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Would like a tutorial

Dean Andrea
in General
Hi Adam,
I am not sure if this is the appropriate place to ask, but here goes. I would like to see your opinions/expertise on the philosophy behind using Ha data. I understand the principle of using narrow band Ha to enhance the red channel, however you are so good at explaining the rationale behind things, like what is the basic approach to use Ha data and when to create an artificial Lum etc. When one acquires data on an object, is Ha simply used when one wants to enhance the red data etc. I know you cover a lot of specifics(specific examples on data sets) in your tutorials, but I have not run across any general discussions as to the whys and wherefores. Maybe I have missed it, or maybe most people are not interested in the subject, but I have never felt like I have grasped the general concepts on this one. Hope this makes sense. Thanks! Dean A.

Comments

  • BlockHeadBlockHead
    edited August 2022
    Hi Dean,

    I am not certain there are many good cases for using Ha as a luminance image. The idea behind a luminance image is that it has strong signal which allows for separate processing and later blending with color information. You mention using Ha to enhance the red-channel- but part of that process is to also put the Ha signal into the Luminance (if constructing an LRGB image). This will make certain that this added signal is colored correctly in the LRGB result (and not the dreaded salmon result). 

    So the question is- why should the Ha (alone) be used a a luminance image? Perhaps there are some bright red targets where this could done. Some might argue that they city-bright skies require them to collect Ha - since the sky is too bright for an unfiltered L image. But these are special circumstances that are both object specific and light pollution dependent. In general it isn't possible to gather enough Ha signal to do significant  deconvolution (for example)- which is where the broadband imagery helps. 

    So, I think this isn't a big "thing"...though I have done something like this in the past. With the usage of Starless images- making Ha a "luminance" image to combine with broadband colors is just kind of an odd thing (unless, again the object is pretty much just red). If Ha is to be used as a "luminance" with other NB images... well, this is just NB stuff.

    Let me know if any of this hits on some part of the answer.

    -the Blockhead
  • Dean Andrea
    Thanks Adam! I guess what I am most interested in is not so much using Ha as ones Luminance frame, but more on the generalities of using, or wanting to use Ha. For example, I feel I really do not understand the general rationale surrounding Ha data. If one does not understand the principles behind something it is very difficult, if not impossible to find answers or solve problems that are out of the norm. I am guessing a lot of your approach to teaching the way you do is to help us(the students) to be able to problem solve on our own. I like to be able to use what I have learned from you to tackle things that are specific to my own images. I hope this makes sense! Thanks, Dean
  • BlockHeadBlockHead
    The basic idea behind using Ha (or any narrowband image) is that doing so makes light from the sky dark. This means the image of the nebula can be shown with much greater contrast. When a picture is taken of the Lagoon nebula there is little difference between a broadband red image and an Ha image of it. It is bright- brighter than the sky. The Lagoon nebula is emitting most of its light in one particular wavelength- the Ha (656nm) wavelength. So a Red filter lets in all of the Red light plus wavelengths on either side of this emission line from the sky. The Ha filter rejects most of the other wavelengths and *narrowly* lets through only those nearly equal to 656nm. But the nebula is bright... so not much difference.

    However, if you take a picture of a fainter red nebula (region near the Cone nebula for example)- the Red image will have equal measures of nebula and sky. The contrast of the red nebula will be low since the nebula is faint. So imaging this nebula (for a long time) in Ha will have much better contrast since the sky photons are rejected BUT the nebula doesn't change much in its brightness in this filter if it is actually passing the same wavelength it is emitting at fully. 

    So now we have the reason that Ha data is useful. It can be used as an image that has high contrast compared to a broadband Red image of faint nebulosity. Thus when you take a broadband image of something that has emission in red at 656nm - it is possible to take Ha data and enhance the parts of the nebula that glows at this particular wavelength by boosting the signal in the broadband image that is being swamped by the sky. 

    It is all about contrast resulting from filtering out the sky and other photons that decrease contrast.

    Is this an explanation that helps?

    -the Blockhead
  • Dean Andrea
    Yes, it helps clarify the reasoning of taking Ha. Is there a general way to use Ha data. I mean, I know that every dataset has its own specifics, but for instance, IN GENERAL, does one combine/blend Ha with red and also add Ha to the lum? If one takes an RGB image, does one simply add blend the red and Ha? Questions like these are the nuts and bolts of what I am looking for. Again, I see specific examples of other astrophotographers image processing/tutorials, but is there a general technique regarding RGBHa and LRGBHa as to how to combine/blend this data. Sorry for being dense here! Thanks! Dean
  • BlockHeadBlockHead
    *IF* you create an LRGBHa image... you *always* create an LBlend(Ha)Rblend(Ha)GB image. Otherwise, you will not color the Ha that is in the L...and it looks goofy. 

    Whether or not you do RBlend(Ha)GB or LBlend(Ha)RBlend(Ha)GB depends on the data. Usually for faint things the LRGB blend variety is best. In the case of brighter objects... RBlendGB might be good.

    At AdamBlockStudios, the primary/preferred blending method is Screen.

    -the Blockhead 

    As an aside... some people add a small portion of the Ha to the Blue channel is well to cover the H-beta emission that usually comes along with Ha (even if not captured directly in NB). This is just for color purposes...but it makes the reds go magenta...which I personally do not prefer.

  • Dean Andrea
    Adam,
    As always, I very much appreciate your time, knowledge and expertise! Thanks, Dean
  • craig smith
    This is a wonderfully succinct description, Adam.  

    Craig
  • Scott Badger
    For me, shooting Ha to blend with LRGB is more just an excuse to image when the moon is out. : ) Anyhow,  a (very red) target where I found Ha worked better as the luminance is IC 63.

    Cheers,
    Scott
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