Imaging Equipment - Light Box
In order to calibrate images and remove uneven illumination and dust motes it is necessary to apply flat frames (flats). Flats need to be shot against evenly illuminated unfocused object. Common techniques used are: shooting sky at dusk or dawn, imaging illuminated wall, imaging a lamp through some kind of fabric (white t-shirt) or using a light box. Light box is a box with an internal source of even light, one side with a circular opening with some kind of semi-transparent screen. The box is attached to the telescope and acts as evenly illuminated object far out of focus. It is an ideal do-it-yourself project (like Bahtinov mask), but of course there are commercial light boxes available.
I built my own light box from a foam core poster board, sides and back from .25in boards, the front (rigid, with an opening) from three .5 in boards glued together. Connections were made with hot glue gun and white duct tape. The light source is 4 flashlight bulbs in sockets from RadioShack, connected serially. Yeah, I know, when one bulb fails, all stop shining, but with one out the light box is not evenly illuminated anyway, so either way it has to be fixed. The wires go outside and are soldered to a 9V battery adapter. The bulbs are positioned on the inside of the front panel in the corners; in order to prevent them shining directly on the front opening four rectangular light baffles few inches tall are attached across each corner directing the light upwards to the back panel. This way the light spreads out inside the box and is very even since before reaching the from opening it reflects from the sides and back panels. Front opening is closed with a fabric glued between 2nd and 3rd part of the from panel, I just cut out a piece from a white bed sheet.
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Originally my light box was designed for C14, later I added a second, removable front panel cut for O10. The removable front panel is attached with Velcro to the outside of the original C14 front panel.
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Over time the lightbox served me well, but sometimes the resulting flats didn't scale well in Nebulosity resulting in slight over- or under-correction. This happened only when applying them to color images, so I figured out the bulbs produce too reddish light. It turned out to be true - histogram shows three distinct peaks, the red being the strongest ... I have retrofitted the lightbox with 4 white LED light sources: white LED clusters (LC26TW8D, 12V) from LC-LED. The original 12V 4-bulb circuit was left in place since removing it would cause more damage than it is worth. I used this opportunity to make some pictures of the internal structure of my lightbox, see pictures below.
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