Light Box For Flat Calibration Frames

Late last summer while taking an image of M45 from my backyard, I noticed a lot of dust motes in my images. After carefully cleaning all of my optics, I realized that the dust was in fact inside the camera body.  Getting the dust out was to be a whole other project on its own and another article for later, but in the meantime I decided to make myself a light box that I could use to acquire flat calibration frames in the middle of the night.  I’d then be able to use these flats to remove any unwanted “dust spots” in my pictures. I initially looked into getting electroluminescent panels that were compact and generated a homogenous source of light that closely resembled light seen by a telescope at twilight.  They were rather expensive at the time though so I decided to make something of my own that would generate this even light source.  The problem however, was that any light source I could think of was more or less a point source of light.  I needed a way to diffuse this light so that by the time it reached the telescope it appeared to be an even source of light over the entire area seen by the telescope.

The key idea came one day when I was given a monitor that had stopped working and was asked to repair it.  While opening it up, I discovered that the back light of the LCD panel uses a thin white film-like piece of plastic to diffuse light generated by two fluorescent tubes behind it.  Bingo!  I figured tracing paper ought to do the same trick, so I went about designing a box using corrugated plastic.  I put it together as can be seen in the image to the right. A few pieces of plastic were cut to form the outside of the light box. For the end that would fit over the telescope, I cut a circle into two of the squares I had cut out.  The circle had to have the same diameter as the outside diameter of my Explore Scientific 127mm telescope.  These two pieces would allow the whole box to fit snugly onto the telescope.  Next I cut a large square into four more squares of plastic and glued a sheet of tracing paper to each side of the resulting plastic frame and then spaced the frames evenly in the box. I chose to use a sheet of tracing paper on both sides of each frame since it diffuses the light better.

Now that the main portion of the box was complete, I started construction on the light source itself.  I found an old LED light in my things and decided to use it.  By placing the light near the end of the light box and observing how much it was diffused from the other end, I realized the box had to be longer.  I constructed a smaller light diffusing box out of a few more pieces of plastic and added in a few layers of tracing paper. This box would be attached to the main box.  It just so happened that the pieces of plastic were scraps from a project used to calibrate a printer.  I found it to be a colorful touch and decided to face the printed sides outward.  I glued the light to the last piece of tracing paper and glued the battery compartment of the LED light horizontally onto the box.  I figured it would be more secure this way, and the end of the compartment had a switch on it anyways so it was useful.

The extra “rainbow” box attached to the main light diffusing box did the trick and the light generated by the LED light was then diffused enough by the time it reached the telescopes objective that it was useful in taking flat calibration frames.