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Astronomik CLS clip Canon 7d very blue images


David010167

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Hi, I am slowly setting up my equipment to try the dark and patient art of astro photography.

Due to light pollution I have bought a Astronomik CLS clip fillet for my 7d. when I take a photo it comes out very blue.

Is this normal?

From what I can see there is virtually no red channel in the image when examined in Photoshop.

The CLS filter looks more like my Baader UHC-S filter than my skywatcher light pollution filter?

You can see what I mean in the attached image. Don't laugh this is a first attempt of 3 x mins at ISO800 using DSS followed by Photoshop curves. Only managed to get 3 frames before the fog rolled in.

regards

David

post-21390-133877508603_thumb.jpg

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The CLS is cutting out orange light so your red channel will take a hit. However there will be red in there. There are different ways to get this back, some more elegant that others.

Looking at your pic you have a blue bias - the background is excessively blue. This is very easy to correct in PS. Use levels and correct the black point in the histogram for each individual channel ie. move the L hand slider to a point just before the start of the histogram.

Looking at your highlights, these are definitely cyan indicating a problem with the red channel. A simple way of correcting this is to bring up the histogram in PS and choose the option which displays the 3 channels. Find the point in each channel where the right side of the histogram drops to close to zero (not the absolute bottom of the histogram). Look at the levels for the 3 chanels. The figure for red will be significantly lower than the others. Use the colour balance tool - ctrlB select highlights and up the red whilst lowering blue and green until the above point on the histograms match up.

This should correct your balance. Although there are fancier ways to do this which don't generate quite as much noise this method is easy and works pretty well.

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  • 2 weeks later...

You could also take a photo of a grey card in daylight, and set it as a custom white balance for the camera, although if you are shooting in raw the white balance adjustment will not be changed on the camera, you will still have the "as shot option" which can be applied in post processing.

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Hi David

I had a play with your image, and I have to say it was not easy getting any kind of 'correct' colour balance. I resorted to using PixInsight, with full use of masks, and then PS. I've pretty much destroyed any quality the original had, but it was only a small jpeg!

David

post-21430-133877512009_thumb.jpg

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Thanks every one. PixInsight looks like an interesting tool, I will check it out.

On the greycard topic. I use lightroom and CS5 and there appears to a limit to how far either will allow a rebalanceing of the raw file. Strangely Canon DPP does a better job. I use an Expodisc for whitebalancing, as I find it accurtate for most lighting conditions.

I have not had another chance to try out any astrophotos since the one above, it has been either very cloudy, misty, foggy or now snow. I am trying to persuade my wife to let me take the scope to wales at christmas for some dark skys, but alas I am not winning that battle at the moment. I guess I need to wait for clear skys in Surrey.

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  • 2 weeks later...

Hi David,

The blue colour is normal with the CLS filter. You'll also find that you have to expose for a bit longer too.

Since you mentioned DSS, you can get it to correct the colour as part of the stacking process. On the "Light" tab of the "Stacking Parameters" window there is a background calibration option (at the bottom left). If you set it to "RGB Channels" then DSS will normalise the three channels to the same value, giving a grey background.

Then when you play with the levels you should find some red stars in the area of M31...

Sara, I bought a CLS filter and I've used it with my unmodified 1000D and 60D cameras. To try and describe the effect it has... by cutting down the sky glow it allows (or requires) me to expose for longer and capture fainter detail than I otherwise could. Also, I can attempt targets in brighter parts of the sky and nearer the horizon.

I'm waiting for them to bring one out that can see through clouds :(

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The Astronomik CLS would help a lot allowing for longer exposures... personally I prefer Hutech IDAS P2 as it give a more natural color balance but they are a more expensive option and depending on the LP sources that you have to deal with may not be so effective...

I have Astronomik CLS-CCD, Hutech IDAS P2, Lumicon Deepsky and various othe LP filters at my disposal...

Billy...

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The only problem at the moment with longer exposures is that my setup is unguided at the moment. Realistically I am looking at max of 2 mins - So I guess I'd probably lose more than I'd gain with such a filter?

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Your image looks normal for a sub taken witht he CLS filter. I use the filter all the time so the blue subs are very familiar to me. This is a problem easily solved. If you stack your subs using Deep Sky Stacker (free) there is an option to Align the RGB channels - your resulting stacked image then has properly balanced colour - much easier than trying to do it manually in Adobe.

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