'Flats', Colour and White Balance

[ArmyAirForce]

Does the colour and white balance of 'flats' matter when processing images. I've made myself a portable 12v flat field light panel using a car bulb ( at present ), which is a bit orange, and to get the histogram around 33%, it is coming out quite pink. I didn't know whether white LED's would be needed instead. Before this, I was using a white T-shirt and pointing the scope at a desk lamp.

My lightbox was made to be used out in the field without upsetting everyone with a white light.

Flat DSLR image.

[IanL]

Two things:

1. Is there sufficient exposure in all the wavelengths you are using? If using a DSLR or One Shot Colour CCD camera that would be somewhere in the red, green and blue wavelengths.

It is an issue because you can only take one exposure for all three colours (as each pixel will have a fixed red, green or blue filter on them). You need to take a look at the histogram of each of the colour channels in your image. Provided they are all exposed you should see three curves well away from the left hand side of the histogram. I'd have thought a car bulb would be fine as it is a continuous spectrum light source, so even if it is biased towards the red end, you should still have plenty of green and blue in there.

It is more of a problem if you are using LEDs, which may look white to the eye, but some only give off light in a narrow range of wavelengths, or an EL Panel for the same reason. You may find that the exposure in one of the three colours is much too low or non-existent.

Also if you use narrowband filters you need to make sure your light source is producing enough light at that specific wavelength to illuminate the pixels. If you are using a mono ccd camera, you can compensate by using a longer exposure (unlike a DSLR where exposing more for one colour may saturate the other colours).

2. If using a DSLR, or OSC how does your processing software deal with flats? The correct process is to deal with each channel separately before the image is debayered. In essence the software should create a separate flat for the red pixels, for the green pixels and for the blue pixels. The normal process is to 'normalise' the range of pixel values to lie between slightly more than zero and one and then divide the corresponding light frame pixels by that value.

Because each colour is dealt with separately and normalised, it doesn't matter if there is a colour cast, so your only concern is that the least exposed channel has a reasonable level of exposure without overexposing the brightest channel. So long as the histogram for each channel is not at the far left or far right of the graph, you should be ok.

If your software is dumb (and some can be), it will debayer the flats and the lights first, then do the normalisation and division processes. This is more of a problem if you do not have a 'white' flat frame, as it will now introduce a colour cast to your image. In this case your pink/orange flat would end up creating an image that is too blue/green. If that happens don't panic, you can still correct the colour balance of the final image by playing with the histogram tools in your preferred package (or using some kind of colour balance adjustment tool if it has one).

[ArmyAirForce]

Thanks for the comprehensive reply. I'm just using an unmodified DSLR for imaging, and no narrowband filters at present. I'm using Deep Sky Stacker 3.3.2 for stacking my images. I've just checked the histogram for the flat and the green and blue are around the 25% mark, with red at around 37%.

[IanL]

You may be okay then, as that means all three colours are exposed. I only used DSS a while ago before I invested in PixInsight, so I can't remember much about the details of its flat-frame process. I have seen a number of posts on here about problems with colour balance when using flats in DSS, so it may be an issue that you will have to deal with in post-processing, or it may just be a setting that people aren't using properly

[ArmyAirForce]

Cheers. I would have liked to try tonight, and while it's a lovely clear sky right now, it's also blowing 25 to 30mph!!

[ArmyAirForce]

Just been reading up on the Deep Sky Stacker website.....

"Is it possible to use colored flat frames?

The short answer is yes.

The overall tint of your flat frames is not really important because DeepSkyStacker is processing each channel separately and is applying the flat frames to each channel accordingly.

Of course if your flat frames are fully red only the red channel of your light frames will be properly calibrated but otherwise as long as as the peak in each channel is between 1/3 and 2/3 of the maximum you should have no problem using slightly colored flat frames."

[ollypenrice]

I used a bulb in a lightbox once which, bizzarely, gave almost no blue signal whatever. Why not just split channels in Ps for a flat and see that you are really getting signal in all three?

Olly

[ArmyAirForce]

Here's my flat info from PS.

[RikM]

I think you'll probably do fine with that.

Best bet is to try it and see. If you shoot flats with the new box and a second set with the old lamp and t-shirt method, you haven't lost anything if it doesn't work.

[ArmyAirForce]

Thanks. I wish I could make a clear sky as easily.