Quote:
|
Originally Posted by Striker
call me stupid but i still dont understand the principal's behind Flats and how to do them with a t'shirt.
Everyone says their easy???
So all I have to do is to throw a white t'shirt over the end of my scope and take some shots..firstly for how long and what settings...what do I do with each shot after this?????? and how do apply them to my images.??
|
Can I really call you stupid?
OK here goes. This is what I do: (for my DSLR)
Set up the scope with the same optical train including focus and camera orientation. I usually use a naturally lit room with the scope pointing at the ceiling. Stretch a white cloth over the objective. Choose a low ISO and adjust shutter speed so as to get a histogram spike 1/3 to 1/2 way along the x axis (when viewed on the cameras LCD). I then take about 15 shots, rotating the tube as I go to account for any uneven illumination in the room. These are then converted, average combined and desaturated. The master flat is now ready for use. It is a picture of the optical defects in your system and it is used to correct those defects in your light frames.
You then need to use software designed for astro image processing to apply the flat to your light frames. The actual process is something like this: An average pixel value of the flat frame is calculated. Then each pixel in the light frame is divided by the corresponding pixel in the flat frame and then multiplied by the average value. In effect, each pixel in the light frame is multiplied by a value close to one. If the pixel in the flat is darker than the average flat pixel, this value will be larger than one, resulting in a brightening of the pixel in the light frame. If the pixel in the flat is lighter than the average this value is less than one, resulting in a darkening of the pixel in the light frame. What you end up with is an evenly illuminated light frame.
This process can be "simulated" in Photoshop, but photoshop doesn't actually do the same "division" described above. There are various packages that will do it correctly such as ImagesPlus, IRIS (free), Keiths image stacker etc.
Hope this helps.