Specifications

AUTOSTAR CCD PHOTOMETRY 91
APPENDIX H
Lightbox Design and Construction
This section briefly discusses the need for flat fields, alternate
methods of imaging flat fields, and the design and construction of
a light box.
Flat Fields
The purpose of taking images of a uniformly illuminated field is to
permit calibration of the photometry images, by eliminating
artifacts due to uneven response across the CCD chip, pixel
defects, and shadows caused by out–of–focus dust particles and
other debris ("dust doughnuts") in the optical train. Examination of
an image taken of an evenly–illuminated field readily shows up
these inevitable problems. The so–called "flat field image(s) are
algebraically subtracted from the data images during calibration
through the software. The technical challenge is in achieving a
uniformly illuminated field, and a proper exposure. The exposure
issues are discussed elsewhere.
There are several methods used in obtaining flat fields, in
increasing order of uniformity and consistency:
(4) Sky Flats – Flat Field images may be made during twilight, as
the sky either grows darker or lighter. Difficulties to be overcome
include gradients in sky brightness, presence of star images, and
the short time period during which the sky illumination is useful,
and remains relatively constant.
(3) Dome Flats – Images may be taken of a suitably placed flat
white surface, illuminated indirectly. This is frequently used by
professionals with large telescopes, enclosed in a dome (or other
enclosure). A flat panel is mounted on the inside of the dome. A
white panel could be used for portable use, provided the
illumination is even, and directed from the side. Care must be
taken not to allow extraneous light to enter the telescope directly.