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Color balancing CLS blue cast

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#1 spectre

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Posted 18 February 2013 - 05:44 PM

I have started imaging using a CLS clip-in filter and need some help with color balancing due to the bluish cast that the filter gives my images. I've attached an image of M-37 that has been stretched enough to spread out the histogram. I've tried processing this further but I always end up with blue-green stars. :bangbangbang: How do I process these images to get 'natural' looking colors? I know it can be done because I've seen many of your images that look great taken with this filter but I must be missing something somewhere.
Help please.

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#2 Tom and Beth

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Posted 18 February 2013 - 05:46 PM

Post deleted by Tom and Beth

#3 Tom and Beth

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Posted 18 February 2013 - 05:48 PM

Have you tried this?

HLVG

#4 jambroseus

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Posted 18 February 2013 - 08:17 PM

Hi spectre,

I downloaded you shot and did a neutralize background, Digital development and levels in ImagesPlus and got this...

there is not much color in the image. You really need to do a custom white balance with your filter in place. Check you camera manual to do this.

John

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#5 fco_star

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Posted 18 February 2013 - 08:20 PM

+1 John

#6 HaleBopper

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Posted 18 February 2013 - 08:33 PM

Like jambroseus said, you need to do a custom white balance. I used to have the same problem with my CLS.

Not being the greatest at processing, I bought myself a gray card and took a picture of it, with the filter in place, outside on a clear day around noon. I made sure not to get any shadows interfering with the shot. After that, I used that photo as a custom white balance. Check your camera manual. I think you could also get away with using a white wall or piece of paper brightly lit by our Mr. Sun.

#7 cclark

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Posted 18 February 2013 - 08:42 PM

I gave it a go in PI with the following steps:

1. check histogram - adjust black point for blue & green channels to match the red channel. The red is lower than the blue & green, which is typical for this filter.
2. run SCNR on the green channel (which is similar to the HLVG step mentioned above - just PI specific) to remove any residual green tint.
3. do a small stretch to bring the brightness back up a little
4. auto background extract to remove slight gradient.

Chris

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#8 Toxic Coolaid

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Posted 18 February 2013 - 08:43 PM

the color will be fine if you are stacking subs. If you are only using single subs do the custom white balance. You should also be able to line up the RGB in levels

#9 avarakin

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Posted 18 February 2013 - 09:11 PM

Spectre,
Could you please elaborate on your processing process?
What did you use for stacking? What did you use for stretching? Once we know this, we can give you more help specific to your tool.

Given information you gave us, I can suggest the following:
don't stretch and then color balance, instead do color balance and then stretch.
Also I hope you are shooting using RAW of your camera, if that is the case then you should be able to bring color balance to normal.

Alex

#10 Samir Kharusi

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Posted 18 February 2013 - 10:09 PM

Ummm, sorry. It's unfortunate that the CLS is/was advertised as capable of a good white balance. It is NOT! An excercise in futility if you are fussy about white stars. There is far too much of intermediate wavelengths missing; and unless you create these wavelengths by synthesis it ends up frustrating. The white balance achievable is nevertheless satisfactory when you are imaging strongly colored nebulae, teal OIII and deep red Ha. All explained here. Shooting Custom White Balance is irrelevant if you are using Raw capture. Just eyedrop white balance on unsaturated stars themselves right at the end, but recall that you cannot get an accurate white because of that large chunk of wavelengths that has been blocked out. It's a compromise to near-white.

#11 spectre

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Posted 19 February 2013 - 06:05 PM

Thanks for all the great ideas. I was under the impression that, if you shoot RAW, a custom white balance doesn't make any difference? So, I should color balance before I do any other stretching or processing?
Alex, I always use RAW subs, stack in DSS, and use PS CS4 for all my processing. I also have HLVG but it didn't seem to do enough. I'll try again using your suggestions.
Thanks again
John

#12 fco_star

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Posted 19 February 2013 - 06:12 PM

I think you are right No custom balance is necessary if you use RAW files.

#13 jambroseus

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Posted 19 February 2013 - 07:46 PM

Francisco, it is my understanding that when you process a raw it uses the stored white balance, custom or whatever.

John

#14 fco_star

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Posted 19 February 2013 - 11:05 PM

Didn't mean to confuse, sorry I'm not an expert so please someone jump on this tread.

I use a custom WB with my modded but I usually process jpg files instead of RAW



#15 avarakin

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Posted 19 February 2013 - 11:13 PM

spectre,
Try to use background calibration of DSS:
Stack Checked Pictures-> Light -> RGB Channels Background Calibration.

Alex

#16 srosenfraz

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Posted 20 February 2013 - 02:08 AM

Hi John -

I gave your image a try:

Posted Image


My adjustments are pretty much the Photoshop equivalent of what Chris did in P.I. You can download my Photoshop .psd file if you're curious to see what I did.

Your histogram can give you some very good hints as to what needs to be done to color balance an image. For astronomical images that do not have nebulosity throughout the entire image, well color balanced histograms will have a few typical features:

1) The color channel black points will be at the same level (i.e., neutral colored background).

2) Color channel peaks will be well aligned.

3) Color channels will usually have similar widths UNLESS the image should have a predominant color. For example, a Pleiades image will typically have a wider blue channel, and a Lagoon Nebula image will typically have a wider red channel.

Now, the above are not rules, but they are very good guidelines and can help you to see the cause of a number of color balance problems.

If you look at your initial histogram, you can see that the red channel peak and black point are significantly below the blue and green channels (which are closely, but not perfectly aligned). This explains the clear cyan cast to your image.

The way I fixed the major part of the color balance is with two simple levels adjustments.

1) Identify a part of the image that should be neutral in color - preferably a part of background sky (you want to be sure there isn't any significant color in this area). I usually put a color sampler point at this spot for easy identification.

2) Add a Levels adjustment layer (or do an Image | Adjustment | Levels). Choose the grey point eyedropper (the middle eyedropper) and then click on the neutral spot. You'll immediately see your color channels align rather well (not necessarily perfectly, but it'll be a big improvement).

3) Add another Levels adjustment layer (you can use the same Levels adjustment, but I like to separate the two so that I can see each effect) or do another Image | Adjustment | Levels. Choose the black point eyedropper and doubleclick on it. This will bring up the "select target shadow color" dialog box. Fill in the RGB values for your preferred black point - I recommend something between 25 and 30 (this is a good starting point for most images). So, you'll have 30, 30, 30 in the RGB fields. Click on OK. When Photoshop asks you if you want to save these as your defaults, tell it Yes. In future Levels adjustments, your black point will be set to 30,30,30 until you change it in the select target shadow color box.

4) Click on the same spot you chose earlier. This will set that spot to 30,30,30 and will give you a neutral background.

You'll find that these two adjustments will go a very long way to giving reasonably good color balance for this and most images.

I added a few more layers to do final tweaks on it. The first one is HLVG (which Tom and Beth recommended) and I would highly recommend you try in on EVERY image (with or without a filter). I find that HLVG virtually never hurts and image, and will usually improve it or have no effect. Most images will benefit from Strong or Medium. For your image, I chose Medium.

The rest of the adjustments I did were just for a little more enhancement - a bit of saturating with non-noise increasing techniques, a slight brightening curve, and a final levels adjustment to bring down the final black point.

I agree with those who have recommended using a custom white balance. It is true that the custom white balance does not effect the data in the raw file. What it does is provide that color balance data in the Exif headers of the image. Many commonly used programs can read this CWB data and apply it to your image. Images Plus is one, and I believe that DSS can do this as well (but don't quote me on that). You can absolutely do all of your color balance adjustments in post processing without a CWB, but you'll find that starting with a CWB will give you better initial color balance. If you aren't experienced at color balancing, its much easier if your image starts out fairly close. That's why I recommend using a CWB.

I also believe that Samir is correct that the CLS and many other LPR filters don't leave the colors as a full spectrum (that's their purpose). But, since I have seen far too many very attractive images taken with a CLS filter, I have to conclude that it is possible to have good looking images through this filter.

The color balancing steps I did on your M37 image are not everything you would need to do on an image with more color throughout it. As it happens, the final red channel is slightly wider than the green and blue channel. I suspect that is a feature of the CLS filter (and/or your cameras response to it), so its quite likely that more complex images will require some destretching of the red channel (ask me how to do that when you cross that bridge). But, I believe that these few steps here (set grey point, set black point, run HLVG) will go a long way to color balance many of your images.

Hope this helps.

#17 spectre

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Posted 20 February 2013 - 07:26 PM

Thanks Scott, that workflow is really helpful. :bow: I'm not anywhere near an expert in PS so everything helps. I took your workflow and applied it to an image of M-67 using the CSL filter and it came out much better than my previous efforts. I used some of Carboni's Actions, HLVG, and some levels and curves. I feel much better about using the filter now.
What do you think? Any other tweaks?

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#18 AddictedToAstro

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Posted 20 February 2013 - 08:47 PM

srosenfraz, thank you very much for posting your PSD file. I downloaded it and played with your setting, and then reset it and followed your instructions and learned a lot.

Much appreciated. I use that same filter and I'm always struggling to get rid of that color cast. DSS does do a decent job itself though.

#19 srosenfraz

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Posted 20 February 2013 - 10:29 PM

Thanks Scott, that workflow is really helpful. :bow: I'm not anywhere near an expert in PS so everything helps. I took your workflow and applied it to an image of M-67 using the CSL filter and it came out much better than my previous efforts. I used some of Carboni's Actions, HLVG, and some levels and curves. I feel much better about using the filter now.
What do you think? Any other tweaks?



Color balance-wise - I think you nailed it. Your histogram looks perfect, and it shows in the colors of the image as well.

As far as tweaks - typically we go through great pains while stretching our images to ensure our stars stay nice and small so that they don't distract from the subject. However, when imaging open clusters, the stars ARE the subject. As such, in my mind, its OK to stretch them aggressively and have them bloat somewhat. That way you can see them more easily.

So, the first thing I would do is to put on a brightening curve (much like I did on your M37 image). Now, when you brighten the stars, the color saturation is going to go down. So, the next step is to increase the color saturation to add color back to the stars. There's a number of ways to do this, but you want to be sure that you use a non-noise increasing method. I typically use Block method (for general saturation increases), Lab Color (particularly for saturating blue, yellow, and magenta), Match Color (selecting the image as the source), and Vibrance (use sparingly). What you don't want to use is a simple saturation adjustment in Photoshop - it'll increase the saturation, but it'll also increase the background noise.

Since you have Carboni's actions ($21.95 well spent), then the "Increase star color" is a great and simple tool to use. You'll probably want to run it more than one time until you can see the star colors well.

The other method that is perfect for stars is Jerry Lodriguss' technique for increasing star colors. If you look at Jerry's recent M41 image, its obvious that Jerry truly knows how to make pretty stars (along with many other astrophotographic talents!).

Finally, for most astrophotographs, black points around 20 to 30 usually look pretty good. The downside to darker black points is that it makes it difficult to see faint nebulosity. Brighter black points tend to leave your images fairly low in contrast. For open cluster images, there usually isn't a lot of faint things you want to show. So commonly lower black points look good, because it'll give you higher contrast. For your image, I would probably do a final levels adjustment to bring the black point down to the low 20s (you're currently in the high 30s).

Great job color balancing your M67 image - this sure is a fun hobby when the images start looking good!

#20 FloridaObserver

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Posted 20 February 2013 - 10:31 PM

Hi Spectre,

I only intended to try my hand at color balancing, but was curious how far I could take a small JPEG with my usual processing set.

For color balancing only, I recommend Russell Croman's Gradient Xterminator with a Coarse/Low combination (make sure the Balance Background Color box is checked). I use this for all my shots.

Hope all these replies helped,

Doug

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#21 srosenfraz

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Posted 20 February 2013 - 10:34 PM

srosenfraz, thank you very much for posting your PSD file. I downloaded it and played with your setting, and then reset it and followed your instructions and learned a lot.

Much appreciated. I use that same filter and I'm always struggling to get rid of that color cast. DSS does do a decent job itself though.


I'm very happy to hear it was helpful, Addict. :-)

I like to post the .psd files because I think it describes what I'm doing a whole lot better than I could ever write. Plus, it gives people the opportunity to turn on and off the layers to see what each adjustment does.

Thanks again - looking forward to seeing your images!

#22 acotiga

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Posted 20 February 2013 - 10:53 PM

I've had this problem for years now; white balance. In short, it's a pain but two days ago I was shown the light (pun intended). Try eXcalibrator. Take a while to ga through the documentation but if you do it step by step it's going to work.
I hope it's going to be the same eye opener it was for me.

#23 spectre

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Posted 21 February 2013 - 06:27 PM

Thanks again Scott. I've tried 'Increase star color' but after about two iterations the star cores get blown out. The color is there but the stars look strange. I'll give Jerry's method a try. I'll also knock down the black point. Your tips are great and I've got both your posts printed and saved.
Thanks again for your help.
John

#24 AddictedToAstro

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Posted 21 February 2013 - 08:21 PM

I just printed them as well :) Great reference material.

I went back to one of my previous images of M35 and your technique helped tremendously!

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#25 srosenfraz

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Posted 22 February 2013 - 01:16 AM

Your M35 looks really good, Addicted - I can even see NGC 2158 rather well.






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