Making Flatfields with a photo-flash
We tried all different methods to achieve good flats. Skyflats are difficult because the time-window where the sky is dark enough but still no stars are visible is very small. Even if you move the telescope during the flats you still have strikes from bright stars on the flat. So we tried to find another method. Here is what we came up with:
Use a simple photo-flashlight and a big white sheet. The sheet should be placed between
2 and 10m away from the telescope and should have 2x the diameter of the telescope. Point
the telescope to the middle of the sheet. Place the flash at the front end of the
telescope, facing towards the sheet. Now use an exposure time like 2s. During this
exposure time, use the lightning of the photoflash to illuminate the sheet. The ccd sensor
should be illuminated by 1/3 to 3/4 of the maximum possible pixel value.
If this value is to large (the first experiment may completely overexpose your ccd) use a mask in front of the flashlight (we have a mask of d=5mm made from black tape on our flash, so most of the flash is masked.)
Most flashlights can be flashed by closing a contact so you may even place it in the center of the optical axis (above the secondary) and remotely flash it from your controll PC. This has some advantage because the illumination is even better. Use a normalized median and median-average at least 6 exposures. Use a normalized median because there is some difference in brightness between each flash.
We think that this method works so fine because photo-flashlights give a very uniform illumination, much better that with a normal flashlight.