Sky & Telescope - December 2020 - 60

BACKYARD SCIENCE

by Richard Berry

ight from the stars reveals their secrets. Gazing at the
sky may satisfy our romantic impulses. But when we
ask hard questions - What are the stars? How hot?
How old? What are they made of? How far away are they? -
simply gazing at them is not enough. To learn about them, we
need to look at their light analytically.
One way we can answer some of these questions is through
the process of photometry. This is the art and science of measuring brightness radiated by astronomical objects - a pursuit
that a growing number of amateur astronomers is undertaking
from backyards, rooftops, balconies, and observatories all over
the world. As a science, many amateur astronomers today
apply photometry to the study of the most interesting stars:
variable ones that change in brightness. Here's how you can
get involved and contribute valuable science yourself.

To capture an image, the telescope focuses light on the
sensor in the camera. Although CCDs and CMOS work
somewhat differently, the process begins the same way. The
camera clears stray charge from the photodiode elements.
The chip is then ready to receive photons. During the exposure, photons strike the sensor, generating electrons. When
the exposure ends, each photodiode in the array holds a
number of electrons proportional to the number of photons
that fell on it.
The camera then converts these stored electrons into a
digital signal and streams it back to the computer. The computer adds information about the image, such as the width
and height of the array, the date, time, length of exposure,
as well as any other information needed to document the
observation. It then saves the data as a FITS ﬁle (a standard
astronomical ﬁle format).

Analyzing Starlight
These days, we can accurately measure the magnitudes of
stars by recording them with digital cameras equipped with
CCD or CMOS detectors. These detectors consist of an array
of millions of tiny photodiodes packed into a convenient chip
package. The camera body provides a safe and clean environment for the light-sensitive chip. It also contains electronics
to power the chip, scan its array, and return a digital signal
representing the image that fell on the chip.

Calibration Is Key
One fundamental problem we face in making accurate
measurements of starlight is that every photodiode on a sensor is slightly different from every other photodiode. These
differences are small but measurable. Once captured in an
image, the signal coming from a photodiode is called a pixel
(short for "picture element"). As captured in a raw image, a
pixel might be a fraction of a percent more sensitive than its

Measuring the Stars

COUNTING PHOTONS Recording the brightness of stars can be easy, particularly when
using modern equipment. Author Richard Berry
is seen capturing light from distant stars in his
backyard observatory in rural Oregon.

DECEMBER 2 02 0 * SK Y & TELESCOPE

ALL IMAG ES COURTESY OF AU THOR
UNLESS OTHERWISE NOTED

Here's how you can contribute
to the science of astronomy.

Sky & Telescope - December 2020

Table of Contents for the Digital Edition of Sky & Telescope - December 2020