Antilhue - Chile
A flat field frame or "flat" is an image of an evenly illuminated field with the CCD camera's shutter open. It shows the optical imperfections in the imaging system (vignetting, dust particles, reflections, etc) and is applied to the light frame (image of the object) in order to remove these imperfections.
After experimenting with a number of different methods and materials to obtain quality flats I decided upon a light box, mainly for these reasons:
- constant light level: independent of variable reflections coming off of external surfaces.
- repeatibility: the same performance every time
- ease of use: no guesswork or need to time and rush as with sky flats.
- easy set-up and take-down: slides on and off dew cap within seconds.
I made two light boxes, one for my refractors and the other one for the 10" Maksutov-Newton. When I received the 14.5" Ritchey-Chrétien it quickly became clear that handling and storing an even bigger light box in the dome was not going to be practical. Still, I wanted to keep the advantages listed above, so I opted for an "open box" design with the diffuser and light sources on the telescope and the reflector on the dome roof panel - much like professional observatories shoot their flats.
Execution was straightforward. I bought a 4mm thick sheet of white translucent acrylic from a local neon sign shop and cut out a 25 mm wide ring that just clears the telescope tube's front diameter. Eight bright white socketed LEDs were mounted around this ring - using 2 parallel circuits of 4 series wired LEDs - and connected to the pulse width regulated AUX output of the scope's TCC (Telescope Command Center - the black box with the red connectors). The ring is positioned about 25mm below the OTA rim to block direct light from reaching the aperture.
Then I used the "leftover" inside disk of the ring to replace the metal front cover that came with the scope. It serves as a diffuser and at the same time seals the front against intruders (which the original metal cover did not). To achieve this I used a short length of 3 inch PVC water pipe with an end cap and glued it in a matching hole, slightly offset from the acrylic disk's center. This way the protruding focuser servo motor is covered and protected. The piece of pipe also makes it easy to handle the diffuser without touching its surfaces.
Finally, I spray-painted a square section of the clamshell roof panel using a can of flat white paint.
Pointing the scope to the reflecting surface is done by using a memorized Az-El position. Light intensity is set by one of the AUX controls of the TCC. As a result the whole process can be remotely controlled.
Final note: I grew up as a "refractor guy" and I guess that's why I like closed tube assemblies. In order to prevent spider, bee, wasp, etc... entry from the back, I used very short lengths of the same 3" PVC pipe and end caps to close off the fan openings in the backplate.