Flat Field Box

Full image calibration is very important to get good results on ccd photography. Flat field can reduce the effect of optical vignetting and dirt. Vignetting is always visible on ccd images, especially when focal reducers are used.
Various techniques exist to get a flat field image:

Dome flats: Only possible if a dome is available.
Sky flats during dusk or dawn: You only have two chances to get good flat field images during one night.
Flats produced by screens illuminated by a torch: Difficult to handle. There are often some light gradients in the flat field image due to non-uniform illumination of the setup.
Flats produce with a flat field box: You need a flat box.

For me, the flat field box technique is the most suitable one. Although the box is somewhat bulky it produces flat fields that are highly uniform and have less artefacts caused by non-uniform illumination.

The construction of a flat box is rather easy: some pieces of thin plywood are screwed and pasted together. On the base side is a circular opening (diameter 35 cm) that is shielded by Plexiglass. This opening fits on the tube of the SCT.

Four 12 V car lamps provide enough light to get flat field images with exposure times of about one second. The lamps are shielded by wooden plates and Plexiglass to avoid light scattering into the tube.

All inner parts of the box are painted white. The size of the box is approx. 45x45x50cm, its weight is approx. 4 kg.

The newest version of my flat box (not shown here) is made from lightweight cupboard (special thanks go to Susanne Buckler for providing this material) and polistyrol plates. The construction is similar to the plywood box. The power lamps have be reduced because of the higher reflection by the cupboard (6V, 0,6 W).

Box with oppening.

12 V car lamp with shield.

Base plate with Plexiglass insert.