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Light Box for imaging Flat Field Frames

Making a light box became a necessity when I began subtracting dark frames from my images and then, on careful examination, could still spot other marks that only flat field frames could remove.

I searched the internet but few sites mentioned how they had made their light boxes. Other alternatives to produce a flat frame did not seem appropriate for my use, i.e. dome, twilight or sky flats.

Don Goldman at Astrodon.com gives instructions and a plan for the light box he had made - mine is a near copy of Don's. The light box is a 14" cube for my Meade LX200GPS - note that the OTA for the 10" telescope has a diameter of 12".

I obtained three sheets of foam-board from my local artists shop but could not find a supplier for the translucent plastic until I suddenly realised that most shop-front signs have acres of the stuff - a local sign maker cut me two off-cuts to size for £1 each.

The image to the left shows an edge-on picture of the foam board.
The design puts two squares of white translucent plastic, separated by about 4", at the telescope end of the light box. Cutting the board with  craft knife was easy: the most difficult operation was making the round hole to fit the dew shield which is permanently mounted onto my telescope. I sealed the exposed edges of this aperture with  a coating of glue to stop any foam fragments landing on the objective when the light box is used.

The sides of the box were secured with white 'gaffer' tape; separators and locating supports within the box were glued to the sides to secure the plastic sheets in place.

As you can see from the work-in-progress shots, I left the green protection covers on the plastic sheets till the last moment of assembly.

I adapted the corner templates from the plan in Don's drawing: the angle between the equal sides of equilateral triangle I made between 45^o - 60^o.

Two corner deflectors were glued in place to give the box some rigidity and then the control box was glued to the outside and the cables to the LEDs routed around the outside of the box to their respective corners.

The LEDs were glued through holes to small foam board squares glued below the corner deflectors.

The electronic control box was made from similar parts to that defined by Don on his site, Astrodon.com. I mounted the lot in a plastic box and used telephone cable for the runs to the LEDs. The parts were available in the UK from Maplin Electronics.

I bought a rotary potentiometer with an on/off switch to save a installing separate on/off switch.

The parts list from Maplin Electronics included the following parts:

When the light box interior was complete and the LEDs proved to be functioning correctly, the green protective light plastic sheet was removed from the translucent plastic sheet and the box top was secured in place with white gaffer tape. Effort was made to ensure the box was sealed to prevent dust and dirt from entering.

The box was then mounted onto the telescope - I have secured the light box to the telescope when it is used by a couple of heavy elastic bands from two small coat hooks glued to the top corners of the box to two of the plastic screws used to secure the dew shield to OTA. A further elastic band secures the lower side to the third screw on the dew shield.

Following the instructions in The Handbook of Astronomical Image Processing by Berry & Burnell, I adjusted the potentiometer to half-fill the photosites  on my CCD with a 10 second exposure. I have marked this position on the control box and use it as the standard for my flat field exposures.

An early attempt to use the light box for flat field production showed the tell-tale dirt 'doughnuts'.

I found that exposing for 10 seconds and using 30% of the potentiometer resistance gave me the required 50% photosite saturation levels.

 

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