Canon Colour Balance Query

[BlueAstra]

I use a unmodified Canon 1000d with a Skywatcher LPF. I set the control to manual, asa800 and daylight white balance. The light pollution will break through on long exposures giving a magenta cast to the image. I generally find that that after DSS processing the blue and green histograms overlap, but the red is off to the right. Adjusting R to the left to overlap the B & G with the simple colour controls in DSS virtually eliminates the colour cast, but I seem to lose any colour in the image (M42 is white!)

I put one RAW sub with colour cast into paintshop and it also showed B&G overlap and R to the right in the Canon RAW editor. However, selected 'colour balance: auto' splits the histograms into separate G,R,B curves in that order. The image looks a bit better and the colour is not lost.

So my question is, if you are using an unmod Canon & LPF, what white balance setting do you use on the camera, and how do you colour balance the image when processing it (in maybe DSS, PS) to get an accurate colour rendition?

[astro_dt]

Hi

I shoot in RAW then use the colour temperature in the Canon photo software to adjust the background and reduce the effects of light pollution. It's quick and easy!

Cheers

Danny

[jgs001]

I leave my 450d in Auto and let DSS take care of the colour cast from the LP filter. In the RAW settings, uncheck both of the White Balance boxes. In the Lights section, select RGB Channels Background Calibration. These two options force DSS to carry out a white balance calculation on the images. I got this from the DSS author on the Yahoo group, but for some reason, it doesn't seem to work for everyone, but give it a try.

[BlueAstra]

OK, thanks I'll try it. I guess I don't really understand what the histograms are telling me if B&G are superimposed and R is to the right. At first I thought I should superimpose all of them, but that seems to just gives you a greyscale image. Found out its best not to touch them.

I've enclosed a typical sub, and after DSS with PS 'auto' colour balance. Still some work to do though.

[Dangerous-Dave]

I leave my 450d in Auto and let DSS take care of the colour cast from the LP filter. In the RAW settings, uncheck both of the White Balance boxes. In the Lights section, select RGB Channels Background Calibration. These two options force DSS to carry out a white balance calculation on the images. I got this from the DSS author on the Yahoo group, but for some reason, it doesn't seem to work for everyone, but give it a try.

Wot John said. If you shoot in RAW, white balance is academic as you can change it on your computer. DSLR white balance never gets a DSO right anyway, its easier to balance the RGB channels in levels.

[BlueAstra]

Can you explain what you mean by balancing the RGB channels in levels in PS? What exactly do you do?

[jgs001]

The Histogram is showing you that you have a strong red cast to the image, although sometimes I've had DSS do odd things to the display with the end points of the colour bars. I don't actually adjust the image with DSS. Just save the image with adjustments embedded, then get to work in PS afterwards.

There's a couple of options for adjusting... The Colour Balance tool... or in levels or curves, you can, by selecting the drop down at the top, adjust each individual colour channel instead of the combined channel. So if you have a strong red tint, you can adjust just the red channel and reduce it a bit to even it out with the blue and green.

If you get the settings in DSS right though, there should be a minimal amount of this required.

[Dangerous-Dave]

I don't think colour balance is the same as balancing the levels John (ie colour bias removal). Simply adjust the histograms in levels so the peaks of Red, Green and Blue channels fall in the same place. You can use the dropper tool on a bit of black sky with the info palette open - you are aiming to have R, G and B show roughly the same value.

[Starflyer]

Do what John says below and the image does look very grey.... But if you then click on the saturation tab in DSS and bump it up a bit (15 - 20 works for me) and apply it. The DSS image should become more colourful, although the colour balance will still be slightly off and will need tweaking in photoshop.

I very rarely use the image file saved from DSS. I use the Autosave.tif file that DSS automatically creates in your RAW files directory. If you have PS CS2 or above you can work on this file, open it and change it to 16bit (Image | Mode | 16 Bits/Channel). I don't know categorically if it makes a difference but I'd rather have PS convert the image than DSS.

I leave my 450d in Auto and let DSS take care of the colour cast from the LP filter. In the RAW settings, uncheck both of the White Balance boxes. In the Lights section, select RGB Channels Background Calibration. These two options force DSS to carry out a white balance calculation on the images. I got this from the DSS author on the Yahoo group, but for some reason, it doesn't seem to work for everyone, but give it a try.

[BlueAstra]

Thanks for all the hints. I'll get back to PS and see what I can do.

A typical 4m sub just shows the target but has a bright magenta cast, as shown earlier. Do you think it would be better to go for more 2m subs, with less LP breaking through?