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Thread: White balance on full spectrum

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    Default White balance on full spectrum



    I'm confused. I have a Canon T4i with a full spectrum mod and supposedly I need to set a custom white balance to get rid of the red cast in my astro images. But people say WB won't change a RAW photo and to always shoot in Daylight WB. To me it seems like that statement should have ended after RAW photo... So why does it matter what the WB is set as if it doesn't affect a RAW image?

    I guess I'll ask the infamous question- Is it possible to get rid of the red cast in my RAW astro images using a custom WB? How do I do it?

    And I know about a pricey Hutech filter that does the trick but its too expensive for me right now since I just bought a new computer yesterday.
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  2. #2
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    Default Re: White balance on full spectrum

    Quote Originally Posted by Jameedon View Post
    I'm confused. I have a Canon T4i with a full spectrum mod and supposedly I need to set a custom white balance to get rid of the red cast in my astro images. But people say WB won't change a RAW photo and to always shoot in Daylight WB. To me it seems like that statement should have ended after RAW photo... So why does it matter what the WB is set as if it doesn't affect a RAW image?

    I guess I'll ask the infamous question- Is it possible to get rid of the red cast in my RAW astro images using a custom WB? How do I do it?

    And I know about a pricey Hutech filter that does the trick but its too expensive for me right now since I just bought a new computer yesterday.
    WB does not change the RAW image data in any way... it writes meta-data in the file header which may be acted upon by software to apply the WB colour shift. Provided that you use software that is configured to ignore WB it makes no difference what you set it to in-camera. If you want some preview jpegs with reasonably "correct" colours, the WB should be set to daylight (unmodded cameras) or a custom white balance (all modded cameras).

    You cannot correct entirely for the "red" caste with a custom WB but it will get you most of the way there if you are using jpegs. In AP, RAW files and proper RGB colour calibration (not just a simple colour temp "tilt") in post-processing are required if you want "good" star colours, regardless of the camera used.

    What post-processing software are you using?
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    Default

    I stack with DSS then I use photoshop but I'm testing the Star Tools demo and soon to try the PixInsight trial as soon as they finish building my custom computer I bought yesterday, once I get a feel for both, then I will decide on which one I will buy. Regardless, I would like to be able to correct the color as much as possible in PS CS5 by itself but having that severely red image makes it harder to do so. I might be trying to correct it the wrong way which could be more of an issue than being so worried about a custom white balance?

    What should I do to correct the color in the final image? Is it a matter of preventing the RAW images from being so red or is it an after the fact approach where red is tamed in post processing, or both?

    And does DSS use the meta-data in the header?
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    Default Re: White balance on full spectrum

    Since the white balance doesn't affect the colour in the raw file, you can't prevent the raw images from being red. THe solution is strictly in post-processing.

    PixInsight has a nice colour calibration tool that is easy to use. You tell it an area of the image where the highlights ought to average out to white, and an area that is "background". Hit "go" and if fixes the colours.

    Or, in Photoshop or any similar app, you can adjust the colour curves separately for each channel (red, green, blue) or use a colour balance tool; there are various ways to deal with the red. Since you are currently using DSS, you can use its auto white balance. It did a pretty good job for me with the colours from my modded Canon 350D.
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    Default Re: White balance on full spectrum

    What Keith says is correct, although that nice expensive Hutech HEUIB filter WILL tame much of the excessive red from a full-spectrum camera without losing any of the valuable HA. In this respect it is unique right now.
    Pixinsight has the most sophisticated tools for colour calibration after the event.

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    Default Re: White balance on full spectrum

    Color calibration is a tricky subject and causes a lot of confusion.
    The short answer is that you will get best results by doing all color calibration in post processing according to 'best practices' for the various applications (read up on them when trialing the different software), based on their capabilities.

    The long answer is that it should be two stage process.
    First bias is corrected for (a persistent background level, e.g. a value that all pixels are 'off' by), establishing a baseline neutral background. Then a multiplier for each channel is applied as each channel will have had a different responsivity to incoming photons.
    The multipliers can be picked by eye, or can be established by various analysis tools (such as calibrating against certain types of galaxies, G2V stars, or using starfield averaging).

    All applications (except StartTools) encourage you to do your calibration close to the start of your work flow when the data is still linear.

    What happens then is kind of a murky field with many ways of representing color, depending on software (in)abilities and/or preferences. Often people start to stretch their color information along with their luminance information (causing arbitrary color shifts and desaturation) causing galaxy and DSO cores to look white and making it hard to see star color and temperatures. (the latter problem is why StarTools does calibration at the end as it is able to compensate for non-linearity of the color data).

    Adding or removing parts of the spectrum complicates things, as white balancing (as humans see it) has no meaning anymore, since you're trying to accomodate data that humans can't see (or if you use an LP filter, trying to make up for data that humans *can* see). Tweaking multipliers (which is what white balancing is) for a part of the spectrum that doesn't belong to the spectrum that these multipliers govern (human visible red, green, blue) simply makes no sense!

    Cheers,
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    Default Re: White balance on full spectrum

    In DSS under the RAW tab there is a color scale and when you change the red value from 1.00 to in the ballpark of .80 the stacked image won't have the excessive red cast. But when you do that, will you be loosing Hα data?

    irwjager - Have you ever used the Hutech HEUIB filter? Would using one of those filters on RAW photos that are then stacked in DSS, would the stack still be considered linear? And wold that stacked photo be considered "murky"?

    And when am I supposed to do color correction in Star Tools? Do I turn off the track before color correction?

    If you don't have one already you should do a tutorial video on how to process RAW data from a modified DSLR using Star Tools. I have data if you would like to use mine.
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    Default Re: White balance on full spectrum

    Quote Originally Posted by Jameedon View Post
    In DSS under the RAW tab there is a color scale and when you change the red value from 1.00 to in the ballpark of .80 the stacked image won't have the excessive red cast. But when you do that, will you be loosing Hα data?
    You don't lose data in the red channel. You only scale it. You do lose 20% of the dynamic range resolution, had lowered the ISO for red.
    irwjager - Have you ever used the Hutech HEUIB filter? Would using one of those filters on RAW photos that are then stacked in DSS, would the stack still be considered linear? And wold that stacked photo be considered "murky"?
    The filter is an actual glass filter that goes in the optical train. It won't do anything to raw data after the image has been captured. The stacked data remain linear until you stretch them. The results i have seen from people using the filter look anything but murky.
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    Default Re: White balance on full spectrum

    Jamee,
    I'll just throw in another vote for Pixinsight. It's a little pricey, but way less than Photoshop, and has a steep learning curve, but now that I have a handle on the basics I wouldn't use anything else. There are a lot of video tutorials available for Pixinsight and they are constantly coming out with updates that are free and Pixinsight automatically detects and asks if you want to download.

    I have a full spectrum T3, and all of the photographs I have posted recently were taken with it and processed with Pixinsight. As others have said, the color all balances out in the post processing with Pixinsight's color calibration tools.

    Also, for what it's worth, before I got the modified T3 I used an unmodified T2i with a light pollution filter that made the images look blue. Pixinsight worked very well on those too, but, of course, it didn't have as much red data as the modified T3 provides.

    As for white balance, I leave my camera on daylight, but that's only because that's where it was when I started using it and I never changed it. It makes no difference if you're processing raw images.

    Good Luck,
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    Default Re: White balance on full spectrum

    Jamee,

    I fought the fight of red caste RAW images for a long time with my modded Nikon. I set a custom white balance using a gray card, and that helped reduce the red cast (yes, the wb info is imbedded in the RAW data).

    When Hutech introduced the HEUIB-II filter, it looked promising and I ordered one up... I won't be taking any images from a modded camera without it, now that I've seen the results.

    The HEUIB-II filter aligns the color channels perfectly in a modded camera, reduces LP effects and enhances Ha. RAW images look like they've been taken with a normal camera.

    Do yourself a long term favor and get one, it's that good.

    Imho,

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