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Version 1.5 rev 26 Sep
2016
Apogee Astronomy
Alta and Ascent Manual
SECTION 7: USING YOUR APOGEE CAMERA
Digital imaging and image processing forms the core of modern astronomy. Much of what we know about
the structure and behaviour of planets, stars, interstellar clouds, galaxies and the universe that surrounds
us has been gleaned from processing pictures exposed through telescopes. This section outlines the basic
things you can accomplish with your instruments. In the end, you are only limited mainly by your patience,
skill and imagination.
7.1 Standard CCD Image Calibration Procedures
Today, most astronomical images taken in visible light (also known as white light pictures) are produced
using a CCD camera such as your Apogee camera. Although these devices have imaging chips similar to
those in many camcorders and personal digital cameras, the CCDs used in astronomical cameras are
characterized by their low noise and ability to capture the full visual spectrum of light in a repeatable, linear
manner that's easily quantified.
However, raw CCD images of the sky must be corrected for a number of problems produced by the
imaging process.
Every pixel value of a raw image taken through the telescope correlates to the number of electrons
collected by that pixel during the exposure. Ideally, the number of electrons is exactly proportional to
number of photons captured by each pixel for its portion of the sky.
In practice the number of electrons is equal to the number that were freed by photons from the sky
impacting on the pixel
and
by the pixel's thermal agitation or
dark current
. Furthermore, even before the
exposure starts, each pixel value is usually
biased
higher than zero by some amount.
The number of electrons captured is also dependent on the
sensitivity
of each pixel- some pixels are more
sensitive than others across the face of the imaging sensor. The standard procedures used to produce
astronomical images is to correct the raw image for dark current, bias, and sensitivity variations.
7.2 Dark Current Correction
Dark current
fills each pixel with electrons at a steady rate based on the temperature of the CCD chip. The
final number of dark current electrons depends on the temperature of the chip and the length of the
exposure.
The standard way to correct for this type of error is to subtract an image that only contains dark current
from the sky image. The image to be subtracted is made by taking an exposure of the same length with the
chip set to the same temperature as the sky exposure. However, during this exposure, the shutter is closed
so no external light is captured. This exposure is known as the dark image.
Because the amount of dark current errors changes very slowly over time, it is possible to produce a dark
image and use it over and over for several weeks or even months.
