Quick question on light frame

[IvanT]

Hi, just a quick one hopefully, am I correct in saying that flat frames do not need to be the same duration as the lights? I'm using one of those illuminated panels with a canon 1000d. If I am correct is it simply x number of flat frames at same temp an no change to optics. I'm currently running 5x10 min subs on M45 just to check guiding really and see if I can capture more of the nebulosity and wonder what lights to take, planning on 2x10 min darks.

Thanks

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[roundycat]

Flats are photographs of a uniform light source exposed at a level that gives good signal but does not run into the non-linear aspect of the chip. Aim for an average exposure of one-half to two-thirds of full well. Imagine in real photography using a Zone V exposure.

Dennis

[IvanT]

Thanks Dennis, excuse my ignorance, but I'm not only trying to get my head around the complexities of astronomy but photography too. I'm new to both subjects, let's just say I like a challenge :-)

When you say two-thirds of full well, does that mean (in layman terms) that the live view histogram should be roughly one-third away from the right hand side?

[roundycat]

I guess that would be a good enough place to start. The best way to determine your flat value is to measure the average pixel value over the whole flat. Something your capture s/w should be able to do. Your camera is non-linear so don't make too big an exposure.

Flats can be difficult. For starters take ten, average them and try the resultant master on a light frame. It should end up a lot cleaner and 'flatter'!

Flats should be properly calibrated to get rid of any thermal pixels (not much of a problem there due to the short exposures) and the bias offset. Best way to do that is to shoot 20 bias and subtract the master from each flat, then combine the flats. Sounds bad but doesn't take long.

If the flat looks to be over or under correcting try another master made at a longer or shorter exposure. Use a light box if you can rather than the sky as that tends to obviate the need to normalise each flat. If you combine by Median that will happen anyway.

Dennis

[jgs001]

Ivan, the simplest option with dSLR's is to put your camera into Av mode, and let the exposure meter work out the correct exposure to use. Make sure you've got exposure compensation set to 0. Turn on the display on the rear and press the DISP button till the single histogram shows (if you get a red, green and blue, you've gone too far, go around again ). Using Av, puts the exposure of the flat at about the 75-80% mark of the histogtam, the right hand bar one back from the right hand end. This works well. I don't know why, but last time I took flats, I ended up with a lower value on the histogram, and the flats made the resulting image worse. Given the histogram display on the camera is weighted towards the right, I think the 75-80% mark on the histogram is in fact about 2/3's point Dennis mentioned.

[IvanT]

Thanks guys. You've confirmed that I'm a complete muppet. For some reason I'd got it into my head that flat duration was the same as dark so what I've been trying to subtract from my images has been a HUGELY over exposed flat.