Flat field light box constructed from electro-luminescent panel
For several years I managed to use an evening sky as the target for taking flat field exposures. At the time, the prospect of constructing a light box using conventional bulbs did not appeal to me. However, the introduction of electro-luminescent panels has made constructing one a lot simpler.
My EL panel light box is constructed from a "low cost" A4 size EL panel purchased from Earlsmann. The panel is supplied with a control unit, which can be powered from a 12v DC supply via a standard 2.5mm power plug. I placed an A4 size piece of frosted perspex in front of the panel. This provides protection to the EL panel and also acts as a diffuser. The EL panel and perspex were then placed between two sheets of metal, one of which has a nosepiece attached to it. As you can see from the photographs, I simply taped the whole thing together and fixed some self-adhesive felt to the nosepiece to prevent it scratching the telescope.
Fortunately I have a friend who works in engineering and he had the metal plates and nosepiece made for me. An alternative and inexpensive way of achieving the same result would be to sandwich the EL panel between two sheets of A4 perspex (readily available on Ebay) and then have a piece of 3" or 4" thick foam professionally cut to be A4 in size with a hole in the centre to suit the diameter of the scope. The foam can then be glued to the front piece of perspex and it will make a very good sleeve to fit over the scope.
Electroluminescent panels are quite bright, resulting in very short exposure times for flat field exposures, which may not be suitable for CCD cameras with a mechanical shutter. I would suggect using either light reduction film or tinted perspex in front of the EL panel to reduce the brightness.
I should add that I find my EL panel is low in narrowband light emission, i.e. .Ha, OIII and SII. Ths is not a problem as such, it just means using longer exposure times for narrowband flats.