Flat images ???

[StevenA (Steven)]

When I try to make a flat frame (using different methods) I always seem to come up with an image which has a tint of colour. Usually a bluish tint. Is this normal? I thought they were meant to be white? After all I use white translusent plastic and white T-Shirts, even a white plastic shopping bag. I have even tried photographing a white wall. Is this normal outcome?

I also use my webcam in the same way to acquire Flat frames but the blue is still there.

What am I doing wrong if it is not normal? F.Y.I, I am using a very simple Kodak digital camera. Pocket type thing. No removable lense's etc Simply can't afford a DSLR. ( thus the Kodak C690 Still I get fair luna images.)
Thanks for imput. Steven A

[Lumen Miner (Mitchell)]

Sorry my friend, I am only up to taking darks, flats I will try next...

I am just speculating here but, I would imagine flats work on a similar principal to darks. You are trying to eliminate the defects in the light. The bluish tinge is probably a defect that will be subtracted, or do you add flats...??? Don't know sorry...

The only other suggestion would be, try changing the light setting in the camera... I get bluish images when I have my compact set to.... one of the settings, try them all.

[DavidU (Dave)]

Prob a white balance issue.

[Benno85 (Ben)]

I can't help personally, but perhaps check out the guide posted here:

http://www.iceinspace.com.au/63-211-0-0-1-0.html

Cheers,

Benno.

[Octane (Humayun)]

Steven,

It is ideal to have grey flats, but, it's not a problem if you have a blue tinge, as they're treated as greyscale images in pre-processing.

The blue cast you're getting may be due to incorrect white balance settings in your camera. See if you can set it to auto white balance.

Mitchell, re: your question about the maths behind flat frames: darks are subtracted from your lights; and your lights are divided by your flat fields.

Regards,
Humayun

[Lumen Miner (Mitchell)]

Originally Posted by DavidU

Prob a white balance issue.

Thanks David, that's the term I was looking for, white balance settings.

You should have about 5, daylight, halogen/incandescent, fluoro, then probably sepia, black and white.

Humayun, thanks for clarifying, though still perplexing. I will have to learn more, thanks.

[Octane (Humayun)]

Mitchell,

If I'm not mistaken, you're using Nebulosity, eh? You should be able to do all that pre-processing in Nebulosity.

Try it out the next time you're out doing your star trails. I think you'll find the calibration images will improve your already good star trails.

Improvements will include removing arbitrary noise (amp glow, read noise, thermal noise) and will also remove the strong vignetting which is often found on wide-angled lenses such as the one you're using.

My recommendation: take about an hour's worth of dark frames immediately after shooting your light frames. For flat frames, make sure you don't move the focus ring on your lens; the optical path has to match what your light frames were gathered with, as well as the same f/ratio. Load up Microsoft Paint, or any other application, which will allow you to have a pure white background on your screen. Rest your camera on the keyboard or somewhere else in front of your screen. Ensure your ISO is set to ISO-100, put your camera in Av (aperture priority) mode and set it to the same aperture as what your light frames were taken with, and snap off a couple of frames. Check the histogram on the LCD after each frame. What you want to do is see your histogram peak between 1/2-way and 2/3rd's of the way across the histogram. Once you think you've got the right exposure, take about 15 flat frames. Put the lens cap on your camera, make sure the viewfinder is covered on the back, and take 15 flat dark frames.

The rest should be a matter of loading the right files into Nebulosity (I'm not sure how to do this as I don't use Nebulosity for pre-processing) to create your master flat dark, master flat, master dark and so on.

Give it a go.

Regards,
Humayun