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Why can you see white through H-alpha filter?

  1. Nov 28, 2004 #1
    I recently bought a Hydrogen-alpha filtered telescope (Coronado PST). Looking through it, the Sun looks mostly red as expected since the H-alpha filter only lets through visible light at a wavelength of exactly 656.3nm which is in the red part of the spectrum.

    And I can see both lighter and darker red in places (filaments are darker), which I assume reflects the intensity of the light (not a different wavelength).

    What I haven't seen yet, but have seen in many H-alpha pictures, are white spots called Plagues.

    My question is, how is it possible to see anything white (or any other color for that matter, other than red) through a filter that only lets through red light?

    Still sleepless in Cincinnati
    :bugeye:
     
  2. jcsd
  3. Nov 28, 2004 #2

    russ_watters

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    Your question answers the question - you can only see red because the filter only lets through red.
     
  4. Nov 28, 2004 #3
    Russ. . . read the question more carefully.

    I'm asking why it is possible to see WHITE in pictures taken through a RED (H-alpha) filter. You should only see RED. I see such pictures all the time. Usually the white area is a solar flare or plague, whereas filaments appear darker red. I heard from someone on another board that the answer is that photographically a high INTENSITY of light will be rendered as lighter on the picture and low intensity as darker, although both at the red wavelength. At the extreme, those would look white and black. Also, some solar photographers enhance the colors of their pictures a bit and may add that whiteness. Sounded plausible. Thanks anyway.
     
  5. Nov 28, 2004 #4

    Astronuc

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    Are you asking about black and white photographs or color ones?

    The image will depend in the wavelength of light and the emulsion which is exposed to produce the picture. Is the picture developed from the filtered sunlight directly or is it a negative which is then processed using some secondary light source (i.e. not sunlight, or H-radiation)?

    What one's eyes sees is different from what is produced through photography. Also, could you be looking at 'false color' reproductions?
     
  6. Nov 28, 2004 #5
    Thanks Astronuc!

    I'm asking about color photographs I see in text books, magazines, and on the internet. So, I'm not sure on any of them how the picture was taken, or developed.

    I'm guessing any and all of the things you mention could be the culprit depending on what picture I'm looking at. And it sounds like you and Russ agree that visually, I should never see true white through the eyepiece with a H-alpha filter. Only in pictures. That makes sense now.

    Thank you both.

    Paul
     
  7. Dec 2, 2004 #6

    Nereid

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    IIRC, the H alpha photos you see in books and on the web are not 'colour photos' of the family holidays kind. A 'photo' taken through a narrowband filter can only 'truthfully' be printed as black and white; however, to make these look sexier, publishers 'adjust' them - the most 'truthful' adjustment is to give the B&W image a colour that's the same as the wavelength of the filter (doesn't work for IR or UV filters of course). However, these aren't very interesting looking, so the images have the saturation stretched, and when that happens the most intense parts become 'white'.

    Another trick is to use two 'colours', say red and yellow, with yellow for high intensity (and white for the most intense) parts of the image.
     
  8. Dec 4, 2004 #7

    Labguy

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    If that is the case, then why do we ( I ) see a lot of white when looking through a PST (the small 40mm one) and even more white when using a larger Coronado solar Ha scope? Happens all the time.
     
  9. Dec 5, 2004 #8

    Nereid

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    Very good question Labguy!

    Let's assume that the filters do indeed allow only a very narrow range of frequencies (wavelengths) through, and that these are just in the 'red' part of the spectrum.

    If that's the case, then your perception of 'white' cannot be simply a mix of wavelengths incident on your retina, so either the G and B cones are being excited, or some other part(s) of the human visual system - deeper towards, or in - the brain are doing some interesting processing (or both). While G cones are sensitive (just) to the Ha wavelength, the rest of the human visual system makes the appropriate adjustment.

    Maybe it's an intensity effect? If you (one) look at the world through a narrow-band (deep) blue filter, do you also perceive 'white'? What happens in the middle of the 'green' (G cones have a peak sensitivity of approx 535 nm)?
     
  10. Dec 5, 2004 #9

    Labguy

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    Here is a new one:
    http://antwrp.gsfc.nasa.gov/apod/ap041206.html
    That is exactly how it looks through a good Ha filter. Notice the white areas. This is a photo, but it looks the same to the eye at an eyepiece, white included.
     
  11. Dec 5, 2004 #10

    Nereid

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    Looks like an intensity effect of some kind, don't you think? As the amount of light (from a region) increases, it gets paler, until it becomes 'white'?

    In principle, this should be easy enough to test; in practice I'm not sure you could get a bright enough light - the headlights of your car?

    I like the date on this APOD (27 December 2004)!

    Edit: looks like the 'author' did some processing to produce the final image, including "Next, under Image | Adjustments | Levels, I’ll adjust the mid-tone point slider to darken the disk detail and then the white point slider to brighten any active regions."
     
    Last edited: Dec 6, 2004
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