Flat Field Light Box
 

Project Goals

This project is construction of a flat field light box for my C8 using a plastic bucket and a LED lamp. The project goals were to construct the light box for less than $20 using only several hours construction time.


Background

Flat field images compensate for uneven illumination due to dust or imperfections in an optical system.  Flat field images are acquired by placing a light box over the telescope objective lens and acquiring an image of the distortions and dust effects in  the optical path. The flat field images are digitally subtracted during image processing to remove the distortions from the telescope optics. Flat field images are acquired with even illumination over the entire objective lens and the imaging camera must be in the same orientation as during imaging. The exposure time is adjusted to give average pixel values at 0.25% - 0.33% of the imager saturation level.

Flat field light boxes direct a light source through a series of diffusers to evenly distribute the light beam across the exit diffuser (pointed at the telescope objective). Typical designs include: light bulbs, LEDs, or electroluminescence technology contained in cardboard or foam-core boxes. The light source can be orientated to shine directly onto the diffusers or indirectly, by reflecting the light off the light box back wall. The light diffusers can be constructed from many types of semitransparent white material: paper, plastics, etc.

Materials

Construction required only a single afternoon and I used the following materials for this project.
  1. 3-LED push button light
  2. Large white plastic bucket with lid
  3. 2 extra bucket lids for fabricating the light diffusers
  4. Tracing paper
  5. Tape
  6. 2 cm square dowel rod
  7. Felt table leg pads  

Light Box and Paper Light Diffusers

The light box was constructed form a large plastic bucket. I cut a circle in the end of the bucket with a slightly larger diameter than the C8 corrector plate, but smaller than the outer diameter of the metal end ring. I placed the bucket over the C8 and verified that the opening was slightly wider than the corrector plate and that the bucket would still sit on top of the C8 (below left photo). The end of the bucket was covered with a light diffuser, cut from a single sheet of tracing paper and taped to the bucket inside (below right photo).



The light diffusers are plastic strips, formed into hoops, that hold tracing paper diffusers in the light path (below photo). I cut strips from the outer perimeter of spare bucket lids and formed them into hoops, secured with electrical tape. The plastic hoop diameter is adjustable, so it's easy to test different diffuser placements in the light path. Two layers of tracing paper were taped into each hoop; additional layers can be added/removed to adjust the light penetration.
 

Since the bucket is tapered, the two light diffusers just wedge in place inside the bucket (below photos). Adjusting the hoop diameter determines where the diffuser sits. The top and bottom light diffusers have larger and smaller diameters (below left and right photos, respectively).        



LED light and Holder

The LED light source is a commonly available 3-LED push button light (below left photo). Pressing the clear plastic LED cover activates the LED. I fabricated a mount to hold the LED,  ensuring there was no chance of the LED light falling through the light box and damaging the C8 corrector plate. The LED sits inside 3 wood blocks and is covered by a scrap of plastic bucket lid (below photos). The plastic holder is covered in aluminum tape to reflect stray light, and is attached with M5 machine screws and lock washers. An external light switch was not necessary: the light is activated by removing the bucket lid, pressing the LED light, and replacing the bucket lid.



Illuminated Light Box

The below photos show a first test illumination. The top photos show the light diffusers inside the bucket and their effect on the light intensity. The below photo shows the end diffuser; the illumination is very uniform across the final light diffuser and the individual LED lights can't be resolved. 

I Wrote a simple program to extract the RGB (red-green-blue) pixel values from a photograph of the light box end diffuser. The below left photo shows a screen shot of the program. Wherever I place the cursor on the photograph, the program returns the RGB values and calculates the pixel intensity (the square root of the sum of squares of the RGB values). The red circles show where I measured the RGB values. The below right photo shows the intensity data placed on the photograph.


C8 Centering Bracket

I fabricated a bracket to simplify positioning the light box over the C8 (below photo). The felt pads on the dowels prevent scratching the C8 optical tube and the plastic ring (cut from a bucket lid) prevents the dowels from sliding inside the C8 OTA and damaging the corrector plate.




The Completed Light Box and C8

The below photos show the completed light box on the C8. The below right photo shows the illuminated light box.





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