Creating Flats for a Wide Angle?

[MarkS]

How can I create flats for a wide angle camera lens (50mm in this case)?

My usual trick (for a telescope) is to point at zenith in a unobstructed sky around sunset.  But for a wide angle lens the sky has too much gradient.

[Mike]

tracing paper in front of the lens? Lightbox?

[MarkS]

Lightbox?  Good idea!  I don;t have one but pointing the camera at a LCD screen displaying white seemed to work quite well.

[Ian]

that's assuming the lcd is evenly illuminated by it's backlight...

[Daniel]

Actually, could anyone tell me a sure fire way of creating flats, I've read a lot of conflicting reports on how to do them,  I'd be taking them at night, so what i did last time was cover the apeture of the scope with a white cloth and shine a very bright torch onto it.

One thing im not sure of is the exposure and ISO, last time I used the same ISO as i was imaging with (ISO 1600) for the same amount of time I was exposing for (30s) I've now read other peoples techniques where they take much shorter exposures on ISO 100, what techniques work for you guy's?

[MarkS]

Ian,  I put the camera right up to the LCD so it was totally out of focus and any unevenness cancelled out (I hope).

Daniel,  darks must be taken at the same ISO, shutter speed & temperature as your images.  But flats can be taken at any ISO, shutter & temperature as long as your optical train is exactly the same (barlows, flatteners, camera etc).  Point at something very evenly illuminated such as the cloud-free zenith sky around sunset.  New flats are needed for every change in the optical train (and for every new or removed dust speck on your CCD!)

[Ian]

To a point short exposures are good. Five minute flats are going to need dark subtractions