Light Box 6

updated: 05/28/2007

 

I have experimented with a lot of light boxes seeking ease of use, uniformity of illumination and ultimately acceptable flat field correction.  This note documents the the sixth iteration design that I use with my 12.5" RC truss telescope.

I had a piece of 1/8" thick white plastic or milk plastic cut 16" x 16" and then drilled a 2.75" hole in the center.  The hole is to accommodate the secondary focuser motor for the RC.  This became the base for the light box shown below.

Figure 2: Finished Light Box

The light box is constructed of 1/4" foam core, available in graphics arts and some hobbyist supply stores.  Each side piece is exactly 15" x 15" (4 pieces).  The top is 14.75" x 14.75".  The assembly is held together by a combination of straight pins (to hold initial alignment) and duct tape.  The completed box is attached to the white plastic plate by hot melt glue.

The illumination source and box proportions are key to a successful design with uniform illumination. 

A detailed analysis had indicated most uniform illumination occurs when the distance from the inside top of the box to the LED's is equal to the separation of the LED's.  The concept is that the top of the box acts as a first diffuser and by locating the illumination sources near the corners of the box, the resultant light combines to a uniform level at the plastic diffuser.  The plastic diffuser smoothes out any remaining gradients.  Flat field to better than 0.5% is possible with this design.

The illumination sources were expertly identified by Don Goldman in his light box design.  He recommended using 4 high brightness white LED's, Radio Shack part number 276-0320.  A second key item were the LED holders, Radio Shack part number 276-080.  These holders support the LED below the holder and act as a baffle to prevent the light from directly illuminating the diffuser.

The LED's and holders are mounted near the center of a 1" x 1" square of foam core, glued to the inside corners of the box with holt melt adhesive and located approximately above the plastic diffuser.  They are located around 4.5" above the plastic.  Spacing is not critical except to maintain the dimensional ratio mentioned above.  I poked leads through the side of the foam core (more straight pins) and did the wiring outside of the box.  I then covered the wiring with duct tape.  Figure 3 is the schematic.

Figure 3: Schematic

(Apologies in advance - I don't have a schematic drawing software package.)  A 5000 ohm pot is shown along with a number of 1/4 watt resistors.  The illumination is designed to be powered by up to 13.8 volts and the current is limited by design to 16 mA per LED.

Here is the light box mounted on the scope.

Figure 4: mounted on the secondary support ring.

The light box is not heavy at all and sits easily on the brackets attached to the secondary support ring.  The potentiometer and knob is visible on the side turned fully up. 

Using this light box with my 12.5" F/9 RC and an ST-10XME, I want to get a count of 20,000 ADU average for the flat frames.  The exposure times were between 8.4 and 12.6 sec. for L binned at 1x1 and RGB binned at 2x2.  These LED's have a strong blue output, unlike incandescent lights and that helps keeps the blue flat field exposure time to 12.6 sec.  I found these durations as long enough to avoid any shutter effects and not too long to be onerous.  I generally take 3 flats for each filter to achieve flat SNR's that don't degrade the light frame SNR.  In my case, the potentiometer is fully out of the circuit.

Thanks to Don Goldman for the information and inspiration to build what I hope will be my final light box - at least for a while. 

As a test, I decided to image NGC4438 and NGC4435 (The Eyes).  I found the faint detail easy to bring out and very little color non-uniformity in the background.

I hope this gives you some ideas to build one of your own.

 

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