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Thread: White balance

  1. #1
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    Default White balance



    I bought a Hutech IDAS LPS-P2 light pollution suppression filter.
    It seems to work nicely visibly reducing color cast caused by light pollution.

    I have a problem though. I captured 4+ hours of light frames of the Leo Trio in a couple of nights and I'm now unable to remove the blue color tint caused by the filter. The color is visible also in the flat frames.
    I tried different settings in the RAW/FITS screen of DSS, but I don't see any improvements. The best I could do was to move the sliders in DSS after completing the stacking, but still the colors where not looking right.
    I also tried to change the white balance in the RAW files I'm using with no difference.

    Unfortunately I didn't image any G2V stars for WB calibration, so I'm stuck with the light frames I have.

    What can I do to fix the problem? The "hamburger galaxy" in blue is awful (and basically everything else in the picture) !
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  2. #2
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    Default

    I also noticed that the red peak in the histogram after stacking the images is much lower than the green and in particular the blue one. How can I solve the problem?
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  3. #3
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    Default

    An update.
    I tried to overlap a transmittance graph of the stock IR filter on the sensor with the one that I received with the IDAS LPS.

    The IR filter on the sensor has its maximum transmittance between 430 and 550 nm. Tansmittances changes consistently between 400 and 430nm and gradually from 500 to roughtly 480nm.
    The IDAS has a peak (82%) at 420nm, decreases to 0% at 430, then goes up to 90% between 440nm and 530nm.
    There is a peak at 560nm, another one at 605nm and again between 640nm and 690nm.

    It seems that what I see in my pictures makes sense when looking at the graph: most of the green and blue light is left untouched by the filter (blue more than green as light around 545nm is cut by the filter), and in any case spectrum is broad enough to let the sensor capture enough data.
    The problem is the red light. Yellow is necessairly cut (that's the purpose of an LPS filter, right? cut the light emitted by Na lamps), but:
    - the red light at 605nm is dimmed to about 60% by the IR filter;
    - light between 640nm and 690nm is dimmed between 30% and 5% by the IR filter.

    This was also clear by looking at the different peaks in the histogram (when debayering with 1:1:1 ratio):
    - blue was the highest peak, roughly twice as high as red;
    - green was half way between the two;
    - red was the lowest.
    I managed to make them almost as high by using 1.55:1:0.83 (roughly calibrated with a gray card in daylight).

    The problem is that even with these settings, that do work to balance the color of the background, I'm unable to extract the color from the galaxies.
    I'm attaching the pictures of the 3 plans (R, G and B) exported: it is evident that the brightest part will be blue in my picture.

    I have the feeling that a light pollution filter is not ideal with an unmodded DSLR.

    The problem is that DSS doesn't let me stretch the channels with different settings, as I can only work on the global luminance.
    Is there any solution I can try to use my exposures to have nice balanced galaxies?
    Attached Images Attached Images
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    Default

    Stefano, I noticed there have been no replies to this post? I think it's because you are light years ahead of the curve when it comes to processing/transmission cut-offs.

    I love the 3d image, if it were mine I would be tickled pink. Professionals like Robert Gendler have captured some color in the triplet, but he's also using equipment that costs the price of a small vacation home in Tuscany also..

    Grats on the Leo triplet image, you're being way too critical of yourself!

    Ciao

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    Default

    I think you need to move away from DSS once you have stacked the photos. You need software that lets you freely manipulate the R/G/B channels with curves (or similar).
    Then you can bump up the red channel.

    I have several software packages that let you do this (Photoshop CS5, Photoshop Elements and PixInsight but none are freeware. I imagine that GIMP (free) will let you do this too.

    If you are using an unmodded DSLR and a LP filter that cuts down red, you are going to need many more exposures to capture red!.

    However, I'd try GIMP first and see if you can balance the red with that.

    DSS is very good at stacking (although PixInsight is far far better) but I would save your file as soon as the DSS stack has finished and then work on it outside of DSS,

    clear skies
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  8. #6
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    Default

    Thanks for your comments.
    I like the details captured, but I used a DSLR and my plan was to capture a color image.
    That is 3 hours and 50 minutes of total exposure time, as a result of the combination of 5 minutes subs (46x300s) taken with my new Tecnosky APO 80/480mm (first light for it!).

    I normally perform post processing in Photoshop after stacking the subs (to fix histogram, curves, color balance, gradient removal, star diameter and sharpness), but I try to have a balanced picture in the beginning, otherwise post production becomes very visible in the final picture.
    The problem here is that the blue channel is very bright after stacking (galaxies in the 3 different pictures clearly show that), that's why I was trying to find a way to change that during the stacking rather than in post production.

    As I'm planning to purchase a CCD I'm also thinking about how to upgrade the software, to get a good program to stack the pictures. Pixinsight, ImagesPlus and MaximDL look like great programs, but they are quite expensive (Nebulosity not too much actually) so I may try the demos in the future (after getting the new camera).

    What is weird is that I found online some reviews about the LPS-P2, tried with an unmodded camera, showing a good color balance (or very slightly off) in the test pictures.
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    Default

    Stacking IS post-production :-)
    All stacking actually does is reduce noise.

    Pushing curves in PS should not produce an unnatural effect.

    Anyway,another way round the problem is to use a custom white balance if your camera supports it... Just shoot a flat with the filter in place and then use that as "white".
    That will remove ve the blue cast from your lights and flats "in camera"

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    Default

    Don't abandon the Filter quite yet.
    The image can be calibrated in PS using the histogram pulling each channel separately to the left and lining them up.
    I use a much more powerful software (Pixinsight) to do color calibration but its the same principle.
    Its a little harder with the DSLR and the LP filter being the the Bayer matrix filtering is 1 Red ,2 greens ,and 1 blue.
    adding a blueish filter really plays with the data but I would leave the WB to daylight and software manipulate the stacked linear image in the initial stretch
    Ken

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  12. #9
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    So after more attempts I'm still fighting with DSS for those colors.
    But there's also a small success: I shot with the DSLR with the filter mounted my computer screen with a diffusion panel to spread out the light. I then opened the image in PS (it was visibly blue-cyan) and I measured the average color value in the center. Obviously blue was the strongest channel and I moved the white pointer of the levels tab for the red and green channels until the R and G value matched the B one.
    I then copied the levels to a picture I took of M51 and I tweaked again the levels for each channel:
    - by moving the black pointer until the beginning of the main part of the histogram;
    - by moving the gray pointer just after the main part of the histogram;
    - by moving the 3 pointers for the combined RGB to fix brightness and contrast of the image.
    The result was an almost color balanced image, and the color of the galaxy looked pretty good.
    I tried the same workflow with M3 and the Leo Trio (that was my initial target).

    Of course the noise in the pictures was awful, but I was just performing a test in single pictures, and it seems to have worked out!

    Tomorrow I'll take some pictures of a white or 18% gray sheet (maybe I can ask the photographer close to where I live if I can borrow it for a couple of shots) and try again to extract the perfect correction parameters.

    Now that I know that the colour in my pictures can be fixed, I still have no clue about how to fix the colors in DSS when I stack the subs.
    I will definitely try to create a custom WB setting and then use the "camera white balance setting" option in DSS, maybe that could work.
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    Default

    don't worry about the colors in DSS or you will end up making the data more difficult to deal with in PS.
    Its a linear image.
    color calibration is done in processing .
    the one thing you can do to better interpret the raw data is darks ,Flats ,and bias files.
    I would still leave the WB at daylight and let the sensor interpret the data ,but then again its rewarding when you get it set right.(its just a lot harder when you have a filter in front of the sensor.)
    If you didnt have the LP filter a custom WB would be order.
    Ken

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