Flat frames

[Astroboy239]

Can someone please help me to get the flat frames in the right way? 

 

[swag72]

This is a good read about all calibration frames http://www.rawastrodata.com/pages/typesofimages.html

 

[alexbb]

I don't know if letting the camera decide the exposure for each frame is a good idea. I let it decide for the evaluation then I set it to manual so all the next flat frames have the same exposure time. The histogram should get to around 50% level, a little less maybe.

How I take them, I put a white T shirt in front of the scope, tie it around the tube so that no folds are in front of the light path and then I put a white image on my TV, make the room as dark as possible and then put the scope in front of the TV, right in front of it and shoot about 30 frames.

Make sure that focus is kept the same and more important that the camera is not rotated after lights are taken.

The idea behind using flat frames is that the flat master (the result after combining all flat frames) is stretched in order that the lightest point will have a value of 1 (0..1 interval) of lightness, the darkest point will have a proportional value and the light frames are divided with the flat's values.
Ie. the lightest point (in center somewhere) has a lightness value of 0.8 and the darkest point (somewhere on the edges) has a value of 0.4. Then the lightest point will be amplified to 1 and the darkest to 0.5. All the pixels' values in the light stacked frame will be then divided with the pixels' values in the master flat on the same position.
All these operations can be automatically done if you're using DSS.
 

Hope this helps you,

Alex