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Thread: Approximate RGB filters.

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  1. #1
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    Approximate RGB filters.

    Hi,

    I have a monocromatic camera and no LRGB filter set. However, I have the Wratten filters #23A (light red), #58 (green) and #80A (blue). Is it possible to achive an approximate color composition with them? Does it worth trying?

    Thanks.
    English is not my first language.

  2. #2
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    It's probably possible but there are many disadvantages. The #23A has relatively high transmission but the data I've seem suggest that you also get quite of IR. Both #58 and #80A has low transmission and #80A is also very wide, it lets both green and red light through. I'm not sure how standardized the transmission curves are between manufacturers.

    I've googled a bit and there are people using Wratten filters for RGB. One guy used Red #25A, Green #58 and Blue #38A for instance but the transmission curves of modern RGB filters are better.

  3. #3
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    What about using a nebula narrowband or light pollution filter to cut the IR, in a luminance channel? (Only when targeting nebulas, of course.)
    English is not my first language.

  4. #4
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    Quote Originally Posted by Jairo View Post
    What about using a nebula narrowband or light pollution filter to cut the IR, in a luminance channel? (Only when targeting nebulas, of course.)
    To include IR is not always a bad idea if your telescope lacks cromatic error or if your camera has an IR-cut filter. Do you have photographic or visual narrowband filters? Narrowband filters designed for visual use are much wider than photographic narrowband filters (and solar narrowband is even narrower). They would certainly cut IR but they would also cut most everything else as well. How different light pollution filters work varies a lot so I can't really help there.

    Which camera and telecope are you using?

  5. #5
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    Quote Originally Posted by glappkaeft View Post
    To include IR is not always a bad idea if your telescope lacks cromatic error or if your camera has an IR-cut filter. Do you have photographic or visual narrowband filters? Narrowband filters designed for visual use are much wider than photographic narrowband filters (and solar narrowband is even narrower). They would certainly cut IR but they would also cut most everything else as well. How different light pollution filters work varies a lot so I can't really help there.

    Which camera and telecope are you using?
    I have a Meade LXD75 ACF8" and a DMK 21AU618.AS. I have experience with RegiStax 6 for planetary imaging with an old Sony Cyber-shot color camera, but I'm an absolute beginner for deep space imaging and color channel composition. I'm making images almost by accident, and I'm already happy with them.
    English is not my first language.

  6. #6
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    Quote Originally Posted by Jairo View Post
    I have a Meade LXD75 ACF8" and a DMK 21AU618.AS.
    Ohh, that is some very nice optics you got there. The camera chip in the DMK 21AU618.AS is good one even if it's rather small for a telescope with 2000mm focal length (you get a field of view of only 8x7 arcminutes). If I had your setup here in Sweden I'd also like slightly larger pixels (206*5.6/2000 = ~0.6"/pixel) since seeing here only very rarely is better than 2" (FWHM) but I don't know what your seeing is like (rule of thumb - pixel scale should be equal or higher than seeing/2). To be able to use the stuff you already got is a big bonus though!

    I have experience with RegiStax 6 for planetary imaging with an old Sony Cyber-shot color camera, but I'm an absolute beginner for deep space imaging and color channel composition. I'm making images almost by accident, and I'm already happy with them.
    I remember those days, fun times. Marveling over a unfocused, unguided and badly tracking image of the Andromeda galaxy or Orion nebula. Nowadays astrophoto has become more mechanical although we still learn stuff almost every time.

    Have you tried "Deep Sky Stacker"? It's a bit clunky until you learn it but it's free and works rather well.

  7. #7
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    I don't know my local average seeing in arc seconds, but in my 8 inches telescope, I always needed to stack frames to do fine colimation. However, after processed, the images seems to show details smaller than 2 arc seconds (Cassini Division?).

    I don't know the Deep Sky Stacker. I just opened the page and I'm reading about it. It seems to be more specialized than the RegiStax. Thanks!
    English is not my first language.

  8. #8
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    There used to be transmission curves for wratten filters online - can't find them now - but I did find this PDF which has the data in numeric form...

    http://www.karmalimbo.com/aro/pics/f...%20filters.pdf

    + And there's this for general filter recommendations:

    http://www.analyticalsci.com/Astrono...ibed_Meade.htm

  9. #9
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    Quote Originally Posted by Jairo View Post
    Is it possible to achive an approximate color composition with them?
    It's worth trying - most of the images you'll see from the surface of Mars are taken with nIR, G, B.

    Why not try it - share the raw images here, I'm sure a few imaging gurus will take the results and make something of it!

    D

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    Quote Originally Posted by djellison View Post
    It's worth trying - most of the images you'll see from the surface of Mars are taken with nIR, G, B.

    Why not try it - share the raw images here, I'm sure a few imaging gurus will take the results and make something of it!

    D
    Hi,

    This is my first try. I used different exposures and gains for each channel to get the best histograms, and combined them with Gimp. I have no focuser nor mask. The green channel was the darkest one, and I clearly didn't get frames enough. I don't remember which values I used, and I adjusted the channel intensities according to what I thought that Saturn should look like. Sorry. I will be more methodic on my next try.

    saturno_cores.jpg
    English is not my first language.

  11. #11
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    By the way, I just added the color layers. How do I use a luminance layer? Should I multiply it instead of adding? I hope it became sharper.
    English is not my first language.

  12. #12
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    Pulled from another thread on 09 Apr 2012

    Quote Originally Posted by Jairo View Post
    I plan to buy a Star Analyzer 100, for spectroscopy with my SC8 telescope and DMK camera (which I already chose with this forum's advice, and I'm very pleased with).
    SC8 = Meade SC 8" f/10

  13. #13
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    Deep Sky Stacker is best for FITS files, mainly Deep Sky Objects while Registax is for many planetary images contained in an AVI run. They are really two very different programs for different purposes.

    Your green data is very poor compared to the red channel. Blue is in-between those two. You need all three channels to be of near equal quality for decent color.

    I've never used GIMP so can't help you. In Photoshop to make an LRGB image (rare in planetary work) the RGB layer is usually placed on the bottom then the L layer placed above it. The two are combined using the Luminosity blend mode. I'd use the Red layer for your Luminance channel as well as the red channel since it is by far the best quality. Green is really too poor to use and is causing the green fringes you see because of this. The Red channel is enough better than the blue that it gives an overall red cast to the image. Not much can be done to help the color until you have three more equal channels. Since the three filters aren't color balanced for this purpose you might want to take a photo of a neutral gray object then note the different intensity adjustments you have to make to result in a white image. Normally green is considered 1 and blue and red are adjusted to match it. Once you know the right ratios to apply you should be close. Atmosphere will take blue and some green from an image. More is lost as the object gets lower in the sky, hence the red or orange sun at sunset and sunrise. This can vary from day to day so the neutral gray object trick (in bright sunlight) is only a start. You will have to adjust some from there unless the object is quite high in the sky.

    When processing the data process the L channel for contrast and detail. The RGB channel need only be processed to get the right colors. Sharpening detail can often alter color balance due to enhancing noise also present in one or more color channels when only a few frames of each color are available. If you have 30 or more good quality AVI files for each channel then processing RGB for detail may be best and you can eliminate the L channel. Just depend on you data which is the better way. Most planetary imagers only use RGB after getting plenty of good quality frames of each color. Using an L channel is done only when one channel is better than the rest. It is then blurred to match the others when making the RGB layer. Or sometimes the other layers can be sharpened by a deconvolution routine to better match the better channel. Often a combination of blur and deconvolution is used to get the best match. But you need to start with more equal data than you have obtained in the example posted.

    Rick

  14. #14
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    I don't know if I understood. Should I picture a gray object with each channel with the respective same exposures and gains, bring the result back to gray, and then aply the same adjustments to the space image?

    The DMK makes images as AVI files. Does the Deep Sky Staker have a way to convert them to FITS?
    English is not my first language.

  15. #15
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    Since you're getting avi files and imaging big bright colorful planets like Saturn, that would mean rick recommends registax.

  16. #16
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    Hi, I've understood this part. However, even though my DMK is a "planetary" type, I got a f/3.3 focal reducer and saw the Orion Nebula's core at shutterspeeds as short as 1/15 s. The field is around 30 arc minutes, so I feel tempted to try deep space imaging for some small and bright targets.
    English is not my first language.

  17. #17
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    Of the two programs, Registax is best for AVI files of planetary images. It has routines to key on planetary features and remove distortions further improving the image. It can search thousands of AVI frames, find and stack the best ones of each color filter then has the tools to process these into a great image. It is designed for planetary imaging.

    Deep Sky Stacker is what its name implies, a program to stack deep sky images, preferably ones taking with dithering. It then calibrates the images (darks, flats, Bias-if needed) and stacks the images using any of several rejection algorithms that removes hot pixels, cosmic ray hits, satellite trails and the like. It works only with a FEW not hundreds of frames and is designed for the FITS format used by deep sky cameras.

    These are two very different programs for two very different purposes. Of course there are several programs of each type, just these are two free ones and as free more limited than the better pay programs for these purposes. Both can do a very good job and are a great starting point but each for its purpose. I'd not use Registax for deep sky work nor would I use Deep Sky Stacker for planetary work with a video camera like the DMK.

    Yes, on the gray paper. I use a medium gray paper stock for my reference. Be sure it is lit by a bright sun with no reflections from trees or other sources that could alter the color. Take it through each filter. Measure the intensity level of each color. It should be the same but this is very unlikely unless using filters designed for the spectral response of your CCD. All CCD's are more sensitive to some colors than others. Some are blue sensitive others red sensitive, none are flat across the spectrum. By taking these reference images you can then adjust the levels of each image to bring them back to the same intensity level which should be exposed for a middle level intensity, say about 127 or so for an 8 bit image, 2047 for a 12 bit image or 32767 for a 16 bit image. No need to be exact 100 to 150 is fine for an 8 bit image, just that all three colors be the adjusted to the same level, whatever it is. I don't know GIMP so can't tell you how to do this. In making the adjustment only the intensity should change, not gamma or again colors of various intensity will be wrong. Trying to do this with Wratten filters not designed for tri color imaging likely will make achieving high quality color balance impossible. This is why special filters for the purpose are made. They are also transmission filters that block light outside the pass band so have sharp cut offs with overlaps designed not to skew colors. They pass nearly 100% of the light in the pass band and block nearly 100% outside it. Wratten filters are very soft on the edges and pass various percentages of light in the pass band and are slow to block colors outside the passband making it impossible to get an even color across the spectrum. Acceptable maybe, depends on how particular you are I suppose but don't expect colors in Saturn or Mars to equal those Jaicoa posts here using filters designed for tri color imaging.
    http://www.bautforum.com/showthread....April-3rd-2012

    Note how his three color channels are quite equal as to data quality. This is necessary for good color imaging. His L channel however is much sharper and clearer as it has been processed for that purpose. The other three for correct color. Since only the color is used to color the image when making the LRGB image it need not be as high resolution as the L channel but does need high color accuracy. Fortunately the human eye is very insensitive to color detail making this possible.

    Looking at the histograms of your three color channels they are no where near aligned. Some of this is due to the difference in data quality. It appears while the green and red channels spread rather well across the entire range the blue is compressed into about half the range. The three should cover the entire range if properly exposed and processed. It appears the blue channel needed more exposure time.

    There are ways to properly color balance the channels in the pay programs I use to adjust for filter and CCD spectral differences. I don't know the free ones you use however. There are forums dedicated to planetary imaging that can give you better information on your software as to how to best adjust the three channels for proper white balance.

    Rick

  18. #18
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    Quote Originally Posted by RickJ View Post
    The three should cover the entire range if properly exposed and processed. It appears the blue channel needed more exposure time.
    I know it is important to spread the histogram, but I actually avoided making them cover all the way to the 255, because that produced regions of pure white with no details on the planets. How do I counter that?

    Besides, if I change and record the exposure to make them cover the entire histogram, how do I use this information to calculate how much I should change the color balance?
    English is not my first language.

  19. 2012-Apr-27, 09:49 PM
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  20. #20
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    With data as unequal as those in your example there's little you can do to salvage a color image. Make a good black and white one from the red channel. Until you can achieve good and equal quality data across the channels color shouldn't be attempted.

    As to how to apply the color balance info I can't help you as I don't know a thing about your software. None of it is software I use. I'm sure that there's a way but not knowing the software I can't help. This is why I suggested going to forums specific to planetary imaging. There you will get software specific help. Putting Registax and White Balance into Google this link came up on top.
    http://www.astronomie.be/registax/5_1features-4.html
    Looks like it may help.
    Rick

  21. #21
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    Hi,

    I made my first attempt with true LRGB filters.

    I used 15 fps during 2 min for each RGB channel. Due mainly to a vibration in the RA axis of my mount, the images were so poor I barely could see the Cassini Division even after processing. So I used the luminance image made from 60 fps during 2 min (I thought I wouldn't need it for planetary photography, but I made it anyway for comparisons).

    I used the "automatic color balance" function from Photoshop, and the result was familiar; I really have seen color patterns like that in magazines and internet. However, I was expecting a full orange disk as I see in the telescope. So what causes this difference? Does Saturn actually change colors with time, or does the processing on separate color channels bring out this kind of contrast?

    (And how do I send TIF files on this forum?)

    Thanks,

    Saturn LRGB.jpg
    Last edited by Jairo; 2012-Jul-04 at 09:47 PM. Reason: Grammar
    English is not my first language.

  22. #22
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    Auto color is designed for ordinary pictures. It is totally worthless for astro images. You're lucky it didn't do a lot more damage.

    You need to image something white, use that to determine the RGB corrections needed then apply those ratios to the planetary image. The moon is close to white and should work as a starting point. Then follow the G2V process I sent you in another post to find the ratios needed.

    Once auto color has been used a lot of data is lost so no way to bring it back. What I did was assume the b ring is white. Pretty close to reality and applied the corrections to red and blue needed to accomplish this. I used the free photoshop plug in at: http://blog.deepskycolors.com/archiv...-Whitecal.html . The result is attached. The results would be better if using the full tri-color image before data was lost with the auto color application.

    Tiff images are far too big for forums like this so are not allowed or limited to very small sizes. Many browsers won't support them which is another reason they are rare on the net except as a download option.

    Rick
    Attached Images Attached Images

  23. #23
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    Thank you! I still have the four videos, so no information is actually lost. I will reprocess them following those instructions.
    English is not my first language.

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