I have just recently completed building myself a couple of light boxes and I am very pleased with the results. If anyone is interested in me building one for them, PM me and I can discuss price etc, one thing is for sure, they are going to be at least a third of the price of imported ones, and IMHO work just as well. The two attached images will give you some sort of idea of my construction. My method of construction allowes the light box to fit over the front of the telescope/dew shield, much like a cover so one does not need to balance it on the scope.
I use a 12v lighter type connector for the power, and each LED is rated at between 6000mcd and 12000mcd depending on the size of the box required.
If interested please feel free to ask any questions.
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For all those who use a light poloution filter such as the sls clip, I use one fo my flats taken with this box, as a custom white balance for taking shots and it works a treat, it's not perfect but it's better than nothing.
This is a brief description of what flat frames are all about.
Flat frames are essential if the faintest parts of a target are to be detected. The basic concept is to point the system to a uniformly illuminated light source and take an exposure. The resultant exposure will capture all of the imperfections in the system - vignetting, dust on the optical surfaces, off-axis optics and other disturbances.
The imperfections etc are then removed in stacking the flats with the other frames. A pretty rough explanation but I think you should get the general gist.
Some people use a white T shirt the next day to achieve this, but it has its limitations. It is much better to take the flats at the same time as the primary image as nothing in the image train should be changed before taking flats. You can then move onto the next image, move your focuser etc without hassles as you already have flats prepared for that particular image.