This project is
construction of a flat field light box for my C8 using a
and a LED lamp. The project goals were to construct the light box for
less than $20 using only several hours construction time.
Flat field images
compensate for uneven illumination due to dust or imperfections in an
optical system. Flat field images are acquired by placing a
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.
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,
required only a single afternoon and I used the
following materials for this project.
- 3-LED push button light
white plastic bucket with lid
extra bucket lids for fabricating the light diffusers
paper or white plastic
- Tape or silicone adhesive
cm square dowel rod
table leg pads
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
plate, but smaller than the outer
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).
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. I later
replaced the tracing paper with white plastic cut from a garbage bag. I
secured the plastic to the light diffusers with silicone adhesive.
bucket is tapered, the two light diffusers
just wedge in place inside the bucket (below photos).
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).
light and Holder
The LED light source is a commonly available
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
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.
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
the end diffuser; the illumination is very uniform across the final
light diffuser and the individual LED lights can't be
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.
I fabricated a bracket to simplify
light box over the C8 (below photo). The felt pads on the
dowels prevent scratching the C8 optical tube and the plastic
(cut from a bucket lid) prevents the dowels from sliding inside the C8
OTA and damaging the corrector plate.
Completed Light Box and C8
The below photos show the completed light
the C8. The below right photo shows the illuminated light box.