFreak131 View Post
    Right now, I've got a Canon 10D DSLR camera. Would there be any benefit to getting RGB or Ha/Sii/Oiii filters? Or is the point and shoot capability enough to capture good DSOs?
    So, with a DSLR of any sort, there is no point in using RGB filters as your camera already has them built in permanently - it is called the Bayer Matrix or Colour Filter Array (CFA).

    Narrowband CAN work with DSLR's and can give worthwhile results (contrary to popular myth) but you really need excellent tracking and guiding as exposures will need to be LONG...

    The "point and shoot" as you put it can capture many things well (globular clusters for example), but many small and dim galaxies and nebulae will not work well I'm afraid - this is why people have IR-modified, cooled DSLR's (with or without narrowband filters) and of course expensive mono CCD's with filter wheels...

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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    Okay, so until I can get my EQ mount to automatically drive itself, and drive well, I won't even consider doing very long exposures.

    So what do you say to IR/UV cutoff and astrophotograhy LP filters? Yay or nay? I live in a light polluted area, so it seems like a no brainer to me, but is there any condition in which it wouldn't make sense for me to invest in the two filters?

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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    You dont need IR/UV cutoff filters on an unmodifed DSLR (the internally fitted filters do the job already). For LP, something like an Astronomik CLS is good but will cost you one stop of exposure so you will need to double your frame exposure lengths.

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    Default Re: Uses and the pros/cons of Hα/OIII/SII vs RGB

    You sir, have saved me a lot of money. Thank you.

 

 

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